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Impedance Spectroscopy on Plant Tissues

Tapani Repo(r, Panu Hiekkala(t, Mikko lrinonen(t, Timo Päåikkönen(t, Aija Ryyppö(t(lUniversity of Joensuu, Faculty of Forestry P.O. Box 1l 1, FIN-80101 Joensuu, Finland; (2The Finnish

Forest Research Institute, Suonenjoki Research Station, FIN-77600 Suonenjoki, Finland

Abstract: The electrical impedance spectra (80H2 tolMHz) of different plant species were measured andanalysed in different stages of the annual development.The spectrum features were dependent on the species andthe organ studied. The typical feature was the stronglydepressed center of the spectrum that was composed ofeither one or two arcs. For certain organs the spectra wereslightly skewed. These features indicate, that the spectrameasured from heterogenous plant tissues are bestrepresented by the distributed circuit models (DCE).Certain circuit parameters changed clearly with annualstage of development and as a result of frost damage,which indicate structural changes in cells.

INTRODUCTION

The electrical impedance spectroscopy can give basic

information about the physiology of different organisms II,2l.In this study impedance spectra of different plant species and

organs were examined. By that method, we studied impedance

parameters which can be used for determining the stage ofacclimation for winter and for determination of cellulardamage caused by artificial frost.

METHOD

The impedance spectral measurements (80H2 to lMHz)were made in 10 or 15 mm long sections of tissues with the

method described previously t3l. By using Ag/AgClelectrodes which were set in contact with a conductive paste

to form a salt bridge between electrode and sample, and byshort-circuit correction of the instrument (HP42844),

electrode/tissue interface polarization was kept at minimum.The tested plant material consisted woody samples, i.e.

current-year stems, needles and roots of Scots pine (Pinussylvestris), current-year stems of willow (Salix wiminalis) and

previous-year stems of Norway spruce (Picea abies L. Karst.),

as well as stems sections from a herbaceous plant, English

rye grass (I"o I iu m p e r e nn e) . The measurements were co nducted

in different stages of the annual cycle of the plants, and after

exposing them to different degrees of frost. The symmetric

impedance spectra were analysed by Cole-Cole function(ZARC response function in [a]) and the asymmetric spectra

by Havriliak-Negami function or Model-A [4,5,6]. The

equivalent circuit parameters were estimated by CNLS-curvefitting program (LEVM v6.0).

RESULTS

In the stems of Scots pine the impedance spectrum was

composed of two arcs, and the proportion of the arcs changed

with the developmental stage. The stem was best described by

a double-DCE model. In willow, the spectrum was composed

of two arcs during the growth season, but during lignificationand frost hardening one arc of the spectrum graduallydisappeared or the arcs became overlapped. Both the double-DCE and the single-DCE model were used for fitting. InNorway spruce, the spectrum typically composed of one arc

although there was slight indications of a second arc at lowfrequencies. However, this was difficult to take into accountin the curve fitting of the double-DCE model. The spectrumwas skewed. also.

The impedance spectrum of the Scots pine needles and

roots, and the stem sections of English ryegrass composed ofone arc. The needles were best modelled by the Havriliak-Negami model or the new Model-A which took into accountthe slight skewness of the spectrum. However, good fittingresults were obtained by the Cole-Cole function, too. The stem

sections of English ryegrass were best modelled by the

Havriliak-Negami model and the roots by the Cole-Colefunction.

The changes in intracellular resistance of stem of Scots

pine and willow coincided with changes in frost hardiness.

Furthermore. the extracellular resistance and the relaxationtime typically decreased with increasing cellular damage.

DISCUSSION

The results of this study prove the diversity in the impedancespectra of plant species and organs. The strongly depressed

spectra suggest the heterogenity of the tissues studied. Thus

the lumped models used to describe homogeneous tissues as

potato tuber and apple fruit, must be discarded in the case ofmore heterogenous tissues. The heterogeneity and the

application of the distributed models make the biologicalinterpretation of the model parameters difficult. The Model-Adeveloped for needles is an exception since the parameters

have a reasonable interpretation. For the plant species tested,

the coincidence of the changes of the intracellular resistance

with frost hardiness is promising for applications.

REFERENCES

tll J.J.Ackmann, and M.A.Seitz, "Methods of compleximpedance measurement in biological tissue", CRC CriticalReview in Biomedical Engineering ll: 281-3 1 1, 1984.

t2l K.S. Cole, Membranes, Ions and Impulses. - University ofCalifornia Press, Berkeley and Los Angeles, 1968.

t3] T. Repo, "Influence of different electrodes and tissues on

the impedance spectra of Scots pine shoots", Electro- and

Magnetobiology 13: l-14, 1994.

t4l J.R. Macdonald, Impedance spectroscopy - EmphasizingSolid Materials and Systems. John Wiley & Sons, New York,1987.

t5l T.Repo, M.I.N. Zhang,A. Ryyppö, E. Vapaavuori, and S.

Sutinen, "Effects of freeze-thaw injury on parameters of

Medical & Biologicat Engineering & computing Vol. 34, supplement 1, part 2, 1996The 1st International Conference on Bioeleclromagnetism, June b-13, 1996, Tampere, Finland 173

distributed electrical circuits of stems and needles of Scotspine seedlings at different stages of acclimation", Journal ofExperimental Botany 45: 823-833, 1994.

t6l M.I.N. Zhang, T. Repo, J.H.M. Willison, and S. Sutinen,"Electrical impedance analysis in plant tissues: on thebiological meaning of Cole-Cole s in Scots pine needles",European Biophysics Journal 24: 99-106, 1995.

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