Pre-treatment of fulvic acid plays a stimulant role in protection of soybean (Glycine max L.) leaves against heat and salt stress

The purpose of this study was to illuminate the effects of fulvic acid in plants’ stress signaling pathway. 2.0 mg/l fulvic acid was sprayed on soybean leaves for 3 days at 12 h intervals, followed by treatment of 150 mM NaCl or exposed to heat stress at 35°C for 2 h over 2 days. Pre-treatment with fulvic acid increased the relative water content (RWC), antioxidant enzyme, isoenzyme activities (SOD, APX, GST), as well as alleviated the stress-induced oxidative damage by decreasing the levels of hydrogen peroxide (H2O2) and malondialdehyde (MDA). In addition, the application of fulvic acid under salt stress induced rubisco expression only at 12 h, while it induced the expression of cytochrome c oxidase at 6 h and 12 h. On the other hand, fulvic acid under heat stress induced significant expression of both rubisco and cytochrome c oxidase at 6 h and 12 h. However, under high salinity conditions, fulvic acid suppressed the transcript levels of Hsp70, while it induced increases in Hsp70 levels under heat treatment at 6 h. As a result, in this study, fulvic acid played the role of a regulator and stimulant in stress response of soybean leaves.