The Ecophysiological Significance of Leaf Movements in Rhododendron Maximum

The patterns of leaf movements and their environmental stimuli have been documented for over a century. Although there have been several studies on the physiological significance of heliotropisms, thermotropic movements have been largely ignored. The objective of this study was to evaluate the ecophysiological significance of thermotropic leaf movements in a subcanopy plant through controlled experiments. Rhododendron maximum L. was selected because of its strong winter season, thermotropic leaf movements, and the ease of identifying even—aged leaf cohorts. A series of leaf manipulations was established to restrict curling, changes in leaf angle, or both. The effects of these treatments on leaf energy budget, chlorophyll content, photosynthesis, and leaf survival were evaluated. Leaves that were maintained in a horizontal position (whether they were allowed to curl or not) had the greatest decrease in chlorophyll content and the most inhibited photosynthetic rate. Reduced photosynthetic potential was due to both lower quantum yield and lower light—saturated nthetic rates. Optimal temperature for photosynthesis was several degrees above air temperature, corresponding to a horizontal leaf temperature in a sunfleck. Leaf angle influenced leaf temperature during daylight and night time; but the influence was small and most likely had no ecophysiological significance. Leaf curling had little to no influence on the ecophysiological parameters measured in this study.