214
F. C. STEWARD AND Η. Y. MOHAN RAM
germination withheld by the effect of both natural and synthetic
substances that act as inhibitors of germination (Evenari, 1949, 1957;
Moewus et al., 1951; Toole et al., 1956). Some of these naturally occurring
inhibitors may be present in the ovary wall (Evenari, 1949), the testa,
the embryo (Kaufmann, 1943) or even in the endosperm (Randolph and
Cox, 1943). Such substances (i.e. inhibitors ard/or their antimetabolites)
may mediate the effects on germination of such external variables as
red and far red light in the case of lettuce seed (Borthwick et al., 1954),
and such temperature effects (reviewed by Hamner, 1948) as those
familiarly known as vernalization.
More recently, tissue culture techniques have been used to detect
inhibitors of growth (a specific case has already been cited in the role of
hydroxyproline as a proline competitor which inactivates the growth
and protein synthesis, which is otherwise induced by coconut milk).
Many organs of perennation may contain such growth inhibitors, and
these have been detected in resting potato tuber, onion bulb scales,
dormant maple buds as contrasted with the non-dormant ones. In fact,
many of the unusual nitrogen compounds now being disclosed in the
soluble nitrogen of storage organs may act in this way as antimetabolites
(e.g. azetidine carboxylic acid in Convallaria rhizomes is also a proline
antagonist; Steward, Pollard, Witkop and Patchett, 1958).
An outstanding case is the contrasted effect of light and darkness on
the suppression of internodal growth in the light and its release in the
dark, as in completely etiolated dicotyledonous shoots. The nature of
the substance which mediates the response due to light, though unknown, produces the equivalent effect of an antiauxin. Sodium
arsenate which acts by uncoupling oxidative phosphorylation specifically inhibits cellular enlargement (Hopkins, 1952). Coumarin, scopoletin, umbelliferone and certain other known naturally occurring unsaturated lactones (Thimann and Bonner, 1949a; Goodwin and Taves,
1950) can function as growth inhibitors at different concentrations.
The inhibition caused by coumarin-like substances due to reaction with
SH groups in plant tissues can be reversed by BAL (2,3-dimercaptopropanol) as shown by Thimann and Bonner (1949a). The recent
applications of unidirectional paper chromatography to the detection
of auxin-like substances in the standard assay method, has disclosed as
many potential auxin inhibitors as auxins (Luckwill, 1956; Nitsch, 1956).
A specific case where a potential auxin inhibitor has been implicated in
bud dormancy is mentioned by Hemberg (1958). Such facts mentioned
above make one wonder, as Thimann (1956) has pointed out,
. .
whether perhaps plants contain a natural growth-inhibiting system,
which is excessively sensitive to SH reagents. If such a system exists,
then normal growth would be due to the balance between it and the
F. C. STEWARD AND Η. Y. MOHAN RAM
germination withheld by the effect of both natural and synthetic
substances that act as inhibitors of germination (Evenari, 1949, 1957;
Moewus et al., 1951; Toole et al., 1956). Some of these naturally occurring
inhibitors may be present in the ovary wall (Evenari, 1949), the testa,
the embryo (Kaufmann, 1943) or even in the endosperm (Randolph and
Cox, 1943). Such substances (i.e. inhibitors ard/or their antimetabolites)
may mediate the effects on germination of such external variables as
red and far red light in the case of lettuce seed (Borthwick et al., 1954),
and such temperature effects (reviewed by Hamner, 1948) as those
familiarly known as vernalization.
More recently, tissue culture techniques have been used to detect
inhibitors of growth (a specific case has already been cited in the role of
hydroxyproline as a proline competitor which inactivates the growth
and protein synthesis, which is otherwise induced by coconut milk).
Many organs of perennation may contain such growth inhibitors, and
these have been detected in resting potato tuber, onion bulb scales,
dormant maple buds as contrasted with the non-dormant ones. In fact,
many of the unusual nitrogen compounds now being disclosed in the
soluble nitrogen of storage organs may act in this way as antimetabolites
(e.g. azetidine carboxylic acid in Convallaria rhizomes is also a proline
antagonist; Steward, Pollard, Witkop and Patchett, 1958).
An outstanding case is the contrasted effect of light and darkness on
the suppression of internodal growth in the light and its release in the
dark, as in completely etiolated dicotyledonous shoots. The nature of
the substance which mediates the response due to light, though unknown, produces the equivalent effect of an antiauxin. Sodium
arsenate which acts by uncoupling oxidative phosphorylation specifically inhibits cellular enlargement (Hopkins, 1952). Coumarin, scopoletin, umbelliferone and certain other known naturally occurring unsaturated lactones (Thimann and Bonner, 1949a; Goodwin and Taves,
1950) can function as growth inhibitors at different concentrations.
The inhibition caused by coumarin-like substances due to reaction with
SH groups in plant tissues can be reversed by BAL (2,3-dimercaptopropanol) as shown by Thimann and Bonner (1949a). The recent
applications of unidirectional paper chromatography to the detection
of auxin-like substances in the standard assay method, has disclosed as
many potential auxin inhibitors as auxins (Luckwill, 1956; Nitsch, 1956).
A specific case where a potential auxin inhibitor has been implicated in
bud dormancy is mentioned by Hemberg (1958). Such facts mentioned
above make one wonder, as Thimann (1956) has pointed out,
. .
whether perhaps plants contain a natural growth-inhibiting system,
which is excessively sensitive to SH reagents. If such a system exists,
then normal growth would be due to the balance between it and the