Genus Rhododendron contains more than 800 species worldwide, currently grouped into eight subgenera. Four of these subgenera—comprising the evergreen azaleas, deciduous azaleas, small scaly-leaved rhododendrons, and large non-scaly leaved rhododendrons—have been the focus of ornamental breeding for over 150 years. As a rule of thumb, species within a subgenus are cross-fertile, and most hybrids are derived from intra-subgeneric crosses. Success with wider (inter-subgeneric) crosses, especially deciduous azaleas × large-leaved rhododendrons, has been occasionally reported in the past, based on the intermediate morphology of the hybrids. I crossed a tetraploid `Ilam group' azalea with R.`Catlalgla' (a selection of the native diploid rhododendron species R. catawbiense) and produced a small population of seedlings that proved to be true `azaleodendron' hybrids, based on shared parental alleles at 2 isozyme loci, Idh-1 and Mdh-3. However, none of the progeny are hybrid in appearance; they share the leaf morphology and deciduous trait of the maternal azalea parent. I attribute this result to a dosage effect in these (probable) triploid hybrids, where the azalea genetic contribution is twice that of the rhododendron parent. Higher copy number can be inferred from stronger band intensities for the azalea gene at diallelic loci (Idh-1), or from triallelic loci (Mdh-3) where the genetic contribution to the hybrid progeny appears to be 2:1, azalea: rhododendron. Previously, azalea-like progeny from azalea × rhododendron crosses were thought to result from parthenogenesis or accidental self-pollination.
Stephen L. Krebs and Michael D. Wilson
Fifty-seven rhododendron cultivars (genus Rhododendron L.) were screened for resistance to root rot caused by Phytophthora cinnamomi, using two levels of inoculum. While a majority (77%) of genotypes was susceptible, six cultivars had moderate resistance, and seven cultivars exhibited a high level of resistance to the disease. In these resistant groupings, the severity of root rot did not increase significantly with a 3-fold increase in inoculum. Comparisons of micropropagated and conventionally propagated plants revealed no significant difference in root rot ratings. The species R. keiskei was identified as a possible source of resistance to P. cinnamomi in two of the rhododendron cultivars.
Michael C. Long*, Stephen L. Krebs, and Stan C. Hokanson
Forty deciduous azalea (Rhododendron sp.) cultivars from commercial sources were evaluated for powdery mildew (Microsphaera sp.) resistance. Plants were established in two duplicate field plantings in Ohio and Minnesota and evaluated in 2002 and 2003. Plants were scored using a disease symptom rating based on the percent of leaf area infected, evaluating both ab- and adaxial leaf surfaces. Highly significant differences were found for cultivar, location, year, cultivar × location and cultivar × year for disease severity. Calendulaceum × speciosum, `Fragrant Star', `Garden Party', `Late Lady', `Millennium', `Parade', and `Popsicle' showed no powdery mildew symptoms in both locations. Another group of plants with only minimal symptoms (<25% leaf area affected) included `Jane Abbott', `Magic', `Northern Hi-Lights' and `Snowbird'. The symptom-free cultivars exhibited glaucous foliage, suggesting a potential, common resistance mechanism. The mean scores for the abaxial and adaxial leaf surfaces were 2.34 and 1.64, respectively, although four cultivars had more disease symptoms on the adaxial surface. `Arneson Gem' showed nearly a two-point difference between abaxial and adaxial scores. Evaluations of azalea powdery mildew susceptibility should consider both leaf surfaces and use the highest score as the best estimate of host resistance.
