Estimating Cold Stress in `Beautiful Arcade' Apple Roots using Electrical Impedance Analysis
2,3,5-triphenyl tetrazolium chloride (TTC) staining, electrical conductivity, and electrical impedance (Z) analyses were used to assess freezing injury of `Beautiful Arcade' apple (Malus ×domestica Borkh.) roots taken in late March from either the field or 3C-refrigerated storage (cold-stored). Lethal temperature (LT50) levels using TTC or electrical conductivity occurred at colder temperatures than those found using Z. Techniques varied in their ability to detect changes in cell viability with increasing cold stress. Listed in order of decreasing responsiveness they are electrical impedance (Z), electrical conductivity, and TTC vital staining. With the most sensitive technique, Z, two parameters—extracellular and total tissue electrical resistance—were about five and eight times lower (indicating more injury) for roots from the field than from cold storage. The smaller values obtained from the field roots were probably due to natural in-field freeze-thaw cycling before the controlled cold-stress tests in the laboratory. More importantly, the impedance technique provided more detailed information than TTC or electrical conductivity about how apple roots respond to cold stress and how Z may provide some insight into freeze-thaw history before injury assessment. Although this technique shows potential, future studies are required to render a complete physiological significance to the impedance parameters involved in calculating Z.