Assessment of grain filling dynamics and yield response of bread wheat (Triticum aestivum L.) under terminal heat stress

Wheat yield is highly influenced by the grain filling duration (GFD), during which grains accumulate assimilates. The duration and rate of grain filling directly affect grain weight, thousand-grain weight and final yield, making this stage especially critical in heat-prone regions. To assess the impact of heat stress on grain filling and yield components, a two-year field study (2023–2024 and 2024–2025) was conducted at Krishi Vigyan Kendra, Banasthali Vidyapith, Tonk, Rajasthan, using diverse wheat genotypes procured from the Indian Institute of Wheat and Barley Research (IIWBR), Karnal. A paired t-test at <0.05 level were carried out between timely and late sown in GFD and in grain weight per plant to check the significant effect of sowing dates on wheat genotypes heat susceptibility index (HIS) values varied significantly among the 16 genotypes, indicating different levels of sensitivity to heat stress. A strong positive Pearson correlation (R2 = 0.7556) in 2023–2024 and (R2 = 0.697) in 2024–2025 was observed between HSI for grain filling duration and HSI for grain weight per plant. Genotypes HD2501, HD2985, HD2177 and DBW90 consistently showed low HSI values for both traits and were identified as heat-tolerant. In contrast, SWATI, UP2425 and DBW222, exhibited high susceptibility and were classified as heat-sensitive. Overall, grain filling duration emerged as a reliable selection trait for identifying heat-tolerant wheat genotypes for breeding and cultivation in heat-affected environments.