Phototropism

These and other experiments by Went led to what has become known as the Cholodny-Went theory of tropic curvature. In terms of phototropism, the Cholodny-Went theory proposes that (a) auxin is synthesized in the coleoptile tip; (b) the coleoptile tip perceives the asymmetric illumination and this causes auxin to move into the un-irradiated side; (c) auxin moves down the coleoptile so that lower regions develop an auxin asymmetry; and (d) the higher auxin concentration on the un-irradiated side causes the coleoptile to bend toward the light source.

There is currently vigorous debate among plant physiologists about the Cholodny-Went theory. Critics have noted that Went and other early researchers never actually measured the auxin concentrations but only relied on bioassays performed with agar blocks. Furthermore, the early studies relied on small sample sizes which were statistically unreliable, and the researchers may have wounded the coleoptiles during tip removal.

In addition, numerous recent experiments indicate that the coleoptile tip is not always necessary for tropic responses and that auxin gradients form in the tissue more slowly than the development of curvature.

Despite these criticisms, many plant physiologists maintain that the basic features of the Cholodny-Went theory have been upheld. The debate about the Cholodny-Went theory has stimulated much new research in phototropism and gravitropism. Many researchers currently are investigating tropic curvature using modern time-lapse photography. Others are examining the role of additional plant hormones in regulating phototropism and gravitropism.