Sulfur is an essential element in determining the productivity and quality of agricultural products. It is also an element associated with tolerance to biotic and abiotic stress in plants. In agricultural practice, sulfur has broad use in the form of sulfate fertilizers and, to a lesser extent, as sulfite biostimulants. When used in the form of bulk elemental sulfur, or micro- or nano-sulfur, applied both to the soil and to the canopy, the element undergoes a series of changes in its oxidation state, produced by various intermediaries that apparently act as biostimulants and promoters of stress tolerance. The final result is sulfate S+6, which is the source of sulfur that all soil organisms assimilate and that plants absorb by their root cells. The changes in the oxidation states of sulfur S0 to S+6 depend on the action of specific groups of edaphic bacteria. In plant cells, S+6 sulfate is reduced to S−2 and incorporated into biological molecules. S−2 is also absorbed by stomata from H2S, COS, and other atmospheric sources. S−2 is the precursor of inorganic polysulfides, organic polysulfanes, and H2S, the action of which has been described in cell signaling and biostimulation in plants. S−2 is also the basis of essential biological molecules in signaling, metabolism, and stress tolerance, such as reactive sulfur species (RSS), SAM, glutathione, and phytochelatins. The present review describes the dynamics of sulfur in soil and plants, considering elemental sulfur as the starting point, and, as a final point, the sulfur accumulated as S−2 in biological structures. The factors that modify the behavior of the different components of the sulfur cycle in the soil–plant–atmosphere system, and how these influences the productivity, quality, and stress tolerance of crops, are described. The internal and external factors that influence the cellular production of S−2 and polysulfides vs. other S species are also described. The impact of elemental sulfur is compared with that of sulfates, in the context of proper soil management. The conclusion is that the use of elemental sulfur is recommended over that of sulfates, since it is beneficial for the soil microbiome, for productivity and nutritional quality of crops, and also allows the increased tolerance of plants to environmental stresses.

This experiment was carried out in a greenhouse to evaluate the effects of selenium application (as Na2SeO3) on mineral concentration (as N, P, K, Ca and Se), biomass, yield and total antioxidant status (TAS) of tomato fruit. The study consisted of two experiments: an irrigation experiment with the application of selenium at 0, 2.5 and 5 mg L−1 on the fertilizer solution in soil and perlite; and the foliar application experiment with selenium application at 0, 10, and 20 mg L−1 in foliar spray every 20 days. Results showed that mineral content (as K, Ca, Mg and P) was not modified by selenium application. However, N decreased due to the Se applied in fertilizer solution 5 mg L−1, and a negative correlation was found between the selenium applied in foliar form and the nitrogen concentration. The Se concentration, TAS, and biomass increased in plants in all Se treatments. However, the best response in TAS and Se in fruits was observed with foliar spray every 20 days at concentrations of 10 mg L−1, without negative responses in biomass or mineral content.

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Iodine, silicon, and selenium are considered elements not essential for the metabolism of plants. However, these elements are vital for humans, and their presence as traces in food is beneficial. The use of I, Si, and Se in the fertilization programs of the plants allows, on the one hand, the mineral biofortification of the crops and, on the other hand, through mechanisms not yet fully understood, the production and accumulation of more antioxidants in the edible organs. This chapter provides an overview about the use of I, Si, and Se both for mineral biofortification and for the increase in the concentration of antioxidants in plants, with an emphasis on redox metabolism adjustments and antioxidant chemical species studied. The scope of the chapter is on horticultural species in the open field and under greenhouse or tunnels.

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