Potassium transport in corn roots : I. Resolution of kinetics into a saturable and linear component

Abstract

Influx isotherms were obtained for (86)Rb(+) uptake into 2-cm corn (Zea mays [A632 x (C3640 x Oh43)] root segments for both low- (0.2 millimolar CaSO(4)) and high-salt (0.2 millimolar CaSO(4) + 5 millimolar KCl) grown roots. Unlike the discontinuous curves usually presented for K(+) influx, our isotherms were smooth, nonsaturating curves that approached linearity at K(+) (Rb(+)) concentrations above 1 millimolar. The kinetics for K(+) transport could be resolved into saturable and linear components. The saturable components yielded K(m) values of 16 and 86 micromolar for low- and high-salt roots, respectively, while V(max) values were 5.62 and 1.85 moles per gram fresh weight per hour. Results of experiments with the penetrating sulfhydryl reagent, N-ethyl maleimide (NEM), and the impermeant reagent, p-chloromercuribenzene sulfonic acid (PCMBS) indicated that the saturable and linear components were independent mechanisms of K(+) transport.Short-term NEM exposures (30 seconds to 5 minutes) selectively inhibited the saturable system, but had little effect on the linear component. Increasing NEM exposures resulted in further inhibition and subsequent abolition of the saturable component; the linear component exhibited limited NEM sensitivity. PCMBS elicited the same general inhibitory trends, although it was less effective as a saturable component inhibitor.The effects of NEM and PCMBS on K(+) efflux were also studied. Short NEM exposures had no effect on cytoplasmic efflux, while inhibiting vacuolar efflux significantly. From these data, it is unclear at which site(s) NEM is acting. A more complex response was obtained with PCMBS, where a monophasic efflux curve was observed. Analysis indicated that the vacuolar efflux was stimulated, while the cytoplasmic component was abolished.The nature of the linear component is discussed, and it is proposed that the mechanism may be more complex than simple facilitated diffusion.