Frost dehardening and rehardening of floral buds of deciduous azaleas are influenced by genotypic biogeography

Temperate-zone woody perennials may resist cold dehardening and reharden effectively after unseasonably warm winter conditions to avoid frost damage. Few controlled experiments have examined dehardening kinetics or the impact of dehardening on rehardening capacity after cold temperatures return. We used nine genotypes of deciduous azalea from eight known provenances to study the influence of biogeographical origin on floral bud dehardening and rehardening after controlled dehardening. Buds cold acclimated in the field were placed in warm conditions to stimulate dehardening. Visual assays were conducted periodically over 11 days of dehardening to evaluate survival of immature florets at subfreezing temperatures. A rehardening regime was applied to three genotypes after 1, 3, 5, and 8 days of dehardening. Dehydrin abundance after dehardening and rehardening was estimated for selected genotypes. Floral buds from warmer-climate azaleas Rhododendron canescens, Rhododendron prunifolium, and Rhododendron viscosum variety serrulatum exhibited lower mid-winter hardiness than did the colder-climate azaleas Rhododendron calendulaceum, Rhododendron canadense, Rhododendron prinophyllum, and Rhododendron viscosum variety montanum. The dehardening rates of the “low dehardening-resistant” R. canadense, R. canescens, and R. viscosum var. serrulatum were at least twice the rates of “high dehardening resistant” Rhododendron arborescens, Rhododendron atlanticum, R. calendulaceum, R. prinophyllum, R. prunifolium, and R. viscosum var. montanum throughout the time-course. Genotypes originating in colder and warmer climates did not always exhibit high and low dehardening-resistance, respectively. Dehardening was associated with declining levels of dehydrins in R. prinophyllum and the two R. viscosum varieties. All tested genotypes rehardened in response to cold even after 8 days of dehardening. The high-altitude variety of R. viscosum had substantially larger rehardening-capacity than the low-altitude variety. Rehardening was associated with increasing levels of dehydrins in both R. viscosum varieties. Mid-winter hardiness ≥26.0 °C, dehardening rates ≤1.0 °C day⁻¹, a capacity to reharden, and the ability to accumulate dehydrins could all be important winter survival strategies for genotypes originating in colder climates.