Effect of ammonium or nitrate nutrition on photosynthesis, growth, and nitrogen assimilation in tomato plants

Nitrogen (N) is taken up by most plant species in the form of nitrate (NO $ _3^- $ ) or ammonium (NH $ _4^+ $ ). Plant response to continuous NH $ _4^+ $ nutrition is species‐dependent. In this study, we compare the responses of tomato ( Solanum lycopersicum L. cv. Rio Grande) plants to N source (NO $ _3^- $ or NH $ _4^+ $ ). To this end, early plant growth, photosynthesis, chlorophyll, carbohydrate, and N‐compound concentrations as well as the activities of main enzymes involved in N metabolism (nitrate reductase, nitrite reductase, glutamine synthetase, and glutamate dehydrogenase) were analyzed. Early plant growth was remarkably ameliorated under NH $ _4^+ $ ‐ in comparison to NO $ _3^- $ ‐based nutrition. Concomitantly, photosynthetic activity, total chlorophyll, and carbohydrate concentrations were significantly increased. With increasing external NH $ _4^+ $ concentration, NH $ _4^+ $ accumulated mainly in roots. In addition, root protein concentration was significantly increased, reflecting high NH $ _4^+ $ incorporation into organic nitrogen. Root glutamine synthetase (GS) activity was enhanced by NH $ _4^+ $ for concentrations below 5 mM, whereas root glutamate dehydrogenase (GDH) activity increased in parallel to NH $ _4^+ $ availability. Together with the positive effect of NH $ _4^+ $ on tomato plant cv. Rio Grande growth, these results reveal that GDH could have, in addition to GS, a possible role in NH $ _4^+ $ detoxification and tolerance of NH $ _4^+ $ ‐based nutrition.