Calcium application enhances yield and phytoremediation efficiency of Pueraria thomsonii Benth in cadmium-contaminated soils

Cadmium (Cd)-contaminated soil presents substantial risks to both agricultural sustainability and human health. Pueraria thomsonii Benth. has emerged as a promising candidate for phytoremediation, owing to its high biomass production and Cd accumulation capacity. However, the physiological mechanisms underlying its Cd stress response and the associated safety of its edible product, arrowroot, remain poorly understood. This study evaluated the potential of exogenous calcium (Ca) to ameliorate Cd toxicity, improve phytoremediation efficiency, and ensure the safety of arrowroot. A two-year pot experiment was conducted using a factorial design comprising three Cd concentrations-control without exogenous Cd addition (0 mg kg⁻¹, Cd0), moderately contaminated (1.5 mg kg⁻¹, Cd1.5), and severely contaminated (15 mg kg⁻¹, Cd15) and four Ca application rates-0 (Ca0), 335 (Ca1), 446 (Ca2), and 670 (Ca3) mg kg⁻¹. Cd stress markedly suppressed plant growth, whereas Ca application significantly mitigated Cd-induced phytotoxicity. The efficacy of Ca was dose-dependent and varied with Cd stress; Ca3 (670 mg kg⁻¹) was most beneficial under control conditions to moderate Cd stress, while Ca2 (446 mg kg⁻¹) was most effective under severe Cd stress. These optimal Ca treatments enhanced yield by 18-58% and biomass production by 13-52%. This enhanced growth was associated with improved photosynthetic performance, reflected in increased net photosynthetic rate, stomatal conductance, chlorophyll content, and SPAD values, and strengthened antioxidant defense, evidenced by elevated peroxidase and superoxide dismutase activities and reduced malondialdehyde content. The Cd concentration in arrowroot consistently remained low (0.08-0.29 mg kg⁻¹) and below acceptable limits across all treatments. These findings demonstrate, for the first time, that strategic Ca application to Cd-contaminated soils can synergistically enhance Cd tolerance, boost phytoremediation potential, and keep the Cd content of arrowroot below the permissible limit, with optimal Ca application rates depending on Cd stress levels.