Fulvic acid‐releasing chitosan nanoparticles promote the growth and salt stress tolerance of soybean plants

Nanotechnology offers several advantages over conventional inputs, with widespread application in agriculture. The current climate change crisis has accelerated the accumulation of salts in soils, which is a major challenge to global food security. Here, we synthesized fulvic acid-releasing chitosan nanoparticles (Ch-FANPs) for promoting soybean growth and salt stress tolerance. In a screening hydroponic experiment, 0.1 mM Ch-FANPs promoted plant growth and enhanced the growth parameters of pot-grown soybean plants significantly and modulated stomatal movement under control as well as salt stress conditions induced by 150 mM NaCl. Salt stress affected overall plant growth and reduced the chlorophyll content. However, plants treated with Ch-FANPs not only accumulated significantly higher chlorophyll under both control and salt conditions but also enhanced several above- and below-ground growth parameters by more than 50%. Interestingly, the Ch-FANP-treated salt-exposed plants accumulated ~30% less soluble proteins than untreated salt-stressed plants. Ch-FANPs-mediated protection against salt stress was related to the activation of antioxidant machinery as the highest ascorbate peroxidase (APX) activity was recorded in Ch-FANPs-treated salt-stressed plants along with significantly low MDA and H₂O₂ contents. ICP-MS analysis showed a tremendously higher accumulation of Na⁺ ions (~35 ppm) in the leaves of salt-stressed plants compared to 19 ppm Na⁺ ions when also treated with Ch-FANPs. Salt-exposed plants treated with Ch-FANPs had the highest K⁺ content (~76 ppm) and Ca²⁺ (62 ppm). Furthermore, Ch-FANPs-mediated protection against salt stress was associated with a significant increase in the expression of salt stress marker genes GmSOS1, GmSOS2, GmNHX1, and GmP5CS1.