Nitrate and ammonium nutrition of plants: Effects on acid/base balance and adaptation of root cell plasmalemma H + ATPase

The increase of rhizosphere pH in the course of nitrate nutrition results from H + consumption in the external medium during uptake of NO 3 − in a H + co‐transport and from internal OH − production during nitrate reduction. Synthesis of organic acids for NH 4 + assimilation as well as strong partial depolarization of membrane potential with NH 4 + uptake are the important reasons for rhizosphere acidification during ammonium nutrition. Despite differences in proton balance depending on N form, cytoplasmic pH changes are small due to physico‐chemical buffering, biochemical pH regulation, H + inclusion in vacuoles, and H + release into the rhizosphere. Because of the large capacity for proton excretion the plasmalemma H + ATPase of root cells plays an essential role during ammonium nutrition. An increase of the kinetic parameter V max after ammonium nutrition relative to nitrate nutrition suggests that the capacity of H + release may be adjusted to the particular requirements of ammonium nutrition. Moreover, H + ATPase is adjusted not only quantitatively but also qualitatively. The increase of the kinetic parameter k m as well as the capability of the plasmalemma vesicles in vitro to establish a steeper pH gradient favours the supposition that H + ATPase isoforms are formed which allow H + release into the rhizosphere under conditions of low pH or poor H + buffering of the soil. In this respect species differences exist, e.g . between maize (efficient adaptation) and faba bean (poor adaptation).