An optimal combination of sowing date and cultivar could mitigate the impact of simultaneous heat and drought on rainfed wheat in arid regions

Drought and heat stress are the most important limiting factors for crop production. Simultaneous impacts of drought and heat stress during crop's sensitive growth phases could be more harmful than their individual impacts. Accordingly, we investigated several management practices including 4 sowing dates, 3 cultivars in combination with 4 initial soil water contents to investigate the simultaneous impact of drought and heat stress on rainfed wheat grain yield in different climates (57 locations) across Iran from 1980 to 2016. A modified version of the wheat module of the Agricultural Production Systems sIMulator (APSIM) modelling framework was adopted. Historical temporal trends showed that grain yield has increased by + 10 kg ha(-1) y(-1) in cold regions and decreased by 14.5 kg ha(-1) y(-1) in mild and hot regions since 1980 for Azar-2 (as common cultivar). Long-term simulations indicated that grain yields of 3.3 t ha(-1) could be obtained in rainfed agro-ecosystems under onlydrought growth conditions. By adding the impact of heat stress, however, average grain yield further declined to 2.9 t ha-1 indicating a reduction of 0.4 t ha(-1) owing to heat stress per se for the common cultivar. The results also showed that the long-term impact of drought and heat stress could be diminished with an optimal combination of sowing date, cultivar, and initial soil water depending upon climate classes. A combination of an earlymaturity cultivar ("Azar-2") with early sowing date (07-Oct) was recognized as the best management practice for cold and mild regions while the interaction of a mid-maturity cultivar (MMC), and an intermediate sowing date (20-Oct) was the best management practice for hot climates.