6. PHOTOTROPISM AND PHOTOTAXIS
283
thionine, quinine sulfate, methylene blue, and even chlorophyll (in fact,
almost any fluorescent substance), can act as sensitizers for IAA photooxidation (78, 121, 122, 125, 126). Secondly, there is no evidence that
riboflavin acts in vivo. Thus if the action spectrum for IAA destruction
by the brei is held to demonstrate photoreception by riboflavin, then
the action spectrum for phototropism shows no such participation. In a
recent re-examination of these effects Mer (127) has found that when
the riboflavin content of coleoptiles is increased by providing an external supply, illumination still does not cause any greater decrease in
growth than in controls. He points out that "in this experiment riboflavin was known to be present in the coleoptile and the absence of
effect must have been due either to a failure of riboflavin to enter the
cells or to the absence of a reaction with the naturally occurring auxin."
He concludes that while auxin and riboflavin might interact in light in
vivo, no experimental evidence supports it. Thirdly, the quantum yields
and auxin studies discussed earlier indicate that auxin is redistributed,
rather than destroyed, in the tip response.
In sum, there are three main criteria for a decision about the probable photoreceptor; the action spectrum, occurrence and distribution in
the plant, and possible modes of action. For the tip response in Avena,
a carotenoid best fits the first two criteria, but there is some in vitro
evidence to connect a flavin with the last; a final decision between one
or the other, or in favor of some quite different, as yet unsuspected,
pigment, will only be possible when all three criteria are satisfied. All
in all, the balance of evidence is still in favor of the conclusion reached
fifteen years ago (128): "it may be concluded tentatively that the primary process of phototropism is mediated through carotenoid pigments."
There is, perhaps, one possible objection to attaching decisive importance to the action spectrum, namely that the relative effectiveness
of different wavelengths might be determined by the light absorption
by pigments other than the effective one. This idea, that the action
spectrum is a function of "masking pigments" rather than of the active
photoreceptor itself, will now be discussed and its possibilities and
limitations presented.
C. INTERACTIONS BETWEEN "ACTIVE" AND "INACTIVE" PIGMENTS IN THE
PHOTORECEPTION PROCESS
1. In Phototropism
It has been argued recently (21, 86, 121, 129-131) that a carotenoidlike action spectrum may be a consequence of a "masking" or "internal
283
thionine, quinine sulfate, methylene blue, and even chlorophyll (in fact,
almost any fluorescent substance), can act as sensitizers for IAA photooxidation (78, 121, 122, 125, 126). Secondly, there is no evidence that
riboflavin acts in vivo. Thus if the action spectrum for IAA destruction
by the brei is held to demonstrate photoreception by riboflavin, then
the action spectrum for phototropism shows no such participation. In a
recent re-examination of these effects Mer (127) has found that when
the riboflavin content of coleoptiles is increased by providing an external supply, illumination still does not cause any greater decrease in
growth than in controls. He points out that "in this experiment riboflavin was known to be present in the coleoptile and the absence of
effect must have been due either to a failure of riboflavin to enter the
cells or to the absence of a reaction with the naturally occurring auxin."
He concludes that while auxin and riboflavin might interact in light in
vivo, no experimental evidence supports it. Thirdly, the quantum yields
and auxin studies discussed earlier indicate that auxin is redistributed,
rather than destroyed, in the tip response.
In sum, there are three main criteria for a decision about the probable photoreceptor; the action spectrum, occurrence and distribution in
the plant, and possible modes of action. For the tip response in Avena,
a carotenoid best fits the first two criteria, but there is some in vitro
evidence to connect a flavin with the last; a final decision between one
or the other, or in favor of some quite different, as yet unsuspected,
pigment, will only be possible when all three criteria are satisfied. All
in all, the balance of evidence is still in favor of the conclusion reached
fifteen years ago (128): "it may be concluded tentatively that the primary process of phototropism is mediated through carotenoid pigments."
There is, perhaps, one possible objection to attaching decisive importance to the action spectrum, namely that the relative effectiveness
of different wavelengths might be determined by the light absorption
by pigments other than the effective one. This idea, that the action
spectrum is a function of "masking pigments" rather than of the active
photoreceptor itself, will now be discussed and its possibilities and
limitations presented.
C. INTERACTIONS BETWEEN "ACTIVE" AND "INACTIVE" PIGMENTS IN THE
PHOTORECEPTION PROCESS
1. In Phototropism
It has been argued recently (21, 86, 121, 129-131) that a carotenoidlike action spectrum may be a consequence of a "masking" or "internal