page_title: Thermotropism
section_title: Thermotropism in leaves
hierarchical_section_title: Thermotropism / Thermotropism in leaves
caption_attribution_description: Rhododendron (Rhododendron maximum), Lackawanna State Forest, Lackawanna County. A cold-hardy species, R. maximum responds to subfreezing temperatures by curling, folding down, and clamping shut its foliage. This action helps protect the shrub from the desiccating effects of cold air. (According to my GPS unit, the air temperature was 9ºF [–13ºC] at the time.)
context_page_description: Thermotropism or thermotropic movement is the movement of a plant or part of a plant in response to a change in temperature. A common example is the curling of Rhododendron leaves in response to cold temperatures. Mimosa pudica also show thermotropism by the collapsing of leaf petioles leading to the folding of leaflets, when temperature drops.
  The term "thermotropism" was originated by French botanist Philippe Van Tieghem in his 1884 textbook Traité de botanique. Van Tieghem stated that a plant irradiated with an optimum growth temperature on one side laterally, and a much higher or lower temperature on the opposite side, would exhibit faster growth on the side exposed to optimum temperature.
  The precise physiological mechanism enabling plant thermotropism is not yet understood. It has been noted that one of the earliest physiological responses by plants to cooling is an influx of calcium ions from the cell walls into the cytosol, which increases calcium ion concentration in the intracellular space. This calcium influx is dependent upon mechanical changes in the actin cytoskeleton that alter the fluidity of the cell membrane, which allows calcium ion channels to open.
context_section_description: Gardening hobbyists have frequently noted the dramatic change in the shape of Rhododendron or "Rhodie" leaves during warm versus cold weather. In warm weather, the leaf has a flat oblong shape. As the temperature of the leaf drops, the blade curls inward, giving the leaf a tubular, cigar-like shape.
  Research on Rhododendron leaf thermotropism suggests that the curling response might help prevent damage to cell membranes caused by rapid thawing after a freeze. During the winter months, wild Rhododendrons in the Appalachian Mountains regularly drop to freezing temperatures at night, then thaw again in the early morning. Because a curled leaf has less of its surface area exposed to the sunlight, the leaf will thaw more slowly than it would if it were unfurled. Slower thawing minimizes damage caused to leaf cell membranes by ice crystal formation.
caption: Rhododendron leaves curling in response to cold temperatures.
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