Efflux of photosynthate and acid from developing seed coats of Phaseolus vulgaris L.: a chemiosmotic analysis of pump-driven efflux

Seed coats of Phaseolus vulgaris</it> L. unload photosynthetic products, mineral ions and acid into the apoplastic space surrounding the embryo. We report measurements, on detached seed coats, of the rates of unloading of photosynthates, ions and acid at different external pH and in the presence of treatments intended to alter the rate of proton pumping. We also report measurements of membrane potential difference (PD</it>) and of cytoplasmic pH under the same conditions, measurements which have allowed us to validate the treatments we used and to investigate functional relationships between membrane processes. A chemiosmotic model of the seed-coat cell membrane is proposed, in which sucrose efflux and acid efflux are both driven by the proton pump. Sucrose efflux is proposed to occur by sucrose/proton antiport driven by the proton-motive force (PMF</it>), and acid efflux to occur by pumped protons accompanied by a passive efflux of anions. We use our measurements to estimate the net efflux of sucrose on the antiporter and the total efflux of protons on the pump. We have tested the model by using experimental treatments designed to manipulate the pump rate as the independent variable. Under these conditions, and assuming the model is correct, the pump rate determines the cytoplasmic pH. Over the range covered by our experiments the net sucrose efflux is dependent on external and cytoplasmic pH, the latter having the major role. The efflux of acid, under the same treatments, depends primarily on the proton pump rate, and was found to be well fitted by a quadratic function of pump rate. This means that, as pump rate increases, an increasing proportion of the pump output is used by acid efflux and a decreasing proportion by sucrose antiport. The membrane PD</it>, although an important component of the PMF</it>, does not appear to function in rate control of net sucrose or of acid efflux, since neither efflux is correlated with membrane PD</it> under our treatments which vary the pump rate. The PD</it> correlates well with external potassium concentration, and seems largely determined by the diffusion of potassium ions and anions.