290
KENNETH V. THIMANN AND GEORGE M. CURRY
tion it is supposed that the organism moves so as to keep the photoreceptor fully illuminated. Hence a change in motion is only possible
when light falling on the photoreceptor becomes intercepted by the
screening substance. In this case, then, the screening substance acts in a
"positive" rather than in a "negative" sense. Change in motion can only
occur in regions of the spectrum where both the photoreceptor and the
screening pigment absorb. If, for instance, the photoreceptor were to
absorb uniformly over a given spectral region, then the sensitivity would
be a direct function of the absorption by the masking pigment (and
vice versa). In general the sensitivity will thus be a product of the
absorption of the two pigments. As discussed in Section VI, A, this fact
provides one possible basis for the lack of correspondence between the
action spectrum and the absorption of any of the pigments known to
be present in the cell.
D. PHOTORECEPTOR FOR THE "BASE RESPONSE" OF Avena
The spectral sensitivity of the "base response" appears to differ from
that of the "tip response," and the specific nature of its photoreceptor
for visible light remains obscure. This is partly due to the fact that the
base response is quite insensitive and difficult to isolate in Avena. Haig
(S3), studying base response as a function of wavelength and using
reaction time as a criterion, found an action spectrum rising steadily
toward the violet, the highest point occurring near 410 m/x. Haig's curve
does not suggest any specific pigment, although both Galston and Baker
(99) and Went (22) say that it indicates riboflavin. If it has any
recognizable structure it is more like that of a carotenoid, for it has
three slight peaks in the blue. Thorning (135), studying the lightgrowth reactions of decapitated Avena coleoptiles, found that 447 m/x
light was more effective than 460 m/x, and 460 m/x more effective than
477 m/x, suggesting that the photoreceptor does not have two absorption
peaks in this region.
In regard to the base response in the ultraviolet, however, the data
are clearer. The action spectrum for ultraviolet-induced base responses
shows two peaks at 280 and 297 m/x (see Fig. 12). It is notable that the
wavelengths absorbed by riboflavin are relatively ineffective; indeed the
370 and 265 m/x peaks of riboflavin coincide with low points in this
action spectrum.
The shape of the action spectrum as a whole is suggestive of that of
IAA itself, though slightly modified around the peak and shifted about
12 m/x towards the red. Since also application of IAA to decapitated
plants reinstates their ability to curve towards ultraviolet it was suggested (26) that the photoreceptor may be a biologically active complex
of IAA. However, other possibilities no doubt exist.
KENNETH V. THIMANN AND GEORGE M. CURRY
tion it is supposed that the organism moves so as to keep the photoreceptor fully illuminated. Hence a change in motion is only possible
when light falling on the photoreceptor becomes intercepted by the
screening substance. In this case, then, the screening substance acts in a
"positive" rather than in a "negative" sense. Change in motion can only
occur in regions of the spectrum where both the photoreceptor and the
screening pigment absorb. If, for instance, the photoreceptor were to
absorb uniformly over a given spectral region, then the sensitivity would
be a direct function of the absorption by the masking pigment (and
vice versa). In general the sensitivity will thus be a product of the
absorption of the two pigments. As discussed in Section VI, A, this fact
provides one possible basis for the lack of correspondence between the
action spectrum and the absorption of any of the pigments known to
be present in the cell.
D. PHOTORECEPTOR FOR THE "BASE RESPONSE" OF Avena
The spectral sensitivity of the "base response" appears to differ from
that of the "tip response," and the specific nature of its photoreceptor
for visible light remains obscure. This is partly due to the fact that the
base response is quite insensitive and difficult to isolate in Avena. Haig
(S3), studying base response as a function of wavelength and using
reaction time as a criterion, found an action spectrum rising steadily
toward the violet, the highest point occurring near 410 m/x. Haig's curve
does not suggest any specific pigment, although both Galston and Baker
(99) and Went (22) say that it indicates riboflavin. If it has any
recognizable structure it is more like that of a carotenoid, for it has
three slight peaks in the blue. Thorning (135), studying the lightgrowth reactions of decapitated Avena coleoptiles, found that 447 m/x
light was more effective than 460 m/x, and 460 m/x more effective than
477 m/x, suggesting that the photoreceptor does not have two absorption
peaks in this region.
In regard to the base response in the ultraviolet, however, the data
are clearer. The action spectrum for ultraviolet-induced base responses
shows two peaks at 280 and 297 m/x (see Fig. 12). It is notable that the
wavelengths absorbed by riboflavin are relatively ineffective; indeed the
370 and 265 m/x peaks of riboflavin coincide with low points in this
action spectrum.
The shape of the action spectrum as a whole is suggestive of that of
IAA itself, though slightly modified around the peak and shifted about
12 m/x towards the red. Since also application of IAA to decapitated
plants reinstates their ability to curve towards ultraviolet it was suggested (26) that the photoreceptor may be a biologically active complex
of IAA. However, other possibilities no doubt exist.