6. PHOTOTROPISM AND PHOTOTAXIS
269
favor either of the latter explanations. Whichever mechanism operates, it
must be some aspect of the auxin-producing system which is lightsensitive. *
This conclusion is supported (in a rather negative way) by some
recent experiments using artificially applied auxin (66, 69, 70). In
these, isotopically labeled indoleacetic acid was applied to the tip of a
coleoptile and then after unilateral illumination the distribution of isotope was followed. In Biinning's case a thin section from the extreme tip
was removed and the auxin then applied to the cut surface in agar,
which, as Brauner (71) points out, may have removed just the tissue
able to carry out auxin redistribution. However, Reisener (67a) repeated the experiment with intact coleoptiles to which a blocklet of IAA
in agar was applied. The results of the three workers were the same,
namely that the C
14 content of the bright and dark sides was identical
within the limits of counting error. Although the authors cited have
drawn different conclusions, the only reasonable one appears to be that
preformed indoleacetic acid is not acted on by the light, and therefore
that the action occurs at the site of auxin production.
For the sake of completeness, it should be mentioned here that in
recent years it has become clear that a second powerful growth-controlling factor, namely gibberellin, operates in plants. However, since the
characteristic of gibberellin-induced growth is that it takes place par
excellence in the light (72), it is highly improbable that gibberellin
plays any major part in phototropism.
VI. Nature of the Photoreceptors
A. PHOTORECEPTORS FOR PHOTOTAXIS
From the biochemical viewpoint, one of the major problems in the
study of a light reaction is to identify the photoreceptor, and this in
turn can best be approached by optical methods through determination
of the action spectrum. But action spectra cannot be used to determine
the absorption spectrum of the light absorber unless they are both
precise and detailed. For this reason many of the early observations
as to the spectral regions effective for phototaxis give little basis for
reliable deductions about the photoreceptor.
The first organisms to be studied carefully for their spectral sensitivity were the purple bacteria, especially Rhodospirillum rubrum. As
* This argument probably does not hold for the base response, since it is possible to apply auxin after illumination of decapitated Avena coleoptiles and obtain
curvature (see Section VII, B). Here the light must act upon a transport system
which remains laterally polarized for some time afterwards.
269
favor either of the latter explanations. Whichever mechanism operates, it
must be some aspect of the auxin-producing system which is lightsensitive. *
This conclusion is supported (in a rather negative way) by some
recent experiments using artificially applied auxin (66, 69, 70). In
these, isotopically labeled indoleacetic acid was applied to the tip of a
coleoptile and then after unilateral illumination the distribution of isotope was followed. In Biinning's case a thin section from the extreme tip
was removed and the auxin then applied to the cut surface in agar,
which, as Brauner (71) points out, may have removed just the tissue
able to carry out auxin redistribution. However, Reisener (67a) repeated the experiment with intact coleoptiles to which a blocklet of IAA
in agar was applied. The results of the three workers were the same,
namely that the C
14 content of the bright and dark sides was identical
within the limits of counting error. Although the authors cited have
drawn different conclusions, the only reasonable one appears to be that
preformed indoleacetic acid is not acted on by the light, and therefore
that the action occurs at the site of auxin production.
For the sake of completeness, it should be mentioned here that in
recent years it has become clear that a second powerful growth-controlling factor, namely gibberellin, operates in plants. However, since the
characteristic of gibberellin-induced growth is that it takes place par
excellence in the light (72), it is highly improbable that gibberellin
plays any major part in phototropism.
VI. Nature of the Photoreceptors
A. PHOTORECEPTORS FOR PHOTOTAXIS
From the biochemical viewpoint, one of the major problems in the
study of a light reaction is to identify the photoreceptor, and this in
turn can best be approached by optical methods through determination
of the action spectrum. But action spectra cannot be used to determine
the absorption spectrum of the light absorber unless they are both
precise and detailed. For this reason many of the early observations
as to the spectral regions effective for phototaxis give little basis for
reliable deductions about the photoreceptor.
The first organisms to be studied carefully for their spectral sensitivity were the purple bacteria, especially Rhodospirillum rubrum. As
* This argument probably does not hold for the base response, since it is possible to apply auxin after illumination of decapitated Avena coleoptiles and obtain
curvature (see Section VII, B). Here the light must act upon a transport system
which remains laterally polarized for some time afterwards.