Effects of the ammonium stress on photosynthesis and ammonium assimilation in submerged leaves of Ottelia cordata - an endangered aquatic plant.

Although ammonium (NH-N) is an important nutrient for plants, increases in soil nitrogen (N) input and atmospheric deposition have made ammonium toxicity a serious ecological problem. In this study, we explored the effects of NH-N stress on the ultrastructure, photosynthesis, and NH-N assimilation of Ottelia cordata (Wallich) Dandy, an endangered heteroblastic plant native to China. Results showed that 15 and 50 mg L NH-N damaged leaf ultrastructure and decreased the values of maximal quantum yield (F/F), maximal fluorescence (F), and relative electron transport rate (rETR) in the submerged leaves of O. cordata. Furthermore, when NH-N was ≥ 2 mg L, phosphoenolpyruvate carboxylase activity (PEPC) and soluble sugar and starch contents decreased significantly. The content of dissolved oxygen in the culture water also decreased significantly. The activity of the NH-N assimilation enzyme glutamine synthetase (GS) significantly increased when NH-N was ≥ 10 mg L and NADH-glutamate synthase (NADH-GOGAT) and Fd-glutamate synthase (Fd-GOGAT) increased when NH-N was at 50 mg L. However, the activity of nicotinamide adenine dinucleotide-dependent glutamate dehydrogenase (NADH-GDH) and nicotinamide adenine dinucleotide phosphate-dependent glutamate dehydrogenase (NADPH-GDH) did not change, indicating that GS/GOGAT cycle may play an important role in NH-N assimilation in the submerged leaves of O. cordata. These results show that short-term exposure to a high concentration of NH-N is toxic to O. cordata.