Abstract
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 ?M). Cadmium-treated wheat roots showed a reduction in growth (RGR) and the osmotic potential (??) and an increase in proline content (Pro). Although 100-?M 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 ?? 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.
Highlights
Excess accumulation of heavy metals in soil has become a critical environmental concern due to their diverse impacts on important metabolic processes
100 μM Cd stress triggered the activation of CAT and ascorbate peroxidase (APX) enzymes, these enzymes did not prevent the increase of H2O2 content
Addition of humic acid (HA) to stress-treated plants led to increased activities of superoxide dismutase (SOD), CAT, APX, monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) and GSH content and GSH/GSSG ratio
Summary
Excess accumulation of heavy metals in soil has become a critical environmental concern due to their diverse impacts on important metabolic processes. As a result of the deleterious effects of heavy metals such as cadmium on energy production and efficiency of other metabolic processes, reactive oxygen species (ROS) are produced in plants. Ertani et al (2013) reported that bio-stimulants increased tolerance to salinity stress in maize plants observed through enhancement of Na+/K+ and increased synthesis of flavonoids. Asli & Neumann (2009) reported that HA found in soil water can reduce root hydraulic conductivity and thereby reduce transpiration and growth of maize seedling leaves when applied in high concentrations. The aims of the present work were to determine: (i) the interaction between growth, the water status and HA treatment; (ii) the effect of HA on photosynthetic parameters; and (iii) the interaction between humic acid application and radical scavenging enzymes in wheat (Triticum aestivum L.) roots grown in a Cd-contaminated medium
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