266
KENNETH V. THIMANN AND GEORGE M. CURRY
vincing; Went's data, for instance (55) showed a much greater decrease
in auxin on the lighted side than increase on the dark side. This agrees
with the growth measurements (62) just mentioned. Van Overbeek's
data with Raphanus tips (17) show the same thing, provided the auxin
is being produced in the tissue; they do not show it for auxin externally
applied to tissue cylinders (Table V). Oppenoorth (20) using ether
extraction and Avena coleoptiles, found less auxin on the lighted side
than in dark controls, but in some experiments the increase on the dark
side began only after two hours, while in others it never did occur,
both sides showing less auxin than in the unilluminated controls. At light
dosages causing negative curvatures he found a greater increase on the
light than on the dark side, but both sides showed more auxin than before illumination. Thus in positive curvatures there appears to be some
over-all destruction by light, or in negative curvatures some over-all
promotion of auxin synthesis. Söding, in his book, concludes: "These two
factors make it difficult to conclude with complete certainty, from relative figures of the auxin content of the dark and light sides, that there
is lateral movement of auxin, since all the differences can theoretically
be equally well ascribed to a reduction or an increase of the auxin content by light" (63). However, Briggs et al. (58) repeated and extended experiments of the type done by Went, but using corn (Zea
mays) coleoptiles bisected at the base. When placed on agar in the
dark or in all-round light the auxin yield from the two sides was satisfactorily the same (Fig. 6) and when illuminated from one side, that
side yielded less auxin than the other, as expected. But if the bisection
went all the way up to the tip (perpendicular to the direction of illumination ), so that lateral movement could not occur, then the auxin yield
from the two sides was the same. This experiment seems quite convincing and one must conclude that the effect of low intensity light is
exerted, not on the auxin itself, but on the system which produces
and/or transports it.
A similar conclusion follows from an entirely different consideration,
namely from the quantum yield of phototropism. Biinning (64) came to
the conclusion that the energy input for tip responses was too low by a
factor of 10
4 (assuming a quantum yield of unity) to account for sufficient auxin destruction. An estimate along similar lines follows.
For highly sensitive coleoptiles a dose of 1 erg/cm.
2 of 440 m/x light
applied on 1 mm.
2 of the apical surface will suffice for a 5° curvature.
This dose represents approximately 2 X 10
11 quanta/cm
2 χ 10~
2 cm.
2
= 2 X 10
9 quanta. Making the generous assumption that 10% of these
quanta are involved in a one-for-one inactivation of auxin molecules on
the front side of the coleoptile, one would expect a difference of 2 X 10
8
KENNETH V. THIMANN AND GEORGE M. CURRY
vincing; Went's data, for instance (55) showed a much greater decrease
in auxin on the lighted side than increase on the dark side. This agrees
with the growth measurements (62) just mentioned. Van Overbeek's
data with Raphanus tips (17) show the same thing, provided the auxin
is being produced in the tissue; they do not show it for auxin externally
applied to tissue cylinders (Table V). Oppenoorth (20) using ether
extraction and Avena coleoptiles, found less auxin on the lighted side
than in dark controls, but in some experiments the increase on the dark
side began only after two hours, while in others it never did occur,
both sides showing less auxin than in the unilluminated controls. At light
dosages causing negative curvatures he found a greater increase on the
light than on the dark side, but both sides showed more auxin than before illumination. Thus in positive curvatures there appears to be some
over-all destruction by light, or in negative curvatures some over-all
promotion of auxin synthesis. Söding, in his book, concludes: "These two
factors make it difficult to conclude with complete certainty, from relative figures of the auxin content of the dark and light sides, that there
is lateral movement of auxin, since all the differences can theoretically
be equally well ascribed to a reduction or an increase of the auxin content by light" (63). However, Briggs et al. (58) repeated and extended experiments of the type done by Went, but using corn (Zea
mays) coleoptiles bisected at the base. When placed on agar in the
dark or in all-round light the auxin yield from the two sides was satisfactorily the same (Fig. 6) and when illuminated from one side, that
side yielded less auxin than the other, as expected. But if the bisection
went all the way up to the tip (perpendicular to the direction of illumination ), so that lateral movement could not occur, then the auxin yield
from the two sides was the same. This experiment seems quite convincing and one must conclude that the effect of low intensity light is
exerted, not on the auxin itself, but on the system which produces
and/or transports it.
A similar conclusion follows from an entirely different consideration,
namely from the quantum yield of phototropism. Biinning (64) came to
the conclusion that the energy input for tip responses was too low by a
factor of 10
4 (assuming a quantum yield of unity) to account for sufficient auxin destruction. An estimate along similar lines follows.
For highly sensitive coleoptiles a dose of 1 erg/cm.
2 of 440 m/x light
applied on 1 mm.
2 of the apical surface will suffice for a 5° curvature.
This dose represents approximately 2 X 10
11 quanta/cm
2 χ 10~
2 cm.
2
= 2 X 10
9 quanta. Making the generous assumption that 10% of these
quanta are involved in a one-for-one inactivation of auxin molecules on
the front side of the coleoptile, one would expect a difference of 2 X 10
8