Thermonastic leaf movements: a synthesis of research with Rhododendron

Thermonastic leaf movements in Rhododendron L. occur in response to freezing temperatures. These movements are composed of leaf curling and leaf angle changes that are distinct leaf movements with different responses to climatic factors. Leaf angle is controlled by the hydration of the petiole, as affected by soil water content, atmospheric vapour pressure, and air temperature. In contrast, leaf curling is a specific response to leaf temperature, and bulk leaf hydration has little effect. The physiological cause of leaf curling is not well understood, but the mechanism must lie in the physiology of the cell wall and/or regional changes in tissue hydration. Available evidence suggests that intercellular freezing is not a cause of leaf curling. Manipulation experiments demonstrate that changes in leaf orientation in Rhododendron most likely serve to protect the leaves from membrane damage due to high irradiance and cold temperatures. In particular, the pendent leaves protect the chloroplast from photoinhibition. Leaf curling may serve to slow the rate of thaw following freezing, a common phenomenon in the Appalachian mountains of the U.S. The thermonastic leaf movements have a greater importance to plants in a dim environment because the potential impact to canopy carbon gain is greater than in high light environments. These leaf movements have several implications for horticultural management. There seems to be a trade-off between water stress tolerance and freezing stress tolerance by leaf movements. Thermonastic leaf movements may be a major mechanism of cold stress tolerance in Rhododendron species. The actual physiological cause of leaf movement has not been elucidated and many more species need to be evaluated to verify the general importance of leaf movements to Rhododendron ecology and evolution.