The photo was taken in january 2004, at Parque Estadual do Rio Preto (Black River State Park), in the Cerrado (savanna-like) of northeastern-central Minas Gerais, Brazil.
The plant was herbaceous/shrub, but maybe it was because of its tender age?
Aflatoxins: Occurrence, Properties and Controls | Plant Diseases
In this article we will discuss about: 1. Introduction to Aflatoxins 2. Occurrence and Distribution of Aflatoxins 3. Fluorescence Production 4. Properties 5. Health Hazards 6. Factors Favouring Aflatoxin Production 7. Analysis for Aflatoxins in Foods and Feeds 8. Control and Management 9. Biological Control.
- Introduction to Aflatoxins
- Occurrence and Distribution of Aflatoxins
- Fluorescence Production of Aflatoxins
- Properties of Aflatoxins
- Health Hazards Caused by Aflatoxins
- Factors Favouring Aflatoxin Production
- Analysis for Aflatoxins in Foods and Feeds
- Control and Management of Aflatoxins
- Biological Control of Aflatoxins
1. Introduction to Aflatoxins:
Mycotoxin (Gk. mykes, mushrooms-fungus) is a toxin produced by an organism of the fungus includes mushrooms, moulds and yeasts. Aflatoxins are naturally occurring mycotoxins that are produced as secondary metabolites by some strains of Aspergillus flavus and A. parasiticus, plus related species A. nomius and A. niger on a varity of food stuff causing health hazards to animals consuming them.
2. Occurrence and Distribution of Aflatoxins:
Aflatoxin in swine has been reported in Georgia USA in 1940. The death of swine was traced as a result of feeding mould maize. Similar incident occurred in 1950 at Alabama. In 1960 more than 100,000 young turkeys on poultry farms in England died in the course of a few months from an apparently new disease that was termed Turkey X disease.
It was soon found that the difficulty was not limited to turkeys. Ducklings and young pheasants were also affected and heavy mortality was experienced. A careful survey of the early outbreaks showed that they were all associated with feeds, namely Brazilian peanut meal.
An intensive investigation of the suspect peanut meal was undertaken and it was quickly found that this peanut meal was highly toxic to poultry and ducklings with symptoms typical of Turkey X disease.
Speculations made during 1960 regarding the nature of the toxin suggested that it might be of fungal origin. In fact, the toxin-producing fungus was identified as Aspergillus flavus in 1961 and the toxin was given the name Aflatoxin by virtue of its origin (A. flavus > Afla).
This discovery has led to a growing awareness of the potential hazards of these substances as contaminants of food and feed causing illness and even death in humans and other mammals.
In India Aflatoxin levels were found in range of 1000 to 5000 ppb (parts per billion) in 12% of groundnut samples in 1965 during a study conducted in Andhra Pradesh, 50% of groundnut cake samples were positive for aflatoxin in 1976 in Madhya Pradesh, similarly a study conducted by National Institute of Nutrition (N.I.N) (Hyderabad) recorded aflatoxin contamination in maize grain up to 1560 ppb which resulted death of 100 people by acute hepatitis in Tribal belts in western India in 1986.
Biology of Aspergillus flavus and Aspergillus Parasiticus:
The two fungi A. flavus Link ex Fr. and A. parasiticus Spear, are closely related and grow as saprophytes on organic material left on and in the soil. They are distributed worldwide, with a tendency to be more common in countries with tropical climates that have extreme ranges of rainfall, temperature and humidity.
Members of the genus Aspergillus are characterized by the production of non-septate conidiophores, which are quite distinct from hyphae and which are swollen at the top to form a vesicle on which numerous specialized spore-producing cells, known as phialides or sterigmata are borne either directly (uniseriate) or on short outgrowths known as metulae (biseriate).
Some time difficulty may arise especially to determine because the primary sterigmata are tiny and are easily obscured by spores or other sterigmata. Colonies of A. flavus are green- yellow to yellow-green or green on Czapek’s agar. They usually have biseriate sterigmata reddish-brown sclerotia are often present, conidia are finely roughened, variable in size and oval to spherical in shape.
Colonies of A. parasiticus dark green on Czepak’s agar, remain green with age. Sterigmata are uniseriate, sclerotia are usually absent conidia are coarsely echinulate, uniform in shape, size and echinulation. There are about 18 different types of aflatoxins identified major members are four plus two additional metabolic products, designated as B1 B2, G1 G2, and M2.
3. Fluorescence Production of Aflatoxins:
The aflatoxins fluorescence strongly in ultraviolet light (365 nm). Designation of aflatoxins B1, and B2 resulted from the exhibition of blue fluorescence of the relevant structures under ultraviolet light, G1 and G2 form yellow green fluorescence under ultraviolet light.
While M1 and M2 were first isolated from milk of lactating animals fed aflatoxin preparations. A. flavus typically produces B1 and B2, whereas A. parasiticus produce G1 and G2 as well as B1 and B2. Four other aflatoxins M1, M2, B2A and G2A (Fig. 20.4) were isolated from cultures of A. flavus and A. parasiticus.
4. Properties of Aflatoxins:
Studies revealed (Table 20.1) that aflatoxins are produced primarily by some strains of A. flavus and by most, if not all, strains of A. parasiticus, plus related species, A. nomius and A. niger. Moreover, these studies also revealed that there are four major aflatoxins: B1, B2, G1, G2 plus two additional metabolic products, M1 and M2, that are of significance as direct contaminants of foods and feeds.
The aflatoxins M1 and M2 were first isolated from milk of lactating animals fed aflatoxin preparations hence, the M designation. Whereas the B designation of aflatoxins B1 and B2 resulted from the exhibition of blue fluorescence under UV-light, while the G designation refers to the yellow- green fluorescence of the relevant structures under UV-light.
These toxins have closely similar structures and form a unique group of highly oxygenated, naturally occurring heterocyclic compounds. Food products contaminated with aflatoxins include cereal (maize, sorghum, pearl millet, rice, wheat etc.), oilseeds (groundnut, soybean, sunflower, cotton), spices (chilies, black pepper, coriander, turmeric, zinger), tree nuts (almonds, pistachio, walnuts, coconuts) and milk.
Aflatoxins are potent toxic, carcinogenic, mutagenic, immunosuppressive agents, produced as secondary metabolites by the fungus Aspergillus flavus and A. parasiticus on variety of food products. Among 18 different types of aflatoxins identified, major members are aflatoxin B1 B2, G1 and G2 (Fig. 20.4). Aflatoxin B1 (AFB1) is normally predominant in amount in cultures as well as in food products.
Pure AFB1 is pale-white to yellow crystalline, odorless solid. Aflatoxins are soluble in methanol, chloroform, acetone, acetonitrile. Aflatoxin M1 and M2 are major metabolites of aflatoxin B1 and B2 respectively, found in milk of animals that have consumed feed contaminated with aflatoxins.
Aflatoxins are normally refers to the group of difuranocoumarins and classified in two broad groups according to their chemical structure the difurocoumarocyclopentenone series (AFB1, AFB2, AFB2A, AFM1, AFM2, AFM2A and aflatoxicol) and the difurocoumarolactone series (AFG1, AFG2, AFG2A, AFGM1, AFGM2, AFGM2A and AFB3).
The aflatoxins display potency of toxicity, carcinogenicity, mutagenicity in the order of AFB1 > AFG1 > AFB2 > AFG2 as illustrated by their LD50 values for day-old ducklings.
Structurally the dihydrofuran moiety, containing double bond, and the constituents liked to the coumarin moiety are of importance in producing biological effects. The aflatoxins fluorescence strongly in ultraviolet light (ca. 365 nm) B1 and B2 produce a blue fluorescence where as G1 and G2 produce green fluorescence.
5. Health Hazards Caused by Aflatoxins:
In 1967, twenty people of Taiwan became ill with apparent food poisoning due to moldy rice that was later confirmed to contain Aflatoxin B1. In 1978 Significant levels of aflatoxin were found in the livers 23 of children who had died of Rye’s syndrome in Thailand. Children who died in Czechoslovakia and New Zealand in 1977 have also been found to have aflatoxin in their liver at autopsy.
The out break of hepatitis that effected 400 people in western states of India in 1974 of whom 100 died by consuming corn contaminated with A. flavus containing about 15 mg/kg aflatoxin. Aflatoxicosis is primarily a hepatic disease. The susceptibility of individual animals to aflatoxins varies considerably depending on species, age, sex, and nutrition.
In fact, aflatoxins cause liver damage, decreased milk and egg production, recurrent infection as a result of immunity suppression (e.g., salmonellosis), in addition to embryo toxicity in animals consuming low dietary concentrations. While the young of a species are most susceptible, all ages are affected but in different degrees for different species.
Clinical signs of aflatoxicosis in animals include gastrointestinal dysfunction, reduced re-productivity, reduced feed utilization and efficiency, anemia, and jaundice. Nursing animals may be affected as a result of the conversion of aflatoxin B1 to the metabolite aflatoxin M1 excreted in milk of dairy cattle.
Fig. 20.6. A rat live fed with high doses of aflatoxins B1. Notice the induced tumors in the liver.
The induction of cancer by aflatoxins has been extensively studied. Aflatoxin B1, aflatoxin M1 and aflatoxin G1 have been shown to cause various types of cancer in different animal species.
However, only aflatoxin B1 is considered by the International Agency for Research on Cancer (IARC) as having produced sufficient evidence of carcinogenicity in experimental animals to be identified as a carcinogen (Fig. 20.5 and 20.6).
Thus aflatoxins were proved to be potent toxic .carcinogenic, mutagenic, immunosuppressive agents. Their toxicity is illustrated by their LD50 values for day old ducklings (Table 20.2).
Effect of A. Flavus and Aflatoxins Contamination:
Deteriorate in grain quality due to A. flavus growth and become unfit for marketing and consumption. In groundnut, seed and non-emerged seedling decay and afla-root disease was observed due to fungus attack. Aflatoxins contamination in grain poses a great threat to human and livestock health as well as international trade.
According to FAO estimates, 25% of the world food crops are affected by mycotoxins every year. And also crop loss due to aflatoxins contamination costs US producers more than $100 million per year on average including $ 26 millions to peanuts ($ 69.34/ha).
6. Factors Favouring Aflatoxin Production:
Fungal growth and aflatoxin contamination are the consequence of interactions among the fungus, the host and the environment. The appropriate combinations of these factors determine the infestation and colonization of the substrate, and the type and amount of aflatoxin produced.
However, a suitable substrate is required for fungal growth and subsequent toxin production, although the precise factor(s) that initiates toxin formation is not well understood.
Water stress, high-temperature stress, and insect damage of the host plant are major determining factors in mold infestation and toxin production. Similarly, specific crop growth stages, poor fertility, high crop densities, and weed competition have been associated with increased mold growth and toxin production.
