Characterization of the Permeability of Excised Apple Tissue 2

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Abstract
Sorbitol uptake from a bathing solution into the compartmented space and into the diffusible or apparent free space of excised parenchyma tissue from apple fruit (Pyrus malus L. cv. Golden Delicious) was investigated. Uptake into the two cell compartments was measured after washing of l4C-loaded tissue for 1 h with an osmoticum-free bathing solution. Compartmental analysis showed that this treatment released sorbitol taken up into the cytoplasm of the cell, which was considered to be part of the apparent free space. Uptake of sorbitol into the apparent free space was dependent on the osmotic concentration of the incubation medium. Using mannitol up to 200 mM, uptake decreased by 60%, and increased again above 600 mM mannitol, the external concentration where turgor was eliminated. Uptake in the compartmented space was about 3 times lower and was hardly affected by the external osmotic concentration. PCMBS inhibited sorbitol transport into the apparent free space by 25% at 100 mM mannitol, but at 600 mM the inhibitor had no effect. The results indicate that sorbitol transport across the plasma membrane is possibly facilitated by a turgor-sensitive carrier. Uptake of l4C-sorbitol into the freely diffusible space of tissue discs also increased by 200% after storage of unripe fruit for 70 d. This increase in aged tissue did not occur when uptake was measured at 4°C or in the presence of 200 mM PEG. Enhanced uptake was concomitant with an increased release of endogenous sugars from aged tissue. It would appear that the effect of a hypotonic bathing solution on the permeability of excised apple tissue is related to structural changes, such as stretching of the plasma membrane. This effect, which becomes more marked as unripe fruit ages, is probably brought about by turgor-driven relaxation of the tissue. It may increase non-specific leakage of sugars but could also be a factor affecting carrier-mediated transport of sorbitol at the plasma membrane.

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