Thermotropism
Thermotropism or thermotropic movement is the movement of an organism or a part of an organism in response to heat or changes from the environment's 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.[1]
The term "thermotropism" was originated by French botanist
The definition of thermotropism can sometimes be confused with the term, thermotaxis, a mechanism by which temperature gradients can alter the behavior of cells, such as moving toward the cold environment.[3] The difference between them is that thermotropism is more commonly used in botany because it could not only represent the movement in organism level, thermotropism could also represent an organ level of movement, such as movement of leaves and roots toward or away from heat; but thermotaxis can only represent locomotion at the organism level, such as the movement of a mouse away from a warm environment.
The precise physiological mechanism enabling plant thermotropism is not yet understood.
Thermotropism in leaves
Gardening hobbyists have frequently noted the dramatic change in the shape of Rhododendron or "Rhodie"
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.[4]
Although there is little known about the molecular mechanisms of this rolling behavior, turgor pressure is responsible for the leaf movement. The exact stimulus for this output is not understood, but it is known that freezing cold temperatures causes an influx of water to the leaf petiole. As the turgor pressure increases, the leaves roll up, making it tighter to the stem. The leaf also droops perpendicular to the ground. There are predictions on the mechanism of this behavior. Regional changes of cell hydration can cause the inward curling. Another prediction is a change in cell wall physiology.[6] These predictions are very broad, indicating the need for further research.
There are currently two hypotheses to why Rhododendrons do this. The first is that the shape is more effective for snow shedding and better protects the more sensitive areas. Another hypothesis for leaf rolling called the desiccation theory, circulating in recent years, is to prevent membrane and light damage.[7]
In a 2017 study about cold stressed Rhododendron leaves showed that photosynthetic proteins decreased, while proteins for cell permeability increased. The same study showed the highest increase in proteins were responsible for transcription and translation regulation.[8] Thermotropic response in rhododendron leaves protects cells by changing leaf shape and protein levels.
Thermotropism in roots
The
Experimentation with maize has demonstrated the existence of thermotropic responses in roots, with stronger responses seen when the thermal gradient increases. Positive thermotropism, or growth towards higher temperatures, was shown to occur at lower temperatures, with the strongest response observed at a temperature of 15 C. As the temperature increases, the strength of the response decreases. With continually temperature increases, a lack of thermotropic response is observed and occurs once a temperature threshold is reached. This threshold is dependent on the thermal gradient, with the threshold being colder with smaller gradients. For example, a gradient of 4.2 C per cm had a threshold value of 30 C while a gradient of 0.5 C per cm had a threshold value of 24 C. It is thought that this lack of thermotropic response is due to the lack of sufficient stimuli to induce root curvature. Negative thermotropic behavior was recorded and was shown to occur at higher temperature, but the conditions to establish such behavior is less defined.[10]
Within the same experiment, roots were capable of undergoing positive thermotropism away from gravitational force. The inhibition of normal gravitropic curvature was seen when temperatures were 18 C and lower, with stronger curvature away from gravity seen with lower temperature. This overriding behavior indicates integration of the plant's gravitropic and thermotropic system and suggests that the sensory systems are an interconnected network of responses rather than separate stimulation response pathways.[11]
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Thermotropism's relation with Heliotropism
References
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- ^ Hooker, Jr., H. D. (1914). "Thermotropism in Roots". The Plant World. 17: 136. Retrieved 23 May 2016.
- PMID 31266655.
- ^ a b Nilsen, Erik Tallak (Winter 1990). "Why do Rhododendron Leaves Curl?" (PDF). Arnoldia. 50 (1): 30–35. Retrieved 23 May 2016.
- PMID 21172632.
- ^ Nilsen, Erik (1990-01-01). "Why Do Rhododendron Leaves Curl?". Arnoldia. 50.
- PMID 33974686.
- PMID 28542212.
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- S2CID 750871.
- ^ McIntosh, Philip (22 February 2012). "Six Ways Plants Grow". Maximum Yield Indoor Gardening. Maximum Yield Publications. Retrieved 23 May 2016.
- PMID 33974686.
- S2CID 206650484.