Aflatoxin formation is also affected by associated growth of other molds or microbes. For example, pre-harvest aflatoxin contamination of peanuts and corn is favored by high temperatures, prolonged drought conditions, and high insect activity while post harvest production of aflatoxins on corn and peanuts is favored by warm temperatures and high humidity.
7. Analysis for Aflatoxins in Foods and Feeds:
Sampling and Sample Preparation:
Sampling and sample preparation remain a considerable source of error in the analytical identification of aflatoxins. Thus, systematic approaches to sampling, sample preparation, and analysis are absolutely necessary to determine aflatoxins at the parts-per-billion level.
In this regard, specific plans have been developed and tested rigorously for some commodities such as corn, peanuts, and tree nuts sampling plans for some other commodities have been modeled after them. A common feature of all sampling plans is that the entire primary sample must be ground and mixed so that the analytical test portion has the same concentration of toxin as the original sample.
Thin layer chromatography (TLC), also known as flat bed chromatography or planar chromatography is one of the most widely used separation techniques in aflatoxin analysis. Since 1990, it has been considered the AOAC official method and the method of choice to identify and quantitative aflatoxins at levels as low as 1 rg/g (Table 20.3). The TLC method is also used to verify findings by newer, more rapid techniques.
Liquid chromatography (LC) is similar to TLC in many respects, including analytic application, stationary phase, and mobile phase. Liquid chromatography and TLC complement each other.
For an analyst to use TLC for preliminary work to optimize LC separation conditions is not unusual. Liquid chromatography methods for the determination of aflatoxins in foods include normal-phase LC (NPLC), reversed-phase LC (RPLC) with pre- or before-column derivatization (BCD), RPLC followed by post column derivatization (PCD), and RPLC with electrochemical detection.
Thin layer chromatography and LC methods for determining aflatoxins in food are laborious and time consuming. Often, these techniques require knowledge and experience of chromatographic techniques to solve separation and interference problems.
Through advances in biotechnology, highly specific antibody-based tests are now commercially available that can identify and measure aflatoxins in food in less than 10 minutes.
These tests are based on the affinities of the monoclonal or polyclonal antibodies for aflatoxins. The three types of immunochemical methods are radioimmunoassay (RIA), enzyme-linked immunosorbent assay (ELISA), and immunoaffinity column assay (ICA).
Manonmani (2006) developed a rapid method for assessment of aflatoxigenic fungi in food using an indigenously designed primers for aflatoxin regulatory gene aflR in PCR shows positive amplification with DNA of aflatoxigenic A. flavus and A. parasiticus can detect up to 100 cfu of target toxigenic fungi.
8. Control and Management of Aflatoxins:
Aflatoxins are considered unavoidable contaminants of food and feed, even where good manufacturing practices have been followed. The FDA has established specific guidelines on acceptable levels of aflatoxins in human food and animal feed by establishing action levels that allow for the removal of volatile lots from commerce.
The action level for human food is 20 ppb total aflatoxins, with the exception of milk which has an action level of 0.5 ppb for aflatoxin M1. The action level for most feeds is also 20 ppb.
However, it is very difficult to accurately estimate aflatoxins concentration in a large quantity of material because of the variability associated with testing procedures. No presentable data is available to form an experimentally valid basis for regulation of aflatoxin levels in food and feed hence, the true aflatoxin concentration in a lot cannot be determined with 100% certainty.
In US and Europe aflatoxins are only mycotoxins jurisdictionally regulated (Table 20.2, and Table 20.3). In Asian countries regulation is primarily introduced to protect export market. On the other hand domestic regulatory measures on aflatoxin received little attention. In India mycotoxin legislation have been introduced but its implementation is inadequate.
Because aflatoxin contamination is unavoidable, numerous strategies for their detoxification have been proposed. These include physical methods of separation, thermal inactivation, irradiation, solvent extraction, adsorption from solution, microbial inactivation, and fermentation. Chemical methods of detoxification are also practiced as a major strategy for effective detoxification.
Structural Degradation Following Chemical Treatment:
A diverse group of chemicals has been tested for the ability to degrade and inactivate aflatoxins. A number of these chemicals can react to destroy (or degrade) aflatoxins effectively but most are impractical or potentially unsafe because of the formation of toxic residues or the perturbation of nutrient content and the organoleptic properties of the product.
Two chemical approaches to the detoxification of aflatoxins that have received considerable attention are ammoniation and reaction with sodium bisulfite.
Many studies provide evidence that chemical treatment via ammoniation may provide an effective method to detoxify aflatoxin-contaminated corn and other commodities. The mechanism for this action appears to involve hydrolysis of the lactone ring and chemical conversion of the parent compound aflatoxin to numerous products that exhibit greatly decreased toxicity.
On the other hand, sodium bisulfite has been shown to react with aflatoxins (B1, G1 and M1) under various conditions of temperature, concentration, and time to form water-soluble products.
Modification of Toxicity by Dietary Chemicals:
The toxicity of mycotoxins may be strongly influenced by dietary chemicals that alter the normal responses of mammalian systems to these substances.
A variable array of chemical factors, including nutritional components (e.g., dietary protein and fat, vitamins, and trace elements), food and feed additives (e.g., antibiotics and preservatives), as well as other chemical factors may interact with the effects of aflatoxins in animals.
Alteration of Bioavailability by Aflatoxin Chemisorbents:
A new approach to the detoxification of aflatoxins is the addition of inorganic sorbent materials, known as chemisorbents, such as hydrated sodium calcium aluminosilicate (HSCAS) to the diet of animals. HSCAS possesses the ability to tightly bind and immobilize aflatoxins in the gastrointestinal tract of animals, resulting in a major reduction in aflatoxin bioavailability.
9. Biological Control of Aflatoxins:
Ciegler (1966) after screening about 1000 microorganisms found Flavobacterium aurantiacum NRRL B-164 ability to remove aflatoxin B1 form solution.
Milk corn oil and peanut butter were artificially contaminated with 600, 700 and 700 micro gram of aflatoxin B1 respectively when viable cellos of Flavobacterium aurantiacum were added to each of these foodstuffs aflatoxin concentration were reduced to about 0.
When viable cells were mixed with soybean, corn and peanut seeds contaminated with aflatoxin were completely removed from corn and peanut and about 86% from soybean. Displacement of toxigenic strains of A. flavus with a toxigenic strains is one of the effective strategy.
This strategy is possible because of great variability of phenotypes of A. flavus in agricultural fields and common occurrence of a toxigenic strains. Use of bio-fungicides can be a better choice for controlling aflatoxin fungi.
Neem extracts have been reported to block aflatoxin biosynthesis up to 98% (Bhatnagar and McCormic, 1988). Essential oils of Thymus eriocalyx and Gracinia indica (extract) have been reported inhibitory against aflatoxin producing fungi and also inhibit aflatoxin production.
The cassava root is long and tapered, with a firm, homogeneous flesh encased in a detachable rind, about 1 mm thick, rough and brown on the outside. Commercial cultivars can be 5 to 10 centimetres (2 to 4 inches) in diameter at the top, and around 15 to 30 cm (6 to 12 in) long. A woody vascular bundle runs along the root's axis. The flesh can be chalk-white or yellowish. Cassava roots are very rich in starch and contain small amounts of calcium (16 mg/100 g), phosphorus (27 mg/100 g), and vitamin C (20.6 mg/100 g).  However, they are poor in protein and other nutrients. In contrast, cassava leaves are a good source of protein (rich in lysine), but deficient in the amino acid methionine and possibly tryptophan. 
Wild populations of M. esculenta subspecies flabellifolia, shown to be the progenitor of domesticated cassava, are centered in west-central Brazil, where it was likely first domesticated no more than 10,000 years BP.  Forms of the modern domesticated species can also be found growing in the wild in the south of Brazil. By 4,600 BC, manioc (cassava) pollen appears in the Gulf of Mexico lowlands, at the San Andrés archaeological site.  The oldest direct evidence of cassava cultivation comes from a 1,400-year-old Maya site, Joya de Cerén, in El Salvador.  With its high food potential, it had become a staple food of the native populations of northern South America, southern Mesoamerica, and the Taino people in the Caribbean islands, who grew it using a high-yielding form of shifting agriculture by the time of European contact in 1492.  Cassava was a staple food of pre-Columbian peoples in the Americas and is often portrayed in indigenous art. The Moche people often depicted yuca in their ceramics. 
Spaniards in their early occupation of Caribbean islands did not want to eat cassava or maize, which they considered insubstantial, dangerous, and not nutritious. They much preferred foods from Spain, specifically wheat bread, olive oil, red wine, and meat, and considered maize and cassava damaging to Europeans.  The cultivation and consumption of cassava were nonetheless continued in both Portuguese and Spanish America. Mass production of cassava bread became the first Cuban industry established by the Spanish.  Ships departing to Europe from Cuban ports such as Havana, Santiago, Bayamo, and Baracoa carried goods to Spain, but sailors needed to be provisioned for the voyage. The Spanish also needed to replenish their boats with dried meat, water, fruit, and large amounts of cassava bread.  Sailors complained that it caused them digestive problems.  Tropical Cuban weather was not suitable for wheat planting and cassava would not go stale as quickly as regular bread.
Cassava was introduced to Africa by Portuguese traders from Brazil in the 16th century. Around the same period, it was also introduced to Asia through Columbian Exchange by Portuguese and Spanish traders, planted in their colonies in Goa, Malacca, Eastern Indonesia, Timor and the Philippines. Maize and cassava are now important staple foods, replacing native African crops in places such as Tanzania.  Cassava has also become an important crop in Asia. While it is a valued food staple in parts of eastern Indonesia, it is primarily cultivated for starch extraction and bio-fuel production in Thailand, Cambodia or Vietnam.  Cassava is sometimes described as the "bread of the tropics"  but should not be confused with the tropical and equatorial bread tree (Encephalartos), the breadfruit (Artocarpus altilis) or the African breadfruit (Treculia africana). This description definitely holds in Africa and parts of South America in Asian countries such as Vietnam fresh cassava barely features in human diets. 
There is a legend that cassava was introduced in 1880-1885 C.E. to the South Indian state of Kerala by the King of Travancore, Vishakham Thirunal Maharaja, after a great famine hit the kingdom, as a substitute for rice.  However, there are documented cases of cassava cultivation in parts of the state before the time of Vishakham Thirunal Maharaja.  Cassava is called kappa or maricheeni in Malayalam. It is also referred to as tapioca in Indian English usage.
In 2018, global production of cassava root was 278 million tonnes, with Nigeria as the world's largest producer, having 21% of the world total (table). Other major growers were Thailand and Democratic Republic of the Congo. 
|Cassava production – 2018|
|Country||Production (millions of tonnes)|
|Democratic Republic of the Congo||30.0|
|Source: FAOSTAT of the United Nations |
Cassava is one of the most drought-tolerant crops, can be successfully grown on marginal soils, and gives reasonable yields where many other crops do not grow well. Cassava is well adapted within latitudes 30° north and south of the equator, at elevations between sea level and 2,000 m (7,000 ft) above sea level, in equatorial temperatures, with rainfalls from 50 to 5,000 mm (2 to 200 in) annually, and to poor soils with a pH ranging from acidic to alkaline. These conditions are common in certain parts of Africa and South America.
