Crosstalk between Selenium and Sulfur Is Associated with Changes in Primary Metabolism in Lettuce Plants Grown under Se and S Enrichment

This study investigated the beneficial effects of selenium (Se) and sulfur (S) enrichment on the primary metabolism in butterhead lettuce. The plants were treated with three levels of Se via foliar application in the presence of two S levels in the nutrient solution under greenhouse conditions. The lettuce plants that were exposed to the lower selenate level (1.3 μM) in combination with the adequate and high S supplies (1 and 2 mM, respectively) accumulated 38.25 ± 0.38 µg Se g(−1) DM and 47.98 ± 0.68 µg Se g(−1) DM, respectively. However, a dramatic increase in the Se concentration (122.38 ± 5.07 µg Se g(−1) DM, and 146.71 ± 5.43 µg Se g(−1) DM, respectively) was observed in the lettuce heads that were exposed to the higher selenate foliar application (3.8 μM) in response to the varied sulfate concentrations (S1 and S2, respectively). Under higher Se and S supplies in the lettuce plants, the levels of organic acids, including malic acid and citric acid, decreased therein to 25.7 ± 0.5 and 3.9 ± 0.3 mg g(−1) DM, respectively, whereas, in the plants that were subjected to adequate S and lower Se fertilization, the malic acid, and citric acid levels significantly increased to 47.3 ± 0.4 and 11.8 ± 0.4 mg g(−1) DM, respectively. The two Se levels (1.3 and 3.8 μM) under the S1 conditions also showed higher concentrations of water-soluble sugars, including glucose and fructose (70.8.4 ± 1.1 and 115.0 ± 2.1 mg g(−1) DM; and 109.4 ± 2.1 and 161.1 ± 1.0 mg g(−1) DM, respectively), compared to the control. As with the glucose and fructose, the amino acids (Asn, Glu, and Gln) exhibited strikingly higher levels (48.7 ± 1.1 μmol g(−1) DM) under higher S and Se conditions. The results presented in this report reveal that the “crosstalk” between Se and S exhibited a unique synergistic effect on the responses to the amino acids and the soluble sugar biosynthesis under Se and S enrichment. Additionally, the Se-and-S crosstalk could have an important implication on the final nutritional value and quality of lettuce plants.