SummaryThe positive effects of stay-green quantitative trait loci on grain yield of sorghum under post-anthesis drought are emergent consequences of their effects on water-use patterns, resulting from changes in pre-anthesis canopy size.

Retention of green leaf area at maturity (GLAM), known as stay‐green, is used as an indicator of postanthesis drought resistance in sorghum [Sorghum bicolor (L.) Moench] breeding programs in the USA and Australia. The critical issue is whether maintaining green leaves under postanthesis drought increases grain yield in stay‐green compared with senescent hybrids. Field studies were undertaken in northeastern Australia on a cracking and self‐mulching gray clay. Nine closely related hybrids varying in rate of leaf senescence were grown under two water‐limiting regimes, post‐flowering water deficit and terminal (pre‐ and postflowering) water deficit, and a fully irrigated control. Under terminal water deficit, grain yield was correlated positively with GLAMtrue(r=0.75**true) and negatively with rate of leaf senescence true(r=−0.74**true) Grain yield also increased by ≈0.35 Mg ha−1 for every day that onset of leaf senescence was delayed beyond 76 DAE in the water‐limited treatments. Stay‐green hybrids produced 47% more postanthesis biomass than their senescent counterparts (920 vs. 624 g m−2) under the terminal water deficit regime. No differences in grain yield were found among eight of the nine hybrids under fully irrigated conditions, suggesting that the stay‐green trait did not constrain yield in the well‐watered control. The results indicate that sorghum hybrids possessing the stay‐green trait have a significant yield advantage under postanthesis drought compared with hybrids not possessing this trait.

Sorghum is an important source of food, feed, and biofuel, especially in the semi-arid tropics because this cereal is well adapted to harsh, drought-prone environments. Post-flowering drought adaptation in sorghum is associated with the stay-green phenotype. Alleles that contribute to this complex trait have been mapped to four major QTL, Stg1-Stg4, using a population derived from BTx642 and RTx7000. Near-isogenic RTx7000 lines containing BTx642 DNA spanning one or more of the four stay-green QTL were constructed. The size and location of BTx642 DNA regions in each RTx7000 NIL were analysed using 62 DNA markers spanning the four stay-green QTL. RTx7000 NILs were identified that contained BTx642 DNA completely or partially spanning Stg1, Stg2, Stg3, or Stg4. NILs were also identified that contained sub-portions of each QTL and various combinations of the four major stay-green QTL. Physiological analysis of four RTx7000 NILs containing only Stg1, Stg2, Stg3, or Stg4 showed that BTx642 alleles in each of these loci could contribute to the stay-green phenotype. RTx7000 NILs containing BTx642 DNA corresponding to Stg2 retained more green leaf area at maturity under terminal drought conditions than RTx7000 or the other RTx7000 NILs. Under post-anthesis water deficit, a trend for delayed onset of leaf senescence compared with RTx7000 was also exhibited by the Stg2, Stg3, and Stg4 NILs, while significantly lower rates of leaf senescence in relation to RTx7000 were displayed by all of the Stg NILs to varying degrees, but particularly by the Stg2 NIL. Greener leaves at anthesis relative to RTx7000, indicated by higher SPAD values, were exhibited by the Stg1 and Stg4 NILs. The RTx7000 NILs created in this study provide the starting point for in-depth analysis of stay-green physiology, interaction among stay-green QTL and map-based cloning of the genes that underlie this trait.

Production of sorghum [Sorghum bicolor (L.) Moench], an important cereal crop in semiarid regions of the world, is often limited by drought. When water 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 extent of genetic variation in green leaf area retention is required. Field studies were undertaken in northeastern Australia on a cracking and self‐mulching 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 fully irrigated control. Under terminal water deficit, hybrids possessing the B35 and KS19 sources of stay‐green retained more GLAM (1260 cm2 plant−1) compared with intermediate (780 cm2 plant−1) and senescent (670 cm2 plant−1) hybrids. RQL12 hybrids (KS19 source of stay‐green) displayed delayed onset and reduced rate of senescence; A35 hybrids displayed only delayed onset. Visual rating of green leaf retention was highly correlated with measured GLAM, although this procedure is constrained by an inability to distinguish among the functional mechanisms determining the phenotype. Linking functional rather than phenotypic differences to molecular markers may improve the efficiency of selecting for traits such as stay‐green.