Early Heat Stress in Wheat: Osmolytes and Antioxidants as Candidate Biochemical Indicators of Tolerance

Heat stress is posing a serious threat to crop physiology due to a consistent increase in global temperatures. In crops, the seedling stage is the most temperature‐sensitive stage. The present study was conducted to identify candidate biochemical indicators associated with the early heat response of 18 wheat ( Triticum aestivum L.) genotypes at seedling stage to find the reason for good seedling establishment and high yield of wheat genotypes grown under early sown conditions. Twelve wheat genotypes, along with six heat tolerant check cultivars, were exposed to two temperature conditions, i.e., 25 °C (control) and 35 °C (high stress). A significant increase in proline content, α‐tocopherol, reducing and nonreducing sugars in 12‐days‐old seedlings genotypes was recorded at 35 °C. In high yielding wheat genotypes, increased total sugar content was accompanied by enhanced α‐amylase activity and reduced starch content. Antioxidative potential was upregulated at 35 °C via enhanced activities of superoxide dismutase (SOD) (47.03%–88.61%), ascorbate peroxidase (APX) (33.03%–88.73%), catalase (CAT) (47.37%–77.38%), peroxidase (POD) (24.10%– 66.64%), and glutathione reductase (GR) (24.01%–85.16%). Reactive oxygen species (ROS) indicators, viz., hydrogen peroxide (91.67% and 93.44%) and malondialdehyde (80.51% and 88.43%) content increased in low yielding genotypes EWS 171 and EWS 433 at 35 °C. Correlation analysis showed strong positive correlations among antioxidant enzymes (GR, APX, SOD, and POD) and sugars (sucrose and reducing sugar), indicating a possible physiological association among these traits under heat stress rather than a demonstrated regulatory pathway, while total free amino acids and H 2 O 2 showed significant negative correlations with these enzymes, suggesting their inhibitory or stress‐indicator roles. Moreover, principal component analysis (PCA) further elaborates the role of enzymatic and nonenzymatic antioxidants in plant defense mechanism under heat stress conditions and clearly indicates the distinct behavioral patterns of wheat genotypes under early sown conditions.