Sorghum breeding

Sorghum, which is grown on approximately 40 million ha worldwide, is an especially important crop for resource-poor farmers in rainfed regions of the developing world as a result of its excellent level of adaptation to drought and heat stress (again related to its C4 metabolism) and its dual-purpose nature, it being used both in cooking and, as the stover, for animal feed.

Maintenance of stay-green under terminal drought conditions is used as a visual selection criterion for sorghum breeding under drought in the USA and Australia (Borrel et al., 2000 and references therein). Stay-green is valuable not only as a selection criterion for yield under drought but also because non-senescent genotypes accumulate more soluble sugars in stems during and after grain filling to improve nutritional and commercial value (McBee et al., 1983). Research has suggested that greater green leaf area duration in sorghum is the product of different combinations of three factors: green leaf area at flowering, time of onset of senescence and rate of senescence, all of which appear to be inherited independently (Borrell et al., 2000; Borrell and Hammer, 2000).

Despite the ease with which stay-green can be selected, a number of doubts have existed among breeders with respect to its merits, including: (i) whether the trait might be associated with smaller panicle size; (ii) that its selection under drought might result in a yield penalty under irrigated conditions; (iii) the magnitude of genetic gains associated with its selection; and (iv) whether it is also involved in lodging resistance. These issues were comprehensively addressed by Borrell and colleagues (2000) in a study of nine closely related hybrids that varied in rate of leaf senescence under contrasting water regimes in north-eastern Australia. While differences in yield among hybrids under well-watered conditions were negligible, under terminal water stress stay-green hybrids produced almost 50% more post-anthesis biomass than senescent lines, and green leaf area at maturity was strongly correlated with grain yield (Borrell et al., 2000). Mechanisms related to supply and demand for N during grain filling contribute to stay-green expression in sorghum and include: (i) a higher initial level of leaf N at flowering; (ii) more N uptake during grain filling; (iii) less remobilization of N from leaves; and (iv) larger retention of chloro-plast proteins until late senescence (Borrel and Hammer, 2000). Genetic studies have identified QTLs associated with stay-green (Harris et al., 2007).

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