Intracellular resistance correlates with initial stage of frost hardening in willow (Salix viminalis)

Distributed model parameters of shoots of five clones of willow (Salix viminalis) were examined with electrical impedance analysis at the end of the growing season and with cold acclimation. The parameters were compared with regard to frost hardiness, linoleic (18:2) and linolenic (18:3) fatty acids, unsaturation/saturation ratio of fatty acids and dry weight content. The intracellular resistance (r(i)) correlated best with changes in frost hardiness. The value of r(i) rose from 1-2 Ohm m in non-hardened to about 12 Ohm m in hardened samples. In the initial stages of frost hardening, a linear relationship was found between r(i) and frost hardiness and levels of 18:2 fatty acid, and an inverse relationship between r(i) and levels of 18:3 fatty acid. The unsaturation/saturation ratio of fatty acids rose fairly rapidly in the initial stage of hardening. The dry weight content increased stepwise during the experimental period, and less steadily than r(i). In addition, equivalent circuit parameters changed in the prehardening phase, and were probably connected with cell differentiation and lignification. Frost hardiness by the visual method and by extracellular resistance, determined after controlled freezing tests, correlated well in the initial stages of hardening until about -10 degrees C but deviated upon further hardening.