Evidence for the coupling of proton extrusion to K+ uptake in pea internode segments treated in fusicoccin or auxin

When pea internode segments are incubated in a K+-deficient medium, a rapid increase of K+ in the medium is observed in the first 3 h, followed by a progressive decrease in the following period. Fusicoccin (FC) and, to a much lesser extent, indole-3-acetic acid (IAA), increase the initial K+ outburst, while in the second phase they accelerate the reabsorption of K+. Since the initial K+ outburst takes place also if the segments are incubated in buffered solutions at progressively decreasing pH, its enhancement by FC is interpreted as a consequence of the acidification of the free space. Such an acidification makes available, for both uptake into the cells and diffusion into the medium, a consistent amount of the cations retained by non-exchangeable anions in the Donnan free space (DFS). The proton extrusion promoted by FC or IAA is differentially affected by the presence of monovalent cations in the medium. K+ and to a lesser extent Na+ stimulate it while Cs+ and Li+ have little activity or inhibit the reaction. The order of activity of these ions roughly corresponds to their capacity to activate a K+ -activated ATPase present in “plasmalemma” preparations from pea internode segments. Dicyclohexycarbodiimide (DCCD), an inhibitor of K+-ATPases, also inhibits growth and proton extrusion. If segments were partially depleted of the K+ in the DFS by preincubation with either growth-promoting substances or in acidic buffers, and then transferred into fresh medium, the FC- and IAA-promoted proton extrusion was strongly stimulated by the presence in the fresh medium of K+, while Na+ appeared much less active. A satisfactory correlation between the stimulatory effect on proton extrusion and that on K+ uptake appears to exist even in the very first phase of treatment with FC, provided the segments are preincubated in buffers at pH 5. This treatment eliminates the initial K+ outburst that otherwise would mask the stimulation of K+ uptake. Under such conditions a proton extrusion/K+ uptake ratio close to unity was found.