Alleviation of High Temperature Stress in Bell Pepper through Foliar Application of Melatonin and Sodium Nitroprusside

The rise in global temperatures, resulting from global warming, imposes severe stress on plants, hindering their growth and development. This study aimed to investigate the effects of melatonin and sodium nitroprusside (SNP) on the growth of California Wonder green bell pepper under heat stress conditions. A factorial experiment using a completely randomized design with three replications was conducted. Plants were exposed to temperatures of 25°C (control), 35°C, and 40°C for 24 hours following foliar application of 0 µM, 50 µM, or 100 µM melatonin and SNP. Results showed that 100 µM melatonin increased shoot dry weight by 13.26% compared to the control. Under heat stress, leaf nitrogen content increased by 32.73% and 37.24% with 50 µM and 100 µM SNP, and by 9.61% and 23.72% with 50 µM and 100 µM melatonin, respectively. At 40°C, leaf potassium levels rose significantly—up to 72% with 100 µM SNP. Additionally, 100 µM SNP increased copper and iron levels by 17.96% and 202.98%, respectively. Foliar spraying with 100 µM melatonin improved photosynthetic traits (carotenoid and carbohydrate contents) and reduced malondialdehyde levels, enhancing stress tolerance. Hydrogen peroxide content decreased by 15.16% and 20.99% with 50 µM and 100 µM SNP, respectively, at 40°C. Both melatonin and SNP significantly enhanced the activity of antioxidant enzymes (superoxide dismutase, ascorbate peroxidase, and guaiacol peroxidase) under heat stress. Overall, 100 µM melatonin was most effective in mitigating heat-induced damage and improving the physiological and biochemical performance of green bell pepper seedlings.