Cadmium phytotoxicity, related physiological changes in Pontederia cordata: antioxidative, osmoregulatory substances, phytochelatins, photosynthesis, and chlorophyll fluorescence

Abstract

Pontederia cordata is a heavy metal accumulator, while the heavy metal tolerance mechanisms of this plant are not well understood. Hydroponic experiments were used to assess the effects of Cd2+ on antioxidative activities, osmoregulatory substances and photosynthesis in leaves. Exposure of 5mgL-1 Cd2+ for 7days, the photosynthetic apparatus functioned normally and sustained a relatively high photosynthetic rate, and good growth was observed. Under 50 and 75mgL-1 Cd2+, accelerated lipid peroxidation and increased peroxidase activity (POD; E.C.1.11.1.7) were detected, while no significant differences were observed in superoxide dismutase (SOD; E.C.1.15.1.1) and catalase (CAT; E.C.1.11.1.6) activities, as well as in lutein, ascorbic acid, and glutathione contains of leaves. Proline content increased, while soluble sugar and soluble protein contents decreased under 75mgL-1 Cd2+. Cd2+ at different concentrations induced a reduction in carotenoid, total carotenoid, and ascorbic acid-dehydroascorbate contents. A significant increase in phytochelatin content was induced by 75mgL-1. Chlorophyll content decreased under Cd stress and disturbed photosynthesis, causing dramatic reductions in photosynthetic parameters. Stomatal closure was responsible for a reduced photosynthetic rate under Cd2+ exposure. Cd2+ concentrations of no less than 25mgL-1 disorganized the photosynthetic apparatus, induced the partial closure, and decreased activity of the photosystem II (PS II) reaction center, thus disturbing light conversion and utilization, thereby decreasing the photosynthetic efficiency in PS II.