280
KENNETH V. THIMANN AND GEORGE M. CURRY
very steep rise in the curve. These results, however, directly confirm the
presence, in the photosensitive part of the coleoptile, of a pigment having light absorption properties close to those indicated by the action
spectrum.
A third explanation for the absence of a peak at 360 m/x in the absorption spectrum might be that the curvature at that wavelength is
due to fluorescence, that is, the 360 m/x light might elicit the emission
of fluorescent light of a wavelength between 425 and 475 m/x. A suggestive example of this is furnished by lumiflavin, a breakdown product of
riboflavin, which fluoresces in a wavelength range between 425 and
550 m/x with a maximum at 462 m/x (110). As yet there is no direct evidence for or against this possibility, but the large number of fluorescent
substances found in plants (e.g., by R. H. Goodwin, 111) makes it not
improbable.
Beside the small discrepancies between absorption and action spectra
noted above, a group of experiments involving artificial or natural
changes in the carotene content of phototropic organs has led some
workers to doubt that a carotenoid can be the photoreceptor. T. W.
Goodwin and Lijinsky (112) found that Phycornyces grown on a medium
containing diphenylamine, which reduces the carotenoid level to something like 3% of the normal, retained its phototropic sensitivity. Similarly, Reinert (113) found that a strain of Phycomyces which only produces carotene when cultured on a medium containing glucose, retained
its full phototropic responsiveness when cultured on lactate, in which
case it was presumably very low in carotene. In the germinating
mycelia of Puccinia and Uromyces, Gettkandt (50) found that the
photosensitive hyphal tips contained no visible carotenoid, while the
part below the tip, which is not photosensitive, is rich in carotenoids
dissolved in oil droplets. Furthermore the correlation between photosensitivity and carotenoid was quite poor; of 18 species studied by
Gettkandt and other workers, 7 were photosensitive and contained
carotenoid, 5 were insensitive and contained none, but 3 were insensitive although containing carotenoid, and 3 more were fully photosensitive without apparently containing any carotenoid. These last, Botrytis
einer ea, another Botrytis sp., and the uredospore mycelium of Puccinia
glumarum, deserve further study.
Similar findings have been reported with seedlings. Bandurski et ah
(114) claimed "normal" photosensitivity in albino corn seedlings from
which they could extract no measurable amounts of carotene, and Galston and co-workers [cited in (23)] have found a similar situation in
albino barley coleoptiles. On the other hand, several workers (22, 115)
have found that carotenoid is still present in such albino strains, though
in small amounts; Virgin's data (115) indicate that it is about 5% of the
KENNETH V. THIMANN AND GEORGE M. CURRY
very steep rise in the curve. These results, however, directly confirm the
presence, in the photosensitive part of the coleoptile, of a pigment having light absorption properties close to those indicated by the action
spectrum.
A third explanation for the absence of a peak at 360 m/x in the absorption spectrum might be that the curvature at that wavelength is
due to fluorescence, that is, the 360 m/x light might elicit the emission
of fluorescent light of a wavelength between 425 and 475 m/x. A suggestive example of this is furnished by lumiflavin, a breakdown product of
riboflavin, which fluoresces in a wavelength range between 425 and
550 m/x with a maximum at 462 m/x (110). As yet there is no direct evidence for or against this possibility, but the large number of fluorescent
substances found in plants (e.g., by R. H. Goodwin, 111) makes it not
improbable.
Beside the small discrepancies between absorption and action spectra
noted above, a group of experiments involving artificial or natural
changes in the carotene content of phototropic organs has led some
workers to doubt that a carotenoid can be the photoreceptor. T. W.
Goodwin and Lijinsky (112) found that Phycornyces grown on a medium
containing diphenylamine, which reduces the carotenoid level to something like 3% of the normal, retained its phototropic sensitivity. Similarly, Reinert (113) found that a strain of Phycomyces which only produces carotene when cultured on a medium containing glucose, retained
its full phototropic responsiveness when cultured on lactate, in which
case it was presumably very low in carotene. In the germinating
mycelia of Puccinia and Uromyces, Gettkandt (50) found that the
photosensitive hyphal tips contained no visible carotenoid, while the
part below the tip, which is not photosensitive, is rich in carotenoids
dissolved in oil droplets. Furthermore the correlation between photosensitivity and carotenoid was quite poor; of 18 species studied by
Gettkandt and other workers, 7 were photosensitive and contained
carotenoid, 5 were insensitive and contained none, but 3 were insensitive although containing carotenoid, and 3 more were fully photosensitive without apparently containing any carotenoid. These last, Botrytis
einer ea, another Botrytis sp., and the uredospore mycelium of Puccinia
glumarum, deserve further study.
Similar findings have been reported with seedlings. Bandurski et ah
(114) claimed "normal" photosensitivity in albino corn seedlings from
which they could extract no measurable amounts of carotene, and Galston and co-workers [cited in (23)] have found a similar situation in
albino barley coleoptiles. On the other hand, several workers (22, 115)
have found that carotenoid is still present in such albino strains, though
in small amounts; Virgin's data (115) indicate that it is about 5% of the