284
KENNETH V. THIMANN AND GEORGE M. CURRY
screening" effect of large concentrations of carotenoid pigments in the
presence of relatively low amounts of the actual photosensitizing pigment (assumed to be a flavin). Brauner (21) has aptly summarized
Reinert's main contention as follows: "the high phototropic sensitivity
of the tip is explained by the optical filter action of the carotene present,
which secures the necessary light absorption across the organ, thus protecting the auxin on the posterior side from destruction. Since the
difference in brightness between both flanks is mainly controlled by the
internal carotene screen, then the phototropic action spectrum must be
determined by the double-peaked absorption curve of the carotene."
Reinert's experiments (121), which were largely responsible for this
theory, showed that when IAA is inactivated by visible light in the
presence of riboflavin and ^-carotene, the carotenoid masking introduces two peaks into the action spectrum, not by addition of the
carotenoid peaks to that of riboflavin, but by depressing part of the
riboflavin curve. He worked with a colloidal suspension of ^-carotene
in water. This had no action alone as a sensitizer for IAA photolysis.
Its absorption spectrum possessed a single, broad peak near 420 m/x.
The riboflavin, which does sensitize IAA photolysis, had its usual broad
peak around 450 m/x, and the action spectrum for riboflavin plus IAA
alone gave a sharp peak at 430 m/x [cf. Galston and Baker (99), who
found a broad peak at 440 m/x for a similar system]. The riboflavin-ßcarotene-IAA system gave rise to a two peaked action spectrum for IAA
photolysis, one peak being at 430 m/x and the other at 450 πΐμ. The
curve rose sharply below 410 m/x. (This rise was also noted by Galston
and Baker in their concentrated brei system, but they found no interference, apparently, by carotenoids or other pigments in the system.)
In Reinert's words: "Das Maximum der Riboflavin Wirksamkeit bei
430 m/x bleibt auch in Reaktionsgemischen mit Carotin erhalten, weil
die benachbarten Messpunkte noch im Bereich des breiten Gipfels der
Carotinabsorption liegen. Der zweite Gipfel bei 450 m/x ergibt sich aus
der stark ansteigenden Carotinabsorption ab 455 m/x. Der Tiefpunkt bei
410 m/x ist eine Folge der sich verringernden Abschirmung des langwelligen UV-Lichtes durch das Carotin."
Thus the carotenoid absorption depresses the action of riboflavin,
presumably by competing for the light. Yet, when the theory is extended
to phototropism, the reverse is presumed to occur ("high phototropic
sensitivity of the tip is explained by the optical filter action of the
carotene"). This reversal is apparently connected with the fact that the
phototropic effect depends on the gradient of light action across the
coleoptile.
Let us now consider a simple system equivalent to Reinert's (34).
KENNETH V. THIMANN AND GEORGE M. CURRY
screening" effect of large concentrations of carotenoid pigments in the
presence of relatively low amounts of the actual photosensitizing pigment (assumed to be a flavin). Brauner (21) has aptly summarized
Reinert's main contention as follows: "the high phototropic sensitivity
of the tip is explained by the optical filter action of the carotene present,
which secures the necessary light absorption across the organ, thus protecting the auxin on the posterior side from destruction. Since the
difference in brightness between both flanks is mainly controlled by the
internal carotene screen, then the phototropic action spectrum must be
determined by the double-peaked absorption curve of the carotene."
Reinert's experiments (121), which were largely responsible for this
theory, showed that when IAA is inactivated by visible light in the
presence of riboflavin and ^-carotene, the carotenoid masking introduces two peaks into the action spectrum, not by addition of the
carotenoid peaks to that of riboflavin, but by depressing part of the
riboflavin curve. He worked with a colloidal suspension of ^-carotene
in water. This had no action alone as a sensitizer for IAA photolysis.
Its absorption spectrum possessed a single, broad peak near 420 m/x.
The riboflavin, which does sensitize IAA photolysis, had its usual broad
peak around 450 m/x, and the action spectrum for riboflavin plus IAA
alone gave a sharp peak at 430 m/x [cf. Galston and Baker (99), who
found a broad peak at 440 m/x for a similar system]. The riboflavin-ßcarotene-IAA system gave rise to a two peaked action spectrum for IAA
photolysis, one peak being at 430 m/x and the other at 450 πΐμ. The
curve rose sharply below 410 m/x. (This rise was also noted by Galston
and Baker in their concentrated brei system, but they found no interference, apparently, by carotenoids or other pigments in the system.)
In Reinert's words: "Das Maximum der Riboflavin Wirksamkeit bei
430 m/x bleibt auch in Reaktionsgemischen mit Carotin erhalten, weil
die benachbarten Messpunkte noch im Bereich des breiten Gipfels der
Carotinabsorption liegen. Der zweite Gipfel bei 450 m/x ergibt sich aus
der stark ansteigenden Carotinabsorption ab 455 m/x. Der Tiefpunkt bei
410 m/x ist eine Folge der sich verringernden Abschirmung des langwelligen UV-Lichtes durch das Carotin."
Thus the carotenoid absorption depresses the action of riboflavin,
presumably by competing for the light. Yet, when the theory is extended
to phototropism, the reverse is presumed to occur ("high phototropic
sensitivity of the tip is explained by the optical filter action of the
carotene"). This reversal is apparently connected with the fact that the
phototropic effect depends on the gradient of light action across the
coleoptile.
Let us now consider a simple system equivalent to Reinert's (34).