Cassava
Manihot esculenta, commonly called cassava, manioc, or yuca, is a woody shrub of the spurge family, Euphorbiaceae, native to South America, from Brazil, Paraguay and parts of the Andes. Although a perennial plant, cassava is extensively cultivated in tropical and subtropical regions as an annual crop for its edible starchy tuberous root. Cassava is predominantly consumed in boiled form, but substantial quantities are processed to extract cassava starch, called tapioca, which is used for food, animal feed, and industrial purposes. The Brazilian farofa, and the related garri of West Africa, is an edible coarse flour obtained by grating cassava roots, pressing moisture off the obtained grated pulp, and finally drying and roasting it.
Cassava is the third-largest source of carbohydrates in food in the tropics, after rice and maize, making it an important staple; more than 500 million people depend on it. It offers the advantage of being exceptionally drought-tolerant, and able to grow productively on poor soil. The largest producer is Nigeria, while Thailand is the largest exporter of cassava starch.
Cassava is grown in sweet and bitter varieties; both contain toxins, but the bitter varieties have them in much larger amounts. Cassava has to be prepared carefully for consumption, as improperly prepared material can contain sufficient cyanide to cause poisoning. The more toxic varieties of cassava have been used in some places as famine food during times of food insecurity. Farmers may however choose bitter cultivars to minimise crop losses.
Etymology
The generic name Manihot and the common name "manioc" both derive from the Guarani name mandioca or manioca for the plant. The specific name esculenta is Latin for 'edible'. The common name "cassava" is a 16th century word from the French or Portuguese cassave, in turn from Taíno caçabi. The common name "yuca" or "yucca" is most likely also from Taíno, via Spanish yuca or juca.Description
Cassava is a perennial species that is usually harvested within one year after planting. The harvested part is the storage root, which is long and tapered, with an easily detached rough brown rind. The white or yellowish flesh is firm and even in texture. Commercial cultivars can be wide at the top, and some long, with a woody vascular bundle running down the middle. The tuberous roots are largely starch, with small amounts of calcium, phosphorus, and vitamin C. Content of total pro-vitamin A carotenoids has been increased through conventional breeding from a maximum of 10.3 to 24.3 υg/g FW basis Cassava roots contain little protein, whereas the leaves are rich in it, except for being low in methionine, an essential amino acid.Cassava is monoecious, with separate male and female flower on the same plant, arranged in inflorescences. These develop at the apex of the growing stem; buds can sprout to form new branches below the inflorescence, enabling continued vegetative growth. Farmers prefer a non-branching plant because it is easier to collect, transport, and store. However, the erect form complicates plant breeding as flowers are scarce or absent; breeders make use of photoperiod extension, pruning, and plant growth regulators to encourage flowering.
Genome
The complete and haplotype-resolved African cassava genome has been reconstructed and made available using the Hi-C technology. The genome shows abundant novel gene loci with enriched functionality related to chromatin organization, meristem development, and cell responses. Differentially expressed transcripts of different haplotype origins were enriched for different functionality during tissue development. In each tissue, 20–30% of transcripts showed allele-specific expression differences with <2% of direction-shifting. Despite high gene synteny, the HiFi genome assembly revealed extensive chromosome rearrangements and abundant intra-genomic and inter-genomic divergent sequences, with significant structural variations mostly related to long terminal repeat retrotransposons.Although smallholders are otherwise economically inefficient producers, they are vital to productivity at particular times. Small cassava farmers are no exception. Genetic diversity is vital when productivity has declined due to pests and diseases, and smallholders tend to retain less productive but more diverse gene pools.
The molecular genetics of starchy root development in cassava have been analyzed and compared to other root and tuber crops, including possible roles for
orthologs.
History
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 ago. Forms of the modern domesticated species can also be found growing in the wild in the south of Brazil. By 4600 BC, 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. It became 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 cassava 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.
Portuguese traders introduced cassava to Africa from Brazil in the 16th century. Around the same period, it was introduced to Asia through Columbian Exchange by Portuguese and Spanish traders, who planted it in their colonies in Goa, Malacca, Eastern Indonesia, Timor and the Philippines. 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 and Vietnam. Cassava is sometimes described as the "bread of the tropics" but should not be confused with the tropical and equatorial bread tree ', the breadfruit ' or the African breadfruit . This description definitely holds in Africa and parts of South America; in Asian countries such as Vietnam fresh cassava barely features in human diets.
Cassava was introduced to East Africa around 1850 by Arab and European settlers, who promoted its cultivation as a reliable crop to mitigate the effects of drought and famine.
There is a legend that cassava was introduced in 1880–1885 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, cassava was cultivated in the state before that time. Cassava is called kappa or maricheeni in Malayalam, and tapioca in Indian English usage.
Cultivation
Optimal conditions for cassava cultivation are mean annual temperatures between, annual precipitation between, and an annual growth period of no less than 240 days. Cassava is propagated by cutting the stem into sections of approximately, these being planted prior to the wet season. Cassava growth is favorable under temperatures ranging from, but it can tolerate temperatures as low as and as high as. These conditions are found, among other places, in the northern part of the Gulf Coastal Plain in Mexico. In this part of Mexico the following soil types have been shown to be good for cassava cultivation: phaeozem, regosol, arenosol, andosol and luvisol.Harvesting
Before harvest, the leafy stems are removed. The harvest is gathered by pulling up the base of the stem and cutting off the tuberous roots.Handling and storage
Cassava deteriorates after harvest, when the tuberous roots are first cut. The healing mechanism produces coumaric acid, which oxidizes and blackens the roots, making them inedible after a few days. This deterioration is related to the accumulation of reactive oxygen species 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. Post-harvest deterioration is a major obstacle to the export of cassava. Fresh cassava can be preserved like potato, using thiabendazole or bleach as a fungicide, then wrapping in plastic, freezing, or applying a wax coating.While alternative methods for controlling post-harvest deterioration have been proposed, such as preventing reactive oxygen species effects by using 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 with improved durability after harvest, achieved by different mechanisms. One approach used gamma rays to try to silence a gene involved in triggering deterioration; another strategy selected for plentiful carotenoids, antioxidants which may help to reduce oxidization after harvest.