Electrical impedance analysis in plant tissues: on the biological meaning of Cole-Cole ? in Scots pine needles

Abstract

Electrical impedance spectra (80 Hz–1 MHz) in Scots pine needles were found to be characterized by spectrum skewness in the Cole-Cole plot. These spectra were subjected to analysis with two distributed models: (i) the Cole-Cole function and (ii) an equivalent circuit which takes account of the presence of air spaces within the needles (Model-A). In analysis with untreated needles (without artificial infiltration with water), Model-A fitted better than the Cole-Cole function to the experimental data. After infiltration of water into the needles, the extent of spectrum skewness was substantially decreased compared with the pre-infiltration condition and the Cole-Cole function fitted better than Model-A to the measured impedance data. The Cole-Cole α decreased from 0.47 in non-infiltrated needles to 0.42 in the infiltrated needles. The exceptionally large value of α in non-infiltrated needles can be explained by the presence of air spaces, which produce transmission line properties in the mesophyll. In support of the validity of Model-A, this new model provided specific membrane resistances of 1190 ± 83 Ω cm2 in cold hardened and non-hardened needles respectively. These specific membrane resistance are comparable with previous reports of membrane resistances in other biological systems. It is concluded that in this exceptionally spongy tissue, Cole-Cole α is likely to be due to the effects of the transmission line properties of cells which are surrounded by air spaces and only thin cell walls outside the insulating cell membranes.