Melatonin Modulates Heat Stress Responses in Pepper Plants Under Variable Nitrogen Supply

Melatonin is a molecule in plants with antioxidant activity, which plays a role in increasing plant tolerance to biotic and abiotic stresses. We studied the effect of exogenous melatonin (100 µM) application on the physiological and biochemical processes in pepper plants (Capsicum annuum L. cv. Espinosa F1), in combination with three levels of nitrate concentration in the nutrient solution (5, 12 and 30 mM NO3−), and under optimum (26 °C) and heat stress (43 °C) conditions. The results showed that heat stress reduced fresh biomass and photosynthesis in all treatments, especially at 30 mM NO3−, indicating greater heat sensitivity under excess nitrogen. However, melatonin partially mitigated these effects, fully restoring fresh weight at 5 mM NO3− and increasing net CO2 assimilation rate (ACO2) by 15.18% compared with the corresponding heat-stressed group without melatonin. Heat stress increased glucose accumulation by 80.8%, 113.4%, and 74.9% at 5, 12, and 30 mM NO3−, respectively, whereas melatonin reduced this accumulation by 13.25%, 27.29%, and 38.47%. Spermidine increased under heat stress by 106.29%, 476.26%, and 348.96% at 5, 12, and 30 mM NO3−, respectively. Melatonin modulated heat stress responses depending on nitrogen supply, suggesting its potential use to improve pepper tolerance under high-temperature conditions.