Abstract: In this study, the effects of foliar spraying different concentration (0.05-1 mmol·L^-1) spermidine (Spd) on the photosynthetic pigments, soluble protein content, antioxidant system, and mineral nutrition of Salvinia natans leaves under cadmium (Cd) stress were investigated, aimed to approach the action mechanisms of exogenous polyamines (PAs) in improving the adaptability of aquatic plants to heavy metals stress. The results showed that exogenous Spd could markedly mitigate the Cd stress to S. natans. It retarded the chlorosis, with chlorophyll a/b and total chlorophyll content increased by 18.97% and 43.96%, respectively, and increased the soluble protein content by 27.83%, compared with the control. SDS-PAGE revealed that the increase of soluble protein content was mainly of the expression increase of 5 polypeptides with apparent molecular weights 202.15, 77.67, 52.69, 41.83, and 25.35 kD. Exogenous Spd increased the activities of antioxidant enzymes SOD, POD and CAT and the contents of low molecular weight protective compounds AsA, GSH and Car, with an average increase of 12.44%, 66.90% and 51.23%, and 11.21%, 17.60% and 24.81%, respectively, and effectively maintained the balanced absorption of P, K, Fe, Na, Mg, Zn and Mn (with exception of Ca). In general, the optimum concentration range of exogenous Spd was 0.05-0.1 mmol·L^-1, which could enhance the resistance of S. natans to Cd toxicity by improving photosynthesis, stimulating the expression of protein, promoting the activity / content of antioxidant system, and keeping the balanced absorption of nutrient elements, etc.

Key words: abrupt change, climate change, variation of runoff and sediment flux, main cycle, human activity, Mun River Basin