A COMPARISON OF THE RELATIVE RATES OF TRANSPORT OF ASCORBATE AND GLUCOSE ACROSS THE THYLAKOID, CHLOROPLAST AND PLASMALEMMA MEMBRANES OF PEALEAF MESOPHYLL-CELLS

Mesophyll protoplasts, chloroplasts and thylakoids were prepared from the leaves of 10-12 day old pea seedlings. They contained 1618 +/- 351 , 197 +/- 30 and 25 +/- 7 nmol ascorbate per milligram of chlorophyll respectively equivalent to average concentrations of 8.4, 9.8 and 3.8 mM ascorbate in the intact protoplasts, intact chloroplasts and thylak oid membranes respectively. Transport of [C-14]-ascorbate and [C-14]-g lucose across the plasmalemma, chloroplast envelope and thylakoid memb ranes was measured in these fractions. Saturation kinetics for [C-14]- ascorbate and [C-14]-glucose uptake suggest that the plasmalemma and c hloroplast envelope membranes contain carriers for both ascorbate and glucose. In contrast, the thylakoid membranes appear to have no transp ort system for ascorbate, permeability resulting from diffusion alone. Dehydroascorbate inhibited ascorbate transport but not glucose transp ort across the chloroplast envelope membrane. Ascorbate stimulated [C- 14]-glucose transport across the plasmalemma and chloroplast envelope membranes. Similarly, glucose had a positive effect: on [C-14] ascorba te transport across these membranes. These observations suggest the pr esence of independent carriers for ascorbate and glucose into both pro toplasts and chloroplasts. Unlike animal membranes where the hexose tr ansporters have been found to transport dehydroascorbate the plant cou nterparts does not appear to do so, either in the chloroplast envelope or the plasmalemma membranes. The ascorbate transporters on these mem branes are hence discrete entities not involved with general hexose tr ansport. However, the observed synergistic effects of either glucose o r ascorbate on the transport of the other metabolite suggest co-ordina tion of the two processes.