Freezing Tolerance in Rhododendron and Its Association with Dehydrin Expression

Abstract

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.