Controlled dehardening and rehardening in floral buds of deciduous azaleas

Kalberer, Scott
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Dehardening resistance and rehardening in woody perennials provide protection against frost that may follow unseasonable warm spells during winter and spring. The effects of biogeography on hardiness transitions and endodormancy were studied using species and/or ecotypes of deciduous azaleas (Rhododendron) originating in diverse habitats. The thesis describes dehardening, rehardening, dehydrin accumulation, and water content after increasing days of dehardening (DOD). Hardiness experiments and bud-break forcing were conducted at different times to relate winter progression with endodormancy and hardiness transitions. Rhododendron canadense exhibited a low chilling requirement (CR) (450 chilling units), whereas nine other genotypes had somewhat larger CR (> 820 chilling units). A high CR that delays bud break until there is less danger of frost may be adaptive in the southeast and Appalachians, but not in the northern habitat of R. canadense where unseasonable warm temperatures are less common. Mid-winter hardiness was correlated with the minimum temperatures of the native habitat; genotypes from colder habitats acclimated more than those from warmer areas. Increasing water content was associated with dehardening in all azaleas tested. Dehydrins declined during dehardening and reaccumulated during reahardening in selected genotypes. Dehardening resistance could not be consistently associated with the minimum temperatures of habitats and provenances or with the temperature ranges at those provenances. However, genotypes originating in the Appalachians typically had high dehardening resistance; mountain temperatures, which fluctuate more in frequency and magnitude than those at the coastal plains, may have promoted development of slow dehardening. Dehardening resistance is a complex adaptation that is probably not explained by a single factor. Dehardening resistance decreased as winter progressed and endodormancy declined in some genotypes (e.g., R. canadense) but not in others (e.g., Rhododendron arborescens). In Dec. 2004 the rehardening occurred even after eight DOD in R. canadense and the Rhododendron viscosum varieties (montanum and serrulatum). However, in Feb. 2006 rehardening did not occur until after 10 DOD in R. viscosum var. montanum and 15 DOD in R. arborescens; this finding was interpreted as evidence for a 'minimum threshold' of dehardening required to induce rehardening.