MECHANISM OF ANION UPTAKE IN PLANT-ROOTS - QUANTITATIVE-EVALUATION OFH+ NO3- AND H+/H2PO4- STOICHIOMETRIES/

Concomitant extracellular pH changes were measured qualitatively and q uantitatively to further characterize the mechanism of the high affini ty systems of nitrate and phosphate uptake in roots of Limnobium stolo rniferum and Zea mays. During NO3- and H2PO4- uptake, the incubation m edium alkalinized with a clear stoichiometry of 1 H+:1 A(-). Simultane ous measurements of K+ fluxes explain deviations from a 1 H+:1 A(-) st oichiometry. Concomitant K+ influx caused antagonistic extracellular a cidification and K+ efflux additive alkalinization. However, a stoichi ometry of 1 H+:1 A(-) would not change the membrane potential, as usua lly observed. This apparent contradiction can now be explained taking into account the strong ion concentration difference concept: H+ fluxe s alone do change the membrane potential (E(m)) but do not change pH. Only concomitant fluxes of strong cations (e.g. K+) or anions (NO3- H2 PO4-) result in pH changes, together with an intrinsic stoichiometry o f 1 H+:1 A(-), because overall electroneutrality is maintained in each compartment. Conclusions from our previous electrophysiological exper iments together with the present ones on ion concentration changes, pr ovide a convincing interpretation of NO3- and H2PO4-; uptake to be a H +:A(-) (probably 2:1) cotransport mechanism. Strong inhibition by carb onylcyanide m-chlorophenylhydrazone (CCCP) confirms that Delta mu H+ i s the driving force. However, determination of the mechanism and actua l stoichiometry cannot be achieved by mere pH measurements, instead se veral different techniques have to be employed.