Agronomic adaptations to heat stress: sowing summer crops early in late winter.

Abstract

Abstract CONTEXT: Summer crops are exposed to heat and drought stresses at critical stages during and after flowering, and their intensity and frequency are likely to increase with climate change. Agronomic stress avoidance offers the opportunity to temporally separate critical crop stages from heat and drought events, though it might require sowing cold sensitive summer crops early in late winter into colder than recommended soil temperatures. There is a need to understand how cold is too cold to sow summer crops early in late winter as well as what are the yield benefits and risks. OBJECTIVE: Here we quantify the likely benefits and trade-offs of sowing sorghum, a summer cereal, early in late winter as an adaptation to the increased frequency and intensity of heat and water stresses during flowering and grain filling. METHODS: Two seasons of multi-environment ( n =32) genotype by management trials were conducted across sub-tropical and semi-arid regions of eastern Australia. Environments (E) consisted of the combination of seasons, sites, three times of sowing (early, spring, and summer), and the use of supplementary irrigation. At each E a factorial combination of four plant populations (M) and eight commercial sorghum hybrids (G) were sown with three replications. Crop growth and yield components were measured, and the APSIM model was used to simulate all trials and treatments to quantify risks, and derive insights into functional relationships between simulated and measured environmental co-variates, and measured crop traits. RESULTS: The tested hybrids showed small differences in cold tolerance during crop establishment. Across the tested environments the G×M combinations produced up to 60% variation in treatment yields across environment yields, that varied between <0.5 to about 10t ha -1 , this translated into a ~8-fold variation in water use efficiency. Significant G×E and M×E interactions were observed for grain yield components. No G×M or G×E×M interactions were observed on yield or yield components. Early sowing was associated to a reduced risk of heat stress, and water use transfer from vegetative to reproductive stages. Early sowing in late winter or early spring resulted in no significant yield gain or yield loss when all sites and seasons are included in the analysis, and between 1-2 t ha -1 yield gains when the hottest sites and seasons are considered separately. The likelihood and severity of terminal drought stresses was also reduced with late winter and early spring sowings, improving yield quality parameters. CONCLUSIONS: Early sowing of sorghum can reduce the likelihood of heat stresses around flowering as well as the likelihood of terminal drought stresses. Advantages include reduced yield losses in the hottest seasons, and a transfer of water use to grain filling stages that resulted in increased grain yield and improved grain quality parameters. IMPLICATIONS: Agronomic adaptations offer opportunity to quickly adapt to the increase in the frequency and intensity of extreme hot events during critical crop stages. However, for the practice to be de-risked there is need to increase cold and chilling tolerance in sorghum, and or identify interventions that enhance seed germination and seedling vigour when the crop is sown early into cold soils.