6. PHOTOTROPISM AND PHOTOTAXIS
297
where M is a monophenol in reduced (red) or oxidized (ox) state.
Subsequently Waygood et al. (142) concluded that the phenol-activated
peroxidase functions in IAA destruction by converting Mn
2+ to Mn
3+
which initiates the oxidation and decarboxylation of IAA, and have proposed this as the mechanism of phototropism (143). Their enzyme, obtained from wheat leaves, is apparently a peroxidase, and it oxidizes
IAA in presence of Mn
2+ and a monophenol. This is strongly inhibited
by riboflavin (2.1χ10
4 Μ) in the dark. Blue or white light completely reverses the inhibition. They conclude that riboflavin in the
Avena coleoptile protects IAA from oxidation by the Mn-phenol-peroxidase system, but that light removes this protection: "the photoreversible
retarding effect of riboflavin on the reaction provides an interesting and
highly probable explanation for phototropic curvatures."
It is difficult to evaluate all these proposals. The inhibition of IAA
oxidation by riboflavin found by Maclachlan and Waygood appears to
conflict with Galston and Baker's data. The concentration of riboflavin
used (2.10
-4 M) is about 100 times that present in the Avena coleoptile.
The levels of riboflavin in pea, bean, corn, and lupine seedlings, of
around 1 /xg. per gm. fresh weight or 2.5 X 10~
6 M, agree well with the
Avena figure (144, 145) so there is some reason to doubt the extent to
which the proposed mechanism might operate in plants as a whole. The
in vivo significance of the reaction is not quite clear, since the addition
of riboflavin does not promote the growth of Avena coleoptiles in the
dark (127). Then, too, the in vivo significance of the Mn-phenol-peroxidase system is also far from clear. The powerful and specific IAA-destroying enzyme from the fungus Omphalia flava is active after exhaustive dialysis, shows only moderate stimulation by Mn and little or
none by monophenols (146). Spectrographic analysis reveals little or no
Mn present. It appears to be an oxidase-peroxidase like that acting on
tryptophan. Admittedly, this enzyme is from lower rather than higher
plants, but there is good reason to believe it is effective under physiological conditions (147). All in all, the biochemical mechanisms so far
proposed for light-growth reactions leave much to be desired. The indoleacetic acid oxidase system has recently been reviewed by Ray
(147a).
Whatever the mechanism is, it might perhaps provide a basis for
phototropic curvatures in stems exposed to continuous daylight and perhaps also for those in roots. Such curvatures are mainly of the localized
type and are characterized by a relatively high energy (or continuous exposure) requirement; indeed, their relative "insensitivity" is perhaps the
main reason they have been so little studied. Wherever auxins are limiting growth, a mechanism of the kind postulated on p. 295 would yield
297
where M is a monophenol in reduced (red) or oxidized (ox) state.
Subsequently Waygood et al. (142) concluded that the phenol-activated
peroxidase functions in IAA destruction by converting Mn
2+ to Mn
3+
which initiates the oxidation and decarboxylation of IAA, and have proposed this as the mechanism of phototropism (143). Their enzyme, obtained from wheat leaves, is apparently a peroxidase, and it oxidizes
IAA in presence of Mn
2+ and a monophenol. This is strongly inhibited
by riboflavin (2.1χ10
4 Μ) in the dark. Blue or white light completely reverses the inhibition. They conclude that riboflavin in the
Avena coleoptile protects IAA from oxidation by the Mn-phenol-peroxidase system, but that light removes this protection: "the photoreversible
retarding effect of riboflavin on the reaction provides an interesting and
highly probable explanation for phototropic curvatures."
It is difficult to evaluate all these proposals. The inhibition of IAA
oxidation by riboflavin found by Maclachlan and Waygood appears to
conflict with Galston and Baker's data. The concentration of riboflavin
used (2.10
-4 M) is about 100 times that present in the Avena coleoptile.
The levels of riboflavin in pea, bean, corn, and lupine seedlings, of
around 1 /xg. per gm. fresh weight or 2.5 X 10~
6 M, agree well with the
Avena figure (144, 145) so there is some reason to doubt the extent to
which the proposed mechanism might operate in plants as a whole. The
in vivo significance of the reaction is not quite clear, since the addition
of riboflavin does not promote the growth of Avena coleoptiles in the
dark (127). Then, too, the in vivo significance of the Mn-phenol-peroxidase system is also far from clear. The powerful and specific IAA-destroying enzyme from the fungus Omphalia flava is active after exhaustive dialysis, shows only moderate stimulation by Mn and little or
none by monophenols (146). Spectrographic analysis reveals little or no
Mn present. It appears to be an oxidase-peroxidase like that acting on
tryptophan. Admittedly, this enzyme is from lower rather than higher
plants, but there is good reason to believe it is effective under physiological conditions (147). All in all, the biochemical mechanisms so far
proposed for light-growth reactions leave much to be desired. The indoleacetic acid oxidase system has recently been reviewed by Ray
(147a).
Whatever the mechanism is, it might perhaps provide a basis for
phototropic curvatures in stems exposed to continuous daylight and perhaps also for those in roots. Such curvatures are mainly of the localized
type and are characterized by a relatively high energy (or continuous exposure) requirement; indeed, their relative "insensitivity" is perhaps the
main reason they have been so little studied. Wherever auxins are limiting growth, a mechanism of the kind postulated on p. 295 would yield