Insights into the effects of reproductive phase thermal stress on physiology and yield‐related traits of rice cultivars

High temperatures lead to a significant drop in rice ( Oryza sativa L.) productivity worldwide, especially during the reproductive stage. This research was piloted to evaluate 10 promising rice cultivars for their ability to withstand high‐temperature stress (HS) during the reproductive phase and to assess the impact on physiology and yield. Rice varieties IR‐6 and KSK‐133 were used as checks. Seven‐day heat shock treatments were imposed by shifting the plant pots in the glasshouse at the 50% flowering stage by maintaining ∼40°C–45°C/30°C day/night temperatures. While most rice lines retained adequate water contents, AYT‐3 demonstrated the best heat tolerance due to its cooler canopy temperature. Potassium concentration decreased under heat stress in all genotypes, with the most significant decline occurring in AYT‐7, whereas Ca 2+ ion concentration demonstrated a fluctuating trend under HS. Genotypes AYT‐1 and AYT‐5 showed maximum production of proline and soluble sugars under stress. Plant height was improved in some lines while suppressed in others under heat stress; however, maximum height was noted in AYT‐1. The highest grain weight per plant and dry biomass accumulation was measured in AYT‐2. Diversity among genotypes was verified by multivariate analysis, principal component analysis, and a heatmap with hierarchical clustering, which classified genotypes as tolerant, moderately tolerant, and susceptible. Four advanced lines were found tolerant to high temperature, five as medium tolerant, and two genotypes as heat sensitive. These results provide insights into the resilience of rice lines to reproductive phase thermal stress and offer valuable data for breeding to develop more heat‐resilient cultivars.