Physiological study of cold acclimation in Rhododendron sp. with emphasis on role of dehydrins

Abstract

In this study we established the significance of 25 kDa dehydrin accumulation during cold acclimation (CA) in a wide array of Rhododendron species. These species (24 in total) belong to two diverse subgenera, Hymenanthes and Rhododendron, native to diverse latitudes and altitudes. The dehydrin of interest is highly conserved in Rhododendron genus and was present and up-regulated during CA in all the species studied with one exception-R. brookeanum-a species adapted to tropics. Some other dehydrins were also found to accumulate in response to cold acclimation in several species, but none of these accumulated consistently. Experimental data show that there is no correlation between the absolute amount of 25 kDa dehydrin and the degree of leaf hardiness in cold acclimated plants. Moreover, a higher number of dehydrin species in a particular genotype does not necessarily translate into more hardy Rhododendron. However, our results suggest that the cold-inducibility of a 25 kDa dehydrin is positively correlated with cold acclimation ability in Rhododendron. The 25 kDa dehydrin appeared not to be specifically regulated by cold-its accumulation was triggered by water stress as well. However, we were unable to detect any accumulation of this dehydrin in response to exogenous abscisic acid (ABA) application. During the first stage of cold acclimation, the accumulation of 25 kDa dehydrin is triggered by short photoperiods. However, the onset of cold temperatures during late Fall and winter overrides the early photoperiod stimulus. Our data also showed that short photoperiod alone (in the absence of low temperature) is sufficient to induce both a small level of cold acclimation and 25 kDa accumulation in Rhododendron leaves. However, plants are unable to cold acclimate to any relevant level when exposed to warm temperatures and extended day-length irrespective of exposure to seasonal shifts over the year. Tissue localization studies indicate that 25 kDa dehydrin accumulates in the leaf lamina with lower accumulation in the midrib. This may be correlated with the primary area of injury in Rhododendron as being the midrib. iii AKNOWLEDGEMENTS First of all I would like to acknowledge the assistance provided by my committee chair, Dr. Rajeev Arora for his guidance during the last two years. Also I would like to thank Dr. Steve Krebs for his contribution to my thesis and the commitment to perform long trips from Ohio to West Virginia whenever was necessary. I thank Dr. Sven Verlinden for his advice and willingness to help at all times and Dr. Barton Baker for his support of my graduate studies. Many thanks to the people at David G. Leach Research Station of the Holden Arboretum, who took good care of the plant material used in this study. My thanks to the greenhouse crew, particularly to Sue Myers and Gail Sikorsky for their help with the plant material. My special thanks to Dr. Attila Eris, my lab collaborator for six moths, who was always a helpful and enjoyable presence. I want to thank also Gloria Nestor for putting together...