6. PHOTOTROPISM AND PHOTOTAXIS
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asymmetry of auxin production in the blade to be so precisely transferred to the petiole.
Thus, in most plants we may conceive of two phototropic mechanisms, one in which light acts on the source of auxin, and the other in
which light acts directly, either on auxin or on some system which combines with auxin, near the point of its utilization in growth. A complete
biochemical interpretation of phototropism awaits, on the one hand, a
more precise identification of the light-sensitive systems and, on the
other hand, further information about the production and transport of
auxin and the initial stages of the growth process.
D. THE RELATION BETWEEN LIGHT ENERGY INPUT AND THE RESPONSE
In the very early work on phototropism the prevailing notion was
that the magnitude of the response initiated by light bore little relation
to the actual energy input (Stimmungs-Theorie).
Then both Loeb for
animals and Blaauw for plants made it quite clear that, for very small
responses, at least, the response is closely dependent on the amount of
energy administered (Reizmengengesetz).
This finding has led some to
think that the light energy is itself the source of the energy required
for the directed movement. In those phototactic movements, such as that
of Rhodospirillum, which depend upon chlorophyll and therefore upon
photosynthesis, it seems conceivable that the light energy could ultimately be converted to energy of motion. An examination of phototropism and phototaxis in the more typical, sensitive cases, however,
indicates that the translational energy must actually derive from the
potential energy within the organism; i.e., from the organism's own
energy reserves.
In the higher plants we have seen that some form of auxin seems to
be implicated in phototropism. Auxins are known to be catalytic in
their action; i.e., each hormone molecule controls thousands of growth
"events." In the relatively simple base response the action of light on
auxin or on an auxin complex may be a stoichiometric photooxidation.
Even in this case, however, the resulting redistribution of growth represents a very large amplification of the light energy input, because the
small change in the numbers of auxin molecules results in a very large
change in the relative volumes of tissue on the two sides of the plant.
In the tip response, where light is presumed to act on an enzyme in the
auxin production-transport system (Section VII, C), the amplification
is even greater because it is two-staged. In the first stage each quantum
of light affects the distribution of more than one auxin molecule, and
in the second stage each auxin molecule affects more than one growth
"event." Thus, an exceedingly small light energy input secures a spatial
301
asymmetry of auxin production in the blade to be so precisely transferred to the petiole.
Thus, in most plants we may conceive of two phototropic mechanisms, one in which light acts on the source of auxin, and the other in
which light acts directly, either on auxin or on some system which combines with auxin, near the point of its utilization in growth. A complete
biochemical interpretation of phototropism awaits, on the one hand, a
more precise identification of the light-sensitive systems and, on the
other hand, further information about the production and transport of
auxin and the initial stages of the growth process.
D. THE RELATION BETWEEN LIGHT ENERGY INPUT AND THE RESPONSE
In the very early work on phototropism the prevailing notion was
that the magnitude of the response initiated by light bore little relation
to the actual energy input (Stimmungs-Theorie).
Then both Loeb for
animals and Blaauw for plants made it quite clear that, for very small
responses, at least, the response is closely dependent on the amount of
energy administered (Reizmengengesetz).
This finding has led some to
think that the light energy is itself the source of the energy required
for the directed movement. In those phototactic movements, such as that
of Rhodospirillum, which depend upon chlorophyll and therefore upon
photosynthesis, it seems conceivable that the light energy could ultimately be converted to energy of motion. An examination of phototropism and phototaxis in the more typical, sensitive cases, however,
indicates that the translational energy must actually derive from the
potential energy within the organism; i.e., from the organism's own
energy reserves.
In the higher plants we have seen that some form of auxin seems to
be implicated in phototropism. Auxins are known to be catalytic in
their action; i.e., each hormone molecule controls thousands of growth
"events." In the relatively simple base response the action of light on
auxin or on an auxin complex may be a stoichiometric photooxidation.
Even in this case, however, the resulting redistribution of growth represents a very large amplification of the light energy input, because the
small change in the numbers of auxin molecules results in a very large
change in the relative volumes of tissue on the two sides of the plant.
In the tip response, where light is presumed to act on an enzyme in the
auxin production-transport system (Section VII, C), the amplification
is even greater because it is two-staged. In the first stage each quantum
of light affects the distribution of more than one auxin molecule, and
in the second stage each auxin molecule affects more than one growth
"event." Thus, an exceedingly small light energy input secures a spatial