Action spectra, phototropism

The defenders of the carotenoid-photoreceptor-hypothesis have always understood the shape of these action spectra in the blue to mean that the bluelight receptor is a carotenoid. Indeed, in Fig. 6 3 it can be observed, that the three-peak absorption spectrum of trans-0-carotenoid (in hexene) agrees well with the observed action spectrum of the avena coleoptile (Fig. 3 5). However, there remains one loose end which has been the crucial point of controversy in this field, ever since Galston and Baker66 suggested in 1949 that the photoreceptor for phototropism might be a flavin Flavin absorbs in the near UV, /3-carotenoid does not. 

Phycocymes shows a positive UV-induced light-growth-response (Fig. 3 3) as well as a negative phototropic curvature, as depicted in Fig. 5 1. The exceptionally good fit of this action spectrum and auxin absorption (Fig. 5 2) might indicate that auxin is the UV-photoreceptor. 

Fig. 8. Action spectra as mimicked by low temperature absorption spectra (liquid nitrogen) of (1) flavin49) and (2) carotene169). These spectra are compared with the phototropic action spectrum (curve (S), same as Fig. 3 5)...

Therefore, the action spectrum for phototropism does not simply reflect the absorption spectrum of the active photoreceptor pigment itself, but instead, its absorption spectrum somehow modified by shading pigments. However, on this basis Thimann and Curry failed to calculate a curve fitting the experimental action spec-... 

Only for the (extrapolated) limit of zero bending the action spectrum reflects the absorption spectrum of the photoreceptor for phototropism (avena coleoptile). Just for this limit the UV-peak of the action spectrum disappears 162). 

Delbriick et al., (1976) have recently sought to determine whether or not the stimulation of Phycomyces involves the excitation of the lowest triplet state of riboflavin. They determined an action spectrum of light-growth response between 575 and 630 nm using a tunable laser beam and taking advantage of the null method described above. This action spectrum was compared with an action spectrum obtained by computer extrapolation of a phototropic action spectrum covering 445—560 nm. 

M. Iseki, S. Wada (1995). Action spectrum in the ultraviolet region for phototropism of Bryopsis rhizoids. Plant Cell. Physiol., 36, 1033-1040. 

Baskin, T.I. and lino, M., An action spectrum in the blue and ultraviolet for phototropism in alfalfa, Photochem. Photobiol, 46,127,1987. 

Kataoka, H., Phototropism in Vaucheria geminata 1. The action spectrum. Plant Cell Physiol, 16, 427, 1975. 

Ishizawa, K. and Wada, S., Action spectrum of negative phototropism in Boergesenia forbesii. Plant Cell. Physiol, 20, 983, 1979. 

Janoudi, A.-K. and Poff, K.L., Action spectrum for enhancement of phototropism by Arabidopsis thaliana seedlings, Photochem. Photobiol, 56, 655,1992. 

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