Modulation of Key Physio-Biochemical and Ultrastructural Attributes after Synergistic Application of Zinc and Silicon on Rice under Cadmium Stress

Excessive industrialization and the usage of pesticides plague the farming soils with heavy metals, reducing the quality of arable land. Assessing phytoavailability of cadmium (Cd) from growth medium to plant system is crucial and necessitates precise and timely monitoring of Cd to ensure food safety. Zinc (Zn) and silicon (Si) have singularly demonstrated the potential to ameliorate Cd toxicity and are important for agricultural production, human health, and environment in general. However, Zn-Si interaction on Cd toxicity alleviation, their effects and underlying mechanisms are still fragmentarily understood. Seven treatments were devised besides control to evaluate the single and combined effects of Zn and Si on the physio-biochemical attributes and ultrastructural fingerprints of Cd-treated rice genotypes, i.e., Cd tolerant “Xiushui-110” and Cd sensitive “HIPJ-1”. Supplementation of both Zn and Si promoted plant biomass, photosynthetic parameters, ionic balance, and improved chloroplast ultrastructure with minimized Cd uptake and malondialdehyde (MDA) content due to the activation of antioxidant enzymes in Cd stressed plants. The combined effects of 10 μM Zn and 15 μM Si on 15 μM Cd displayed a greater reduction in Cd uptake and root-leaf MDA content, while enhancing photosynthetic activity, superoxide dismutase (SOD) activity and root-leaf ultrastructure particularly in HIPJ-1, whilst Xiushui-110 had an overall higher leaf catalase (CAT) activity and a higher root length and shoot height was observed in both genotypes compared to the Cd 15 µM treatment. Alone and combined Zn and Si alleviation treatments reduced Cd translocation from the root to the stem for HIPJ-1 but not for Xiushui-110. Our results confer that Zn and Si singularly and in combination are highly effective in reducing tissue Cd content in both genotypes, the mechanism behind which could be the dilution effect of Cd due to improved biomass and competitive nature of Zn and Si, culminating in Cd toxicity alleviation. This study could open new avenues for characterizing interactive effects of simultaneously augmented nutrients in crops and provide a bench mark for crop scientists and farmers to improve Cd tolerance in rice.