The effects of exogenously-sourced NO (nitric oxide, as 100 µM SNP) and proline (50 mM) in the protection of the photosynthetic performance of wheat (Triticum aestivum L.) plants against heat stress were investigated. The study focused on the mechanisms of proline accumulation, activity, gene expression of antioxidant enzymes, and NO generation. Plants were exposed to a temperature of 40 °C for 6 h per day over 15 days, then allowed to recover at 28 °C. Heat-stressed plants showed increased oxidative stress, with higher levels of H2O2 and TBARS (thiobarbituric acid reactive substances) and increased proline accumulation, ACS activity, ethylene evolution, and NO generation, which in turn leads to increased accumulation of antioxidant enzymes and reduced photosynthetic attributes. In the tested wheat cultivar, the exogenous application of SNP and proline under heat stress improved the photosynthesis and reduced oxidative stress by enhancing the enzymatic antioxidant defense system. Potentially, the promoter AOX (alternative oxidase) played a role in maintaining redox homeostasis by lowering H2O2 and TBARS levels. The genes for GR antioxidant and photosystem II core protein encoding psbA and psbB were highly up-regulated in nitric oxide and proline treated heat-stressed plants, indicating that ethylene positively impacted photosynthesis under high temperature stress. Moreover, nitric oxide supplementation under high temperature stress optimized ethylene levels to regulate the assimilation and metabolism of proline and the antioxidant system, lowering the adverse effects. The study showed that nitric oxide and proline increased high temperature stress tolerance in wheat by increasing the osmolytes accumulation and the antioxidant system, resulting in enhanced photosynthesis.
Keywords: antioxidant enzymes; gas exchange; heat stress; nitric oxide; osmolytes.