Date of Award
Doctor of Philosophy
Two Rhododendron species (R. catawbiense and R. ponticum) are used to study their different photoprotective strategies during seasonal cold acclimation at morphological, anatomical, physiological, biochemical and molecular levels. In addition, this dissertation also studied whether or not RcPIP2s (R. catawbiense PIP2s) expression patterns were associated with thermonastic leaf curling in rhododendrons.
Microscopic data revealed that the two Rhododendron species evolved distinct leaf anatomy which could have potential adaptive significance to high light at subfreezing temperatures (chapter 2). Compared with R. catawbiense, R. ponticum has significantly thicker leaf blades but thinner cuticles. There is one layer of upper epidermis and three layers of palisade mesophyll in R. catawbiense, whereas R. ponticum has two distinct layers of upper epidermis and two layers of palisade mesophyll. I suggest that the additional layer of upper epidermis in R. ponticum and thicker cuticle and extra palisade layer in R. catawbiense represent structural adaptations for reducing light injury in leaves and could serve a photoprotective function in winter when leaf photochemistry is generally sluggish. The overall higher opening of stomatal pores per unit leaf area in R. catawbiense indicates that R. catawbiense may be more prone to winter desiccation and that thermonasty may be a particularly beneficial trait in this species by serving as desiccation-avoidance strategy in addition to a photoprotection role.
Comparative study of photoprotection strategies at biochemical and molecular levels during seasonal cold acclimation between the two Rhododendron species suggests that the two species respond differently to winter conditions and have evolved various strategies to reduce and / or tolerate photooxidative stress in winter (chapter 3). In particular, both species have evolved some common strategies including downregulation of photosynthesis capacity, accumulation of early light-induced proteins (ELIPs) and antioxidant systems. R. ponticum accumulates overall higher antioxidant metabolites and enzyme activities, whereas, R. catawbiense has more efficient upregulation of ELIPs and antioxidant systems during seasonal cold acclimation; R. catawbiense also exhibits thermonastic leaf movements. Both species undergo photoinhibition during winter with R. ponticum leaves being relatively less photoinhibited. Such difference may be related to differential sensitivity to excess light in winter and protection efficiencies of ELIPs and antioxidants in the two species, among other adaptations. Although, thermonasty did not seem to provide a clear added advantage to resist photoinhibition in R. catawbiense, its adaptive significance in photoprotection cannot be ruled out. The differences in photoinhibition may also be associated with the distinct leaf anatomies (as shown in chapter 2) in the two species. The additional layer of upper epidermis in R. ponticum, and extra palisade layer and waxy cuticle in R. catawbiense might represent leaf structural adaptations for reducing light injury in winter in these species, and together with ELIPs, antioxidant system and thermonasty constitute photoprotection system in rhododendrons.
To investigate whether or not RcPIP2s expression patterns were associated with leaf curling, the leaves of the thermonastic and nonthermonastic species (R. catawbiense and R. ponticum, respectively) were exposed to a series of temperature regimes in controlled conditions (chapter 4). Progressively cooler temperatures in the subfreezing range employed in this study induced thermonastic leaf curling in R. catawbiense (and uncurling upon warming), whereas R. ponticum leaves showed no curling. Data of gene expression at the transcriptional level using real-time RT-PCR showed similar expression patterns of RcPIP2s across the two species, suggesting that there is no apparent association between RcPIP2s expression and leaf curling / no curling in the two Rhododendron species. However, further investigations involving AQP studies at the protein level as well as at the transcriptional level corresponding to the adaxial and abaxial sides of the leaf with more thermonastic and nonthermonastic Rhododendron species may provide further insights into the role of AQPs in leaf curling in Rhododendron.
Wang, Xiang, "Photoprotective strategies in overwintering rhododendrons" (2009). Graduate Theses and Dissertations. 11128.