Seasonal responses to cold and light stresses by two elevational ecotypes of Rhododendron catawbiense: A comparative study of overwintering strategies

Abstract

For winter survival, evergreens not only need cold acclimation (CA), but also photoprotection. This study was conducted to examine how two elevational ecotypes of Rhododendron catawbiense respond to cold and light stresses in a common garden. Seasonal changes in leaf freezing tolerance (LFT), photosystem II efficiency, photosynthesis, carotenoids (xanthophyll cycle pigments, lutein, beta-carotene, neoxanthin) were investigated from summer to winter. Parallel changes in the expression of violaxanthin de-epoxidase (VDE) gene were examined to explore its association with violaxanthin de-epoxidation. High elevation (HE) ecotype attained overall greater LFT while low elevation (LE) ecotype was more responsive to short-days for the first stage of CA. In summer, both ecotypes predominantly employed reversible xanthophyll cycle (RXC) to dissipate excess light energy, but LE used it more robustly. In autumn, only HE exhibited slight damage to the reaction centers (RCs), i.e. photosystem II degradation (PSD). In winter, both ecotypes suspended photosynthesis, were photoinhibited, and employed primarily sustained de-epoxidation (SD) for photoprotection; but LE still used RXC more effectively than HE. Overall, LE employed xanthophyll cycle more robustly in summer or winter. Results suggest that microclimates of R. catawbiense ecotypes inform their differential physiological response to cold and light stresses. VDE expression data indicate violaxanthin de-epoxidation in R. catawbiense may not be solely transcriptionally regulated.