Exploring spikelet fertility and HSP70 expression as indicators of high temperature tolerance in rice

Developing heat tolerant rice varieties is imperative for safeguarding global food security against climate change. This study establishes the heat shock protein HSP70 as a key molecular biomarker for heat tolerance by linking its expression dynamics to physiological and agronomic resilience. We evaluated four rice cultivars under heat stress (40 ± 2 °C) by analyzing relative membrane permeability (RMP), hydrogen peroxide (H2O2), and malondialdehyde (MDA) to assess membrane thermostability and oxidative damage, alongside spikelet fertility as a yield component. A novel, integrative metric the Heat Tolerance Coefficient (HTC) was developed to quantify resilience based on fertility retention under stress. Our results revealed profound genotypic variation. The heat tolerant cultivars (K-95, IR-6) exhibited robust, early induction of HSP70, which correlated strongly with superior membrane stability (RMP < 35%), minimal oxidative damage (H2O2 ~ 30–34 µM/g FW; MDA ≤ 0.6 µM/g FW), and high spikelet fertility retention (82%), resulting in a high HTC (up to 91). In contrast, sensitive cultivars (DR-92, DR-83) showed delayed HSP70 expression, culminating in severe membrane damage (RMP >64%), significant oxidative stress, and a drastic decline in fertility (to 60%), reflected by a low HTC (as low as 65). The HTC effectively integrates molecular, physiological, and agronomic data into a single powerful index for selection. We conclude that early and sustained HSP70 expression is a hallmark of heat tolerance and propose that the HTC provides a robust, practical framework for accelerating the breeding of climate-resilient rice.