6. PHOTOTROPISM AND PHOTOTAXIS
289
masking). This function has also been plotted in Fig. 11 to show that
it, too, cannot account for the observed action spectrum.
The absolute concentrations of riboflavin and carotene in Avena are
known; the riboflavin content is 20-30 /Ag. per gm. dry weight (cf. Section VI, B), and Biinning (131) estimated that the apical mm. contains 100 /Ag. lutein per gm. dry weight, enough for about 10% absorption of Z 0 . It is important to note that over a wide range of intermediate
concentrations of riboflavin and carotene like these, the postulated
masking of riboflavin by carotene will be very small, because increases
in c, though increasing the gradient, at the same time decrease the
fraction of I 0 absorbed by riboflavin in both sides of the organ.*
In many respects the A-M-A arrangement discussed above is just
the situation examined in certain of Brauner's (71, 129) experiments.
He found that the presence of an "inactive" light absorber between the
"actively" absorbing regions could augment the phototropic response;
e.g., introduction of (100% absorbing) india ink into the hollow of the
coleoptile approximately doubled the curvature obtained on four hours'
illumination of the base. For processes involving the subapical regions,
this effect seems entirely reasonable according to the above interpretation. The gradient of light action is increased, but nothing further is
implied about (nor is there any direct evidence for) specific changes in
the action spectrum due to the introduction of the dye. Similarly, in the
near-ultraviolet, the scattering and absorption losses across the tip will
increase with decreasing wavelength and thus increase the gradient;
this will distort the observed action spectrum in this region. As the
wavelength becomes shorter, the effect of the ultraviolet will be more
and more exerted on the front side (cf., Carlile, 134).
It must be concluded that while the presence of an inactive pigment
will increase the gradient it will not "determine" the action spectrum.
It can only modify it to a limited extent, even in the extreme case, and
will not transform it in the manner proposed by Reinert.
2. In Phototaxis
A particular case of a masking or screening action has long been
discussed as a basis for phototactic movement. The stigma or other
screening material shades the photoreceptor (which is considered to lie
at the base of the flagellum) as in the case of M overlying A above.
This might be expected to lead to "negative masking," but in this situa* Note that the high phototropic sensitivity of the carotenoid-low mutants
described in Section VI, B is impossible to reconcile with the masking theory, because on this theory the sensitivity ought to depend on the gradient imposed by the
masking pigment.
289
masking). This function has also been plotted in Fig. 11 to show that
it, too, cannot account for the observed action spectrum.
The absolute concentrations of riboflavin and carotene in Avena are
known; the riboflavin content is 20-30 /Ag. per gm. dry weight (cf. Section VI, B), and Biinning (131) estimated that the apical mm. contains 100 /Ag. lutein per gm. dry weight, enough for about 10% absorption of Z 0 . It is important to note that over a wide range of intermediate
concentrations of riboflavin and carotene like these, the postulated
masking of riboflavin by carotene will be very small, because increases
in c, though increasing the gradient, at the same time decrease the
fraction of I 0 absorbed by riboflavin in both sides of the organ.*
In many respects the A-M-A arrangement discussed above is just
the situation examined in certain of Brauner's (71, 129) experiments.
He found that the presence of an "inactive" light absorber between the
"actively" absorbing regions could augment the phototropic response;
e.g., introduction of (100% absorbing) india ink into the hollow of the
coleoptile approximately doubled the curvature obtained on four hours'
illumination of the base. For processes involving the subapical regions,
this effect seems entirely reasonable according to the above interpretation. The gradient of light action is increased, but nothing further is
implied about (nor is there any direct evidence for) specific changes in
the action spectrum due to the introduction of the dye. Similarly, in the
near-ultraviolet, the scattering and absorption losses across the tip will
increase with decreasing wavelength and thus increase the gradient;
this will distort the observed action spectrum in this region. As the
wavelength becomes shorter, the effect of the ultraviolet will be more
and more exerted on the front side (cf., Carlile, 134).
It must be concluded that while the presence of an inactive pigment
will increase the gradient it will not "determine" the action spectrum.
It can only modify it to a limited extent, even in the extreme case, and
will not transform it in the manner proposed by Reinert.
2. In Phototaxis
A particular case of a masking or screening action has long been
discussed as a basis for phototactic movement. The stigma or other
screening material shades the photoreceptor (which is considered to lie
at the base of the flagellum) as in the case of M overlying A above.
This might be expected to lead to "negative masking," but in this situa* Note that the high phototropic sensitivity of the carotenoid-low mutants
described in Section VI, B is impossible to reconcile with the masking theory, because on this theory the sensitivity ought to depend on the gradient imposed by the
masking pigment.