Which Hormones in Plants Cause Tropism?
The word tropism, from the Greek "tropos," or "turn," describes the turning or directional growth of an organism. It most frequently describes the directed growth of plants. This directional growth is controlled at a distance from the bending tissue using plant hormones. Plant physiologists are still working out the exact relationship between certain tropisms and the controlling hormones. Several tropisms have been described, including phototropism, gravitropism, thigmotropism and thermotropism.
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Facts
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In plants, a tropism refers to the growth of a plant in response to an external stimulus. Plants can grow either towards or away from stimuli. The curving of plants is frequently due to differential growth, where different sides of a stem or root grow at different rates. Growth in plants is controlled by plant hormones, like auxins. Auxins are important plant hormones that can control the elongation of cells, both by promoting and impeding it.
Phototropism
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Phototropism describes the growth of a plant towards a source of light. There are multiple theories that describe this reaction in plants. One theory is that on the shaded side of the plant, the hormone auxin causes elongation of the cells. As the cells on the shaded side elongate and the cells on the light do not, the growth of the plant is bent toward the light. The converse could also be true: growth inhibitors could be differentially shortening cells on the lit portion of the plant.
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Gravitropism
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Gravitropism describes the growth of a plant either towards or away from gravity. Roots tend to exhibit positive gravitropism, or growth towards gravity; shoots tend to exhibit negative gravitropism, or grow away from gravity. Plant physiologists believe that roots may detect which direction is down via the accumulation of dense plastids at the lowest point of the cell. These plastids would organize the concentration of auxin in the cells. A high concentration of auxin slows down plant growth, while a moderate concentration induces cell elongation. The lowest part of the root slows down until the root straightens up and is pointing straight down.
Thigmotropism
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Thigmotropism is the growth of plants in response to physical contact. This describes the growth of vines and tendril-bearing plants towards things with which they come into mechanical contact. This happens when the cells opposite the mechanical contact exhibit increased growth compared the cells being contacted. Some plants also show contraction of cells experiencing the contact. Thigmotropism should not be confused with the rapid, non-permanent motion of plants like Venus Flytraps or Mimosa. These movements are caused by changes in turgor pressure.
Thermotropism
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Thermotropism describes the movement of a plant in response to heat. A well-studied example of thermotropism occurs in Rhododendron. The leaves of these cold-tolerant plants curl in response to cold temperatures. The mechanism of this curling is not understood. It occurs in temperatures dangerously cold to the plants. The thermotropism seems to prevent the leaves from freezing. Models suggest that being uncurled would cause them to loose more heat.
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