Cassava is a highly productive crop when considering food calories produced per unit land area per day (250,000 cal/hectare/day, as compared with 156,000 for rice, 110,000 for wheat and 200,000 for maize). 
Cassava, yams (Dioscorea spp.), and sweet potatoes (Ipomoea batatas) are important sources of food in the tropics. The cassava plant gives the third-highest yield of carbohydrates per cultivated area among crop plants, after sugarcane and sugar beets.  Cassava plays a particularly important role in agriculture in developing countries, especially in sub-Saharan Africa, because it does well on poor soils and with low rainfall, and because it is a perennial that can be harvested as required. Its wide harvesting window allows it to act as a famine reserve and is invaluable in managing labor schedules. It offers flexibility to resource-poor farmers because it serves as either a subsistence or a cash crop. 
Worldwide, 800 million people depend on cassava as their primary food staple.  No continent depends as much on root and tuber crops in feeding its population as does Africa. In the humid and sub-humid areas of tropical Africa, it is either a primary staple food or a secondary costaple. In Ghana, for example, cassava and yams occupy an important position in the agricultural economy and contribute about 46 percent of the agricultural gross domestic product. Cassava accounts for a daily caloric intake of 30 percent in Ghana and is grown by nearly every farming family. The importance of cassava to many Africans is epitomised in the Ewe (a language spoken in Ghana, Togo and Benin) name for the plant, agbeli, meaning "there is life".
In Tamil Nadu, India, there are many cassava processing factories alongside National Highway 68 between Thalaivasal and Attur. Cassava is widely cultivated and eaten as a staple food in Andhra Pradesh and in Kerala. In Assam it is an important source of carbohydrates especially for natives of hilly areas.
In the subtropical region of southern China, cassava is the fifth-largest crop in terms of production, after rice, sweet potato, sugar cane, and maize. China is also the largest export market for cassava produced in Vietnam and Thailand. Over 60 percent of cassava production in China is concentrated in a single province, Guangxi, averaging over seven million tonnes annually.
Alcoholic beverages Edit
Alcoholic beverages made from cassava include cauim and tiquira [ what language is this? ] (Brazil), kasiri (Guyana, Suriname), impala (Mozambique), masato (Peruvian Amazonia chicha), parakari or kari (Guyana), nihamanchi (South America) also known as [ what language is this? ] (Ecuador and Peru), ö döi (chicha de yuca, Ngäbe-Bugle, Panama), sakurá (Brazil, Suriname), and tarul ko [ what language is this? ] (Darjeeling, Sikkim, India).
Cassava-based dishes are widely consumed wherever the plant is cultivated some have regional, national, or ethnic importance.  Cassava must be cooked properly to detoxify it before it is eaten.
Cassava can be cooked in many ways. The root of the sweet variety has a delicate flavor and can replace potatoes. It is used in cholent in some households. [ citation needed ] It can be made into a flour that is used in breads, cakes and cookies. In Brazil, detoxified manioc is ground and cooked to a dry, often hard or crunchy meal known as farofa used as a condiment, toasted in butter, or eaten alone as a side dish.
Raw cassava is 60% water, 38% carbohydrates, 1% protein, and has negligible fat (table).  In a 100-gram ( 3 + 1 ⁄ 2 -ounce) reference serving, raw cassava provides 670 kilojoules (160 kilocalories) of food energy and 25% of the Daily Value (DV) of vitamin C, but otherwise has no micronutrients in significant content (i.e. above 10% of the relevant DV). Cooked cassava starch has a digestibility of over 75%. 
Cassava, like other foods, also has antinutritional and toxic factors. Of particular concern are the cyanogenic glucosides of cassava (linamarin and lotaustralin). On hydrolysis, these release hydrogen cyanide (HCN). The presence of cyanide in cassava is of concern for human and for animal consumption. The concentration of these antinutritional and unsafe glycosides varies considerably between varieties and also with climatic and cultural conditions. Selection of cassava species to be grown, therefore, is quite important. Once harvested, bitter cassava must be treated and prepared properly prior to human or animal consumption, while sweet cassava can be used after boiling.
Comparison with other major staple foods Edit
A comparative table shows that cassava is a good energy source. In its prepared forms, in which its toxic or unpleasant components have been reduced to acceptable levels, it contains an extremely high proportion of starch. Compared to most staples however, cassava is a poorer dietary source of protein and most other essential nutrients. Though an important staple, its main value is as a component of a balanced diet.
Comparisons between the nutrient content of cassava and other major staple foods when raw must be interpreted with caution because most staples are not edible in such forms and many are indigestible, even dangerously poisonous or otherwise harmful.  For consumption, each must be prepared and cooked as appropriate.
In many countries, significant research has begun to evaluate the use of cassava as an ethanol biofuel feedstock. Under the Development Plan for Renewable Energy in the Eleventh Five-Year Plan in the People's Republic of China, the target was to increase the production of ethanol fuel from nongrain feedstock to two million tonnes, and that of biodiesel to 200 thousand tonnes by 2010. This is equivalent to the replacement of 10 million tonnes of petroleum.  This push for non-grain ethanol was further increased to a goal of 300 million tons of cellulosic and non-grain based ethanol combined by 2020.  As a result, cassava (tapioca) chips have gradually become a major source of ethanol production. On 22 December 2007, the largest cassava ethanol fuel production facility was completed in Beihai, with annual output of 200 thousand tons, which would need an average of 1.5 million tons of cassava. In November 2008, China-based Hainan Yedao Group invested US$51.5 million in a new biofuel facility that is expected to produce 120 million litres (33 million US gallons) a year of bioethanol from cassava plants. 
Animal feed Edit
Cassava tubers and hay are used worldwide as animal feed. Cassava hay is harvested at a young growth stage (three to four months) when it reaches about 30 to 45 cm (12 to 18 in) above ground it is then sun-dried for one to two days until its final dry matter content approaches 85 percent. Cassava hay contains high protein (20–27 percent crude protein) and condensed tannins (1.5–4 percent CP). It is valued as a good roughage source for ruminants such as cattle. 
Laundry starch Edit
Manioc is also used in a number of commercially available laundry products, especially as starch for shirts and other garments. Using manioc starch diluted in water and spraying it over fabrics before ironing helps stiffen collars.
Potential toxicity Edit
Cassava roots, peels and leaves should not be consumed raw because they contain two cyanogenic glucosides, linamarin and lotaustralin. These are decomposed by linamarase, a naturally occurring enzyme in cassava, liberating hydrogen cyanide (HCN).  Cassava varieties are often categorized as either sweet or bitter, signifying the absence or presence of toxic levels of cyanogenic glucosides, respectively. The so-called sweet (actually not bitter) cultivars can produce as little as 20 milligrams of cyanide (CN) per kilogram of fresh roots, whereas bitter ones may produce more than 50 times as much (1 g/kg). Cassavas grown during drought are especially high in these toxins.   A dose of 25 mg of pure cassava cyanogenic glucoside, which contains 2.5 mg of cyanide, is sufficient to kill a rat.  Excess cyanide residue from improper preparation is known to cause acute cyanide intoxication, and goiters, and has been linked to ataxia (a neurological disorder affecting the ability to walk, also known as konzo).  It has also been linked to tropical calcific pancreatitis in humans, leading to chronic pancreatitis.  
Symptoms of acute cyanide intoxication appear four or more hours after ingesting raw or poorly processed cassava: vertigo, vomiting, and collapse. In some cases, death may result within one or two hours. It can be treated easily with an injection of thiosulfate (which makes sulfur available for the patient's body to detoxify by converting the poisonous cyanide into thiocyanate). 
"Chronic, low-level cyanide exposure is associated with the development of goiter and with tropical ataxic neuropathy, a nerve-damaging disorder that renders a person unsteady and uncoordinated. Severe cyanide poisoning, particularly during famines, is associated with outbreaks of a debilitating, irreversible paralytic disorder called konzo and, in some cases, death. The incidence of konzo and tropical ataxic neuropathy can be as high as three percent in some areas."  
During the shortages in Venezuela in the late 2010s, dozens of deaths were reported due to Venezuelans resorting to eating bitter cassava in order to curb starvation.  
Societies that traditionally eat cassava generally understand that some processing (soaking, cooking, fermentation, etc.) is necessary to avoid getting sick. Brief soaking (four hours) of cassava is not sufficient, but soaking for 18–24 hours can remove up to half the level of cyanide. Drying may not be sufficient, either. 
For some smaller-rooted, sweet varieties, cooking is sufficient to eliminate all toxicity. The cyanide is carried away in the processing water and the amounts produced in domestic consumption are too small to have environmental impact.  The larger-rooted, bitter varieties used for production of flour or starch must be processed to remove the cyanogenic glucosides. The large roots are peeled and then ground into flour, which is then soaked in water, squeezed dry several times, and toasted. The starch grains that flow with the water during the soaking process are also used in cooking.  The flour is used throughout South America and the Caribbean. Industrial production of cassava flour, even at the cottage level, may generate enough cyanide and cyanogenic glycosides in the effluents to have a severe environmental impact. 
Food preparation Edit
A safe processing method known as the "wetting method" is to mix the cassava flour with water into a thick paste, spread it in a thin layer over a basket and then let it stand for five hours at 30°C in the shade.  In that time, about 83% of the cyanogenic glycosides are broken down by the linamarase the resulting hydrogen cyanide escapes to the atmosphere, making the flour safe for consumption the same evening. 
The traditional method used in West Africa is to peel the roots and put them into water for three days to ferment. The roots are then dried or cooked. In Nigeria and several other west African countries, including Ghana, Cameroon, Benin, Togo, Ivory Coast, and Burkina Faso, they are usually grated and lightly fried in palm oil to preserve them. The result is a foodstuff called gari. Fermentation is also used in other places such as Indonesia (see Tapai). The fermentation process also reduces the level of antinutrients, making the cassava a more nutritious food.  The reliance on cassava as a food source and the resulting exposure to the goitrogenic effects of thiocyanate has been responsible for the endemic goiters seen in the Akoko area of southwestern Nigeria.  
A project called "BioCassava Plus" uses bioengineering to grow cassava with lower cyanogenic glycosides combined with fortification of vitamin A, iron and protein to improve the nutrition of people in sub-Saharan Africa.  
Cassava is harvested by hand by raising the lower part of the stem, pulling the roots out of the ground, and removing them from the base of the plant. The upper parts of the stems with the leaves are plucked off before harvest. Cassava is propagated by cutting the stem into sections of approximately 15 cm, these being planted prior to the wet season.  Cassava growth is favorable under temperatures ranging from 25 to 29 °C (77 to 84 °F), but it can tolerate temperatures as low as 12 °C (54 °F) and as high as 40 °C (104 °F). 
