Involvement of Divalent Cations in Maintaining Cell Membrane Integrity in Stressed Apple Fruit Tissues

The effects of divalent cations on the response of apple fruit tissue to accelerated ageing, temperature stress, or osmotic stress, were evaluated by measuring ethylene evolution, electrolyte leakage, and membrane microviscosity. Apple tissue slices, incubated in an isotonic sorbitol solution at 25°C for 24 hours, underwent rapid ageing, as expressed by a sharp drop in ethylene production (70–90%)and leakage of potassium from the tissue. At a higher temperature and in hypotonic medium these symptoms were accelerated and enhanced. Addition of Ca²⁺ to the isotonic incubation medium inhibited ethylene production during the first 6 hours but thereafter partially prevented the drop in ethylene production that took place in the aged tissue slices, and completely prevented K⁺ leakage beyond 4 hours of incubation. The influence of Ca²⁺ on the physical state of the membranes was observed by an increase in membrane microviscosity relative to control, as determined by the fluorescence depolarization technique. Using fluorescent probes that bind to different sites in the membrane, it was found that the effect of Ca²⁺ was more pronounced at the membrane surface and diminished toward the hydrophobic region of the membrane bilayer.