298
KENNETH V. THIMANN AND GEORGE M. CURRY
a light-growth reaction and phototropic curvature if the light is distributed asymmetrically.
The weakness of all such proposals is that they require the auxin to
be present at the same time as the illumination. Many years ago Brauner
and others (see reference 19 for review) showed that if unilluminated
tips were placed on previously illuminated coleoptile stumps the latter
would curve, i.e., the effect of the light persists in the tissue in the subsequent dark period. Recently von Guttenberg (147b) has greatly extended this type of experiment; coleoptile stumps were exposed to
about 7.10
6 MCS of white light, then again decapitated and placed in
the dark and indoleacetic acid 0.5 mg. per liter subsequently applied
symmetrically in an agar block. Large curvatures resulted. Even when
the blocks were not applied until after 8 hours in darkness, curvatures
could still result; after 12 hours the effect of the light had finally disappeared. These experiments further increase the improbability that the
action could be a direct photoinactivation; it is difficult to believe that
photoproducts capable of reacting with and destroying IAA would
persist for hours. The polarity of an auxin-transporting system, however, might well persist in the absence of anything for it to transport.
C. TIP RESPONSE
To account for the apparently high quantum yields involved in the
tip response in terms of auxin redistribution, we need to postulate some
kind of amplifying mechanism. It has recently been shown (148, 149)
that the products of IAA destruction by bright light inhibit the growth
of Avena coleoptile sections. The experiment, in principle, was to apply
an agar block containing IAA which had been illuminated with riboflavin, (or pure compounds which have been identified as among the
photolysis products), and after two hours to replace it with another
block containing IAA. The subsequent growth was less than that of
controls which had been given either water or unilluminated IAA during
the first two hours. If, however, similar experiments were done with
sections floating on solution no such inhibition was observed. It was
therefore suggested that the photolytic products act by inhibiting the
transport of IAA through the section. Such an inhibition of transport by
the breakdown products of IAA might provide the required amplification of the light effect. Unfortunately the light energies required to
cause accumulation of an inhibiting level of the photolytic products are
several orders of magnitude greater than those causing the tip response.
Even with added riboflavin as sensitizer 9.10
6 MCS were used in the experiment cited (149). In sections illuminated directly, Meyer points out
KENNETH V. THIMANN AND GEORGE M. CURRY
a light-growth reaction and phototropic curvature if the light is distributed asymmetrically.
The weakness of all such proposals is that they require the auxin to
be present at the same time as the illumination. Many years ago Brauner
and others (see reference 19 for review) showed that if unilluminated
tips were placed on previously illuminated coleoptile stumps the latter
would curve, i.e., the effect of the light persists in the tissue in the subsequent dark period. Recently von Guttenberg (147b) has greatly extended this type of experiment; coleoptile stumps were exposed to
about 7.10
6 MCS of white light, then again decapitated and placed in
the dark and indoleacetic acid 0.5 mg. per liter subsequently applied
symmetrically in an agar block. Large curvatures resulted. Even when
the blocks were not applied until after 8 hours in darkness, curvatures
could still result; after 12 hours the effect of the light had finally disappeared. These experiments further increase the improbability that the
action could be a direct photoinactivation; it is difficult to believe that
photoproducts capable of reacting with and destroying IAA would
persist for hours. The polarity of an auxin-transporting system, however, might well persist in the absence of anything for it to transport.
C. TIP RESPONSE
To account for the apparently high quantum yields involved in the
tip response in terms of auxin redistribution, we need to postulate some
kind of amplifying mechanism. It has recently been shown (148, 149)
that the products of IAA destruction by bright light inhibit the growth
of Avena coleoptile sections. The experiment, in principle, was to apply
an agar block containing IAA which had been illuminated with riboflavin, (or pure compounds which have been identified as among the
photolysis products), and after two hours to replace it with another
block containing IAA. The subsequent growth was less than that of
controls which had been given either water or unilluminated IAA during
the first two hours. If, however, similar experiments were done with
sections floating on solution no such inhibition was observed. It was
therefore suggested that the photolytic products act by inhibiting the
transport of IAA through the section. Such an inhibition of transport by
the breakdown products of IAA might provide the required amplification of the light effect. Unfortunately the light energies required to
cause accumulation of an inhibiting level of the photolytic products are
several orders of magnitude greater than those causing the tip response.
Even with added riboflavin as sensitizer 9.10
6 MCS were used in the experiment cited (149). In sections illuminated directly, Meyer points out