As a paradoxical nutrient, ammonium can promote plant growth under moderate concentration, while toxicity will happen when it is supplied exceeding. Toxicity on plants caused by excess ammonium has become a hotspot in recent years, especially on terrestrial plants, plants respond through several mechanisms, such as the regulation of C-N balance and transformation of metabolic pathways. However, submerged macrophytes have much differences between terrestrial plants, most submerged macrophytes can survive in water without soil, furthermore, they can absorb ammonium through the leaves from water column directly. In such cases, whether the mechanism of ammonium detoxication in submerged macrophytes is the same as terrestrial plants is still unknown. 

In order to explore the ammonium detoxication mechanisms on submerged macrophytes, researchers from Key Laboratory of Aquatic Botany and Watershed Ecology of Wuhan Botanical Garden and Sino-Africa Joint Research Center worked together. They firstly exploited six common submerged macrophytes and selected the most ammonium-tolerant species Myriophyllum spicatum and -sensitive species Potamogeton lucens; then further explored and compared the mechanisms underlying ammonium detoxification. 

The results showed the ammonium-tolerant species, M. spicatum had a higher plasticity on carbon metabolism, and the activity of glutamate dehydrogenase (GDH) performed a dose-response curve (increased 169% for NADH-dependent GDH and 103% for NADPH-dependent GDH) with the [NH₄⁺-N] increasing while glutamine synthetase (GS) activity slightly changed. But for the ammonium-sensitive species, P. lucens, the activity of GDH recorded no major changes, while the GS increased slightly (17%).  

Based on this, the researchers conclude that the alternative pathway of GDH plays an important role in ammonium detoxification by submerged macrophytes. This research not only provides identified species in phytoremediation, but also provides a potential tool to screen other species for ammonium tolerance. 

This work is supported by Strategic Priority Research Program of the Chinese Academy of Sciences, the National Key Research and Development Programme of China and the National Natural Science Foundation of China. It has been published in Science of The Total Environment with the title of “Glutamate dehydrogenase plays an important role in ammonium detoxification by submerged macrophytes”.  

 Selection of different macrophytes and comparison of assimilation pathways of ammonium-tolerant and sensitive species (Image by WBG)