The use of electrical impedance spectroscopy to assess the physiological condition of kiwifruit

The electrical impedance of kiwifruit [Actinidia deliciosa (A. Chev) C.F. Liang et A.R. Ferguson, cv. Hayward] was studied during fruit ripening. Measurements were made on whole fruit, and tissues excised from the outer pericarp, inner pericarp and core. Alternating current at frequencies between 50 Hz and 1 MHz was passed through fruit and tissue samples, and complex impedance spectra were separated into the resistances of the apoplast, cytoplasm and vacuole, and capacitances of the plasma membrane and tonoplast. The differences in R-50 Hz and R-1 MHz between tissues (representative of apoplast resistance and total tissue resistance, respectively) were explained in terms of the anatomy and composition of the respective tissues. Some variations were seen from one year to the other. During ripening, there was little change in the impedance characteristics of the fruit, despite a 10-fold decrease in firmness. This was unexpected since previous studies with nectarine, persimmon and tomato fruit have shown a considerable reduction in impedance during ripening. The failure to observe any impedance change was checked using a number of different methods for measuring impedance, by two different laboratories, and confirmed by measuring electrolyte leakage from tissue discs. All the results suggested that the mobility of electrolytes within the cell wall did not change during kiwifruit ripening. We speculate that physicochemical interactions that take place within the cell wall may have a major impact on the impedance of kiwifruit tissue. (C) 2000 Published by Elsevier Science B.V. All rights reserved.