Zinc conferred cadmium tolerance in Lemna minor L. via modulating polyamines and proline metabolism

To investigate the alleviating effects of zinc (Zn) against gradually increasing cadmium (Cd) stress in aquatic environment, dry weight, polyamines and proline contents as well as metabolic enzymes were studied in Lemna minor L. after 4 days exposure. Dry weight was significantly decreased as the concentration of Cd increased. Cd stress also increased the putrescine (Put) content, while decreasing spermidine (Spd) content, whereas no significant change was observed in spermine (Spm) content. Hence, the ratio of (Spd + Spm)/Put rapidly reduced. In addition, the activities of arginine decarboxylase (ADC), ornithine decarboxylase and polyamine oxidase (PAO) enhanced accordingly. Cd treatment also induced a continuous accumulation of proline. Meanwhile, pyroline-5-carboxylate synthetase (P5CS) activity increased initially only to decline later and ornithine delta-aminotransferase (OAT) activity was only significantly stimulated at 4 mu M Cd, while the proline dehydrogenase (PDH) activity declined. However, Zn supplementation lowered accumulation of Put and proline contents and raised the Spd content, via decreasing the activities the ADC and PAO and keeping the activities of P5CS, OAT and PDH at the control levels, but failed to generate a statistically significant difference in content of dry weight. These results suggested that Zn application can maintain polyamines and proline homeostasis, thus conferring the tolerance of L. minor to Cd.