Comparison of Sorbitol Transport in Excised Tissue Discs and Cortex Tissue of Intact Apple Fruit

Abstract

Summary Transport of [ 14 C]sorbitol from a bathing solution into isolated tissue discs of apple fruit was compared with transport of sorbitol supplied to wells cut in tissue of intact fruit. After uptake of sorbitol, labelled tissue was excised from fruit and, as for isolated discs, the tissue was washed in a hypotonic solution to remove label from the apoplast. This treatment also released radioactive label from the cytoplasm. The procedure allowed the separation of radioactivity located in the apparent free space, comprising apoplast and cytoplasm, from the radioactivity remaining in the compartmented space of the tissue. Transport of sorbitol from an osmotically adjusted medium into the apparent free space of tissue discs was linear with respect to substrate concentration. Uptake was not affected by competitors such as sucrose or mannitol, and was not inhibited by the SH-reagent PCMBS and the uncoupler CCCP. Transport showed a pH-optimum around pH 7.0. Exchange diffusion of [ 14 C]sorbitol previously incorporated in tissue discs was not stimulated by external sorbitol. These findings indicate that in the presence of an osmoticum diffusion of sorbitol across the plasma membrane is the most likely form of transport. Under the same conditions uptake of sorbitol into the compartmented space of excised tissue discs showed a hiphasic response with a saturable component at low concentrations and a non-saturable, linear, diffusion-like component at higher concentrations (up to 100 mM), indicating carrier mediated transport at the tonoplast. In intact fruit, transport of sorbitol into the apparent free space was linear with respect to substrate concentration and had a pH-optimum of pH 7.0. These results were obtained using a hypotonic uptake medium and confirmed the observations made on tissue discs. However, unlike isolated tissue, uptake in intact fruit was not increased by a low osmotic solute concentration of the uptake medium. In intact fruit, transport of sorbitol under hypotonic conditions was further reduced 57% by sucrose, 49% by mannitol and 30% by PCMBS. These observations, made only on intact fruit, would point to the presence of a carrier for sorbitol located in the plasma membrane. It is assumed that in intact fruit the integrity of the plasma membrane is maintained, allowing sorbitol to interact with a carrier. This ability may be lost when tissue is excised from the fruit and kept in solutions of high osmotic activity. A likely location for such a carrier would be within channels of the plasma membrane and its activity might be controlled by pressure sensitive gating of such structures.