Sulfur Supply Modulates Selenium Biofortification, Yield, and Nutritional Quality in Leafy Greens Grown in an Indoor Vertical Farm

Selenium (Se) is essential for human health, but its dietary intake remains insufficient in many regions, increasing interest in biofortification strategies. Indoor hydroponic systems offer a controlled and resource-efficient approach for producing Se-enriched leafy greens. Sulfur (S), an essential macronutrient for plants, affects Se uptake and metabolism due to their chemical similarity. In this study, we investigated the effects of Se supplementation (2 µM Na2SeO4) under two S levels (0.65 and 1.3 mM, supplied as MgSO4) on Se accumulation, yield, and nutritional quality in lettuce, rocket, and basil grown in an indoor nutrient film technique (NFT) system. High S supply increased biomass in lettuce and basil by 16% and 25%, respectively, while rocket remained unaffected. The effect of Se on biomass depended on S status and species. Under low S conditions, Se increased lettuce biomass but reduced basil biomass, whereas no significant effects were observed under high S. Sulfur strongly reduced Se accumulation in all species, leading to lower contributions to the recommended daily allowance (RDA). Under low S conditions, Se-biofortified lettuce, rocket, and basil provided 111%, 179%, and 37% of the RDA per serving, respectively, whereas these values decreased to 56%, 64%, and 20% under high S. Sulfur and Se treatments also influenced macro- and micro-nutrient composition in a species-dependent manner. Se supplementation consistently reduced total phenolic content and antioxidant capacity (DPPH and FRAP) across all species. Total ascorbic acid was affected only in rocket, with the highest levels observed under high S without Se. These findings highlight a clear antagonistic interaction between S and Se in hydroponic systems and demonstrate the need to optimize S supply to balance yield and Se biofortification without compromising nutritional quality in leafy greens grown in indoor systems.