The relationship of cold acclimation and extracellular ice formation to winter thermonasty in two Rhododendron species and their F hybrid.

Premise: Thermonastic leaf movements in evergreen Rhododendron species have been used to study plant strategies for winter photoprotection. To add to the current fundamental understanding of this behavior, we addressed the following questions: (1) Is the cold-acclimated (CA) state necessary for thermonasty, and do cold-induced leaf movements also occur in non-acclimated (NA) plants? (2) Which of the two movements, leaf rolling versus curling, is more responsive to freezing, if any, in a non-thermonastic species? (3) What is the temporal relationship between extracellular freezing and thermonasty? (4) What genetic inferences can be drawn from leaf movement in an F hybrid relative to its parents?

Methods: A temperature-controlled, gradual cooling regime was used to quantify freeze-induced leaf movements. Infrared thermography was used to confirm extracellular ice-formation in leaves.

Cold hardiness increases with age in juvenile Rhododendron populations.

Winter survival in woody plants is controlled by environmental and genetic factors that affect the plant's ability to cold acclimate. Because woody perennials are long-lived and often have a prolonged juvenile (pre-flowering) phase, it is conceivable that both chronological and physiological age factors influence adaptive traits such as stress tolerance. This study investigated annual cold hardiness (CH) changes in several hybrid Rhododendron populations based on T max, an estimate of the maximum rate of freezing injury (ion leakage) in cold-acclimated leaves from juvenile progeny.

RcDhn5, a cold acclimation-responsive dehydrin from Rhododendron catawbiense rescues enzyme activity from dehydration effects in vitro and enhances freezing tolerance in RcDhn5-overexpressing Arabidopsis plants.

Dehydrins (DHNs) are typically induced in response to abiotic stresses that impose cellular dehydration. As extracellular freezing results in cellular dehydration, accumulation of DHNs and development of desiccation tolerance are believed to be key components of the cold acclimation (CA) process. The present study shows that RcDhn5, one of the DHNs from Rhododendron catawbiense leaf tissues, encodes an acidic, SK(2) type DHN and is upregulated during seasonal CA and downregulated during spring deacclimation (DA).

Phylogenetic analysis and seasonal cold acclimation-associated expression of early light-induced protein genes of Rhododendron catawbiense.

The early light-induced proteins (ELIPs) are nuclear-encoded, light stress-induced proteins located in thylakoid membranes and related to light-harvesting Chl a/b-binding proteins. Recent evidence from physiological and genetic (mutant) studies supports a photoprotective function for ELIPs, particularly when green tissues are exposed to high light intensities at suboptimal temperatures. Broad-leaved evergreens belonging to genus Rhododendron are often exposed to a combination of low temperatures and high light in their natural habitat as the understory plants in deciduous forests and, therefore, are expected to employ photoprotective strategies during overwintering phase.

Comparative analysis of expressed sequence tags from cold-acclimated and non-acclimated leaves of Rhododendron catawbiense Michx.

An expressed sequence tag (EST) analysis approach was undertaken to identify major genes involved in cold acclimation of Rhododendron, a broad-leaf, woody evergreen species. Two cDNA libraries were constructed, one from winter-collected (cold-acclimated, CA; leaf freezing tolerance -53 degrees C) leaves, and the other from summer-collected (non-acclimated, NA; leaf freezing tolerance -7 degrees C) leaves of field-grown Rhododendron catawbiense plants. A total of 862 5'-end high-quality ESTs were generated by sequencing cDNA clones from the two libraries (423 from CA and 439 from NA library).

Dehydrin variability among rhododendron species: a 25-kDa dehydrin is conserved and associated with cold acclimation across diverse species.

•  Here we examine the accumulation pattern of dehydrins in non- vs cold-acclimated leaves of 21 species comprising two divergent groups of Rhododendron, Subgenus Hymenanthes and Subgenus Rhododendron. Individuals from five other Ericaceous genera were also evaluated in the same way. Quantitative comparisons of cold-inducibility of a 25-kDa dehydrin and cold acclimation ability in six Rhododendron species were also performed.