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Response of a plant to the force of gravity. Also known as gravitropism.



the ability of the organs of plants to assume a certain position under the influence of the earth’s gravity. Geotropism causes the plant’s axial organs to grow vertically: the main root is directed straight downward (positive geotropism), and the main stem is directed straight upward (negative geotropism). If the main stem of the plant is displaced from its typical vertical position because of some external influence, for example, if it is bent or lodged by the wind, the young, still growing part begins to curve and its upper part rises and is again correctly oriented. The tip of the main root, when displaced from its vertical position, curves downward. Geotropic bending is closely linked to growth, and it occurs because in stems that have been displaced from a vertical state the lower side begins to grow more rapidly and the upper side slows down its growth. The unequal rate of growth of the upper and lower sides of horizontal stems is linked with the shifting of auxins to the lower side of the stem or root under the influence of gravity. The parts of plants that have completed their growth are not capable of geotropic bending. Therefore, when plants have been lodged by wind or rain, only the young, growing top of the stem rises. The grasses are an apparent exception: their ganglia retain the ability to renew growth for a long time. When lodging occurs, the lower side of the lower nodes begins to proliferate and raises the part of the stem located above.

In addition to stems growing vertically under the influence of geotropism (orthotropic), there are also horizontally growing stems (plagiotropic). For the most part these are rhizomes and runners (tendrils). A change in the geotropic reaction may occur because of external influences, such as a lower temperature, causing shoots to press close to the ground in alpine and polar plants. The geotropic reaction may also be altered under the influence of certain gases, such as ethylene.

References in periodicals archive ?
Radicles and secondary roots exhibit geotropism that causes them to grow downward, toward the center of the earth.
The response of auxin to gravity and the resulting growth reaction in plants is termed geotropism.
When a cut flower such as a gladiolus or snapdragon (Antirrhinum) bends upward in response to the force of gravity, it is called geotropism, from the Greek words geo (earth) and trope (turning).
Tropism--A growth reaction of a plant to various external or internal stimuli such as phototropism, the increased growth toward or away from light; geotropism, growth in response to gravity; chemotropism, plant response to chemicals; hydrotropism, plant response to water.
This technique [Miquel 1976] also takes advantage of the negative geotropism of D.
The scientists believe that a mechanism called hydrotropism can actually override geotropism.