Cold hardiness of diploid and corresponding autotetraploid rhododendrons

Summary

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 15to 108C in a greenhouse, the acclimation regime in a phytotron comprised three consecutive weeks at 15, 11 and 228C. 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 LT<sub>50</sub> of the `PJM' leaves was 247.18C and that of NSB 234.28C. The diploid `Cunningham's White' leaves hardened to 235.08C and the tetraploid ones to 222.78C. The LT<sub>50</sub> of the stems of `PJM' and NSB was between 2338C and 2408C 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 211.18C and of the NSB florets 28.18C. 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.