Seasonal patterns in freezing tolerance of five Rhododendron cultivars that vary in feezing tolerance were estimated. Electrolyte leakage was used, and raw leakage data were transformed to percent leakage, percent injury, and percent adjusted injury. These data were compared with visual estimates of injury. Percent adjusted injury was highly correlated (0.753) to visual estimates. Two asymmetric sigmoid functions—Richards and Gompertz—were fitted to the seasonal percent adjusted injury data for all cultivars. Two quantitative measures of leaf freezing tolerance—Lt50 and Tmax (temperature at maximum rate of injury)—were estimated from the fitted sigmoidal curves. When compared to the General Linear Model, the Gompertz function had a better fit (lower mean error sum of squares) than Richards function. Correlation analysis of all freezing tolerance estimates made by Gompertz and Richards functions with visual LT50 revealed similar closeness (0.77 to 0.79). However, the Gompertz function and Tmax were selected as the criteria for comparing relative freezing tolerance among cultivars due to the better data fitting of Gompertz function (than Richards) and more descriptive physiological representation of Tmax (than LT50). Based on the Tmax (°C) values at maximum cold acclimation of respective cultivars, we ranked `Autumn Gold' and `Grumpy Yellow' in the relatively tender group, `Vulcan's Flame' in intermediate group, and `Chionoides' and `Roseum Elegans' in the hardy group. These relative rankings are consistent with midwinter bud hardiness values reported by nurseries.

The objectives of this study were to develop systems for mass proliferation, rooting, and reestablishment of microcuttings of Helleborus orientalis and Aconitum uncinatum. Basal medium for H. orientalis contained 1/2 MS with 0.4 mg thiamine–HCl/liter, 0.5 mg pyridoxine–HCI/liter, 0.5 mg nicotinic acid/liter, 100 mg i-inositol/liter, 5 mg BA/liter, 2% sucrose, and 0.7% Phytagar. There was no effect of GA (1 mg–liter–1) or TDZ (0.1, 1 mg–liter–1) on axillary shoot proliferation. Helleborus orientalis rooted in vermiculite, Redi-Earth, or 4 perlite: 1 peat with 50% to 56% survival. A field plot containing 18 clonal H. orientalis has been established. Basal medium for A. uncinatum contained WPM with 2% sucrose, 2.5 mg BA/liter, 150 ppm ascorbic acid, 150 ppm citric acid, and 0.7% Phytagar. There was no effect of photoperiod (8, 12, 14 h, 52.5 μmol–m–2–s–1 photosynthetic active radiation) or banana extract on axillary shoot proliferation. Significantly more axillary shoots were generated in the presence of BA (10 mg–liter–1) + kinetin (10 mg–liter–1). Medium containing 500 ppm of PVPP resolved blackening of microcutting bases. More than 500 in vitro-rooted microcuttings (1 mg IBA/liter) survived and grew when transplanted into MetroMix 510 and placed under humidity domes for 6 weeks in the mist.

This study examines whether dehydrin expression in leaves is associated with varying levels of cold hardiness among evergreen rhododendrons. Initially, differences in leaf freezing tolerance (LFT) were determined within three groups of plants: 1) a cultivar group; `Chionoides' (CND), `Grumpy Yellow' (GY), and `Vulcan's Flame' (VF) 2) a segregating F2 population derived from a super coldhardy (R. catawbiense) × less-hardy (R. fortunei) cross, and 3) juvenile seedlings and mature plants of wild-collected R. maximum. LFTs in fully acclimated cultivars corresponded with their USDA hardiness zone ratings—CND (zone 4, –32 °C) GY (zone 7, –16 °C), and VF (zone 6, –19 °C). F2 segregation was characterized by a continuous, normal distribution of LFT values, with groups of progeny at the “tails” differing in their mean LFT by 20 °C. Juvenile seedlings of R. maximum exhibited LFTs that were 12 °C lower than LFTs from mature plants. Western blots of leaf proteins revealed a common 50-kDa dehydrin that accumulated during cold acclimation in all three cultivars and appeared to be quantitatively associated with LFTs. Isoelectric focusing of the 50-kDa Rhododendron dehydrin revealed two isoforms (pI 5.5 and 6.5). The more acidic isoform was detected only in the hardiest (CND) cultivar. Experiments are underway to examine qualitative and quantitative expression of dehydrins and its association with LFT in the segregating F2 population and in the group of R. maximum plants differing in physiological age.

A time-course study was conducted to characterize seasonal patterns of cold hardiness (CH) and protein profiles in the leaf tissue of five Rhododendron cultivars (`Grumpy Yellow', `Vulcan's Flame', `Autumn Gold', `Chionoides', and `Roseum Elegans'). Leaf samples were collected monthly (starting in mid September) and leaf discs were subjected to controlled freezing and thawing regimes. CH (LT50 defined as temperature causing 50% injury) was assessed by electrolyte leakage and visual observations. Data indicate that cultivars varied in their CH in nonacclimated state and in their ability to cold acclimate. Results obtained in September showed `Grumpy Yellow' to be least hardy (about –3°C) and `Roseum Elegans' to be most hardy (about –7°C). All cultivars exhibited successive increases in CH during fall and winter. Maximum CH in all cultivars occurred by December/January with `Chionoides' being most hardy (about –31°C) while `Grumpy Yellow' was least hardy (about –20°C). LT50 based on electrolyte leakage was highly correlated with visual rating. Seasonal changes of protein profiles and relationship of specific stress proteins to cultivars' CH and cold acclimation ability are discussed.

Winter survival in woody plants is controlled by environmental and genetic factors that affect the plant's ability to cold-acclimate. A juvenile period in woody perennials raises the possibility of differences in cold-acclimating ability between juvenile vs. mature (flowering) phases. This study investigated the yearly cold hardiness (CH) changes of rhododendron populations and examined the relationship between leaf freezing tolerance (LFT) and physiological aging. Naturally acclimated leaves (January) from individual plants (parents-R. catawbiense and R. fortunei, F1, F2, and backcross) and F1 population generated from R. catawbiense and R. dichroanthum cross were subjected to controlled freeze-thaw regimes. LFT was assessed by measuring freeze-thaw-induced ion leakage from leaf discs frozen over a range of treatment temperatures. Data were then plotted with a sigmoidal (Gompertz) curve by SAS, to estimate Tmax—the temperature causing maximum rate of injury. Tmax for the 30- to 40-year-old parental plants (catawbiense, fortunei, and dichroanthum) and the F1 `Ceylon' (catawbiense × fortunei) were estimated to be about -52, -32, -16, and -43 °C, respectively. These values were consistent over the 3-year evaluation period. Data indicated the F2 (50 seedlings) and backcross (20 seedlings) populations exhibited significant, yearly Tmax increment (of ≈5-6 °C) from 1996 to 1998 as they aged from 3 to 5 years old. A similar yearly increase was observed in the 12 F1 progenies (compared 2 to 3 years old) of catawbiense × dichroanthum cross. The feasibility of identifying hardy phenotypes at juvenile period and research implications of age-dependent changes in CH will be discussed.