Xiang Wang, Rajeev Arora, Harry T. Horner, and Stephen L. Krebs
Evergreen rhododendrons (Rhododendron L.) are important woody landscape plants in many temperate zones. During winters, leaves of these plants frequently are exposed to a combination of cold temperatures, high radiation, and reduced photosynthetic activity, conditions that render them vulnerable to photooxidative damage. In addition, these plants are shallow-rooted and thus susceptible to leaf desiccation when soils are frozen. In this study, the potential adaptive significance of leaf morphology and anatomy in two contrasting Rhododendron species was investigated. R. catawbiense Michx. (native to eastern United States) exhibits thermonasty (leaf drooping and curling at subfreezing temperatures) and is more winter-hardy [leaf freezing tolerance (LT50) of containerized plants ≈–35 °C], whereas R. ponticum L. (native to central Asia) is less hardy (LT50 ≈–16 °C), and nonthermonastic. Thermonasty may function as a light and/or desiccation avoidance strategy in rhododendrons. Microscopic results revealed that R. ponticum has significantly thicker leaf blades but thinner cuticle than R. catawbiense. There is one layer of upper epidermis and three layers of palisade mesophyll in R. catawbiense compared with two distinct layers of upper epidermis and two layers of palisade mesophyll in R. ponticum. We suggest that the additional layer of upper epidermis in R. ponticum and thicker cuticle and extra palisade layer in R. catawbiense represent structural adaptations for reducing light injury in leaves and could serve a photoprotective function in winter when leaf photochemistry is generally sluggish. Results also indicate that although stomatal density of R. ponticum is higher than that of R. catawbiense leaves, the overall opening of stomatal pores per unit leaf area (an integrated value of stomatal density and pore size) is higher by approximately twofold in R. catawbiense, suggesting that R. catawbiense may be more prone to winter desiccation and that thermonasty may be a particularly beneficial trait in this species by serving as a desiccation-avoidance strategy in addition to a photoprotection role.
Michael C. Long, Stephen L. Krebs, and Stan C. Hokanson
Forty-one deciduous azalea (Rhododendron subgen. Pentanthera G. Don) cultivars were assessed for powdery mildew (PM) resistance in a two-location, 3-year field trial. Disease severity (percent leaf area affected) on abaxial leaf surfaces was used to rate the level of field resistance. This measure was proportional to (r = 0.83) but higher than estimates from corresponding adaxial surfaces. Eleven of these cultivars (27%) appeared to be highly resistant under field conditions, i.e., evidence of PM on the leaves was zero or near zero. Twenty-three of the cultivars evaluated in the field experiment were also evaluated in a growth chamber experiment. In contrast to the field study, PM was more severe on the adaxial leaf surface in the growth chamber but still highly correlated with the abaxial response (r = 0.93). Based on adaxial disease scores, no cultivars in the growth chamber experiments were completely resistant. Growth chamber disease ratings based on either leaf surface were predictive of field performance (r 2 = 0.62), suggesting use of the chambers could serve as a low-cost, off-season, early selection component of a deciduous azalea PM resistance breeding program.
Chon C. Lim, Rajeev Arora, and Stephen L. Krebs
Few genetic studies have been conducted on the inheritance of cold hardiness (CH) in woody plants. An understanding of the genetic control of CH can greatly assist the breeder in reducing winter injury. This study was initiated to evaluate the distribution of CH phenotypes in segregating populations of evergreen rhododendrons. Naturally acclimated leaves from individual plants (parents, F1 and 47 F2 progeny) were subjected to controlled freeze–thaw regimes. Using slow cooling rates, leaf discs were cooled over a range of treatment temperatures from –10°C to –52°C. Freezing injury of leaf tissue was assessed by measuring ion-leakage and non-linear regression analysis (data fitted to Gompertz functions) was used to estimate Tmax, the temperature causing the maximum rate of injury. Tmax for the parent plants (R. catawbiense & R. fortunei) and the F1 cultivar Ceylon, were estimated to be –51.6°C, –30.1°C, and –40.4°C, respectively. CH estimates among F2 progeny (Ceylon, selfed) were normally distributed from –14.8°C to –41.5°C, with mean of –27.6°C. Most F2 progeny were less cold-hardy than the tender parent, R. fortunei. The apparent reduction in F2 CH may be caused by the differences in age between the parents (20-year-old mature plants) and F2 progenies (3-year-old juvenile seedlings). Currently, we are testing age-dependent CH responses in rhododendrons, and are also characterizing CH distributions in a backcross population.
Chon C. Lim, Rajeev Arora, and Stephen L. Krebs
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.
