Geotropism

Geotropism is the term applied to the orientation response of growing plant parts to Earth’s gravitational field. Roots are positively geotropic, that is, they will bend and grow downwards, towards the center of Earth. In contrast, shoots are negatively geotropic, that is, they will bend and grow upwards, or away, from the surface.

Geotropism can be demonstrated with seedlings grown entirely in darkness. A seedling with its radicle (or seedling root) and shoot already in the expected orientation can be turned upside down, or placed on its side, while kept in darkness. The root will subsequently bend and grow downwards, and the shoot upwards. Because the plant is still in darkness, photo-tropism (a growth movement in response to light) can be eliminated as an explanation for these movements.

Several theories about the manner by which plants perceive gravity have been advanced, but none of them is entirely satisfactory. To account for the positive geotropism of roots, some researchers have proposed that under the influence of gravity, starch grains within the cells of the root fall towards the “bottom” of the cell. There they provide signals to the cell membrane, which are translated into growth responses.

However, there have been many objections to this idea. It is likely that starch grains are in constant motion in the cytoplasm of living root cells, and only sink during the process of fixation of cells for microscopic examination. Roots can still be positively geo-tropic and lack starch grains in the appropriate cells.

A more promising hypothesis concerns the transport of auxin, a class of plant-growth regulating hormones. Experiments since 1929 have shown that auxin accumulates on the “down” side of both shoots and roots placed in a horizontal position in darkness. This gradient of auxin was believed to promote bending on that side in shoots, and to do the opposite in roots. Confirmation of the auxin gradient hypothesis came in the 1970s. When seeds are germinated in darkness in the presence of morphactin (an antagonist of the hormonal action of auxin), the resulting seedlings are disoriented–both the root and shoot grow in random directions. Auxin gradients are known to affect the expansion of plant cell walls, so these observations all support the idea that the transport of auxin mediates the bending effect that is an essential part of the directional response of growing plants to gravity.

See also Gravity and gravitation.

Geotropism

Plants can sense the Earth's gravitational field. Geotropism is the term applied to the consequent orientation response of growing plant parts. Roots are positively geotropic, that is, they will bend and grow downwards, towards the center of the Earth . In contrast, shoots are negatively geotropic, that is, they will bend and grow upwards, or away, from the surface.

These geotropisms can be demonstrated easily with seedlings grown entirely in darkness. A seedling with its radicle (or seedling root) and shoot already in the expected orientation can be turned upside down, or placed on its side, while kept in darkness. The root will subsequently bend and grow downwards, and the shoot upwards. Because the plant is still in darkness, phototropism (a growth movement in response to light ) can be eliminated as an explanation for these movements.

Several theories about the manner by which plants perceive gravity have been advanced, but none of them is entirely satisfactory. To account for the positive geotropism of roots, some researchers have proposed that under the influence of gravity, starch grains within the cells of the root fall towards the "bottom" of the cell . There they provide signals to the cell membrane , which are translated into growth responses. However, there have been many objections to this idea. It is likely that starch grains are in constant motion in the cytoplasm of living root cells, and only "sink" during the process of fixation of cells for microscopic examination. Roots can still be positively geotropic and lack starch grains in the appropriate cells.

A more promising hypothesis concerns the transport of auxin, a class of plant-growth regulating hormones . Experiments since 1929 have shown that auxin accumulates on the "down" side of both shoots and roots placed in a horizontal position in darkness. This gradient of auxin was believed to promote bending on that side in shoots, and to do the opposite in roots. Confirmation of the auxin gradient hypothesis came in the 1970s. When seeds are germinated in darkness in the presence of morphactin (an antagonist of the hormonal action of auxin), the resulting seedlings are disoriented—both the root and shoot grow in random directions. Auxin gradients are known to affect the expansion of plant cell walls, so these observations all support the idea that the transport of auxin mediates the bending effect that is an essential part of the directional response of growing plants to gravity.

See also Gravity and gravitation.

geotropism (gravitropism) The growth of plant organs in response to gravity. A main root is positively geotropic and a main stem negatively geotropic, growing downwards and upwards respectively, irrespective of the positions in which they are placed. For example, if a stem is placed in a horizontal position it will still grow upwards. It is thought that mobile membrane-bound clusters of starch grains (amyloplasts) sink to the bottom of a plant cell, under the influence of gravity, and somehow create a physiological asymmetry within the cell. This results in differential growth of the upper and lower sides of the cell, and hence upward or downward curvature of the organ concerned, depending on whether it is a root or a shoot. The biochemical mechanisms of this differential growth remain largely unknown, although auxins are required for geotropism in shoots, and also possibly in roots, and there is evidence that calcium ions play a role too. See tropism.

ge·ot·ro·pism / jēˈätrəˌpizəm/ • n. Bot. the growth of the parts of plants with respect to the force of gravity. The upward growth of plant shoots is an instance of negative geotropism; the downward growth of roots is positive geotropism.DERIVATIVES: ge·o·trop·ic / ˌjēəˈträpik; -ˈtrō-/ adj.

geotropism A directional movement of a plant in response to the stimulus of gravity. Primary tap roots show positive geotropism; vertical primary shoots show negative geotropism; horizontal stems and leaves are diageotropic (see DIAGEOTROPISM); and branches and secondary roots at oblique angles are plagiogeotropic.