Effects of ABA on the distribution of sucrose and protons across the plasmalemma of pea mesophyll protoplasts. - Suggesting a sucrose/proton symport

The effects of abscisic acid (10(-9)-10(-6) mol/L) were studied on light in duced CO2-dependent O-2 evolution, sucrose synthesis, efflux of sucrose and changes in cytosolic proton concentration in mesophyll protoplasts of Pisu m sativum L. cv. Fenomen. Photosynthesis did not change with increased apop lastic ABA or sucrose concentrations although sucrose synthesis decreased i n the presence of ABA. In the absence of apoplastic sucrose the sucrose eff lux increased and was most pronounced at 10(-7) mol/L ABA. The sucrose effl ux also increased in the presence of ABA and sucrose together and was most pronounced at 10(-9) mol/L ABA and 20 mmol/L apoplastic sucrose. The cytoso lic proton concentration decreased upon addition of ABA with or without suc rose, but increased upon addition of pchloromercuribenzenesulfonic acid, a sucrose transport inhibitor. The cytosolic proton concentration decreased i n the presence of 10(-9) mol/L ABA in combination with various apoplastic s ucrose concentrations. In the presence of 10(-7)-10(-6) mol/L ABA in combin ation with apoplastic sucrose levels higher than 1 mmol/L, the cytosolic co ncentration of proton increases. The results indicate that there is a relat ionship between the external environment and the sucrose efflux and cytosol ic proton concentration in pea mesophyll protoplasts and that there is a su crose/proton symport at the plasmalemma of pea mesophyll cells. They also s how that sucrose increases the sensitivity to ABA in its action on sucrose efflux and cytosolic proton concentration.