Sorghum

Sorghum is closely related to maize within the PACMAD clade of grasses, and more distantly to the cereals of the BOP clade such as wheat and barley.[23]

Sorghum is in the grass family, Poaceae, in the subfamily Panicoideae, in the tribe Andropogoneae – the same as maize (Zea mays), big bluestem (Andropogon gerardi), and sugarcane (Saccharum spp.).

Accepted species recorded include:[24]

Sorghum flour on sale in a street market, Ethiopia, 2013

A plate of sorghum grain

Piece of sorghum bread contained in basket, Predynastic Egypt, c. 3100 BC (5,100 years ago). Egyptian Museum, Turin[25]

S. bicolor was domesticated from its wild ancestor more than 5,000 years ago in Eastern Sudan in the area of the Rivers Atbara and Gash.[26][27] It has been found at an archaeological site near Kassala in eastern Sudan, dating from 3500 to 3000 BC, and is associated with the neolithic Butana Group culture.[28] Sorghum bread from graves in Predynastic Egypt, some 5,100 years ago, is displayed in the Egyptian Museum, Turin, Italy.[25]

The first race to be domesticated was bicolor; it had tight husks that had to be removed forcibly. Around 4,000 years ago, this spread to the Indian subcontinent; around 3,000 years ago it reached West Africa.[26] Four other races evolved through cultivation to have larger grains and to become free-threshing, making harvests easier and more productive. These were caudatum in the Sahel; durra, most likely in India; guinea in West Africa (later reaching India), and from that race mageritiferum that gave rise to the varieties of Southern Africa.[26]

Domestication and races of sorghum

In the Middle Ages, the Arab Agricultural Revolution spread sorghum and other crops from Africa and Asia across the Arab world as far as Al-Andalus in Spain.[29] Sorghum remained the staple food of the medieval kingdom of Alodia and most Sub-Saharan cultures prior to European colonialism.[30]

Sweet sorghum was important to the sugar trade in the 19th century.[31] The price of sugar was rising because of decreased production in the British West Indies and more demand for confectionery and fruit preserves, and the United States was actively searching for a sugar plant that could be produced in northern states. The "Chinese sugar-cane", sweet sorghum, was viewed as a plant that would be productive in the West Indies.[32]

Most varieties of sorghum are drought- and heat-tolerant, nitrogen-efficient,[46] and are grown particularly in arid and semi-arid regions where the grain is one of the staples for poor and rural people. These varieties provide forage in many tropical regions. S. bicolor is a food crop in Africa, Central America, and South Asia, and is the fifth most common cereal crop grown in the world.[47][48] It is most often grown without application of fertilizers or other inputs by small-holder farmers in developing countries.[49]

Sorghum grows in a wide range of temperatures. It can tolerate high altitude and toxic soils, and can recover growth after some drought.[12] Optimum growth temperature range is 12–34 °C (54–93 °F), and the growing season lasts for ~ 115-140 days. [50] It can grow on a wide range of soils, such as heavy clay to sandy soils with the pH tolerance ranging from 5.0 to 8.5.[51] It requires an arable field that has been left fallow for at least two years or where crop rotation with legumes has taken place in the previous year.[52] Diversified 2- or 4-year crop rotation can improve sorghum yield, additionally making it more resilient to inconsistent growth conditions.[53] In terms of nutrient requirements, sorghum is comparable to other cereal grain crops with nitrogen, phosphorus, and potassium required for growth.[54]

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  Sorghum harvest at the shore of Lake Hayq, Ethiopia, 2012
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  Harvesting sorghum in Oklahoma, USA, with a combine harvester
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  Drying sorghum in the open air, Uganda, 2020
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  Women drying sorghum seeds by tossing them in trays, 2022

Insect damage is a major threat to sorghum plants. Over 150 species damage crop plants at different stages of development, resulting in significant biomass loss.[55] Stored sorghum grain is attacked by other insect pests such as the lesser grain borer beetle.[56] Sorghum is a host of the parasitic plant Striga hermonthica, purple witchweed; it can seriously reduce production.[57] Sorghum is subject to a variety of plant pathogens. The fungus Colletotrichum sublineolum causes anthracnose.[58] The toxic ergot fungus parasitises the grain, risking harm to humans and livestock.[59] Sorghum produces chitinases as defensive compounds against fungal diseases. Transgenesis of additional chitinases increases the crop's disease resistance.[60]

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  The lesser grain borer is a serious pest of sorghum.
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  Acervuli of Colletotrichum sublineolum, the cause of anthracnose, on sweet sorghum
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  Sorghum leaves showing anthracnose damage

The genome of S. bicolor was sequenced between 2005 and 2007.[61][62] It is generally considered diploid and contains 20 chromosomes,[63] however, there is evidence to suggest a tetraploid origin for S. bicolor.[64] The genome size is approximately 800 Mbp.[65]

Paterson et al., 2009 provides a genome assembly of 739 megabase. The most commonly used genome database is SorGSD maintained by Luo et al., 2016. A gene expression atlas is available from Shakoor et al., 2014 with 27,577 genes. For molecular breeding (or other purposes) an SNP array has been created by Bekele et al., 2013, a 3K SNP Infinium from Illumina, Inc.[66]

Agrobacterium transformation can be used on sorghum, as shown in a 2018 report of such a transformation system.[67] A 2013 study developed and validated an SNP array for molecular breeding.[66][68]

In 2013, China began purchasing American sorghum as a complementary livestock feed to its domestically grown maize. It imported around $1 billion worth per year until April 2018, when it imposed retaliatory tariffs as part of a trade war.[70] By 2020, the tariffs have been waived, and trade volumes increased again[71] before declining again as China began buying sorghum from other countries.[72] As of 2020, China is the world's largest sorghum importer, importing more than all other countries combined.[71]