Leaf Ultrastructure and Metabolic Responses to Nickel Stress in Calopogonium mucunoides and Canavalia ensiformis

Nickel (Ni) is an essential micronutrient for plants, but its excess can disrupt physiological functions and impair growth. This study aimed to evaluate the morphophysiological responses of Calopogonium mucunoides and Canavalia ensiformis under increasing Ni concentrations (0, 120, 240, and 360 mg dm⁻³). Gas exchange, antioxidant enzyme activity, and leaf micromorphology were assessed using infrared gas analysis and scanning electron microscopy. C. mucunoides exhibited stable trichome and stomatal densities, increased chlorophyll concentration, and elevated peroxidase activity at 240 mg dm⁻³ Ni, but showed membrane instability and reduced water use efficiency. In contrast, C. ensiformis displayed higher morphological and physiological plasticity, evidenced by pronounced variation in stomatal conductance and superoxide dismutase activity (up to 54% RDPI), while maintaining photosynthetic rate and pigment stability across treatments. This enhanced plasticity under Ni stress suggests a greater acclimation capacity and highlights C. ensiformis as a promising candidate for phytoremediation of Ni-contaminated soils. These findings advance understanding of legume acclimation to nickel stress and provide valuable insights for sustainable agriculture in contaminated environments.