Physiological and Biochemical Bases of Foliar Silicon-Induced Alleviation of Cadmium Toxicity in Wheat

Silicon (Si) is a beneficial nutrient for crop plants, and is gaining increased attention because of its involvement in regulation of abiotic stress tolerance. The present study was carried out to explore the underlying physiological and biochemical mechanisms of foliar Si-induced enhancement in wheat tolerance against cadmium (Cd) stress. Surface sterilized seeds of two wheat cultivars, viz., Inqalab-91 (sensitive) and Sahar-2006 (tolerant), were sown in plastic pots. Foliar application of Si (3 mM) was done at 20 days after sowing under control (-Cd) and Cd stress (25 mg kg(-1)soil) conditions. Plants were harvested after 3 weeks of foliar spray, and data regarding morphological growth, oxidative metabolism, photosynthetic pigments, gas exchange attributes, metabolite accumulation, and ion homeostasis were recorded. The experimental treatments were arranged in completely randomized design with four replicates per treatment. Cadmium stress was found to significantly decrease the morphological growth, photosynthesis, and photosynthetic pigments in both wheat cultivars, while significantly increased malondialdehyde (MDA) content, hydrogen peroxide (H2O2) content, and electrolyte leakage (EL) that triggered oxidative stress. The Cd-induced changes in superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), glutathione S-transferase (GST), and glutathione reductase (GR) activities varied with cultivar and Si application. Plants treated with Si revealed better growth, higher accumulation of total soluble proteins, anthocyanins, total soluble sugar, flavonoids, and phenolics contents, and efficient antioxidant defense system as indicated by increased ascorbic acid and glutathione pools, and higher activities of SOD, POD, CAT, and APX particularly under Cd stress. Application of Si reduced Cd-mediated oxidative stress and increased photosynthetic pigments and net photosynthetic rate in both wheat cultivars. Further, Si application enhanced mineral ions in normal (-Cd) and Cd-stressed plants, and significantly decreased the Cd uptake. The Si-induced effects were more prominent in Sahar-2006 than that in Inqalab-91 for most of the studied traits, particularly under stress conditions. In short, foliar application of Si alleviates the adverse effects of Cd on wheat growth by strengthening antioxidant defense system, enhancing metabolite accumulation, improving plant nutrient status, and decreasing the Cd uptake.