Photoreceptor - Phototropism
The molecular nature of the photoreceptor is unsolved. Initially in the 1930s, some form of carotenoid was thought to be the phototropic receptor. General opinion at present, however, favors some kind of flavin as phototropic receptor, largely on the basis of the spectral match between phototropic action and flavin absorption in the ultra violet region. The action spectra for phototropism compare with the absorption spectrum of riboflavin and carotene, clearly favors riboflavin as the absorbing pigment. In some cases it can also be a flavoprotein.