Suitable Humic Acid Application Methods to Maintain Physiological and Enzymatic Properties of Bean Plants Under Salt Stress

The effects of different humic acid (HA) application methods (control, soil, foliar and soil+foliar) and increasing salt levels (0, 50, 100 and 150 mM) on relative water content (RWC), loss of turgidity (LOT), electrolyte leakage (EL), lipid peroxidation-malondealdehit (MDA) content, some antioxidant enzyme activities and micronutrient contents of bean plants were investigated. Salt treatments caused a decrease in RWC of leaves and increased LOT, EL and MDA. However, HA applications, especially soil+foliar, contributed to maintaining leaf RWC and turgor pressure, and reduced EL and MDA. The enzyme activities caused by salt stress differed with the HA applications. Soil HA application increased antioxidant enzyme activities, while foliar and soil+foliar HA application increased superoxide dismutase (SOD) and glutathione reductase (GR) activities, and decreased catalase (CAT) and ascorbate peroxidase (APX) activities. The concentrations of Zn, Fe, Cu, and Mn decreased with salt applications and the responses to HA applications were different for micronutrients in aerial parts (APs) and roots. However, micronutrient concentrations in APs increased with the HA applications. Soil and soil+foliar applications of HA, considering the increase in root micronutrients, improved uptake of microelements under salt stress. Negative relationships between physiological properties and antioxidant enzyme activities and microelement concentrations reveal that HA reduces the damage of salt stress by increasing the micronutrient concentrations in bean APs. The results revealed that soil+foliar HA application improves microelement uptake, cell and oxidative damage due to salt stress reduces in bean plants, as well as provides multiple benefits against stress as maintaining RWC and turgor pressure.