Postharvest handling and storage Edit
Cassava starch processing
Cassava starch flour processing
Cassava starch wet-processing
Spreading Casabe burrero (cassava bread) to dry, Venezuela
Cassava starch being prepared for packaging
Cassava starch packaged and ready for shipping
Frozen cassava leaves in a Los Angeles market
Cassava undergoes post-harvest physiological deterioration (PPD) once the tubers are separated from the main plant. The tubers, when damaged, normally respond with a healing mechanism. However, the same mechanism, which involves coumaric acids, starts about 15 minutes after damage, and fails to switch off in harvested tubers. It continues until the entire tuber is oxidized and blackened within two to three days after harvest, rendering it unpalatable and useless. PPD is related to the accumulation of reactive oxygen species (ROS) initiated by cyanide release during mechanical harvesting. Cassava shelf life may be increased up to three weeks by overexpressing a cyanide insensitive alternative oxidase, which suppressed ROS by 10-fold.  PPD is one of the main obstacles preventing farmers from exporting cassavas abroad and generating income. Fresh cassava can be preserved like potato, using thiabendazole or bleach as a fungicide, then wrapping in plastic, coating in wax or freezing. 
While alternative methods for PPD control have been proposed, such as preventing ROS effects by use of plastic bags during storage and transport, coating the roots with wax, or freezing roots, such strategies have proved to be economically or technically impractical, leading to breeding of cassava varieties more tolerant to PPD and with improved durability after harvest.  Plant breeding has resulted in different strategies for cassava tolerance to PPD.   One was induced by mutagenic levels of gamma rays, which putatively silenced one of the genes involved in PPD genesis, while another was a group of high-carotene clones in which the antioxidant properties of carotenoids are postulated to protect the roots from PPD. 
A major cause of losses during cassava storage is infestation by insects.  A wide range of species that feed directly on dried cassava chips have been reported as a major factor in spoiling stored cassava, with losses between 19% and 30% of the harvested produce.  In Africa, a previous issue was the cassava mealybug (Phenacoccus manihoti) and cassava green mite (Mononychellus tanajoa). These pests can cause up to 80 percent crop loss, which is extremely detrimental to the production of subsistence farmers. These pests were rampant in the 1970s and 1980s but were brought under control following the establishment of the "Biological Control Centre for Africa" of the International Institute of Tropical Agriculture (IITA) under the leadership of Hans Rudolf Herren.  The Centre investigated biological control for cassava pests two South American natural enemies Anagyrus lopezi (a parasitoid wasp) and Typhlodromalus aripo (a predatory mite) were found to effectively control the cassava mealybug and the cassava green mite, respectively.
The African cassava mosaic virus causes the leaves of the cassava plant to wither, limiting the growth of the root.  An outbreak of the virus in Africa in the 1920s led to a major famine.  The virus is spread by the whitefly and by the transplanting of diseased plants into new fields. Sometime in the late-1980s, a mutation occurred in Uganda that made the virus even more harmful, causing the complete loss of leaves. This mutated virus spread at a rate of 80 kilometres (50 miles) per year, and as of 2005 was found throughout Uganda, Rwanda, Burundi, the Democratic Republic of the Congo and the Republic of the Congo. 
Cassava brown streak virus disease has been identified as a major threat to cultivation worldwide. 
A wide range of plant parasitic nematodes have been reported associated with cassava worldwide. These include Pratylenchus brachyurus, Rotylenchulus reniformis, Helicotylenchus spp., Scutellonema spp. and Meloidogyne spp., of which Meloidogyne incognita and Meloidogyne javanica are the most widely reported and economically important.  Meloidogyne spp. feeding produces physically damaging galls with eggs inside them. Galls later merge as the females grow and enlarge, and they interfere with water and nutrient supply.  Cassava roots become tough with age and restrict the movement of the juveniles and the egg release. It is therefore possible that extensive galling can be observed even at low densities following infection.  Other pests and diseases can gain entry through the physical damage caused by gall formation, leading to rots. They have not been shown to cause direct damage to the enlarged storage roots, but plants can have reduced height if there was loss of enlarged root weight. 
Research on nematode pests of cassava is still in the early stages results on the response of cassava is, therefore, not consistent, ranging from negligible to seriously damaging.     Since nematodes have such a seemingly erratic distribution in cassava agricultural fields, it is not easy to clearly define the level of direct damage attributed to nematodes and thereafter quantify the success of a chosen management method. 
The use of nematicides has been found to result in lower numbers of galls per feeder root compared to a control, coupled with a lower number of rots in the storage roots.  The organophosphorus nematicide femaniphos, when used, did not affect crop growth and yield parameter variables measured at harvest. Nematicide use in cassava is neither practical nor sustainable the use of tolerant and resistant cultivars is the most practical and sustainable management method.  
Methods of Pollination
Pollination by Insects
Figure 1. Insects, such as bees, are important agents of pollination. (credit: modification of work by Jon Sullivan)
Bees are perhaps the most important pollinator of many garden plants and most commercial fruit trees (Figure 1). The most common species of bees are bumblebees and honeybees. Since bees cannot see the color red, bee-pollinated flowers usually have shades of blue, yellow, or other colors. Bees collect energy-rich pollen or nectar for their survival and energy needs. They visit flowers that are open during the day, are brightly colored, have a strong aroma or scent, and have a tubular shape, typically with the presence of a nectar guide. A nectar guide includes regions on the flower petals that are visible only to bees, and not to humans it helps to guide bees to the center of the flower, thus making the pollination process more efficient. The pollen sticks to the bees’ fuzzy hair, and when the bee visits another flower, some of the pollen is transferred to the second flower. Recently, there have been many reports about the declining population of honeybees. Many flowers will remain unpollinated and not bear seed if honeybees disappear. The impact on commercial fruit growers could be devastating.
Many flies are attracted to flowers that have a decaying smell or an odor of rotting flesh. These flowers, which produce nectar, usually have dull colors, such as brown or purple. They are found on the corpse flower or voodoo lily (Amorphophallus), dragon arum (Dracunculus), and carrion flower (Stapleia, Rafflesia). The nectar provides energy, whereas the pollen provides protein. Wasps are also important insect pollinators, and pollinate many species of figs.
Figure 2. A corn earworm sips nectar from a night-blooming Gaura plant. (credit: Juan Lopez, USDA ARS)
Butterflies, such as the monarch, pollinate many garden flowers and wildflowers, which usually occur in clusters. These flowers are brightly colored, have a strong fragrance, are open during the day, and have nectar guides to make access to nectar easier. The pollen is picked up and carried on the butterfly’s limbs. Moths, on the other hand, pollinate flowers during the late afternoon and night. The flowers pollinated by moths are pale or white and are flat, enabling the moths to land. One well-studied example of a moth-pollinated plant is the yucca plant, which is pollinated by the yucca moth. The shape of the flower and moth have adapted in such a way as to allow successful pollination. The moth deposits pollen on the sticky stigma for fertilization to occur later. The female moth also deposits eggs into the ovary. As the eggs develop into larvae, they obtain food from the flower and developing seeds. Thus, both the insect and flower benefit from each other in this symbiotic relationship. The corn earworm moth and Gaura plant have a similar relationship (Figure 2).
Pollination by Bats
In the tropics and deserts, bats are often the pollinators of nocturnal flowers such as agave, guava, and morning glory. The flowers are usually large and white or pale-colored thus, they can be distinguished from the dark surroundings at night. The flowers have a strong, fruity, or musky fragrance and produce large amounts of nectar. They are naturally large and wide-mouthed to accommodate the head of the bat. As the bats seek the nectar, their faces and heads become covered with pollen, which is then transferred to the next flower.
Pollination by Birds
Figure 3. Hummingbirds have adaptations that allow them to reach the nectar of certain tubular flowers. (credit: Lori Branham)
Many species of small birds, such as the hummingbird (Figure 3) and sun birds, are pollinators for plants such as orchids and other wildflowers. Flowers visited by birds are usually sturdy and are oriented in such a way as to allow the birds to stay near the flower without getting their wings entangled in the nearby flowers. The flower typically has a curved, tubular shape, which allows access for the bird’s beak. Brightly colored, odorless flowers that are open during the day are pollinated by birds. As a bird seeks energy-rich nectar, pollen is deposited on the bird’s head and neck and is then transferred to the next flower it visits. Botanists have been known to determine the range of extinct plants by collecting and identifying pollen from 200-year-old bird specimens from the same site.
Pollination by Wind
Figure 4. A person knocks pollen from a pine tree.
Most species of conifers, and many angiosperms, such as grasses, maples and oaks, are pollinated by wind. Pine cones are brown and unscented, while the flowers of wind-pollinated angiosperm species are usually green, small, may have small or no petals, and produce large amounts of pollen. Unlike the typical insect-pollinated flowers, flowers adapted to pollination by wind do not produce nectar or scent. In wind-pollinated species, the microsporangia hang out of the flower, and, as the wind blows, the lightweight pollen is carried with it (Figure 4).
The flowers usually emerge early in the spring, before the leaves, so that the leaves do not block the movement of the wind. The pollen is deposited on the exposed feathery stigma of the flower (Figure 5).
Figure 5. These male (a) and female (b) catkins are from the goat willow tree (Salix caprea). Note how both structures are light and feathery to better disperse and catch the wind-blown pollen.
Pollination by Water
Some weeds, such as Australian sea grass and pond weeds, are pollinated by water. The pollen floats on water, and when it comes into contact with the flower, it is deposited inside the flower.
Pollination by Deception
Figure 6. Certain orchids use food deception or sexual deception to attract pollinators. Shown here is a bee orchid (Ophrys apifera). (credit: David Evans)
Orchids are highly valued flowers, with many rare varieties (Figure 6). They grow in a range of specific habitats, mainly in the tropics of Asia, South America, and Central America. At least 25,000 species of orchids have been identified.
Flowers often attract pollinators with food rewards, in the form of nectar. However, some species of orchid are an exception to this standard: they have evolved different ways to attract the desired pollinators. They use a method known as food deception, in which bright colors and perfumes are offered, but no food. Anacamptis morio, commonly known as the green-winged orchid, bears bright purple flowers and emits a strong scent. The bumblebee, its main pollinator, is attracted to the flower because of the strong scent—which usually indicates food for a bee—and in the process, picks up the pollen to be transported to another flower.
Other orchids use sexual deception. Chiloglottis trapeziformis emits a compound that smells the same as the pheromone emitted by a female wasp to attract male wasps. The male wasp is attracted to the scent, lands on the orchid flower, and in the process, transfers pollen. Some orchids, like the Australian hammer orchid, use scent as well as visual trickery in yet another sexual deception strategy to attract wasps. The flower of this orchid mimics the appearance of a female wasp and emits a pheromone. The male wasp tries to mate with what appears to be a female wasp, and in the process, picks up pollen, which it then transfers to the next counterfeit mate.
