Sulfur-induced salinity tolerance in lettuce is due to a better P and K uptake, lower Na/K ratio and an efficient antioxidative defense system

Lettuce is widely grown in several regions of the globe, even where the availability of good quality water is scarce. Thus, it is essential to develop research and techniques that allow this vegetable growth using water with high levels of salts. Therefore, this study aimed to evaluate the effects of sulfur foliar feeding on physiological and biochemical characteristics of lettuce plants grown in a hydroponic system under salt stress. The experiment was carried out in a 2 x 3 factorial arrangement, in which the plants were grown under two salinity levels in the nutrient solution (0 and 40 mM NaCl) and three doses of sulfur foliar fertilization (0.0, 1.5 and 3.0 g L-1), with four replicates. Salt stress reduced the growth, photosynthesis and stomatal conductance of lettuce, but these decreases were attenuated in sulfur-supplemented plants. Similarly, supplemental sulfur at 1.5 g L-1 also resulted in the best growth conditions for non-stressed plants. The salinity increased the Na+/K+ ratio in the leaves, but this behavior was less pronounced in sulfur-fertilized plants. From the group of plants grown under salt stress, we observed lower membrane damage in those supplemented with sulfur, which also presented higher photosynthetic rates, lower hydrogen peroxide levels and higher activity of antioxidant enzymes such as ascorbate peroxidase and catalase. In general, the salinity decreased the plant nutrient contents, but for potassium and phosphorus, we verified a significantly lower reduction in sulfur-fertilized plants. Overall, it may be said that supplemental sulfur attenuated the deleterious effects of salinity on growth and gas exchange of lettuce plants. Our study proved that those results are related, at least in part, to a more efficient antioxidative system, associated with a better uptake of phosphorus and potassium and a lower Na+/K+ ratio.