Humic acid protects against oxidative damage induced by cadmium toxicity in wheat (Triticum aestivum) roots through water management and the antioxidant defence system

Humic compounds like humic acid (HA) promote ecosystem health by stabilising soil structure and promoting plant development. However, the amount needed is a limiting factor. The use of biostimulants based on HA is an effective way to eliminate oxidative damage caused by heavy metals such as cadmium (Cd) in plants. The aim of this study was to assess the effects of humic acid (HA; 750 and 1500 mg L-1) on growth, the osmotic potential, the antioxidant system, radical content and lipid peroxidation in wheat (Triticum aestivum) roots treated alone or in combination with Cd stress (100 and 200 μM). Cadmium-treated wheat roots showed a reduction in growth (RGR) and the osmotic potential (ψπ) and an increase in proline content (Pro). Although 100-μM Cd stress induced the activities of catalase (CAT) and ascorbate peroxidase (APX), hydrogen peroxide (H2O2) accumulation in roots exposed to stress was not prevented. The membrane of roots showed stress-dependent lipid peroxidation (TBARS content). Application of HA in combination with stress alleviated RGR and ψπ by promoting water intake. Humic acid reduced levels of H2O2 and TBARS through activation of superoxide dismutase (SOD) and CAT. Application of HA under stress also induced enzymes and non-enzymatic substances included in the ascorbate-glutathione cycle such as APX, monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) and glutathione (GSH), in addition to which it increased GSH/GSSG ratios. These results indicate that HA alleviated the negative effects of Cd-induced oxidative damage in wheat roots through regulation of growth, osmotic adjustment, radical accumulation and the action of antioxidant systems.