RIPENING OF NECTARINE FRUIT - CHANGES IN THE CELL-WALL, VACUOLE, AND MEMBRANES DETECTED USING ELECTRICAL-IMPEDANCE MEASUREMENTS

Electrical impedance measurements were used to characterize changes in intracellular and extracellular resistance as well as changes in the condition of membranes during ripening of nectarines (Prunus persica [L.] Batsch cv Fantasia). These measurements were related to changes in fruit texture assessed by flesh firmness and apparent juice content. An electrical model indicated that, during ripening (d 1-5) of freshly harvested fruit, the resistance of the cell wall and vacuole declined by 60 and 26%, respectively, and the capacitance of the membranes decreased by 9%. Accurate modeling of the impedance data required an additional resistance component. This resistance, which declined by 63% during ripening, was thought to be associated with either the cytoplasmic or membrane resistance. Changes in tissue resistance measured using low frequencies of alternating current were closely related to flesh firmness. After storage at 0 degrees C for 8 weeks, the nectarines developed a woolly (dry) texture during ripening at 20 degrees C. The main difference between these chilling-injured nectarines and fruit ripened immediately after harvest was the resistance of the cell wall, which was higher in woolly tissue (4435 Ohm after 5 d at 20 degrees C) than in nonwoolly tissue (2911 Ohm after 5 d at 20 degrees C). The results are discussed in relation to physiological changes that occur during the ripening and development of chilling injury in nectarine fruit.