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. From this information, a hypothesis has formed that the plant cell plasma membrane is an important site of plant temperature perception.
Thermotropism in leaves
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.
Thermotropism in roots
The roots of some plants, including Zea mays, have been shown to bend differently when exposed to different temperature conditions. In general, growing roots tend to bend away from warmer temperatures, and towards cooler temperatures, within a normal range. It has been suggested that this growth behavior is beneficial because in most natural environments, soil closer to the ground's surface is warmer in temperature, while deeper soil is cooler.
Young corn roots have been shown to grow upwards, against the normal gravitropic orientation, in order to bend towards an artificially created cooler temperature zone at the surface of the growth medium. This overriding behavior suggests integration between the plants' gravitropic and thermotropic systems.
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