The Exogenous Application of Non-Toxic Sulfur Contributes to the Growth-Promoting Effects of Leaf Lettuce ( Lactuca sativa L. var. crispa )

Sulfur is an essential nutrient—along with nitrogen, phosphorus, and potassium—for plant growth and development. Sulfur is mostly supplied to crops through soil fertilizers. However, chemical fertilizers are overused to increase crop yields despite environmental threats. The proper use of chemical fertilizers positively affects crop growth and yield increase. Regardless, residues from misuse threaten not only the soil ecosystem, but also the marine ecosystem. Therefore, the need to minimize chemical fertilizer abuse is imperative. This article reports that sulfur can be applied to crop leaves as nontoxic sulfur (NTS) in trace amounts to positively affect plant hormones, chloroplast content, and ROS scavenging system, thereby promoting growth, and increasing crop yields. Furthermore, NTS and microelements, the micronutrients calcium and magnesium, produced a synergistic effect when applied together, and NTS enhanced the expression of auxin and gibberellin-related genes. Additionally, chlorophyll content was increased, and ROS scavenging ability was greatly improved. Therefore, NTS can effectively deliver potent growth-promoting functions of plants faster and safer than did soil fertilizers and consequently increase crop yield. This finding is a new strategy to replace soil chemical fertilizers in supplying sulfur. It is potentially valuable for increasing crop yields and can be applied to other crops.