Understanding the Role of Gibberellic Acid and Paclobutrazol in Terminal Heat Stress Tolerance in Wheat

Understanding the physiological mechanism of tolerance under stress conditions is an imperative aspect of the crop improvement programme. The role of plant hormones is well-established in abiotic stress tolerance. However, the information on the role of gibberellic acid (GA) in abiotic stress tolerance in late sown wheat is still not thoroughly explored. Thus, we aimed to investigate the role of endogenous GA(3) level in stress tolerance in contrasting wheat cultivars, viz., temperature-tolerant (HD 2643 and DBW 14) and susceptible (HD 2189 and HD 2833) cultivars under timely and late sown conditions. We created the variation in endogenous GA(3) level by exogenous spray of GA(3) and its biosynthesis inhibitor paclobutrazol (PBZ). Tolerant genotypes had higher antioxidant enzyme activity, membrane stability, and photosynthesis rate, lower lipid peroxidase activity, and better growth and yield traits under late sown conditions attributed to H(2)O(2) content. Application of PBZ escalated antioxidant enzymes activity and photosynthesis rate, and reduced the lipid peroxidation and ion leakage in stress, leading to improved thermotolerance. GA(3) had a non-significant effect on antioxidant enzyme activity, lipid peroxidation, and membrane stability. However, GA(3) application increased the test weight in HD 2643 and HD 2833 under timely and late sown conditions. GA(3) upregulated GA biosynthesis and degradation pathway genes, and PBZ downregulated kaurene oxidase and GA(2)ox gene expression. GA(3) also upregulated the expression of the cell expansins gene under both timely and late sown conditions. Exogenous GA(3) did not increase thermotolerance but positively affected test weight and cell expansins gene expression. No direct relationship existed between endogenous GA(3) content and stress tolerance traits, indicating that PBZ could have conferred thermotolerance through an alternative mechanism instead of inhibiting GA(3)biosynthesis.