Adapting to heat stress by sowing summer grain crops early in late winter: Sorghum root growth, water use, and yield

CONTEXTDrought and extreme heat at flowering are common stresses limiting the yield of summer crops, which are likely to intensify and become more frequent as projected under climate change. OBJECTIVEThis study explores the idea that adaptation to these stresses could be increased by sowing summer crops early in late winter or spring, to avoid the overlap with critical crop stages around flowering. Here we report on the impacts of early sowing i.e., in late winter and spring on sorghum crop and root growth and function (i.e., water use), and final grain yield. METHODSTwo seasons of on-farm genotype (G) by environment (E) by management (M) sorghum experimentation were conducted in the Darling Downs region of Queensland, Australia. Each trial consisted of a factorial combination of three times of sowing (TOS, referred to as late winter, spring, and summer), two levels of irrigation, four plant populations, and six commercial genotypes. Treatments were replicated three times. Crop roots and shoot were sampled at the flag leaf stage for each TOS. Crop water use across the growing season was monitored using time-lapse electromagnetic induction (EMI) surveys. EMI was also used to calculate a root activity factor. Final grain yield and yield components were determined at maturity. RESULTSResults showed that TOS, irrigation levels, and their interactions significantly influenced crop root and shoot traits, water use, and yield, though results were not always consistent across seasons. In the first season which was dry and had large temperature contrasts between TOS, crop growth in the early sown crops was primarily limited by temperature. In contrast, the second season was much warmer and crop growth was instead primarily limited by water availability. Cold air and soil temperatures in the early sowing dates i.e., late winter and spring during the first season, lead to smaller crops with smaller rooting systems and root-to-shoot ratios, and roots having a larger average root diameter. In general terms, root length and root length density responded positively to increasing pre-flowering mean air temperatures ranging between 16 and 20{degrees}C, while root average diameters were larger below 19 {degrees}C or above 21{degrees}C. Early sowing advanced flowering and therefore decreased the risk of extreme heat during the critical stages around flowering and affected water use before and after flowering. The root activity factor was directly related to the crop root length density. The early sown crops increased yield by transferring water use from vegetative to reproductive stages. The larger yield of the early sown crop was associated with larger grain numbers, particularly for the tillers, and a larger water use efficiency. As expected, irrigated and summer-sown crops exhibited lowest water use efficiency. The early-sown crops left more water in the soil profile at maturity, particularly under irrigated conditions and with small plant populations. CONCLUSIONSWe conclude that early sown sorghum is a potential option to increase crop adaptation to hotter and drier environments. Here we propose that in the race to increase crop adaptation to heat stresses, plant breeding efforts should consider cold tolerance traits during crop germination, emergence, and early vegetative stages so that sorghum sowing windows could be significantly advanced.