How to Identify Warts
This article was co-authored by Heather Richmond, MD. Dr. Heather Richmond, MD is a board certified Dermatologist at Dermatology and Laser Surgery Center in Houston, Texas. With over nine years of experience, Dr. Richmond specializes in comprehensive dermatology including medical, surgical, and cosmetic procedures. She graduated cum laude from Yale University with a BA in Molecular, Cellular, and Developmental Biology. She earned her MD from the University of California, Irvine School of Medicine, where she was inducted into the Alpha Omega Alpha Honor Medical Society. She completed her Internal Medicine internship at Cedars-Sinai Medical Center and her Dermatology residency at The University of Texas MD Anderson Cancer Center in Houston. Dr. Richmond is a fellow of the American Academy of Dermatology and is a member of the American Society for Dermatologic Surgery, American Society for Laser Medicine and Surgery, and the Texas and Houston Dermatological Societies.
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Warts may seem strange or embarrassing, but they're a common and treatable skin issue. If you noticed an unusual bump or cluster of growths, check the size, shape, texture, and color. Unlike blisters or pimples, warts aren't filled with liquid, and they feel fleshy and hard. Usually, you won't notice any symptoms unless the wart is on a weight bearing area, such as your feet. Warts also grow slowly, so any bumps that developed suddenly probably aren't warts. Since they're caused by viruses and can spread easily, wash your hands after examining a suspected wart, and avoid touching or scratching it.
Characteristics and Traits
The seven characteristics that Mendel evaluated in his pea plants were each expressed as one of two versions, or traits. The physical expression of characteristics is accomplished through the expression of genes carried on chromosomes. The genetic makeup of peas consists of two similar or homologous copies of each chromosome, one from each parent. Each pair of homologous chromosomes has the same linear order of genes. In other words, peas are diploid organisms in that they have two copies of each chromosome. The same is true for many other plants and for virtually all animals. Diploid organisms utilize meiosis to produce haploid gametes, which contain one copy of each homologous chromosome that unite at fertilization to create a diploid zygote.
For cases in which a single gene controls a single characteristic, a diploid organism has two genetic copies that may or may not encode the same version of that characteristic. Gene variants that arise by mutation and exist at the same relative locations on homologous chromosomes are called alleles. Mendel examined the inheritance of genes with just two allele forms, but it is common to encounter more than two alleles for any given gene in a natural population.
Phenotypes and Genotypes
Two alleles for a given gene in a diploid organism are expressed and interact to produce physical characteristics. The observable traits expressed by an organism are referred to as its phenotype. An organism’s underlying genetic makeup, consisting of both physically visible and non-expressed alleles, is called its genotype. Mendel’s hybridization experiments demonstrate the difference between phenotype and genotype. When true-breeding plants in which one parent had yellow pods and one had green pods were cross-fertilized, all of the F1 hybrid offspring had yellow pods. That is, the hybrid offspring were phenotypically identical to the true-breeding parent with yellow pods. However, we know that the allele donated by the parent with green pods was not simply lost because it reappeared in some of the F2 offspring. Therefore, the F1 plants must have been genotypically different from the parent with yellow pods.
The P0 plants that Mendel used in his experiments were each homozygous for the trait he was studying. Diploid organisms that are homozygous at a given gene, or locus, have two identical alleles for that gene on their homologous chromosomes. Mendel’s parental pea plants always bred true because both of the gametes produced carried the same trait. When P0 plants with contrasting traits were cross-fertilized, all of the offspring were heterozygous for the contrasting trait, meaning that their genotype reflected that they had different alleles for the gene being examined.
Dominant and Recessive Alleles
Our discussion of homozygous and heterozygous organisms brings us to why the F1 heterozygous offspring were identical to one of the parents, rather than expressing both alleles. In all seven pea-plant characteristics, one of the two contrasting alleles was dominant, and the other was recessive. Mendel called the dominant allele the expressed unit factor the recessive allele was referred to as the latent unit factor. We now know that these so-called unit factors are actually genes on homologous chromosome pairs. For a gene that is expressed in a dominant and recessive pattern, homozygous dominant and heterozygous organisms will look identical (that is, they will have different genotypes but the same phenotype). The recessive allele will only be observed in homozygous recessive individuals (Table 4).
|Table 4. Human Inheritance in Dominant and Recessive Patterns|
|Dominant Traits||Recessive Traits|
|Huntington’s disease||Duchenne muscular dystrophy|
|Widow’s peak||Sickle-cell anemia|
|Wooly hair||Tay-Sachs disease|
Several conventions exist for referring to genes and alleles. For the purposes of this chapter, we will abbreviate genes using the first letter of the gene’s corresponding dominant trait. For example, violet is the dominant trait for a pea plant’s flower color, so the flower-color gene would be abbreviated as V (note that it is customary to italicize gene designations). Furthermore, we will use uppercase and lowercase letters to represent dominant and recessive alleles, respectively. Therefore, we would refer to the genotype of a homozygous dominant pea plant with violet flowers as VV, a homozygous recessive pea plant with white flowers as vv, and a heterozygous pea plant with violet flowers as Vv.
The Punnett Square Approach for a Monohybrid Cross
When fertilization occurs between two true-breeding parents that differ in only one characteristic, the process is called a monohybrid cross, and the resulting offspring are monohybrids. Mendel performed seven monohybrid crosses involving contrasting traits for each characteristic. On the basis of his results in F1 and F2 generations, Mendel postulated that each parent in the monohybrid cross contributed one of two paired unit factors to each offspring, and every possible combination of unit factors was equally likely.
To demonstrate a monohybrid cross, consider the case of true-breeding pea plants with yellow versus green pea seeds. The dominant seed color is yellow therefore, the parental genotypes were YY for the plants with yellow seeds and yy for the plants with green seeds, respectively. A Punnett square, devised by the British geneticist Reginald Punnett, can be drawn that applies the rules of probability to predict the possible outcomes of a genetic cross or mating and their expected frequencies. To prepare a Punnett square, all possible combinations of the parental alleles are listed along the top (for one parent) and side (for the other parent) of a grid, representing their meiotic segregation into haploid gametes. Then the combinations of egg and sperm are made in the boxes in the table to show which alleles are combining. Each box then represents the diploid genotype of a zygote, or fertilized egg, that could result from this mating. Because each possibility is equally likely, genotypic ratios can be determined from a Punnett square. If the pattern of inheritance (dominant or recessive) is known, the phenotypic ratios can be inferred as well. For a monohybrid cross of two true-breeding parents, each parent contributes one type of allele. In this case, only one genotype is possible. All offspring are Yy and have yellow seeds (Figure 4).
Figure 4. In the P0 generation, pea plants that are true-breeding for the dominant yellow phenotype are crossed with plants with the recessive green phenotype. This cross produces F1 heterozygotes with a yellow phenotype. Punnett square analysis can be used to predict the genotypes of the F2 generation.
A self-cross of one of the Yy heterozygous offspring can be represented in a 2 × 2 Punnett square because each parent can donate one of two different alleles. Therefore, the offspring can potentially have one of four allele combinations: YY, Yy, yY, or yy (Figure 4). Notice that there are two ways to obtain the Yy genotype: a Y from the egg and a y from the sperm, or a y from the egg and a Y from the sperm. Both of these possibilities must be counted. Recall that Mendel’s pea-plant characteristics behaved in the same way in reciprocal crosses. Therefore, the two possible heterozygous combinations produce offspring that are genotypically and phenotypically identical despite their dominant and recessive alleles deriving from different parents. They are grouped together. Because fertilization is a random event, we expect each combination to be equally likely and for the offspring to exhibit a ratio of YY:Yy:yy genotypes of 1:2:1 (Figure 4). Furthermore, because the YY and Yy offspring have yellow seeds and are phenotypically identical, applying the sum rule of probability, we expect the offspring to exhibit a phenotypic ratio of 3 yellow:1 green. Indeed, working with large sample sizes, Mendel observed approximately this ratio in every F2 generation resulting from crosses for individual traits.
Mendel validated these results by performing an F3 cross in which he self-crossed the dominant- and recessive-expressing F2 plants. When he self-crossed the plants expressing green seeds, all of the offspring had green seeds, confirming that all green seeds had homozygous genotypes of yy. When he self-crossed the F2 plants expressing yellow seeds, he found that one-third of the plants bred true, and two-thirds of the plants segregated at a 3:1 ratio of yellow:green seeds. In this case, the true-breeding plants had homozygous (YY) genotypes, whereas the segregating plants corresponded to the heterozygous (Yy) genotype. When these plants self-fertilized, the outcome was just like the F1 self-fertilizing cross.
The Test Cross Distinguishes the Dominant Phenotype
Beyond predicting the offspring of a cross between known homozygous or heterozygous parents, Mendel also developed a way to determine whether an organism that expressed a dominant trait was a heterozygote or a homozygote. Called the test cross, this technique is still used by plant and animal breeders. In a test cross, the dominant-expressing organism is crossed with an organism that is homozygous recessive for the same characteristic. If the dominant-expressing organism is a homozygote, then all F1 offspring will be heterozygotes expressing the dominant trait (Figure 5). Alternatively, if the dominant expressing organism is a heterozygote, the F1 offspring will exhibit a 1:1 ratio of heterozygotes and recessive homozygotes (Figure 5). The test cross further validates Mendel’s postulate that pairs of unit factors segregate equally.
Figure 5. A test cross can be performed to determine whether an organism expressing a dominant trait is a homozygote or a heterozygote.
In pea plants, round peas (R) are dominant to wrinkled peas (r). You do a test cross between a pea plant with wrinkled peas (genotype rr) and a plant of unknown genotype that has round peas. You end up with three plants, all which have round peas. From this data, can you tell if the round pea parent plant is homozygous dominant or heterozygous? If the round pea parent plant is heterozygous, what is the probability that a random sample of 3 progeny peas will all be round?
Many human diseases are genetically inherited. A healthy person in a family in which some members suffer from a recessive genetic disorder may want to know if he or she has the disease-causing gene and what risk exists of passing the disorder on to his or her offspring. Of course, doing a test cross in humans is unethical and impractical. Instead, geneticists use pedigree analysis to study the inheritance pattern of human genetic diseases (Figure 6).
Figure 6. Pedigree Analysis for Alkaptonuria
Alkaptonuria is a recessive genetic disorder in which two amino acids, phenylalanine and tyrosine, are not properly metabolized. Affected individuals may have darkened skin and brown urine, and may suffer joint damage and other complications. In this pedigree, individuals with the disorder are indicated in blue and have the genotype aa. Unaffected individuals are indicated in yellow and have the genotype AA or Aa. Note that it is often possible to determine a person’s genotype from the genotype of their offspring. For example, if neither parent has the disorder but their child does, they must be heterozygous. Two individuals on the pedigree have an unaffected phenotype but unknown genotype. Because they do not have the disorder, they must have at least one normal allele, so their genotype gets the “A?” designation.
What are the genotypes of the individuals labeled 1, 2 and 3?
