Effect Of Cadmium On Growth And Oxidative Metabolism As Well As Role Of Salicylic Acid In Alleviating Cadmium Toxicity In Two Leguminous Species With Different Tolerance

Cadmium (Cd) is one of the most toxic environmental pollutants for plants. Cd can interfere with numerous biochemical and physiological processes including photosynthesis, respiration, nitrogen and protein metabolism, and nutrient uptake. In the present study, ten species were compared in solution culture experiments to investigate the resistance of leguminous plants to excessive Cd. Based on the seed germination and seeding growth, common vetch (Vicia Sativa) and mung bean (Phaseolus aureus) were chosen as two different Cd-resistant and Cd-sensitive species.Compared with control, the seed soaking with10-100μM Cd (CdCl2) for12h significantly decreased protease activities and the contents of free amino acid in the germinating seeds of mung bean and common vetch, and increased the content of soluble protein and the electrolyte leakages of germinating seeds.100μM Cd decreased the root vitality, chlorophyll content, photosynthetic rate, nitrate reductase activity, and concentrations of nitrate nitrogen, Ca, Mg, Fe, Zn and Mo in leaves of both species. The decrease of these parameters was more pronounced in seedlings of mung bean than those in common vetch.It was studied that the changes in activity of catalase (CAT), ascorbate peroxidase (APX) superoxide, dismutase (SOD), guaiacol peroxidase (GPOD) and glutathione reductase (GR) and contents of reduced glutathione and ascorbic acid in leaves of mung bean and common vetch seedlings under Cd stress. Results showed that the activities of CAT, APX and SOD initially increased and then decreased and the activities of GPOOD significantly increased in leaves of both species during the9-day of100μM Cd treatment. For the activity of GR, similar change was observed in the leaves of mung bean. In the leaves of common vetch, GR activity increased with the increasing Cd exposure time and reduced glutathione initially decreased and then increased. In contrast, the contents of reduced glutathione and ascorbic acid decreased with Cd treatment time in mung bean. Compared with those in mung bean, the activities of CAT, APX and SOD were higher in common vetch leaves than in mung bean leaves under Cd stress.Excess Cd treatment induced oxidative stress. The effects of Cd on the accumulation of H2O2and O2’-in leaves of mung bean and common vetch were investigated using both histochemical and cytochemical methods. Cadmium at100μM obviously increased the production of O2·-and H2O2, and the activities of plasma membrane-bound NADPH oxidases and the symplastic and apoplastic activities of SOD and APX in the leaves of both species. Apoplastic GPOD activity was significantly induced in the leaves of both species, particularly in mung bean with100μM Cd. In the leaves of both species treated with100μM Cd, H2O2-dependent CeCl3precipitate was mainly localized in cell walls and the corners of the extracellular space as well as a small amount in chloroplasts of the mesophyll cells, and O2·-dependent DAB precipitates were observed mainly in chloroplasts, cell walls, and extracellular spaces. It was evident that more H2O2and O2·-were produced in the chloroplasts and extracellular spaces of mung bean than in those of common vetch. Experiments with diphenyleneiodonium as an inhibitor of NADPH oxidase and NaN3as an inhibitor of peroxidase showed that the majority of Cd-induced ROS production in the leaves of both species may be from plasma membrane-bound NADPH oxidase and apoplastic GPOD. Compared with those in the species common vetch, Cd-induced O2·-and H2O2production, NADPH oxidase and apoplastic GPOD activity increase were more pronounced in mung bean. In contrast, common vetch had higher leaf symplastic SOD and APX activities than mung bean.The possible mediatory role of salicylic acid (SA) in protecting plants from Cd toxicity was studied. Treatment with50μM Cd significantly increased the electrolyte leakages and total thiobarbituric acid reactive substances content in roots of mung bean and common vetch. Seeds soaking with100μM SA for16h or seedlings pretreatment with100μM SA for16h alleviated the negative effect of Cd on growth of both species. The alleviation of Cd toxicity was more pronounced by seed soaking with SA than seedlings pretreatment with SA. Seeds soaking with100μM SA decreased the production of O2·-and H2O2in root apoplast of both species. This effect was more evident in the roots of mung bean than in roots of common vetch. The data suggest that the beneficial effect of SA during an earlier growth could be related to up-regulation of antioxidant enzyme SOD and APX.