Vol. 74 No: 5

Title:
Cold hardiness of diploid and corresponding autotetraploid rhododendrons

Authors:
ANU VÄINÖLÄ and TAPANI REPO

pp: 541-546

Abstract:
Freezing tolerance was studied in two diploid rhododendron cultivars, Rhododendron 'PJM' and R. 'Cunningham's White', and their tetraploid derivatives, R. 'Northern Starburst' (NSB) and CW4. After the growing season and initial hardening at +5 to 10°C in a greenhouse, the acclimation regime in a phytotron comprised three consecutive weeks at +5, +1 and -2°C. The hardiness was studied with controlled freezing tests before each decrease in temperature and at the end of the experiment. The injury on leaves and stem sections was visually assessed and the data analysed using logit models. Electrolyte leakage tests provided a comparison. Flower bud hardiness was studied by differential thermal analysis. Leaves of the diploid clones attained better cold hardiness than those of the tetraploid clones. At the end of the experiment, the LT₅₀ of the 'PJM' leaves was -47.1°C and that of NSB -34.2°C. The diploid 'Cunningham's White' leaves hardened to -35.0°C and the tetraploid ones to -22.7°C. The LT₅₀ of the stems of 'PJM' and NSB was between -33°C and -40°C depending on test and date. There was no statistically significant difference in the stem hardiness between the ploidy levels. The mean hardiness of the 'PJM' florets was -11.1°C and of the NSB florets -8.1°C. Since in other studies 'PJM' florets have been much more cold tolerant, it seems that the conditions in the phytotron did not allow full acclimation.

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