RESPIRATION RATE IN PLANTS
257
congruities. Under the conditions employed by Thimann and W. D.
Bonner he was able to obtain the same growth responses with inorganic
salts of potassium as with the potassium salts of the organic acids, whereas sodium salts of whatever type failed to stimulate growth similarly.
Potassium was found to stimulate growth even in the presence of
iodoacetate. The potassium salts of the organic acids, malonate included,
therefore appeared to alleviate iodoacetate inhibition when the growth
of sections in these salts was compared to the growth of iodoacetatetreated sections in water. In brief, Cooil has suggested that the apparent
response of Avena coleoptile sections to organic acids given in low concentration (0.001 M) at pH 6.5 is in fact a response to potassium ion,
a response which Thimann and Schneider (1938) had actually recognized
earlier. A further nonspecific effect of the organic acids was shown to
result from their buffering action, for the effectiveness of iodoacetate as
an inhibitor increases with a decrease in pH, and the pH drops with time
in unbuffered solutions of iodoacetate containing Avena sections.
When the potassium requirement for growth is first met, and growth
is subsequently inhibited with iodoacetate, the organic acids given at
pH 5.0 in relatively high concentration (0.01 Μ) restore growth. In such
cases malonate not only fails to stimulate growth, but rather inhibits
growth drastically. The growth inhibition educed by malonate is completely reversed by members of the tricarboxylic acid cycle. In general,
when organic acids are given as potassium salts, the response due to
potassium can be elicited at considerably lower salt concentrations, and
at a higher pH, than can the response due to the organic acid per se.
Cooil makes the point that, when a stimulation of growth is experimentally evoked in an inhibited system, it does not follow that the inhibition has been reversed. Reversal of an inhibition properly implies
a diminution in the percentage of inhibition. When growth is inhibited
by iodoacetate the subsequent growth stimulation evoked by potassium
does not represent a reversal of the inhibition, since the inhibitory effect
of iodoacetate percentagewise remains unchanged. On the other hand,
when growth is inhibited by malonate, the growth response elicited by
the organic acids represents a true reversal of the inhibition, since the
percentage inhibition of growth by malonate approaches zero in the
presence of organic acids. The effect of the organic acids in reversing
growth inhibition by malonate is to be compared to their effect in reversing the malonate-induced inhibition of respiration (Bonner, 1948).
Although the organic acids can also reverse iodoacetate growth inhibition under conditions where the potassium requirement has been satisfied, the metabolic implications of this phenomenon are largely obscure,
since iodoacetate inhibits any number of sulfhydryl-containing enzymes,
257
congruities. Under the conditions employed by Thimann and W. D.
Bonner he was able to obtain the same growth responses with inorganic
salts of potassium as with the potassium salts of the organic acids, whereas sodium salts of whatever type failed to stimulate growth similarly.
Potassium was found to stimulate growth even in the presence of
iodoacetate. The potassium salts of the organic acids, malonate included,
therefore appeared to alleviate iodoacetate inhibition when the growth
of sections in these salts was compared to the growth of iodoacetatetreated sections in water. In brief, Cooil has suggested that the apparent
response of Avena coleoptile sections to organic acids given in low concentration (0.001 M) at pH 6.5 is in fact a response to potassium ion,
a response which Thimann and Schneider (1938) had actually recognized
earlier. A further nonspecific effect of the organic acids was shown to
result from their buffering action, for the effectiveness of iodoacetate as
an inhibitor increases with a decrease in pH, and the pH drops with time
in unbuffered solutions of iodoacetate containing Avena sections.
When the potassium requirement for growth is first met, and growth
is subsequently inhibited with iodoacetate, the organic acids given at
pH 5.0 in relatively high concentration (0.01 Μ) restore growth. In such
cases malonate not only fails to stimulate growth, but rather inhibits
growth drastically. The growth inhibition educed by malonate is completely reversed by members of the tricarboxylic acid cycle. In general,
when organic acids are given as potassium salts, the response due to
potassium can be elicited at considerably lower salt concentrations, and
at a higher pH, than can the response due to the organic acid per se.
Cooil makes the point that, when a stimulation of growth is experimentally evoked in an inhibited system, it does not follow that the inhibition has been reversed. Reversal of an inhibition properly implies
a diminution in the percentage of inhibition. When growth is inhibited
by iodoacetate the subsequent growth stimulation evoked by potassium
does not represent a reversal of the inhibition, since the inhibitory effect
of iodoacetate percentagewise remains unchanged. On the other hand,
when growth is inhibited by malonate, the growth response elicited by
the organic acids represents a true reversal of the inhibition, since the
percentage inhibition of growth by malonate approaches zero in the
presence of organic acids. The effect of the organic acids in reversing
growth inhibition by malonate is to be compared to their effect in reversing the malonate-induced inhibition of respiration (Bonner, 1948).
Although the organic acids can also reverse iodoacetate growth inhibition under conditions where the potassium requirement has been satisfied, the metabolic implications of this phenomenon are largely obscure,
since iodoacetate inhibits any number of sulfhydryl-containing enzymes,