Electrical impedance analysis in plant tissues: The effect of freeze-thaw injury on the electrical properties of potato tuber and carrot root tissues

Electrical impedances in the range 100 Hz to 800 KHz were measured in potato tubers and carrot root cortex both before and after freeze-thaw cycles. These impedance data were analyzed using equivalent circuit modelling based on complex nonlinear least squares (CNLS) (Zhang and Willison 1991a). After freezing to and thawing from −3, −6, −9 and −12 °C, carrot root tissues were best characterized by a double shell model composed of extracellular resistance, cytoplasmic resistance, plasma membrance capacitance, vacuole interior resistance, and tonoplast capacitance. Although freeze–thaw cycles to −3 or −6 °C did not kill the carrot tissues, extracellular resistance and vacuole interior resistance were halved relative to control tissues, and cytoplasmic resistance decreased to a third of the control value. Plasma membrane capacitance and tonoplast capacitance were not affected by the −3 and −6 °C noninjurious stresses. After carrot root or potato tuber tissues had been killed by freezing (−3 °C for potato tuber and −18 °C for carrot), the tissues were best represented by a single-shell model composed of extravesicular resistance, intravesicular resistance, and vesicle membrane. Compared with unfrozen controls, freeze-killed potato tubers were characterized by a 30-fold decrease in extracellular resistance, a 7-fold decrease in intracellular resistance, and a 10-fold decrease in plasma membrane capacitance. Freeze-killed carrot roots were characterized by a 30-fold decrease in extracellular resistance, 3-fold decrease in intracellular resistance, and 3.5-fold decrease in plasma membrane capacitance. These results are compatible with massive rupturing of protoplasts during lethal freeze–thaw injury.Key words: Solanum tuberosum L., Daucus carota L., freeze–thaw injury, electrical impedance