Heat shocks increasingly impede grain filling but have little effect on grain setting across the Australian wheatbelt

While sustained warmer temperatures accelerate crop phenology, short exposures to supra-optimal temperatures (‘heat shocks’) can stress wheat crops, with drastic impact on grain yield and quality. Crop models are useful to evaluate crop response to heat at various spatial-temporal scales. A modified version of the Agricultural Production Systems sIMulator (APSIM) was used to investigate wheat response to increased average temperature and heat shocks for three cultivars of contrasting maturity sown from April to July across the Australian wheatbelt over 1985–2017. Increasing average temperatures have resulted in shortening the crop cycle by 1.6 days per decade since 1985, for standard management practices (i.e. mid-maturing cultivar Janz sown on 15 May). While the occurrence of warm nights (Tmin>17 °C) has not significantly changed in most tested conditions, daytime heat shocks occur significantly more often across the wheatbelt, with an extra 0.6 and 1.2 hot days (Tmax>26 °C) per decade occurring around anthesis and during early-mid grain filling, respectively. This warming has resulted in losses in grain number and individual grain weight rising by 1.0 and 4.0% per decade nationally, with an overall yield loss of 4.6% per decade. Heat events were found to have a greater effect on individual grain weight than grain number in Australia. Nationally, for Janz sown on 15 May, heat shocks have caused on average a 3.6% loss in grain number and an 18.1% loss in individual grain weight, resulting in a 20.8% yield loss. Considering the impact of other abiotic stresses like drought, simulated yield of Janz sown on 15 May has decreased by 242 kg ha⁻¹ per decade on average, out of which 62 kg ha⁻¹ was due to heat shocks. As temperatures are projected to keep rising with increased heat events, adapting wheat crops to warmer environments appears to be a priority to maintain or enhance yield.