Physiological responses of rice varieties to heat stress: exploring photosynthetic efficiency, oxidative stress, and yield resilience

Heat stress is a critical limiting factor in rice production, and a comprehensive understanding of the physiological traits contributing to yield under such conditions remains incomplete. This study evaluated the effects of heat stress on six rice varieties: Nagina22 (N22), NERICA4 (NER), Nipponbare (NPB), IR64, Berenj-I-Mahin (BIM), and Berenj-I-Luk (BIL) exposed to maximum temperatures exceeding 40 °C from flowering to the grain-filling stage. Results demonstrated a strong correlation between the photosynthetic rate and both 1000-grain weight and transpiration rate at the flowering stage across all varieties. Under heat stress, NER, BIM, BIL, and IR64 exhibited elevated levels of malondialdehyde, indicative of oxidative damage, and sustained significant heat injury compared to ambient conditions. Heat stress notably impaired pollen viability and spikelet fertility, reducing crop yields. In contrast, the heat-tolerant variety N22 maintained the highest spikelet fertility rate, effectively regulated transpiration cooling, and significantly enhanced yield by maintaining a lower leaf temperature. The findings indicate that photosynthetic efficiency and oxidative stress during the flowering stage critically influence spikelet fertility and overall yield in rice under heat stress. This research provides valuable insights for rice breeders and agronomists, encouraging the development of heat-tolerant varieties and adaptive strategies to mitigate the adverse impacts of elevated temperatures during critical reproductive stages.