Photosynthetic Responses of Pontederia cordata to Cadmium Stress: Anatomical Structure, Ultrastructure, Physiology, and Gene Expression

<i>Pontederia cordata</i>, a horticulturally valuable ornamental plant, exhibits cadmium (Cd) tolerance, but its photosynthetic response to Cd stress has not been fully elucidated. Here, we employed hydroponics to investigate the effects of varying Cd concentrations on the leaf morphology, anatomy, photosynthetic physiology, and carbon metabolism enzymes in <i>P. cordata</i>. At 0.1 mM Cd, the plants grew well and showed no toxicity, with a normal chloroplast ultrastructure and chlorophyll <i>a</i> fluorescence parameters. Higher Cd concentrations (0.2-0.4 mM) disrupted chloroplasts, reduced chlorophyll content, and suppressed photosynthetic enzyme expression, thereby impairing light energy conversion efficiency and photosynthetic performance. In response, <i>P. cordata</i> adapted by maintaining the thickness of the palisade tissue, increasing the ratio of palisade tissue thickness to spongy tissue thickness, stabilizing carotenoid levels, enhancing non-photochemical quenching processes, and increasing the content of key photosynthetic enzymes and soluble sugars. These findings advance the theoretical understanding of photosynthetic adaptation mechanisms to heavy metal stress.