256
GEORGE G. LATIES
B. Intermediate Metabolism Functionally Related to Growth
Commoner and Thimann (1941) were first to suggest that growth in
Avena is particularly related to a fraction of the respiration in which the
dicarboxylic organic acids participate. This concept was extended to
growth in pea epicotyls as well. The role of the organic acids was shown
to be that of intermediates in the tricarboxylic acid cycle (see Thimann,
1951, 1952, 1954).
Various respiratory inhibitors were shown to affect the growth of
Avena and of pea sections. Fluoride, arsenite, fluoroacetate, iodoacetate,
and a variety of organic arsenicals and mercurials in varying degrees
affect growth (Thimann and W. D. Bonner, 1949; W. D. Bonner and
Thimann, 1950), as well as the water-absorbing capacity of potato
disks (Hackett and Thimann 1952a, 1953). Particularly interesting is
iodoacetate, which at certain concentrations inhibits the growth of Avena
sections completely while diminishing respiration less than 10% (Commoner and Thimann, 1941). The latter observation permits at least two
interpretations: either a sulfhydryl enzyme active in growth but not
directly associated with respiration is particularly susceptible to iodoacetate; or, a small fraction of the total respiration, different in kind
from the rest, is both directly linked to growth and readily inhibited
by iodoacetate.
The second of these alternatives was adopted by Commoner and Thimann on the basis of experiments which first showed that a number of
organic acids of the tricarboxylic acid cycle stimulate the growth of uninhibited coloeoptile sections, and secondly suggested that the organic
acids can reverse iodoacetate-induced growth inhibition. Since, furthermore, auxin and the organic acids appeared to exert a synergistic
effect upon growth and respiration, Commoner and Thimann deduced
that a small and variable fraction of Avena respiration having to do with
the metabolism of the organic acids was directly associated with growth.
Considerable evidence has since borne out this deduction, in spite of
the fact that the original observations in all probability did not have the
significance imputed to them.
Thus, in a series of further experiments designed to ascertain whether
growth is particularly dependent upon the operation of the tricarboxylic
acid cycle, Thimann and W. D. Bonner (1948) showed that malonate
(and maléate) relieved the growth inhibition by iodoacetate as effectively as did malate. If the response to malonate were to be considered a true reversal of the growth inhibition, the operation of the tricarboxylic acid cycle would be ruled out as a metabolic prerequisite
to growth, since malonate is itself a very effective inhibitor of one step
in that cycle. Cooil (1952) has elegantly reconciled these apparent in-
GEORGE G. LATIES
B. Intermediate Metabolism Functionally Related to Growth
Commoner and Thimann (1941) were first to suggest that growth in
Avena is particularly related to a fraction of the respiration in which the
dicarboxylic organic acids participate. This concept was extended to
growth in pea epicotyls as well. The role of the organic acids was shown
to be that of intermediates in the tricarboxylic acid cycle (see Thimann,
1951, 1952, 1954).
Various respiratory inhibitors were shown to affect the growth of
Avena and of pea sections. Fluoride, arsenite, fluoroacetate, iodoacetate,
and a variety of organic arsenicals and mercurials in varying degrees
affect growth (Thimann and W. D. Bonner, 1949; W. D. Bonner and
Thimann, 1950), as well as the water-absorbing capacity of potato
disks (Hackett and Thimann 1952a, 1953). Particularly interesting is
iodoacetate, which at certain concentrations inhibits the growth of Avena
sections completely while diminishing respiration less than 10% (Commoner and Thimann, 1941). The latter observation permits at least two
interpretations: either a sulfhydryl enzyme active in growth but not
directly associated with respiration is particularly susceptible to iodoacetate; or, a small fraction of the total respiration, different in kind
from the rest, is both directly linked to growth and readily inhibited
by iodoacetate.
The second of these alternatives was adopted by Commoner and Thimann on the basis of experiments which first showed that a number of
organic acids of the tricarboxylic acid cycle stimulate the growth of uninhibited coloeoptile sections, and secondly suggested that the organic
acids can reverse iodoacetate-induced growth inhibition. Since, furthermore, auxin and the organic acids appeared to exert a synergistic
effect upon growth and respiration, Commoner and Thimann deduced
that a small and variable fraction of Avena respiration having to do with
the metabolism of the organic acids was directly associated with growth.
Considerable evidence has since borne out this deduction, in spite of
the fact that the original observations in all probability did not have the
significance imputed to them.
Thus, in a series of further experiments designed to ascertain whether
growth is particularly dependent upon the operation of the tricarboxylic
acid cycle, Thimann and W. D. Bonner (1948) showed that malonate
(and maléate) relieved the growth inhibition by iodoacetate as effectively as did malate. If the response to malonate were to be considered a true reversal of the growth inhibition, the operation of the tricarboxylic acid cycle would be ruled out as a metabolic prerequisite
to growth, since malonate is itself a very effective inhibitor of one step
in that cycle. Cooil (1952) has elegantly reconciled these apparent in-