Impedance Spectroscopy in Frost Hardiness Evaluation of Rhododendron Leaves

Abstract

Impedance spectroscopy was used in studying frost hardiness of leaves of two diploid rhododendron cultivars, Rhododendron L. ‘PJM’ and R. ‘Cunningham's White’, and their tetraploid derivatives, R. ‘Northern Starburst’ (NSB) and CW4. After the growing season and initial hardening in a greenhouse, plants were subjected to an acclimation regime in a phytotron: 3 consecutive weeks at +5, +1 and −2°C each. Hardiness was studied with controlled freezing tests before each decrease in temperature and at the end of the experiment, based on data of extracellular resistance reand relaxation time τ of the frost-exposed leaves. The correlation of the two estimates was 0.92. Generally, the diploid clones had better frost hardiness than the tetraploid clones. At the end of the experiment, frost hardiness of the diploid ‘PJM’ was −28.7°C and that of the tetraploid NSB −20.6°C. Leaves of the diploid ‘Cunningham's White’ and of the tetraploid CW4 hardened to −32.0°C and −20.9°C, respectively. Frost hardiness estimated by impedance spectroscopy correlated well with earlier results based on visual scoring (r=0.81–0.86) and electrolyte leakage tests (r=0.84–0.90), but results from impedance spectroscopy indicated weaker hardiness than the other tests. The difference between the results from impedance spectroscopy and the other tests was smaller and more coherent within the ‘Cunningham's White’ clones than within ‘PJM’ and NSB. Changes in extracellular and intracellular resistance of non-frozen leaves during the acclimation correlated with the changes in frost hardiness of ‘Cunningham's White’ clones, but not with those of ‘PJM’ and NSB, which belong to another subspecies.