NET SIMULTANEOUS HYDROGEN AND POTASSIUM-ION FLUX KINETICS IN STERILE AEROPONIC SUNFLOWER SEEDLING ROOTS - EFFECTS OF POTASSIUM-ION SUPPLY, VALINOMYCIN, AND DICYCLOHEXYLCARBODIIMIDE

Variation of pH and potassium ion (K+) concentration in a medium bathi ng 48 h sterile, aeroponic dark-grown sunflower (Helianthus annuus L.) seedling roots were simultaneously monitored using specific non-combi ned high-sensitivity electrodes for pH and K+. Net K+ influx rates for different K+ concentrations (0.25, 0.5, 1, 2 and 5 mM) lagged by appr oximate to 60 min with respect to the, hydrogen ion (H+) efflux, and s howed the biphasic saturable kinetics (Epstein's Systems I and II) des cribed by other authors. However, in our close to natural conditions ( H+ extrusion and K+ uptake systems are operating simultaneously), the range of K+ concentrations at which each of the two K+ uptake systems operated was also dependent on the time that the H+ extrusion system w as operating too, and hence on the Delta pH(in,out) created. The K+ co ncentration in the medium had negligible effects on the net root H+ ef flux rate: only 5 mM K+ slightly enhanced in approximate to 14% the ma ximum H+ efflux rate. Valinomycin caused a sharp K+ efflux, followed b y a transitory enhancement of the K+ influx rates, but it had no effec t on the H+ efflux rate. On the contrary, dicyclohexylcarbodiimide (DC CD) quickly annulled H+ efflux, actually reverting it to net H+ influx towards the end of the kinetics, but it only slowly and not completel y inhibited K+ influx. The conclusion from our results is that H+ extr usion and K+ uptake by our roots were not mediated by a single carrier : they could be better explained if only an indirect coupling linked t hem, i.e., if H+-ATPase provided the driving force for secondary K+-tr ansport carriers and/or channels.