Interactive effects of heat and drought on wheat yield change from synergistic to antagonistic as their severity increases

• Combined drought and heat stress cause greater yield losses compared to individual stress. • Interaction between drought and heat vary from antagonisms to synergism. • A meta-regression model can predict yield losses from combined stress. Wheat yield is increasingly threatened by the co-occurrence of drought and heat stress, particularly under climate change. While both stresses independently reduce yield, their combined effects on wheat production remain under-explored. We meta-analysed 141 factorial combinations of heat and drought stress in field and controlled environment experiments. On average, combined stress led to 49% yield loss, indicating no between-stressor interaction as independent heat and drought caused 27% and 22% losses. However, combined stress impacts varied widely around this average value, but were predictable using a meta-regression model based upon the yield losses from independent stressors. Independent stressors causing relatively moderate yield losses led to greater losses in combination than expected from their independent effects. Conversely, severe stressors, more commonly applied in controlled environment experiments and resulting in extreme yield failure, interacted antagonistically. Our results therefore show that the impacts of combined stress will vary between contexts, requiring site-specific adaptation to future climate change, which can be informed by relatively simple experiments focusing on the independent stressors.