258
GEORGE G. LATIES
and it is not evident in many instances why organic acids should reverse
such an inhibition (Cooil, 1952). However, certain organic acids may
prevent, rather than reverse, the inactivation of sulfhydryl-containing
enzymes (Ames and Elvehjem, 1944), and since in the foregoing experiments iodoacetate was in each case given either concomitantly with, or
subsequent to, the addition of the organic acids, the protective action
of the organic acids may explain their effectiveness in precluding growth
inhibition by iodoacetate. One qualifying observation must be mentioned
respecting the generalization that the inhibitory effect of iodoacetate
upon growth is due to a selective effect of iodoacetate upon that fraction of the respiration which is particularly related to growth. Newcomb
(1954) has shown that in tobacco pith cells growth may be suppressed
by iodoacetate at the same time that the respiration is stimulated by
auxin.
Cooil's experiments clearly related growth to the operation of the tricarboxylic acid cycle in Avena. There remains the question whether this
type of respiration constitutes a fraction or all of the total respiration in
Avena. Albaum and Eichel (1943) have demonstrated that the growth
of Avena seedlings is almost unaffected by iodoacetate in the first 72
hours, but becomes increasingly susceptible to the inhibitor thereafter.
Between 72 and 96 hours (when most section growth tests are carried
out), the iodoacetate-sensitive metabolism represents a small but increasing fraction of the whole. [The coleoptile sections used by Commoner and Thimann (1941) and by Bonner (1948) appear to have been
in the transitory stage. ]
At the time that iodoacetate sensitivity becomes evident in intact
Avena seedlings, the inhibitory effect of azide becomes negligible (Albaum and Eichel, 1943). The close agreement in the degree of inhibition
of the respiration of 4-day-old Avena sections by iodoacetate on the
one hand (Commoner and Thimann, 1941), and by malonate on the
other (Bonner, 1948), suggests that the extent of iodoacetate sensitivity
reflects the contribution of the tricarboxylic acid cycle to the respiration.
At 4 days, homogenates of Avena coleoptiles still show cytochrome oxidase activity (Bonner, 1948) even though the seedlings are indifferent
to sodium azide at this time. Furthermore, as has already been indicated,
growth continues to be dependent upon a terminal oxidase which is inhibited by CO, and the inhibition of which is reversible by light. There
is therefore no compelling reason to presume that the oxidations of the
tricarboxylic acid cycle are mediated by other than the cytochrome oxidase system in Avena. The situation is akin to that in aged potato
disks where, while there is no demonstrable participation of cytochrome
oxidase in the respiration, it can be shown that growth remains dependent
GEORGE G. LATIES
and it is not evident in many instances why organic acids should reverse
such an inhibition (Cooil, 1952). However, certain organic acids may
prevent, rather than reverse, the inactivation of sulfhydryl-containing
enzymes (Ames and Elvehjem, 1944), and since in the foregoing experiments iodoacetate was in each case given either concomitantly with, or
subsequent to, the addition of the organic acids, the protective action
of the organic acids may explain their effectiveness in precluding growth
inhibition by iodoacetate. One qualifying observation must be mentioned
respecting the generalization that the inhibitory effect of iodoacetate
upon growth is due to a selective effect of iodoacetate upon that fraction of the respiration which is particularly related to growth. Newcomb
(1954) has shown that in tobacco pith cells growth may be suppressed
by iodoacetate at the same time that the respiration is stimulated by
auxin.
Cooil's experiments clearly related growth to the operation of the tricarboxylic acid cycle in Avena. There remains the question whether this
type of respiration constitutes a fraction or all of the total respiration in
Avena. Albaum and Eichel (1943) have demonstrated that the growth
of Avena seedlings is almost unaffected by iodoacetate in the first 72
hours, but becomes increasingly susceptible to the inhibitor thereafter.
Between 72 and 96 hours (when most section growth tests are carried
out), the iodoacetate-sensitive metabolism represents a small but increasing fraction of the whole. [The coleoptile sections used by Commoner and Thimann (1941) and by Bonner (1948) appear to have been
in the transitory stage. ]
At the time that iodoacetate sensitivity becomes evident in intact
Avena seedlings, the inhibitory effect of azide becomes negligible (Albaum and Eichel, 1943). The close agreement in the degree of inhibition
of the respiration of 4-day-old Avena sections by iodoacetate on the
one hand (Commoner and Thimann, 1941), and by malonate on the
other (Bonner, 1948), suggests that the extent of iodoacetate sensitivity
reflects the contribution of the tricarboxylic acid cycle to the respiration.
At 4 days, homogenates of Avena coleoptiles still show cytochrome oxidase activity (Bonner, 1948) even though the seedlings are indifferent
to sodium azide at this time. Furthermore, as has already been indicated,
growth continues to be dependent upon a terminal oxidase which is inhibited by CO, and the inhibition of which is reversible by light. There
is therefore no compelling reason to presume that the oxidations of the
tricarboxylic acid cycle are mediated by other than the cytochrome oxidase system in Avena. The situation is akin to that in aged potato
disks where, while there is no demonstrable participation of cytochrome
oxidase in the respiration, it can be shown that growth remains dependent