Aside from its importance in human and animal health, low levels of foliar-applied selenate (SeO4) can be advantageous in the presence of sulfur (S), contributing to improved growth, nutrient uptake, and crop quality. A hydroponic experiment in a growth chamber explored the interactive influence of Se and S on micronutrients and several quality indices, such as soluble sugars, organic acids, and total protein concentrations in spinach (Spinacia oleracea L.). Three levels of S (deprivation, adequate, and excessive) with varying quantities of Se (deficient, moderate, and higher) were examined in combination. Under S starvation and along with S nourishment in plant parts, Se treatments were found to cause noticeable variations in plant biomass and the concentrations of the examined elements and other quality parameters. Both Se levels promoted S accumulation in S-treated plants. Although the Se treatment had the opposite effect in shoots, it had a favorable impact on minerals (apart from Mn) in roots grown under S-limiting conditions. The S and Se relationship highlighted beneficial and/or synergistic effects for Mn and Fe in edible spinach portions. Reducing sugars were synergistically boosted by adequate S and moderate Se levels in roots, while in shoots, they were accumulated under moderate-or-higher Se and excessive S. Furthermore, the concentration of the quantified organic acids under S-deficient conditions was aided by various Se levels. In roots, moderate Se under high S application enhanced both malic acid and citric acid, while in the edible parts, higher Se under both adequate and elevated S levels were found to be advantageous in malic acid accumulation. Moreover, by elevating S levels in plant tissues, total protein concentration increased, whereas both moderate and high Se levels (Se1 and Se2) did not alter total protein accumulation in high S-applied roots and shoots. Our findings show that the high S and medium Se dose together benefit nutrient uptake; additionally, their combinations support soluble sugars and organic acids accumulation, contributing ultimately to the nutritional quality of spinach plants. Moreover, consuming 100 g of fresh red spinach shoot enriched with different Se and S levels can contribute to humans’ daily micronutrients intake.
