Silicon-mediated alleviation of combined salinity and cadmium stress in date palm (Phoenix dactylifera L.) by regulating physio-hormonal alteration

We investigated the physio-molecular effects of separate and combined cadmium (Cd; 200 μM) and salinity (NaCl; 100 mM) stress on date palm during silicon (Si; 1.0 mM) applications. The results showed that exogenous Si led to significant improvements in plant growth, as well as physiology when compared with non-Si-treated seedling under stressed (Cd/NaCl) conditions. Interestingly, Si application led to lower metal (Cd) uptake and enhanced plant macronutrient uptake under combined stress, in turn, alleviating the combined salinity- and Cd-induced oxidative stress by lowering the lipid peroxidation rate, and peroxidase and catalase activities. Furthermore, ascorbate peroxidase level and the cytosolic Cu/Zn superoxide dismutase expression were significantly enhanced by Si application under combined stress. We further analyzed the effect of Si on modulation of stress-related hormonal crosstalk. Si markedly downregulated endogenous salicylic acid, jasmonic acid, and abscisic acid under NaCl stress and combined NaCl–Cd stress. However, during Cd toxicity alone, Si showed varying accumulation of these phytohormones. The results suggest that hindering the Cd uptake and enhancing silicon accumulation ultimately led to improvement of biomass and efficiency of the antioxidant system for alleviating combined stress. Moreover, higher transcript accumulation of PROLINE TRANSPORTER 2 and GAPDH and downregulation of ABA RECEPTOR by Si treatment under combined stress in date palm seedlings indicate the stress-ameliorative role of Si. The study provides evidence of the positive influence of Si on alleviating the combined toxicity of Cd and NaCl in date palm and can be further extended for field trials in Cd- and salinity-affected areas.