Does maintaining green leaf area in sorghum improve yield under drought? I. Leaf growth and senescence

Production of sorghum [Sorghum bicolor (L.) Moench], an important cereal cr op in semiarid regions of the world, is often limited by drought. When wate r is limiting during the grain-filling period, hybrids possessing the stay- green trait maintain more photosynthetically active leaves than hybrids not possessing this trait. To improve yield under drought, knowledge of the ex tent of genetic variation in green leaf area retention is required. Field s tudies were undertaken in north-eastern Australia on a cracking and self-mu lching gray clay to determine the effects of water regime and hybrid on the components of green leaf area at maturity (GLAM). Nine hybrids varying in stay-green were grown under a fully irrigated control, postflowering water deficit, and terminal (pre- and postflowering) water deficit. Water deficit reduced GLAM by 67% in the terminal drought treatment compared with the fu lly irrigated control. Under terminal water deficit, hybrids possessing the B35 and KS19 sources of stay-green retained more GLAM (1260 cm(2) plant(-1 )) compared with intermediate (780 cm(2) plant(-1)) and senescent (670 cm(2 ) plant(-1)) hybrids. RQL12 hybrids (KS19 source of stay-green) displayed d elayed onset and reduced rate of senescence; A35 hybrids displayed only del ayed onset. Visual rating of green leaf retention was highly correlated wit h measured GLAM, although this procedure is constrained by an inability to distinguish among the functional mechanisms determining the phenotype. Link ing functional rather than phenotypic differences to molecular markers may improve the efficiency of selecting for traits such as stay-green.