Does the source of nitrogen affect the response of subterranean clover to prolonged root hypoxia?

Nitrogen (N) is taken up by most plant species in the form of nitrate (NO $ _3^- $ ) or ammonium (NH $ _4^+ $ ). The plant response to continuous ammonium nutrition is species‐dependent. In this study, the effects of the source of N nutrition (NO $ _3^- $ , NH $ _4^+ $ , or the mixture of NO $ _3^- $ and NH $ _4^+ $ ) on the response of clover ( Trifolium subterraneum L. cv. 45C) plants to prolonged root hypoxia was studied. Under aerobic conditions, plant growth was strongly depressed by NH $ _4^+ $ , compared to NO $ _3^- $ or mixed N nutrition, as indicated by the significant decrease in root and shoot‐dry‐matter production (DW), root and shoot water contents (WC), leaf chlorophyll concentration, and chlorophyll fluorescence parameters ( F 0 , F v / F m ). However, the N source had no effect on chlorophyll a–to–chlorophyll b ratio. Under hypoxic conditions, the negative effects of root hypoxia on plant‐growth parameters (DW and WC), leaf chlorophyll concentration, and chlorophyll fluorescence parameters were alleviated by NH $ _4^+ $ rather than NO $ _3^- $ supply. Concomitantly, shoot DW–to–root DW ratio, and root and leaf NH $ _4^+ $ concentrations were significantly decreased, whereas root and leaf carbohydrate concentrations, glutamine synthetase activities, and protein concentrations were remarkably increased. The present data reveal that the N source (NO $ _3^- $ or NH $ _4^+ $ ) is a major factor affecting clover responses to hypoxic stress, with plants being more tolerant when NH $ _4^+ $ is the N form used. The different sensitivity is discussed in terms of a competition for energy between nitrogen assimilation and plant growth.