6. PHOTOTROPISM AND PHÖTOTAXlS
281
amount in normal leaves. Turian (116) found with a Mycobacterium
that diphenylamine never lowers the carotenoid content to less than 5
to 10% of normal. Since quantum yields in a photodynamic system are
virtually independent of the concentration of the photodynamic agent
(117), these arguments cannot be considered final.
Evidence in the opposite sense is furnished by unpublished experiments in our own laboratory with a Helianthus mutant which is not
only pure white to the eye but which loses its chlorophyll by bleaching
on exposure to weak continuous light (thus probably containing excessively low levels of carotene (cf. reference 118). These plants showed
abnormally low phototropic sensitivity. Thus it is difficult to assess the
negative claims above, unless one determines: (a) the quantity of
carotenoid necessary for light reception and (b) the lower limit of
sensitivity of the analytical methods used.
Another argument, of a negative type, against the carotenoid hypothesis is that light absorption by carotenoids has not been shown to
affect the auxin system, or in particular to act on indoleacetic acid
(IAA). Schuringa (119) and Kögl and Schuringa (120) reported that
carotenoids, either in alcoholic solutions or in aqueous suspensions,
could sensitize the photoinactivation of auxin-a-lactone, but these experiments have not been confirmed and the role of auxin a and its
lactone as plant hormones is now in doubt. Attempts to find a carotenoid-sensitized photoinactivation of IAA in vitro have failed (121,
122). In view of the argument above, however, that it is the system
producing and transporting auxin which is light-sensitive (rather than
the auxin itself), this failure need not surprise us.
In 1949, Galston and Baker suggested that riboflavin, rather than a
carotenoid, might be the light receptor in phototropism, and in recent
years many investigators have come to believe this. This view developed
from Galston's (123) discovery that IAA is rapidly inactivated in vitro
by light when in the presence of riboflavin. The diffusible auxin of
Avena coleoptiles is similarly inactivated (99). The yellow flavins absorb
in the same part of the blue region as do the carotenoids (Fig. 9, A),
they are present in the phototropically sensitive tissues—in fact universally—and they are known to act photocatalytically in certain systems. Galston and Baker (124) found that riboflavin could sensitize the
photoinactivation of various enzymes, possibly accounting for the required interference in the auxin production-distribution system during
phototropism. Avena coleoptiles contain 28-33 pg. riboflavin per gram
dry weight,* the concentration being about the same in the tip as in the
base (98).
* Our own data give 22 μg., which is comparable though probably significantly
less.
281
amount in normal leaves. Turian (116) found with a Mycobacterium
that diphenylamine never lowers the carotenoid content to less than 5
to 10% of normal. Since quantum yields in a photodynamic system are
virtually independent of the concentration of the photodynamic agent
(117), these arguments cannot be considered final.
Evidence in the opposite sense is furnished by unpublished experiments in our own laboratory with a Helianthus mutant which is not
only pure white to the eye but which loses its chlorophyll by bleaching
on exposure to weak continuous light (thus probably containing excessively low levels of carotene (cf. reference 118). These plants showed
abnormally low phototropic sensitivity. Thus it is difficult to assess the
negative claims above, unless one determines: (a) the quantity of
carotenoid necessary for light reception and (b) the lower limit of
sensitivity of the analytical methods used.
Another argument, of a negative type, against the carotenoid hypothesis is that light absorption by carotenoids has not been shown to
affect the auxin system, or in particular to act on indoleacetic acid
(IAA). Schuringa (119) and Kögl and Schuringa (120) reported that
carotenoids, either in alcoholic solutions or in aqueous suspensions,
could sensitize the photoinactivation of auxin-a-lactone, but these experiments have not been confirmed and the role of auxin a and its
lactone as plant hormones is now in doubt. Attempts to find a carotenoid-sensitized photoinactivation of IAA in vitro have failed (121,
122). In view of the argument above, however, that it is the system
producing and transporting auxin which is light-sensitive (rather than
the auxin itself), this failure need not surprise us.
In 1949, Galston and Baker suggested that riboflavin, rather than a
carotenoid, might be the light receptor in phototropism, and in recent
years many investigators have come to believe this. This view developed
from Galston's (123) discovery that IAA is rapidly inactivated in vitro
by light when in the presence of riboflavin. The diffusible auxin of
Avena coleoptiles is similarly inactivated (99). The yellow flavins absorb
in the same part of the blue region as do the carotenoids (Fig. 9, A),
they are present in the phototropically sensitive tissues—in fact universally—and they are known to act photocatalytically in certain systems. Galston and Baker (124) found that riboflavin could sensitize the
photoinactivation of various enzymes, possibly accounting for the required interference in the auxin production-distribution system during
phototropism. Avena coleoptiles contain 28-33 pg. riboflavin per gram
dry weight,* the concentration being about the same in the tip as in the
base (98).
* Our own data give 22 μg., which is comparable though probably significantly
less.