The possible role of extra magnesium and nitrogen supply to alleviate stress caused by high irradiation and temperature in lemon trees

Aims Citrus trees are subjected to variation of environmental conditions throughout the annual cycle, especially during the flowering and fruit set. Heat waves have affected the environmental conditions required for optimum crop growth and have reduced fruit yield and quality worldwide. This study aimed to evaluate an extra supply of magnesium (Mg) and/or nitrogen (N) to increase citrus tolerance to excessive increased irradiation and consequent air temperature. Methods Young lemon trees were grown in a greenhouse with extra Mg, N and Mg + N supply for five months and then exposed to heat for 12 days. After imposing stress condition, photosynthetic and enzymatic antioxidant system parameters were assessed to evaluate the impact of the extra nutrient supply in alleviating stress induced by combination of elevated irradiance and air temperature. Results Extra nutrient supply increased plant tolerance to the environmental stress, allowing maintenance of high photosynthetic and transpiration rates and low ratio of apparent electron transport rate per photosynthetic carbon assimilated. Enhanced Mg supply also increased activity of the antioxidant enzyme system and decreased the oxidative stress of plants. Trees treated with extra N exhibited increased chlorophyll concentrations and ability to harvest light energy. Conclusions Citrus trees under stressed condition, such as elevated air temperature and radiation, associated to heat waves, exhibited less damage in the photosynthetic mechanism and high activity of the antioxidant enzymatic system when supplied with extra nutrient. It was concluded that extra Mg and N supply are important tools to prevent damages and enhance lemon trees biochemical and physiological responses to alleviate the harmful effects of unfavorable environmental condition.