Effect of dwarfism and endogenous gibberellin modulation via growth regulators on heat stress tolerance in wheat

Wheat yield is hampered by heat stress during the reproductive stage, and climate change is expected to worsen this effect. Wheat semi-dwarf alleles reduced height, and improved productive tillers ensured food security. However, it is unclear how semi-dwarfism, and exogenous application of growth regulators interact to alleviate heat stress. Therefore, the present study attempted to understand the effect of gibberellic acid 3 (GA₃) and paclobutrazol (PBZ) on semi-dwarf (GA-insensitive) and tall (GA-sensitive) wheat genotypes under stressed environments. The exogenous application of GA₃ and PBZ both influenced endogenous GA₃ levels in leaves; however, PBZ had a more pronounced effect on GA-sensitive genotypes compared to GA-insensitive genotypes under heat stress. It was found that the reduction in photosynthesis rate under stress could be effectively mitigated by PBZ application in GA-insensitive genotypes. Our study also concludes that GA₃ plays a minor role in conferring heat tolerance, as neither GA-sensitivity nor exogenous GA₃ application significantly affected antioxidant enzymes activities or reactive oxygen species (ROS). In contrast, PBZ application enhanced heat stress tolerance by increasing antioxidant enzyme levels and membrane stability, while reducing ROS and lipid peroxidation. Additionally, PBZ's inhibition of kaurene oxidase suggests the activation of alternative pathways for the biosynthesis of defense compounds, such as diterpenoid phytoalexins, which further promote stress tolerance. Although GA₃ had a negligible effect on stress tolerance, it significantly increased 1000-grain weight in GA-sensitive genotypes compared to GA-insensitive ones. This suggests that the greater yield reduction in GA-insensitive genotypes under stress was primarily due to a decrease in grain and tiller numbers rather than test weight.