Sowing summer grain crops early in late winter or spring: effects on root growth, water use, and yield

Context Drought and extreme heat at flowering are common stresses limiting the yield of summer crops. Adaptation to these stresses could be increased by sowing summer crops early in late winter or early spring, to avoid overlap of drought and heat with critical crop stages around flowering. Though little is known about the effects of cold weather on root growth, water use and final grain yield in sorghum. Objective This study aims to explore the effects of cold conditions in early sowing sorghum on crop and root growth and function (i.e., water use), and final grain yield. Methods Two years of field experiments were conducted in the Darling and Eastern Downs region of Qld, Australia. Each trial consisted of three times of sowing (late winter, spring, and summer), two levels of irrigation (i.e., rainfed and supplementary irrigated), four plant population densities (3, 6, 9 and 12 pl m(-2)), and six commercial sorghum hybrids. Roots and shoots were sampled at the flag leaf stage on three times of sowing, two levels of irrigation, and three replications, for a single hybrid and a single plant population density (9 pl m(-2)). Crop water use and functional root traits were derived from consecutive electromagnetic induction (EMI) surveys around flowering. At maturity crop biomass, yield and yield components were determined across all treatments. Results The combinations of seasons, times of sowing and levels of irrigation created large variations in growth conditions that affected the growth and production of the crops. Early sowing increased yield by transferring water use from vegetative to reproductive stages and increasing grain numbers in tillers. Cold temperatures in the early sowing times tended to produce smaller crops with smaller rooting systems, smaller root-to-shoot ratios, and larger average root diameters. Total root length and root length density increased with increasing pre-flowering mean air temperatures up to 20 degrees C. Linear relationships were observed between an EMI derived index of root activity and the empirically determined values of root length density (cm cm(-3)) at flowering. Conclusions Sowing sorghum, a summer crop, early in late winter or spring transferred water use from vegetative stages to flowering and post-flowering stages increasing crop water use later in the season. Root length and root length density were reduced by pre-flowering mean temperatures lower than 20 degrees C, indicating a need to increase cold tolerance for early sowing. The higher grain numbers in early sown crops were related to higher grain numbers in tillers. The EMI derived index of root activity has a potential in the development of high throughput root phenotyping applications.