Interactions between Inorganic Nitrogen Nutrition and Root Development

Root development responds not only to the quantity of inorganic nitrogen in the rhizosphere, but to its form, NH4+or NO3−. Root growth of tomato showed a hyperbolic response to soil levels of inorganic nitrogen: very few roots were found in soil blocks depleted in inorganic nitrogen, roots proliferated as soils increased to 2 μg NH4+‐N g−1soil or 6 μg NO3−‐N g−1soil, and root growth declined in soils with the higher levels of inorganic nitrogen. High NH4+concentrations inhibited root growth, but low concentrations promoted the development of an extensive, fine root system. Supply with NO3−as the sole nitrogen source led to a more compact root system. These differences in root morphology under NH4+and NO3−nutrition may be mediated through pH. Rice and maize roots absorbed NH4+most rapidly right at the apex and appeared to assimilate this NH4+in the zone of elongation. During NH4+assimilation, root cells must release protons, and the resulting acidification around the walls of cells in this region should stimulate root extension. By contrast, NO3−absorption reached a maximum in the maturation zone of rice and maize roots, and this NO3−was probably assimilated in more basal regions. Absorption of NO3−requires proton efflux, whereas NO3−assimilation requires proton influx. The net result under NO3−nutrition was only subtle shifts in rhizosphere pH that probably would not influence root elongation. The signal through which roots detect changes in rhizosphere NH4+and NO3−levels is still obscure. It is proposed that a product of nitrogen metabolism such as nitric oxide serves as a signal.