Ammonium supply induces differential metabolic adaptative responses in tomato according to leaf phenological stage.

Nitrate (NO3  -) and ammonium (NH4  +) are the main inorganic nitrogen sources available to plants. However, exclusive ammonium nutrition may lead to a stress situation characterized by growth inhibition, generally associated with a profound metabolic reprogramming. In this work, we aimed at studying how the metabolism adapts according to leaf position in the vertical axis of tomato (Solanum lycopersicum cv. M82) plants grown with NH4  +, NO3  - or NH4NO3 supply. To do so, we dissected leaf biomass composition and metabolism through an integrative analysis of metabolites, ions and enzyme activities. Under ammonium nutrition, carbon and nitrogen metabolism was more perturbed in mature leaves than in young ones, overall suggesting a trade-off between NH4  + accumulation and assimilation to preserve young leaves from ammonium stress. Moreover, NH4  +-fed plants exhibited a rearrangement of carbon partitioning, accumulating sugars and starch at the expense of organic acids, with respect to plants supplied with NO3  -. We explain such reallocation by the action of the biochemical pH-stat to compensate the differential proton production that depends on the nitrogen source provided. This work also underlines that the regulation of leaf primary metabolism is dependent on both the leaf phenological stage and the nitrogen source provided.