Introduction and Impacts
Brazilian pepper-tree is native to Argentina, Paraguay, and Brazil. The species was brought into Florida in mid-1800 for use as an ornamental plant. Its bright red berries and brilliant green foliage are used frequently as Christmas decorations.
This species is an aggressive woody weed. It displaces native vegetation and rapidly invades disturbed sites. It has a high growth rate, wide environmental tolerance, is a prolific seed producer, has a high germination rate, produces shade tolerant seedlings, and has the ability to form dense thickets.States where Brazilian pepper-tree occurs.
Courtesy of EDDMapS. 2018. Early Detection & Distribution Mapping System. The University of Georgia - Center for Invasive Species and Ecosystem Health.
Can someone identify this Brazilian flower? - Biology
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U.S. DEPARTMENT OF AGRICULTURE
Plant Science Research: Raleigh, NC
Effects of Ozone Air Pollution on Plants
Ambient ozone inury to sensitive and tolerant snap beans
Ozone injury in a pumpkin leaf
Tropospheric Ozone Pollution
Ozone is formed in the troposphere when sunlight causes complex photochemical reactions involving oxides of nitrogen (NOx), volatile organic hydrocarbons (VOC) and carbon monoxide that originate chiefly from gasoline engines and burning of other fossil fuels. Woody vegetation is another major source of VOCs. NOx and VOCs can be transported long distances by regional weather patterns before they react to create ozone in the atmosphere, where it can persist for several weeks.
Seasonal exposures at low elevations consist of days when ozone concentrations are relatively low or average, punctuated by days when concentrations are high. Concentrations of ozone are highest during calm, sunny, spring and summer days when primary pollutants from urban areas are present. Ozone concentrations in rural areas can be higher than in urban areas while ozone levels at high elevations can be relatively constant throughout the day and night.
Seasonal mean of ambient ozone concentrations between 09:00 and 16:00 h over the continental United States from 1 July to 31 September 2005 (Tong et al. 2007Atmos. Environ. 41:8772). Areas shown in brown, orange and red can experience significant crop yield loss and damage to ecosystem function from ambient ozone.
Description of Ozone Injury
Ozone enters leaves through stomata during normal gas exchange. As a strong oxidant, ozone (or secondary products resulting from oxidation by ozone such as reactive oxygen species) causes several types of symptoms including chlorosis and necrosis. It is almost impossible to tell whether foliar chlorosis or necrosis in the field is caused by ozone or normal senescence. Several additional symptom types are commonly associated with ozone exposure, however. These include flecks (tiny light-tan irregular spots less than 1 mm diameter), stipples (small darkly pigmented areas approximately 2-4 mm diameter), bronzing, and reddening.
Ozone symptoms usually occur between the veins on the upper leaf surface of older and middle-aged leaves, but may also involve both leaf surfaces (bifacial) for some species. The type and severity of injury is dependent on several factors including duration and concentration of ozone exposure, weather conditions and plant genetics. One or all of these symptoms can occur on some species under some conditions, and specific symptoms on one species can differ from symptoms on another. With continuing daily ozone exposure, classical symptoms (stippling, flecking, bronzing, and reddening) are gradually obscured by chlorosis and necrosis.
Studies in open-top field chambers have repeatedly verified that flecking,stippling, bronzing and reddening on plant leaves are classical responses to ambient levels of ozone. Plants grown in chambers receiving air filtered with activated charcoal to reduce ozone concentrations do not develop symptoms that occur on plants grown in nonfiltered air at ambient ozone concentrations. Foliar symptoms shown on this web site mainly occurred on plants exposed to ambient concentrations of ozone.
Yield Loss Caused by Ozone
Field research to measure effects of seasonal exposure to ozone on crop yield has been in progress for more than 40 years. Most of this research utilized open-top field chambers in which growth conditions are similar to outside conditions. The most extensive research on crop loss was performed from 1980 to 1987 at five locations in the USA as part of the National Crop Loss Assessment Network (NCLAN). At each location, numerous chambers were used to expose plants to ozone treatments spanning the range of concentrations that occur in different areas of the world. The NCLAN focused on the most important agronomic crops nationally.
The strongest evidence for significant effects of ozone on crop yield comes from NCLAN studies (Heagle 1989). The results show that dicot species (soybean, cotton and peanut) are more sensitive to yield loss caused by ozone than monocot species (sorghum, field corn and winter wheat).
Ainsworth EA, A Rogers, ADB Leakey. 2008. Targets for crop biotechnology in a future high-CO 2 and high-O 3 world. Plant Physiology, 147: 13-19.
Bell, JNB and M Treshow. 2002. Air Pollution and Plant Life. 2nd ed. Cinchester:John Wiley & Sons, Inc. 465 pp.
Booker, FL, R Muntifering, M McGrath, KO Burkey, D Decoteau, EL Fiscus, W Manning, S Krupa, A Chappelka, DA Grantz. 2009. The ozone component of global change: Potential effects on agricultural and horticultural plant yield, product quality and interactions with invasive species. Journal of Integrative Plant Biology 51:337-351.
Fiscus, EL, FL Booker, KO Burkey. 2005. Crop responses to ozone: uptake, modes of action, carbon assimilation and partitioning. Plant, Cell and Environment 28:997-1011.
Fishman, J, JK Creilson, PA Parker, EA Ainsworth, GG Vining, J Szarka, FL Booker and X Xu. 2010. An investigation of widespread ozone damage to the soybean crop in the upper Midwest determined from ground-based and satellite measurements. Atmospheric Environment 44:2248-2256.
Heagle, AS. 1989. Ozone and crop yield. Annual Review of Phytopathology 27:397-423.
Heck, WW, AS Heagle, DS Shriner. 1986. Effects on Vegetation: Native, Crops, Forests. In: Air Pollution. 3rd Ed., Vol. VI. Supplement to Air Pollutants, Their Transformation, Transport and Effects. AC Stern, ed., pp. 248-333. Academic Press, New York, NY.
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Techniques used in Molecular Biology
Some of the most important techniques used in molecular biology are as follows:
Molecular Biology techniques include characterization, isolation and manipulation of the molecular components of cells and organisms.
These components include DNA, the repository of genetic information RNA, functional and structural part of the translational apparatus and proteins, the major structural and enzymatic type of molecule in cells.
One of the most basic techniques of molecular biology to study protein function is expression cloning.
In this technique, DNA coding for a protein of interest is cloned (using PGR and/or restriction enzymes) into a plasmid (known as an expression vector).
This plasmid may have special promoter elements to drive production of the protein of interest, and may also have antibiotic resistance markers to help follow the plasmid.
( ii ) Polymerase chain reaction:
The polymerase chain reaction is an extremely versatile technique for copying DNA. PCR allows a single DNA sequence to be copied (millions of times), or altered in predetermined ways. PGR has many variations, like reverse transcription PGR (RT-PGR) for amplification of RNA, and, more recently, real-time PGR (QPGR) which allow for quantitative measurement of j DNA or RNA molecules.
(iii) Gel electrophoresis:
Gel electrophoresis is one of the principal tools of molecular biology. The basic principle is that DNA, RNA, and proteins can all be separated by means of an electric field. In agarose gel electrophoresis, DNA and RNA can be separated on the basis of size by running the DNA through an agarose gel. Proteins can be separated on the basis of size by using SDS-PAGE (polyacrylamide ) gel.
(iv) Macromolecule blotting and probing Southern Blots:
The Southern blot is a method for probing for the presence of a specific DNA sequence within a DNA sample. These tools are widely used in forensic laboratories to identify individuals who have left blood or other DNA-containing material at the scene of crimes. The number of bands that hybridize to a short probe gives an estimate of the number of closely related genes in an organism.
The Northern blot is used to study the expression patterns of a specific type of RNA molecule as relative comparison among a set of different samples of RNA. The RNAs on the blot can be detected by hybridizing them to a labeled probe. The intensities of the band reveal the relative amounts of specific RNA in each sample.
Immunoblots (Western Blots):
Proteins can be detected and quantified in complex mixtures using immunoblots (or Western blots). Proteins are electrophoresed, then blotted on a membrane and the proteins on the blot are probed with specific antibodies that can be detected with labeled secondary antibodies or protein.
(v) DNA microarray:
A DNA array is a collection of spots attached to a solid support such as a microscope slide where each spot contains one or more single-stranded DNA oligonucleotide fragment. Arrays make it possible to put down a large quantity of very small (100 micrometre diameter) spots on a single slide. Each spot has a DNA fragment molecule that is complementary to a single DNA sequence (similar to Southern blotting).
A variation of this technique allows the gene expression of an organism at a particular stage in development to be qualified (expression profiling).
(vi) Antiquated technologies:
In molecular biology, procedures and technologies are continually being developed and older technologies abandoned. For example, before the advent of DNA gel electrophoresis (agarose or polyacrylainide), the size of DNA molecules was typically determined by rate sedimentation in sucrose gradients, a slow and labour-intensive technique requiring expensive instrumentation prior to sucrose gradients, viscometry was used.
Phylogenetic System of Plant Classification | Botany
List of six eminent botanists who contributed to the phylogenetic system of plant classification:- 1. Adolf Engler (1844-1930) 2. John Hutchinson (1884-1972) 3. Armen Takhtajan (1911) 4. Arthur Cronquist (1919-1992) 5. Rolf Dahlgren (1932-1987) 6. Robert F. Thorne (1920).
Botanist # 1. Adolf Engler (1844-1930):
The best known and widely accepted phylo­genetic system is that by Adolf Engler, Professor of Botany, University of Berlin. In 1892, he pub­lished a system of classification mainly based on August Wilhelm Eichler in the book ‘Syllabus der Vorlesungen’ as a guide to study the plants avai­lable in the Breslau Botanic Garden.
During 1887-1915, Engler and his associate Karl Prantl made a monographic work, the “Die naturlichen Pflanzenfamilien” in 20 volumes, including all the known genera of plants from algae to the phanerogams along with key to identify the plants.
Engler, in collaboration with Gilg, and later with Diels, published the works in a single volume ‘Syllabus der Pflanzenfamilien’. After his death , the book was revised by followers in several editions and the latest (12th) one in 2 volumes in 1954 and 1964.
The system of Engler has been widely used in the American and Europian continents. Engler divided the plant kingdom into thir­teen (13) Divisions.
The thirteenth (13) Division is the Embryo­phyta Siphonogama (the seed-bearing plants i.e., Spermatophyta). It is divided into two Sub­divisions, Gymnospermae and Angiospermae. The Angiospermae is divided into two Classes — Monocotyledonae and Dicotyledonae. The Class Monocotyledonae is divided directly into 11 Orders.
On the other hand, the Class Dicotyledonae is divided into two Subclasses — Archichlamydeae i.e., lower dicotyledons, and Metachlamydeae or Sympetalae i.e., higher dicotyledons. The Archichlamydeae is further divided into 33 Orders, and Metachlamydeae into 11 Orders. The Orders are divided into Suborders, Families, Genera and finally into Species.
