268
KENNETH V. THIMANN AND GEORGE M. CURRY
oxidation of IAA, Galston (66a) reported a quantum yield of approximately 0.7. Burkholder and Johnston (67) found significant amounts of
light-destruction of growth substances in intact and excised tips of
Avena, and of Zea mays and Nicotiana growth substance diffused into
agar, but only by the application of relatively tremendous doses (10
6 to
10
s ergs/cm.
2 ). In similar experiments the authors have found that
"natural" auxin from Avena tips is inactivated by ultraviolet light, but
with efficiencies no greater than the inactivation of IAA. Recently
Reisener (66), using C
14 -labeled IAA introduced into the coleoptile
by diffusion, could find no light-destruction with a white light dosage
about equal to 10,000 ergs/cm.
2 of blue light. There was also no destruction of IAA in solution with this dosage. All these facts argue
strongly against the possibility that significant amounts of growth
substance could be inactivated by light in doses of a few ergs per square
centimeter.
In connection with the postulated effect of light on auxin distribution, an important generalization must be made: Wherever auxin is produced by the plant, polar transport away from the production site also
occurs. If production were more rapid than transport, auxin would
accumulate locally and tumors and swellings would result. This seems
to happen only when the auxin is produced by agencies other than those
normally in the plant, i.e., by microorganisms. Accumulation does not
occur (except in fruits) when the auxin production is by the plant's
own normal tissues. Correspondingly, in the absence of auxin production, polar transport disappears; thus it has been found that in Pinus
palustris, which does not elongate for the first seven years of life, auxin
is not destroyed, but both the auxin production and the polar transport
system are absent (68). It seems, then, that production and transport
are interlinked, and the molecules of auxin are transported away from
the site of formation in a process which is integral with the formation
reaction. Under the influence of light, it must be this system which is
modified in such a way that the molecules of auxin, as they are formed,
are transported abnormally. This might mean that under the influence of
light the "polarity" in the tissue changes from longitudinal to lateral,
or it might only mean that light paralyzes, or retards, the transport
system; as a result the produced auxin would then accumulate and some
of the excess would find its way to the dark side and be transported
downwards there. Alternatively, light might paralyze or retard the system which forms the auxin from a precursor. As a result, excess precursor would accumulate and some of this would be converted to auxin
on the dark side. The fact that the total auxin yield does not change
when the tips are illuminated and bisected, however (58), does not
KENNETH V. THIMANN AND GEORGE M. CURRY
oxidation of IAA, Galston (66a) reported a quantum yield of approximately 0.7. Burkholder and Johnston (67) found significant amounts of
light-destruction of growth substances in intact and excised tips of
Avena, and of Zea mays and Nicotiana growth substance diffused into
agar, but only by the application of relatively tremendous doses (10
6 to
10
s ergs/cm.
2 ). In similar experiments the authors have found that
"natural" auxin from Avena tips is inactivated by ultraviolet light, but
with efficiencies no greater than the inactivation of IAA. Recently
Reisener (66), using C
14 -labeled IAA introduced into the coleoptile
by diffusion, could find no light-destruction with a white light dosage
about equal to 10,000 ergs/cm.
2 of blue light. There was also no destruction of IAA in solution with this dosage. All these facts argue
strongly against the possibility that significant amounts of growth
substance could be inactivated by light in doses of a few ergs per square
centimeter.
In connection with the postulated effect of light on auxin distribution, an important generalization must be made: Wherever auxin is produced by the plant, polar transport away from the production site also
occurs. If production were more rapid than transport, auxin would
accumulate locally and tumors and swellings would result. This seems
to happen only when the auxin is produced by agencies other than those
normally in the plant, i.e., by microorganisms. Accumulation does not
occur (except in fruits) when the auxin production is by the plant's
own normal tissues. Correspondingly, in the absence of auxin production, polar transport disappears; thus it has been found that in Pinus
palustris, which does not elongate for the first seven years of life, auxin
is not destroyed, but both the auxin production and the polar transport
system are absent (68). It seems, then, that production and transport
are interlinked, and the molecules of auxin are transported away from
the site of formation in a process which is integral with the formation
reaction. Under the influence of light, it must be this system which is
modified in such a way that the molecules of auxin, as they are formed,
are transported abnormally. This might mean that under the influence of
light the "polarity" in the tissue changes from longitudinal to lateral,
or it might only mean that light paralyzes, or retards, the transport
system; as a result the produced auxin would then accumulate and some
of the excess would find its way to the dark side and be transported
downwards there. Alternatively, light might paralyze or retard the system which forms the auxin from a precursor. As a result, excess precursor would accumulate and some of this would be converted to auxin
on the dark side. The fact that the total auxin yield does not change
when the tips are illuminated and bisected, however (58), does not