Postharvest Biology and Technology 18 2000 9 – 18
The use of electrical impedance spectroscopy to assess the physiological condition of kiwifruit
Anne D. Bauchot
a,1
, F. Roger Harker
a,
, W. Michael Arnold
b
a
The Horticulture and Food Research Institute of New Zealand Ltd., Mt Albert Research Centre, Pri6ate Bag
92 169
, Auckland, New Zealand
b
Industrial Research Ltd., Gracefield Research Centre, P.O. Box
31
-
310
, Lower Hutt, New Zealand Received 30 December 1998; received in revised form 9 August 1999; accepted 10 August 1999
Abstract
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. © 2000 Published by Elsevier Science B.V. All rights reserved.
Keywords
:
Apoplast; Cell wall; Fruit ripening; Kiwifruit; Reactance; Resistance www.elsevier.comlocatepostharvbio
1. Introduction
Electrical impedance spectroscopy EIS has been widely used to assess the in vivo condition of
animal and plant tissues Cole, 1972; Settle et al., 1980; Lewis et al., 1989. Recently, there has been
an increasing interest in the use of this technique to assess fruit ripening Harker and Dunlop, 1994;
Corresponding author. Tel.: + 64-9-8493660; fax: + 64-9- 8154202.
E-mail address
:
rharkerhort.cri.nz F.R. Harker
1
Present address: Department of Agricultural Botany, School of Plant Sciences, The University of Reading,
Whiteknights, PO Box 221, Reading, Berkshire RG6 6AS, UK.
0925-521400 - see front matter © 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 5 2 1 4 9 9 0 0 0 5 6 - 3
Harker and Maindonald, 1994; Inaba et al., 1995. The advantage of EIS is that electrical
circuit diagrams
can be
used to
interpret impedance spectra in terms of components of
resistance and capacitance associated with struc- tures at the cellular level Zhang et al., 1990;
Zhang and Willison, 1991.
Electrical impedance studies have provided a useful insight into ripening of nectarines Harker
and Dunlop, 1994; Harker and Maindonald, 1994, persimmon Harker and Forbes, 1997 and
tomato Varlan and Sansen, 1996. For example, the resistance of the apoplast declined by 60
when nectarine fruit were ripened immediately after harvest, and differences in cell wall resis-
tance were associated with the presence and ab- sence of woolly texture Harker and Maindonald,
1994. Furthermore, the development of chilling injury during storage of New Zealand-grown
‘Fuyu’ persimmon fruit was indicated by changes in the cytoplasmic resistance Harker and Forbes,
1997. In the present study, we have extended this research to kiwifruit; which has been used as a
model for studying fruit softening MacRae and Redgwell, 1992. The kiwifruit consists of four
distinct tissue zones: skin, outer pericarp, inner pericarp and seeds, and the core. Each of these
tissues differs in firmness Jackson and Harker, 1997, mineral and sugar contents Ferguson,
1980; MacRae et al., 1989, cell wall composition Redgwell et al., 1991, 1992 and cell characteris-
tics Hallett et al., 1992. The core is composed of a single population of sphericalellipsoidal cells
0.1 and 0.2 mm diameter, whereas the outer pericarp is composed of a population of large cells
0.5 – 0.8 mm diameter dispersed in a matrix of smaller cells 0.1 – 0.2 mm diameter Hallett et al.,
1992. The inner pericarp consists of the locules, which are enclosed within locule walls Hallett et
al., 1992. Each locule is composed of large radi- ally elongated thin-walled cells 0.2 – 0.4 mm × \
1 mm and seeds, while the locule wall is a narrow region
composed of
smaller thicker
walled cells Hallett et al., 1992. In this study, the
impedance characteristics of individual tissues, outer pericarp, inner pericarp and core, as well as
whole fruit, have been examined during fruit ripening.
2. Material and methods
