Cholodny–Went model

In botany, the Cholodny–Went model, proposed in 1927, is an early model describing tropism in emerging shoots of monocotyledons, including the tendencies for the shoot to grow towards light (phototropism) and the roots to grow downward (gravitropism). In both cases the directional growth is considered to be due to asymmetrical distribution of auxin, a plant growth hormone. Although the model has been criticized and continues to be refined, it has largely stood the test of time.

The model was independently proposed by the Russian scientist Nikolai Cholodny of the University of Kiev in 1927 and by Frits Warmolt Went of the California Institute of Technology in 1928, both based on work they had done in 1926. The basic elements of the theory are that auxin is the sole hormone that controls growth in gravitropism and phototropism; the rate of growth depends on the concentration of auxin; and both gravity and unidirectional light affect the movement of auxin. The original theory predicts that since the growth factor would move from the lighted side to the shady side, growth would slow on the lighted side and speed up on the shady side, so the stem will begin to bend toward the light source.

Went's 1926 experiment appeared to demonstrate that auxin moved towards the shady side of the tip of the coleoptile, the pointed protective sheath covering the emerging shoot. Cholodny and Went proposed that auxin is synthesized in the coleoptile tip, which senses light and sends the auxin down the shady side of the coleoptile, causing asymmetric growth with the shoot bending towards the light source. Later experiments by Dolk in 1930 showed that auxin moved from a source along a horizontal coleoptile section, concentrating along the bottom of the section and thus causing the shoot to bend upward. The Cholodny–Went model for the phototropic movement of shoots was later extended to gravitropism of roots, where auxin was thought to inhibit rather than stimulate growth and to accumulate in the lower side of a root section, causing the root to bend downward.

The theory was widely accepted when first proposed, but began to receive serious criticism in the mid-1980s. Arguments against the model have included views that growth regulators other than auxin may be involved, and that there is no difference in the concentration of auxin on the light and shady sides, or not enough difference to explain the difference in growth rates. A 1987 paper reported results indicating that geotropic curvature of both roots and shoots could be explained by local migration of auxin from one side to another rather than by movement along the whole length of the organ. Other studies showed that sometimes tropic response do not depend on the coleoptile tip, and the development of the shoots' bend may be greater than the auxin gradient of that shoot. Critics have also had problems with the reliability of Went's small sample sizes and use of agar blocks instead of actual auxin concentration measurements.

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