In this system, the Plant Kingdom contains 309 families. The Class Monocotyledonae starts with the family Typhaceae and ends in Orchidaceae, while the class Dicotyledonae starts with the family Casuarinaceae and ends in Compositae.
In this system, Engler considered that in Embryophyta Siphonogama the flower without perianth is the primitive one. Thus, plants like Oak, Willow etc., with woody stem and uni­sexual apetalous flowers (Amentiferae), are treated as primitive Dicotyledons.
The main distinctive features of Engler’s sys­tem that separate it from that of Bentham and Hooker’s system are:
1. The Polypetalae and Monochlamydeae of Bentham and Hooker are amalgama­ted and placed into a single group (Subclasses) Archichlamydeae.
2. The families of the flowering plants are arranged in ascending order with the increasing complexity of the flowers (mainly on floral envelope).
3. Monocotyledons are placed before Dicotyledons.
4. The term Natural order has been replaced by Family.
5. The term Series or Cohort has been replaced by Order.
Merits and Demerits:
1. The entire Plant Kingdom was broadly treated with excellent illustrations, and phylogenetic arrangement of many groups of plants was made.
2. The amalgamation of Polypetalae and Monochlamydeae into Archichlamy­deae is justified.
3. Consideration and placing of Orchidaceae at the end of Monocotyledons and Compositae at the end of Dicoty­ledons are justified — since they are most highly evolved.
4. Juncaceae, Amaryllidaceae and Iridaceae are placed judiciously nearer to Liliaceae.
1. The placement of Amentiferae and Centrospermae almost at the beginning of Dicotyledones, even before Ranales, are not justified.
2. The assemblage of all sympetalous mem­bers under Metachlamydeae increased the distance of closely related orders.
3. The placing of Monocotyledons before Dicotyledons is not appropriate, because it is generally agreed that monocots have arisen from dicoty­ledons by reduction.
4. The placing of the order Helobiae between the advanced orders Pandanales and Glumiflorae is questionable. Araceae was placed much earlier than Liliaceae, from which it has been derived.
5. Fossil evidences gave little support to this system.
Botanist # 2. John Hutchinson (1884-1972):
John Hutchinson was a British botanist asso­ciated with Royal Botanic Gardens, Kew, England. He developed and proposed his system based on Bentham and Hooker and also on Bessey. His phylogenetic system first appeared as “The Families of Flowering Plants” in two volumes.
The first volume contains Dicotyledons (published in 1926) and second volume contains Monocotyledons (published in 1934). He made several revisions in different years. The final revi­sion of “The Families of Flowering Plants” was made just before his death on 2nd September 1972 and the 3rd i.e., the final edition, was pub­lished in 1973.
The following principles were adopted by Hutchinson to classify the flowering plants:
1. Evolution takes place in both upward and downward direction.
2. During evolution all organs do not evolve at the same time.
3. Generally, evolution has been consistent.
4. Trees and shrubs are more primitive than herbs in a group like genus or family.
5. Trees and shrubs are primitive than climbers.
6. Perennials are older than annuals and biennials.
7. Terrestrial angiosperms are primitive than aquatic angiosperms.
8. Dicotyledonous plants are primitive than monocotyledonous plants.
9. Spiral arrangement of vegetative and floral members are primitive than cyclic arrange­ments.
10. Normally, simple leaves are more primitive than compound leaves.
11. Bisexual plants are primitive than unisexual plants and monoecious plants are primitive than dioecious plants.
12. Solitary flowers are primitive than flowers on inflorescence.
13. Types of aestivation gradually evolved from contorted to imbricate to valvate.
14. Polymerous flowers precede oligomerous flowers.
15. Polypetalous flowers are more primitive than gamopetalous flowers.
16. Flowers with petals are more primitive than apetalous flowers.
17. Actinomorphic flowers are more primitive than zygomorphic flowers.
18. Hypogyny is considered as more primitive from which perigyny and epigyny gradually evolved.
19. Apocarpous pistil is more primitive than syncarpous pistil.
20. Polycarpy is more primitive than gynoecium with few carpels.
21. Flowers with many stamens are primitive than flowers with few stamens.
22. Flowers with separate anthers are primitive than flowers with fused anthers and/fila­ments.
23. Endospermic seeds with small embryo is primitive than non-endospermic one with a large embryo.
24. Single fruits are primitive than aggregate fruits.
He divided the Phylum Angiospermae into two Subphyla Dicotyledones and Monocotyledones. The Dicotyledones are further divided into two divisions — Lignosae (arboreal) and Herbaceae (herbaceous).
The Lignosae includes, fundamentally, the woody representatives derived from Magnoliales and Herbaceae includes most of the predominantly herba­ceous families derived from Ranales. The subphylum Monocotyledones are divided into three divisions — Calyciferae, Corolliferae and Glumiflorae.
1. The division Lignosae was further divided into 54 orders beginning with Magnoliales and ending in Verbenales.
2. The division Herbaceae was divided into 28 orders beginning with Ranales and ending in Lamiales.
3. The division Calyciferae was divided into 12 orders beginning with Butamales and ending in Zingiberales.
4. The division Corolliferae was divided into 14 orders beginning with Liliales and ending in Orchidales.
5. The division Glumiflorae was divided into 3 orders beginning with Juncales and ending in Graminales.
So in the latest system of Hutchinson, the Dicotyledones consists of 83 orders and 349 families and Monocotyledones consists of 29 orders and 69 families.
Merits and Demerits Merits:
1. Hutchinson proposed the monophyletic origin of angiosperms from some hypo­thetical Proangiosperms having Bennettitalean characteristics.
2. He made a valuable contribution in phylogenetic classification by his care­ful and critical studies.
3. Monocots have been derived from Dicots.
4. According to him, the definitions of orders and families are mostly precise, particularly in case of subphylum Monocotyledones.
1. There is undue fragmentation of fami­lies.
2. Too much emphasis is laid on habit and habitat. Thus, creation of Lignosae and Herbaceae is thought to be a defect reflecting the Aristotelean view.
3. The origin of angiosperms from Bennettitalean-like ancestor is criticised by many, because the anatomical struc­tures of the early dicotyledons are not tenable with such ancestry.
Botanist # 3. Armen Takhtajan (1911):
Takhtajan was a reputed palaeobotanist of Komarov Botanical Institute of Leningrad, U.S.S.R. (now in Russia). He also made great contributions in the field of angiosperm taxo­nomy. In 1942, he proposed preliminary phylo­genetic arrangement of the orders of higher plants, based on the structural types of gynoecium and placentation.
After 12 years i.e., in 1954, the actual system of classification was published in “The Origin of Angiospermous Plants” in Russian language. It was translated in English in 1958. Later on, in 1964, he proposed a new sys­tem in Russian language. To trace the evolution of angiosperm, he was particularly inspired by Hallier’s attempt to develop a synthetic evolu­tionary classification of flowering plants based on Darwinian philosophy.
The classification was published in ‘Flowering Plants: Origin and Dispersal’ (1969) in English language. Later on, in 1980, a new revision of his system was pub­lished in “Botanical Review”.
Takhtajan (1980) included the angiospermic plants under the Division Magnoliophyta. The Magnoliophyta is divided into two classes Magnoliopsida (Dicotyledons) and Liliopsida (Monocotyledons). The class Magnoliopsida consists of 7 subclasses, 20 superorders, 71 orders and 333 families.
On the other hand, Liliopsida comprises of 3 subclasses, 8 super- orders, 21 orders and 77 families. The class Magnoliopsida starts with the order Magnoliales and ends in Asterales and the class Liliopsida begins with Alismatales and ends in Arales.
The class Magnoliopsida is considered to be monophyletic in origin, probably derived from Bennettitales-like ancestors or stocks ancestral to them. On the other hand, the Liliopsida have been considered to be originated from the stocks ancestral to Nymphaeales. He considered Magnoliopsida more primitive than the Liliopsida. The principles as adopted by Takhtajan (1980) for interpreting the evolutionary lineages in higher plants are mentioned in Table 4.2.
1. The classification of Takhtajan is more phylogenetic than that of earlier sys­tems.
2. This classification is in a general agree­ment with the major contemporary systems of Cronquist, Dahlgren, Thorne, and others. Both phylogenetic and phenetic informations were adopted for delimination of orders and families.
3. Due to the abolition of several artificial groups like Polypetalae, Gamopetalae, Lignosae, Herbaceae, many natural taxa came close together, viz. Lamiaceae (earlier placed under Herbaceae) and Verbenaceae (placed under Lignosae) are brought together under the order Lamiales.
4. Nomenclature adopted in this system is in accordance with the ICBN, even at the level of division.
5. The treatment of Magnoliidae as a primitive group and the placement of Dicotyledons before Monocotyledons are in agreement with the other con­temporary systems.
6. The derivation of monocots from the extinct terrestrial hypothetical group of Magnoliidae is found to be logical.
1. In this system, more weightage is given to cladistic information in comparison to phenetic information.
2. This system provides classification only up to the family level, thus it is not suitable for identification and for adop­tion in Herbaria. In addition, no key has been provided for identification of taxa.
3. Takhtajan recognised angiosperms as division which actually deserve a class rank like that of the systems of Dahlgren (1983) and Throne (2003).
4. Numerous monotypic families have been created in 1997 due to the further splitting and increase in the number of families to 592 (533 in 1987), resulting into a very narrow circumscription.
5. Takhtajan incorrectly suggested that smaller families are more “natural”.
6. Although the families Winteraceae and Canellaceae showed their 99-100% relationship by multigene analyses, Takhtajan placed these two families in two separate orders.
Botanist # 4. Arthur Cronquist (1919-1992):
Arthur Cronquist was the Senior Curator of New York Botanic Garden and Adjunct Professor of Columbia University. He presented an elabo­rate interpretation of his concept of classification in “The Evolution and Classification of Flowering Plants”(1968). The further edition of his classi­fication was published in “An Integrated System of Classification of Flowering Plants” (1981).
The latest revision was published in the 2nd edition in 1988 in “The Evolution and Classification of Flowering Plants”. He discussed a wide range of characteristics important to phylogenetic system. He also provided synoptic keys designed to bring the taxa in an appropriate alignment.
He also represented his classification in charts to show the relationships of the orders within the various subclasses. His system is more or less parallel to Takhtajan’s system, but differs in details.
He considered that the Pteridosperms i.e., the seed ferns as probable ancestors of angiosperm.
The following principles were adopted by Cronquist (1981) to classify the flowering plants:
1. The earliest angiosperms were shrubs rather than trees.
2. The simple leaf is primitive than compound leaf.
3. Reticulate venation is primitive than parallel venation.
4. Paracytic stomata is primitive than the other types.
5. Slender, elongated, long tracheids with numerous scalariform pits are primitive. Further specialisation leads to shorter broad vessels with somewhat thinner walls and transverse end walls with few larger perfo­rations. Later on, the perforation becomes single and large.
