Ion fluxes from protoplasts and split segments of oat coleoptiles relating to the mode of action of growth substances

Indole 3 acetic acid (IAA) is the major plant growth substance that causes normal plant elongation and the bending responses to light and gravity. The mechanism by which IAA causes cell wall loosening to produce elongation growth of plant stems is still in controversy. The most accepted hypothesis for the mode of action of IAA is the acid growth hypothesis, which states that auxin regulates wall loosening by causing a pH drop in the cell wall. Many questions are left in relation to the involvement of ions in the mechanism of wall loosening. The investigation of ion fluxes from plant cells in the presence and absence of the cell walls may contribute to the explanationof the involvement of these ions. The study of proton fluxes on opposite sides of protoplasts shows an asymmetric pattern and polarity of the flux. This leads to the conclusion that the protoplasts must have polarity to allow pH regulation in the cytoplasm. Estimation of the change in the cytoplasmic pH due to observed influx or efflux of protons shows that the cytoplasmic buffering capacity alone is not capable of neutralising the observed proton flux. IAA and fusicoccin (FC) stimulate proton extrusion from protoplasts by factors of about three and two times the control. It must be stimulation of active proton transport that is caused by these substances. IAA also enhances the calcium efflux which may be a way to maintain the cytoplasmic free calcium at a low level. As with protoplasts, IAA and FC also enhance proton and calcium effluxes from epidermal and parenchymal cells of split oat coleoptiles. Ion fluxes from the epidermal cells are larger than those from the parenchymal cells. It could be that the epidermal cells are more sensitive to the growth substances. However, the effects of IAA and FC on ion fluxes from the parenchymal cells are still observable. There is no time delay in enhancement of proton and calcium fluxes from protoplasts and segments by IAA and FC. This is in contrast with results of other workers in observing the effect of IAA. However, enhancement of proton and calcium fluxes by FC is larger than for IAA. The calcium appearing in the external medium may come from a compartment inside the cell and from the cell wall. The cell walls contribute about 75 to 90 % of the total amount of the emerging calcium. The calcium from the walls results from the exchange of the extruded protons from inside the cell with the condensed calcium in the wall. This is in agreement with the Weak Acid Donnan Manning (WADM) model for cell wall ion exchange.