Calin O. Marian, Atilla Eris, Stephen L. Krebs, and Rajeev Arora
The influence of photoperiod and temperature on the seasonal (fall to winter) cold acclimation and accumulation of a 25 kDa dehydrin in Rhododendron `Chionoides' was studied by exposing two groups of plants each in the greenhouse or outdoors to either a natural photoperiod (or short days) or an extended photoperiod (or long days) regime. Results suggest that the shortening daylength alone is sufficient to trigger both the first stage of cold acclimation and concomitant 25 kDa dehydrin induction. Exposure of the plants to natural photoperiod and temperatures induced the greatest cold hardiness and 25 kDa accumulation, while exposure to extended photoperiods (long days) and warmer temperatures (in the greenhouse) failed to induce any significant freezing tolerance in leaves. Whereas short days trigger the cold acclimation process initially, low inductive temperatures can eventually replace the photoperiod stimulus. Seasonal accumulation of 25 kDa dehydrin, on the other hand, appears to be predominantly effected by short photoperiods. Data indicated that the leaf water content of outdoor plants maintained under natural photoperiod was lower than that of plants grown under extended photoperiod. This was also true for the greenhouse plants at the first (September) and the last (January) sampling. It is hypothesized that early 25 kDa dehydrin accumulation may be due to short-day-induced cellular dehydration. Accumulation of two other dehydrins of 26 kDa and 32 kDa molecular masses does not appear to be associated with short day (SD)-induced first stage of cold acclimation. Results show that their accumulation may be regulated by low, subfreezing temperatures and may be associated with the second and/or third stage of cold acclimation of `Chionoides' rhododendron leaves.
Melvyn L. Lacy, Rebecca Grumet, Karen F. Toth, Stephen L. Krebs, Brian D. Cortright, and Elizabeth Hudgins
Scott R. Kalberer, Rajeev Arora, Norma Leyva-Estrada, and Stephen L. Krebs
Dehardening resistance and rehardening capacity in late winter and spring are important factors contributing to the winter survival of woody perennials. Previously the authors determined the midwinter hardiness, dehardening resistance, and rehardening capacities in deciduous azalea (Rhododendron L.) floral buds in early winter. The purpose of this study was to investigate how these parameters changed as winter progressed and to compare rehardening response at three treatment temperatures. Experiments were also conducted to measure bud water content during dehardening and chilling accumulation of 10 azalea genotypes. Buds of R. arborescens (Pursh) Torr., R. canadense (L.) Torr., R. canescens (Michx.) Sweet, and R. viscosum (L.) Torr. var. montanum Rehd. were acclimated in the field and were dehardened in the laboratory at controlled warm temperatures for various durations. Dehardened buds were rehardened for 24 hours at 2 to 4 °C, 0 °C, or –2 °C. Bud hardiness (LT50) was determined from visual estimates of freeze injury during a controlled freeze–thaw regime. The midwinter bud hardiness in the current study was ≈4 to 8 °C greater than in early winter. R. canadense and R. viscosum var. montanum dehardened to a larger extent in late winter than in the early winter study whereas R. arborescens and R. canescens did not. The rehardening capacities were larger in early than in late winter. Even though rehardening occurred throughout the first 8 days of dehardening (DOD) in early winter in the previous study, in the current study it was only observed after 10 DOD (R. viscosum var. montanum) or 15 DOD (R. arborescens). There was no difference among the rehardening capacities at the three rehardening temperatures for any genotype. Water content decreased throughout dehardening in all four genotypes examined. R. canadense had the lowest chilling requirement (CR) [450 chilling units (CU)], followed by R. atlanticum (Ashe) Rehd., R. austrinum (Small) Rehd., R. canescens, and R. calendulaceum (Michx.) Torr. with intermediate CR [820, 830, 830, and 1000 CU respectively). The CR of R. arborescens, R. prinophyllum (Small) Millais, R. prunifolium (Small) Millais, R. viscosum var. montanum, and R. viscosum var. serrulatum (Small) Millais exceeded 1180 CU. Results of this study indicate that the dehardening kinetics (magnitude and rate) and the rehardening capacity of azalea buds are influenced by the progression of winter and the depth of endodormancy.