Exogenous Application of Melatonin Affects Sulfur Metabolism in Soybean Crop

Melatonin (MT) may be related to plant growth and act on stress tolerance, and there is evidence of its role in the acquisition and assimilation of sulfur (S) remains with little information. The research hypothesis is that the application of melatonin helps in the mitigation of the stress of sulfur in soybean metabolism. In this research, we report that low S conditions affect growth by reducing chlorophyll content, photosynthesis, and soybean yield. One of the key nutrients for soybean cultivation is S; however, the management of S fertilization in tropical soils is a major challenge for agricultural producers because the currently adopted production systems can cause deficiencies in this essential element, thereby affecting crop productivity. S is the primary structural component of plants, some amino acids, and vegetable proteins, and it is involved in the formation of chlorophyll. The application of MT is a new alternative strategy for S stress reduction in plants as it is directly related to S metabolism, and recent studies have shown its potential in attenuating S stress. However, the role of MT in S translocation remains unclear. In the present study, we hypothesized that the application of MT to the soil can increase S translocation in soybean plants, thereby mitigating its deficiency. This study aimed to investigate the effects of MT on chlorophyll content, photosynthesis, transpiration, and soybean productivity in the presence of S. A 2 x 2 factorial scheme with four repetitions was used: the first factor was MT (presence and absence) and the second factor was S (presence and absence), with six repetitions. The presence of MT and S favored plant growth, chlorophyll content, photosynthetic rates, S translocation, and soybean productivity. We conclude that exogenous MT application can improve S metabolism, due to greater tolerance to S stress, which is a common condition in tropical soils, thus increasing S utilization efficiency and soybean crop productivity. However, given the complexity of soil-plant-MT-S-environment interactions, more research needs to be carried out to better investigate these effects of exogenous MT application on soil and plants.