Effects of monovalent cations on IAA- and FC-stimulated proton-cation exchange in pea stem segments

1.(1) At relatively low (1–10 mM) salt concentration in the medium, proton extrusion in normal, auxin (IAA)- and fusicoccin (FC)-treated pea internode segments is markedly stimulated by K+ and, at a lesser extent, by Rb+, while Na+ and other monovalent cations show little or no effect. At high (100 mM) concentration also Na+ stimulates H+ extrusion, and the effects of K+, Rb+ and Na+ become quantitatively similar. The stimulation of H+ extrusion by monovalent cations is clearly synergistic with the one induced by either IAA or FC. 2.(2) Both IAA and (much more) FC markedly enhance the rates of K+ and of Na+ uptake in pea internode segments. Both growth promoters strongly increase the selectivity of the uptake system for K+, as compared with Na+, at low but not at high salt concentrations. Under all conditions tested a qualitative correlation exists between K+ and Na+ uptake rate and the stimulating effect of these cations on proton extrusion. The cation uptake/titratable H+ extrusion ratio is decreased by either IAA or FC. The value of this ratio is always larger than 1, and increases with the increase of salt concentration in the medium. This may suggest that re-absorption and/or reassociation of an aliquot of extruded protons with extruded −OH or weak base mask a consistent fraction of the proton efflux. These results are consistent with the hypothesis that IAA and FC, although acting on different primary receptors, activate the same proton/monovalent cation antiport mechanism endowed with a high affinity for K+.