Physiological mechanisms regulating source-sink interactions and grain yield formation in heat-stressed wheat

• Reproductive and grain filling phases of wheat crops are highly sensitive to heat stress. • Degree of damage to grain yield components of wheat depend on heat intensity, duration and the crop developmental phase. • High temperature influences both sink capacity and source strength to reduce wheat yield. • Inhibited carbon and water supplies to developing pollen in heat stressed wheat impair fertilization and grain setting. • High temperatures during grain-filling phase of wheat crops disrupt carbon assimilation, transport and starch deposition affecting final grain size and quality. Climate change and increasing atmospheric temperatures significantly challenge global wheat productivity and food security. Unpredictable weather patterns and frequent heatwaves, particularly during reproductive and grain-filling phases of wheat crops significantly reduce grain yield and quality. This review examines current literature on the impact of heat intensity and duration on grain yield components during these sensitive growth phases. Using the published literature, we quantified grain yield losses in response to varying heat intensity and duration during different developmental phases of wheat crops. The data suggest that grain number loss in wheat is poorly correlated with heat intensity and timing (0 to 15 days before anthesis) alone but it strongly responds ( r ²=0.45) to the number of hot days, with a 0.2 % loss of grains for each additional hot day with a temperature above optimum (16–22 °C). Further, for every 1 °C increase in mean temperature above optimum during sensitive phases (from -5 to 15 days since anthesis), individual grain weight decreases by approximately 2.1 %. This review also discusses how changes in source-sink regulation, particularly carbon assimilation, storage, transport and sugar metabolism in wheat under terminal heat are associated with grain yield losses. It also identifies the research gaps in heat wheat interaction, discussing potential opportunities (e.g., breeding and management) for sustaining wheat production under future hot environments.