Phototropism

There has also been an active search for the identity of the photoreceptor pigment, an aspect of phototropism not covered by the Cholodny-Went theory. In the 1930s, many researchers believed the photoreceptor was a carotenoid, a class of mostly orange plant pigments. They argued that carotenoids strongly absorb blue light and phototropism is most effectively elicited by blue light. Furthermore, retinal, a carotenoid derivative, was identified as the photoreceptive pigment controlling vision in humans and other animals.

However, more recent experiments appear to rule out a carotenoid as the photoreceptor. In particular, when seedlings are treated with norflurazon, a chemical inhibitor of carotenoid synthesis, they still exhibit phototropism. In addition, mutants of plants and fungi which have greatly reduced amounts of carotenoids are unaffected in their phototropic responses.

A great variety of different experiments now indicate that a flavin (vitamin B-2) is the photoreceptor pigment. Like carotenoids, flavins strongly absorb blue light. However, unlike most carotenoids, they also strongly absorb radiation in the near-ultraviolet (370 nm) region. Radiation in the near-ultraviolet region of the spectrum is also highly effective in phototropism.