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, NH 4 + or NO 3 − . 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 NH 4 + ‐N g −1 soil or 6 μg NO 3 − ‐N g −1 soil, and root growth declined in soils with the higher levels of inorganic nitrogen. High NH 4 + concentrations inhibited root growth, but low concentrations promoted the development of an extensive, fine root system. Supply with NO 3 − as the sole nitrogen source led to a more compact root system. These differences in root morphology under NH 4 + and NO 3 − nutrition may be mediated through pH. Rice and maize roots absorbed NH 4 + most rapidly right at the apex and appeared to assimilate this NH 4 + in the zone of elongation. During NH 4 + assimilation, root cells must release protons, and the resulting acidification around the walls of cells in this region should stimulate root extension. By contrast, NO 3 − absorption reached a maximum in the maturation zone of rice and maize roots, and this NO 3 − was probably assimilated in more basal regions. Absorption of NO 3 − requires proton efflux, whereas NO 3 − assimilation requires proton influx. The net result under NO 3 − nutrition was only subtle shifts in rhizosphere pH that probably would not influence root elongation. The signal through which roots detect changes in rhizosphere NH 4 + and NO 3 − levels is still obscure. It is proposed that a product of nitrogen metabolism such as nitric oxide serves as a signal.