Interplaying roles of silicon and proline effectively improve salt and cadmium stress tolerance in Phaseolus vulgaris plant.

The interplaying defensive roles of silicon (Si) and proline (Pro) in improving growth and yield attributes, physio-biochemical attributes, and antioxidant defense systems in common bean plant grown under saline (NaCl) and/or cadmium (Cd²⁺) stress were assessed. Seed were sown in plastic pots filled with sand-free ions as a growing medium that watered with a ½-strength Hoagland's nutrient solution. Twenty five days after planting, pots were split into 4 plots; control (no stress), 150 mM NaCl (salt stress), 1.5 mM Cd²⁺ in CdCl₂ (Cd²⁺ stress), and 100 mM NaCl + 1.0 mM Cd²⁺ (salt + Cd²⁺ stress). Four treatments; foliar spray with distilled water, 6 mM Si (in K₂SiO₃.nH₂O) solution, 6 mM Pro solution, and a combination of Si and Pro were allotted under each of the 4 plots. The experimental layout was a completely randomized design with 15 replicates. Compared to control, NaCl or Cd²⁺ stress significantly (P ≤ 0.05) reduced plant growth and yield attributes, leaf contents of chlorophylls, carotenoids, N, P, and K⁺, K⁺/Na⁺ ratio, RWC, MSI, Pn and Tr, while elevated significantly leaf EL, leaf contents of proline, soluble sugar, glutathione, MDA, Na⁺, and root, leaf and pod contents of Cd²⁺. The activities of antioxidant enzymes were also raised. The combined stress (NaCl + Cd²⁺) was more influential. Addition of Si and/or Pro for common bean plants under NaCl and/or Cd²⁺ stress significantly enhanced all investigated attributes of physiology, morphology, and biochemistry, and further increased the activities of antioxidant enzymes. Supplementation of Si + Pro was the best treatment having more positive influential, especially reducing the Cd²⁺ content in Phaseolus vulgaris pods to the limits (0.27 mg kg^-1) for legumes. Therefore, this combined treatment is recommended to use for alleviating environmental stress effects, especially salinity and Cd²⁺ for common bean production.