Sorghum - Bioenergy Crop Plants - page 484

1988). Sorghum, like maize and sugarcane, carries out C 4 photosynthesis, a specialization that
makes these grasses well adapted to environments subject to high temperature and water limitation
(Edwards et al. 2004). Sorghum is an important target of genome analysis among the C 4 grasses
because the sorghum genome is relatively small (730 Mbp) (Paterson et al. 2009), the cultivated
species is diploid (2 n = 20), and the sorghum germplasm is diverse (Dje et al. 2000; Menz et al.
2004; Casa et al. 2005). As a consequence, numerous sorghum genetic, physical, and comparative
maps have been constructed (Tao et al. 1998; Boivin et al. 1999; Peng et al. 1999; Klein et al. 2000,
2003; Haussmann et al. 2002; Menz et al. 2002; Bowers et al. 2003, 2005), a sorghum expressed
sequence tag (EST) project (Pratt et al. 2005) and associated microarray analyses of sorghum gene
expression have been carried out (Buchanan et al. 2005; Salzman et al. 2005), a comprehensive
analysis of sorghum chromosome architecture has been completed (Kim et al. 2005), and an
8 × draft sequence of the sorghum genome (about twice the size of rice) has been completed by the
U.S. Department of Energy Joint Genome Sequencing program (Paterson et al. 2009; http://www.
phytozome.net/sorghum). In addition, genetic maps have been assembled at Texas A&M University
and the University of Georgia (Menz et al. 2002; Bowers et al. 2003). The U.S. Department of
Agriculture (USDA) germplasm system maintains 42,614 accessions, of which more than 800 exotic
landraces have been converted to day length-insensitive lines to facilitate their use in breeding
programs. In total, there are approximately 168,000 sorghum accessions held at repositories
around the world. A set of mutation stocks developed by the USDA Plant Stress and Germplasm
Development Unit in Lubbock, TX (Xin et al. 2008) is extensive enough to provide mutations in all
of the genes in the sorghum genome. Such genomic tools, already in place, will greatly facilitate
the introduction of traits required to optimize sweet sorghum for bioenergy production schemes.

19.2 BIoloGy oF sorGhum

19.2.1 o rigin
The sorghum plant has undergone selection, domestication, and hybridization by humans to become
a crop that can produce grain and forage in low-rainfall, high-temperature environments, thereby
meeting the nutritional needs of people living in marginal areas. Vavilov (1951; cited by Mann et al.
1983), Snowden (1936; cited by Mann et al. 1983), Harlan and de Wet 1972, and Mann et al. (1983)
have all theorized that Africa is the center of origin for sorghum. Although the exact location is
debatable, domestication can be attributed to the selection for sorghums without grain shattering
to improve harvest ability (Mann et al. 1983). Early records of sorghum show its existence in India
in the first century AD (Bennett et al. 1990), China in the 13th century (Undersander et al. 1990),
and in the United States in the Seventeenth century (Agyeman et al. 2002). Currently sorghum is
the fifth most important crop produced worldwide (Rooney and Awika 2004) and has widespread
production in sub-humid and semi-arid regions in both tropical and temperate climates.

19.2.2 c laSSification and d omEStication
Sorghum is a self-pollinating species consists of cultivated and wild species. S. bicolor subsp. bicolor
(2 n = 20) is the taxon that includes the agronomically important grain races bicolor, caudatum,
durra, guinea, and kafir and ten intermediate races. Additionally, hybrid races can be identified from
crosses among the basic races. There are more than 35,000 accessions of sorghum that are been
maintained at the germplasm collection centers in the United States with similar numbers being
maintained at the International Crops Research Institute for the Semi Arid Tropics (ICRISAT). The
U.S. and ICRISAT collections have been evaluated phenotypically, and the USDA has developed
a description list of more than 75 descriptors (http://www.ars-grin.gov/cgi-bin/npgs/html/desclist.
pl?69). Very high levels of genetic diversity exist among and within races. Most U.S. sorghums are
derived from Kafirn × milo crosses.