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 resu lts from H+ consumption in the external medium during uptake of NO3- i n a H+ co-transport and from internal OH- production during nitrate re duction. Synthesis of organic acids for NH4+ assimilation as well as s trong partial depolarization of membrane potential with NH4+ uptake th e important reasons for rhizosphere acidification during ammonium nutr ition. Despite differences in proton balance depending on N form, cyto plasmic pH changes are small due to physico-chemical buffering, bioche mical pH regulation, H+ inclusion in vacuoles, and H+ release into the rhizosphere. Because of the large capacity for proton excretion the p lasmalemma H+ ATPase of root cells plays an essential role during ammo nium nutrition. An increase of the kinetic parameter V-max after ammon ium nutrition relative to nitrate nutrition suggests that the capacity of H+ release may be adjusted to the particular requirements of ammon ium 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 establ ish a steeper pH gradient favours the supposition that H+ ATPase isofo rms are formed which allow H+ release into the rhizosphere under condi tions 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).