Increased input of nitrate and ammonium to ecosystems is mainly responsible for eutrophication, which is related to biological invasion of aquatic exotic plant species. In order to determine the effects of different nitrogen forms on its growth and physiological responses in eutrophic water, Eichhornia crassipes plants were grown for 28 days in 5mmol/L nitrogen-contained nutrient solutions varying in NH4+:NO3- ratio (100:0, 75:25, 50:50, 25:75, 0:100) in laboratory. The results showed that the NH+:N03- ratio of nutrient solution dramatically affected the plant performance of E. crassipes including relative growth rate, number of generated ramets, nitrate concentration in root and leaf. Furthermore, nitrate reductase activity increased with reduction of NH4+: NO3- ratio in culture solution demonstrating preferences of N sources as nitrate in E. crassipes. However, ammonium concentration and glutamine synthetase activity in leaf did not significantly change, and those in root significantly increased when proportion of NH4+ in nutrient solution increased indicating that E. crassipes plants are able to resist ammonium toxicity by regulation of ammonium transport and assimilation. In a word, the results suggested that establishment and expansion of E. crassipes was not likely to be limited by nitrogen forms.

Key words: ammonium tolerance, Eichhornia crassipes, NH4+-N, NOb--N, physiological response

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