The interaction between selenium (Se) and sulfur (S) was investigated in green and red lettuce grown in a hydroponic system contain-ing a basal mineral complement at contrasting levels of Se and S. The experiment aimed to evaluate the impact of Se and S supplementation on the quality traits of the lettuce, including soluble sugars, organic acids, total protein and nitrate (NO3−) to result in better lettuce yield with improving quality. The Se and S concentrations determined in the lettuce leaves showed complex dependence on the various levels of selenate (SeO42−) and sulfate (SO42−) supplied via the foliar application and the nutrient solution, respectively.
The foliar application of Se resulted in a synergistic interaction between both elements. With elevated Se and S (Se2/S2), Se accumulation was enhanced drastically in red lettuce, and in green lettuce, the S concentration increased significantly. Regarding carbohydrate accumulation, including water-soluble sugars, a lower Se dose under adequate S conditions enhanced glucose levels significantly by 3.2 and 2.1-fold in green and red lettuce, respectively. A synergistic interaction between Se and S was found following higher Se and S treatment (Se2/S2), resulting in a significant (p ≤ 0.05) reduction in glucose and fructose concentrations. However, higher S strikingly increased the accumulation of the reducing sugars (glucose and fructose) by 5.3 and 3.0-fold for glucose and 3.7 and 5.6-fold for fructose in green and red lettuce, respectively. Meanwhile, sucrose increased by 2.0-fold in red lettuce and remained unaltered in green lettuce. A low nitrate level (NO3−) was maintained in response to adequate or elevated S and Se levels. Se levels (Se1 and Se2) did not affect total protein concentration under contrasting sulfate/selenate. However, potential effects in terms of protein accumulation were associated with an adequate or increased S concentration. The data suggest that lower Se and sufficient S doses significantly increase glucose levels in both pigmented lettuce cultivars. Additionally, the synergistic interaction between Se and S could benefit the final nutritional value and quality of lettuce, especially for nitrate, where Se and S enrichment can ensure low nitrate levels