6. Long and slender sieve elements with very oblique end walls where the sieve areas scattered along the longitudinal wall with groups of minute pores are primitive. Whereas, the phloem with short sieve tube elements where end walls having a single transverse sieve plate with large openings is a derived condition.
7. The area and activity of cambium and also the length of fusiform initial is more in primitive form which gradually becomes reduced in advanced one.
8. Plants with vascular bundles arranged in a ring are primitive rather than scattered vas­cular bundle as found in monocots.
9. Plants with large and terminal flowers are primitive, those may arrange in monochasia or dichasia and the other type of inflores­cences have been derived from these types.
10. Flowers with many large, free and spirally arranged petals many linear and spirally arranged stamens and free carpels as found in Magnoliaceae are primitive, and other types got evolved through gradual reduc­tion, aggregation, elaboration and differenti­ation of floral members.
11. Plants with unisexual flowers are evolved from bisexual floral ancestors.
12. The large and indefinite number of floral members are primitive than the small and definite numbers.
13. Androecium with many stamens is primitive than the reduced numbers.
14. Linear stamens with embedded pollen sacs as found in some Magnolian genera are considered more primitive than the others.
15. Uniaperturate pollen grains are considered as primitive and the triaperturate type are derived from it.
16. Insect pollinated plants are considered as primitive from which wind pollinated plants got evolved.
17. The gynoecium comprising of many carpels arranged spirally on a more or less elonga­ted receptacle is considered as primitive. Further evolution leads to the reduction of the number of carpels which are arranged in a single whorl and then undergo further fusion.
18. Axial placentation is primitive from which other types have been evolved.
19. Anatropous ovule is primitive from which other types have been evolved.
20. Ovule with two integuments (bitegmic) is primitive and, either by fusion or abortion, unitegmic condition has been evolved.
21. Embryo-sac with 8-nuclei (Polygonum-type) is primitive from which embryo-sac with 4- nuclei (Oenothera-type) has been derived through reduction.
22. Monocotyledons have been developed from dicotyledons through abortion of one cotyledon.
23. The follicle (fruit) is considered as primitive. Further, dry and dehiscent fruit is more primitive than fleshy and indehiscent fruit.
According to him “many of the evolutionary trends bear little apparent relation to survival value and that there are some reversals”.
In 1981, he divided the Division Magnoliophyta (Angiosperms) into two classes Magnoliatae (Dicotyledons) and Liliatae (Monocotyledons). He divided Magnoliatae into 6 subclasses and 55 orders, of which magnoliales is the primitive and Asterales is the advanced taxa.
On the other hand, the class Liliatae has been divided into 4 subclasses and 18 orders, of which Alismatales is the primitive and Orchidales is the advanced taxa. The class Magnoliatae consists of 291 families and Liliatae with 61 families.
Merits and Demerits:
1. There is general agreement of Cronquist’s system with that of other contemporary systems like Takhtajan, Dahlgren and Thorne.
2. Detailed information on anatomy, ultra- structure phytochemistry and chromo­some — besides morphology — was presented in the revision of the classi­fication in 1981 and 1988.
3. The system is highly phylogenetic.
4. Nomenclature is in accordance with the ICBN.
5. The family Asteraceae in Dicotyledons and Orchidaceae in Monocotyledons are generally regarded as advanced and are rightly placed towards the end of respective groups.
6. The relationships of different groups have been described with diagrams which provide valuable information on relative advancement and size of the various subclasses.
7. The family Winteraceae (vessel-less wood present similar to Pteridosperms) placed at the beginning of dicotyledons is favoured by many authors.
8. The subclass Magnoliidae is considered as the most primitive group of Dicotyledons. The placement of Dicotyledons before Monocotyledons finds general agreements with modern authors.
9. As the text is in English, the system has been readily adopted in different books.
1. Though highly phylogenetic and popu­lar in U.S.A., this system is not very use­ful for identification and adoption in Herbaria since Indented keys for genera are not provided.
2. Dahlgren (1983, 89) and Thorne (1980, 83) treated angiosperms in the rank of a class and not that of a division.
3. Superorder as a rank above order has not been recognised here, though it is present in other contemporary classifi­cations like Takhtajan, Thorne and Dahlgren.
4. The subclass Asteridae represents a loose assemblage of several diverse sympetalous families.
5. Ehrendorfer (1983) pointed out that the subclass Hamamelidae does not represent an ancient side branch of the subclass Magnoliidae, but is remnant of a transition from Magnoliidae to Dilleniidae, Rosidae, and Asteridae.
6. There is a difference in opinion with other authors regarding the systematic position of some orders like Typhales, Arales, Urticales etc.
Botanist # 5. Rolf Dahlgren (1932-1987):
Rolf Dahlgren, working at the Botanical Museum in the University of Copenhagen, Denmark, published a new method in Danish in 1974 to illustrate an angiosperm system in a text book of angiosperm taxonomy. Later on in 1975 he published “A System of Classification of Angiosperms to be Used to Demonstrate the Distribution of Characteristics” in Botanische Notiser in English.
The revised and improved version of his system gradually appeared in the subsequent years:
i. In 1980 in “Botanical Journal of the Linnean Society”.
ii. In 1981, in “Phytochemistry and angio­sperm Phytogeny” (Edited by Young and Siegier).
iii. In 1983, in “Nordiac Journal of Botany”.
In his system he included the information at different levels as much as possible. In his classi­fication he extensively used the chemical cha­racteristics. He considered the following mor­phological and chemical characteristics in his classification.
1. Morphological characteristics:
i. Chloripetalae and Sympetalae.
ii. Apocarpous, syncarpous and monocarpellate condition.
iii. Types of microsporogenesis.
iv. Bi- and trinucleate pollen grain.
v. Tenuinucellate, pseudocrassinucellate and crassinucellate.
vi. Bi- or unitegmic ovules etc.
2. Chemical characteristics:
i. Benzylisoquinoline alkaloids.
ii. Pyrrolisidine alkaloids.
v. Ellagic acid and ellagitannins
vi. Various groups of flavonoids etc.
He did not consider angiosperms to be originated polyphyletically from different gymno­sperms, but believed that the combination of different characteristics like 8-nucleate embryo sac, secondary endosperm etc. would hardly have evolved independently from different groups of gymnosperms.
According to Dahlgren (1980), the class Magnoliopsida (Angiosperms) has been divided into two subclasses, Magnoliidae and Liliidae. The Magnoliidae includes 24 Superorders, those start with Magnoliiflorae and end in Lamiflorae 80 orders, those start with Annonales and end in Lamiales and 346 families.
On the other hand, the subclass Liliidae includes 7 Superorders, those start with Alismatiflorae and end in Areciflorae 26 orders, those start with Hydrocharitales and end in Pandanales and 92 families.
Merits and Demerits:
1. Detailed information on morphology, phytochemistry and embryology was presented in the classification of Dahlgren.
2. This system is highly phylogenetic where angiosperms are ranked as a class like other recent systems.
3. The arrangement of taxa in the form of a bubble diagram gives an idea about the relationship of superorders, orders and even families.
4. The use of superorders and the suffixanae are in accordance with the other modern systems like those of Takhtajan, Thorne, etc.
1. This system provides classification of angiosperms only up to the family level, thus it is not suitable for identification and for adaptation in Herbaria.
2. Dahlgren classified angiosperms into dicots and monocots, which shows inconformity with the recent classifica­tion of APG II (2003) and Throne (2003).
3. Dahlgren placed monocots in-between dicots, while in modern classification monocots are placed in-between primi­tive angiosperms and the eudicots.
4. Although the families Winteraceae and Canellaceae showed their 99-100% relationship by multigene analysis, yet Dahlgren placed these two families in two separate orders.
Botanist # 6. Robert F. Thorne (1920- ):
Robert F. Thorne, an American taxonomist, associated with the Rancho Santa Ana Botanic Garden, California, U.S.A., initially published the principles of his classification in 1958 and 1963. Later, in 1968, he published “Synopsis of a putatively phylogenetic classification of the flow­ering plants” in Aliso. The subsequent revisions were published in 1974, 1976, 1981, 1983, 1992 and 2000. The electronic version of his classification was published in 1999 which was finally revised in 2003.
Thorne gave much emphasis on phytochemical approach. In addition to the above, many other different aspects were also considered by him.
6. Host-parasite relationship.
He believed that the Angiospermae are monophyletic.
In 1983, he divided the Class Angiospermae (Annonopsida) into two subclasses Dicotyledoneae (Annonidae) and Monocotyledoneae (Liliidae). The Dicotyledoneae is further divided into 19 superorder’s which start with Annoniflorae and ends in Asteriflorae 41 orders, which start with Annonales and ends in Asterales and 297 families.
On the other hand, Monocotyledoneae is further divided into 9 superorders, which start with Liliiflorae and end in Commeliniflorae 12 orders, which start with Liliales and ends in Zingiberales.
Thus, he preferred the name Annonopsida for angiosperms, Annonidae for dicots, replacing Magnoliflorae by Annoniflorae and Magnoliales by Annonales. He, however, abandoned these nomenclatures and adopted the conventional names Magnoliopsida, Magnoliidae and Magnoliales since 1992.
Robert F. Thorne (1992) revised his classi­fication (“Classification and Geography of Flowering Plants”) and published it in Botanical Review. He followed the arrangements of different taxa in descending order such as subclasses (-idae), superorders (-anae), orders (-ales), sub­orders (-inae), families (-aceae), subfamilies (-oideae), and tribes (-ineae). He treated the flow­ering plants as Class with an initial bifurcation into two Subclasses, Magnoliidae (Dicots) and Liliidae (Monocots).
The subclass Magnoliidae has been divided into 19 superorders, 52 orders and the subclass Liliidae has been divided into 9 super- orders, 24 orders. The Magnoliidae starts with the order Magnoliales and ends in Lamiales, whereas the Liliidae starts with Triuridales and ends in Restionales.
1. Detailed information on molecular systematics and chemotaxonomy was presen­ted in the classification of Throne.
2. This system is highly phylogenetic where angiosperms are ranked as a class like those of other recent systems.
3. The traditional groups, dicots and mono­cots have been abolished and angio­sperms are divided into 10 subclasses which are in conformity with the recent phylogenetic thinking.
4. Several closely related taxa are placed nearer to one another, viz. the orders Malvales, Urticales, Rhamnales and Euphorbiales have been included under the superorder Malviflorae.
1. This system has no practical utility for identification and for adoption in Herbaria, because identification keys for genera are not provided.
2. The systematic position of the five genera — namely Emblingia, Guametela, Haptanthus, Heteranthia and Pteleocarpa — ha not been mentioned.
3. The placement of Asteridae before Lamiidae is not justified.
4. Segregation of Grewiaceae from Tiliaceae is highly questionable, as in the recent APG classification all the members of Tiliaceae, Bombacaceae and Sterculiaceae are placed under Malvaceae.