RESPIRATION RATE IN PLANTS
259
upon cytochrome oxidase activity, and the latter can be demonstrated
in homogenates. Considerably more must be known about the ontogenetic changes of respiratory pattern before the metabolic reactions
directly responsible for growth can be stated explicitly.
C. The Relation of Growth to Oxidative Phosphorylation and to
Respiration Rate
The question has already been raised regarding the nature of the
dependence of growth upon aerobic respiration. Numerous experiments
dealing with the effect of dinitrophenol on growth lead definitely to the
conclusion that the energy requirement for growth is supplied by the
aerobic respiration. Whether the aerobic metabolism supplies requisite
structural elements as well is less certain.
Dinitrophenol, which dissociates the greater part of the phosphorylative process from the oxidative reactions of the cell, has been shown
to abolish water uptake in potato (Hackett and Thimann, 1953) and
in artichoke disks (Hackett and Thimann, 1952b), and to preclude
growth in Avena sections (Bonner, 1949), pea epicotyls (Thimann,
1951), and rice coleoptiles (Yamada, 1954). Arsenate, which is an uncoupler of a different type from dinitrophenol, replaces phosphate in the
normally phosphorylative reactions (Crane and Lipmann, 1953). Subsequently the arsenylated product spontaneously undergoes hydrolysis.
In a sense, arsenate is a more effective uncoupler than dinitrophenol,
since the substrate-level phosphorylations accompanying the oxidation of
phosphoglyceraldehyde and a-ketoglutarate, which are not affected by
dinitrophenol, are susceptible to arsenolysis. In Avena, arsenate as
well as dinitrophenol inhibits growth and the respiratory increment
which is induced by indoleacetic acid and which accompanies growth.
Both inhibitions are reversed by phosphate (Bonner, 1950).
In general, auxin elicits a respiratory response only when growth is
engendered (Bonner, 1949; Commoner and Thimann, 1941; Bonner
et al, 1953). An interesting exception to this generalization is offered by
Newcomb (1954), who has shown that auxin may bring about a pronounced increase in the respiration of tobacco pith under selected conditions without causing enlargement. In Newcomb's view, a variety of
metabolic transformations, which precede and are responsible for overt
growth, bring about an increase in the respiration. The means whereby
this is accomplished is presumably by the utilization of ATP or its
equivalent in reactions leading to growth. In this way the growth process
results in the formation of the phosphate acceptor, ADP, which in turn
stimulates the respiration. Thus visible growth is in a sense a secondary
phenomenon which follows the primary events just alluded to.
259
upon cytochrome oxidase activity, and the latter can be demonstrated
in homogenates. Considerably more must be known about the ontogenetic changes of respiratory pattern before the metabolic reactions
directly responsible for growth can be stated explicitly.
C. The Relation of Growth to Oxidative Phosphorylation and to
Respiration Rate
The question has already been raised regarding the nature of the
dependence of growth upon aerobic respiration. Numerous experiments
dealing with the effect of dinitrophenol on growth lead definitely to the
conclusion that the energy requirement for growth is supplied by the
aerobic respiration. Whether the aerobic metabolism supplies requisite
structural elements as well is less certain.
Dinitrophenol, which dissociates the greater part of the phosphorylative process from the oxidative reactions of the cell, has been shown
to abolish water uptake in potato (Hackett and Thimann, 1953) and
in artichoke disks (Hackett and Thimann, 1952b), and to preclude
growth in Avena sections (Bonner, 1949), pea epicotyls (Thimann,
1951), and rice coleoptiles (Yamada, 1954). Arsenate, which is an uncoupler of a different type from dinitrophenol, replaces phosphate in the
normally phosphorylative reactions (Crane and Lipmann, 1953). Subsequently the arsenylated product spontaneously undergoes hydrolysis.
In a sense, arsenate is a more effective uncoupler than dinitrophenol,
since the substrate-level phosphorylations accompanying the oxidation of
phosphoglyceraldehyde and a-ketoglutarate, which are not affected by
dinitrophenol, are susceptible to arsenolysis. In Avena, arsenate as
well as dinitrophenol inhibits growth and the respiratory increment
which is induced by indoleacetic acid and which accompanies growth.
Both inhibitions are reversed by phosphate (Bonner, 1950).
In general, auxin elicits a respiratory response only when growth is
engendered (Bonner, 1949; Commoner and Thimann, 1941; Bonner
et al, 1953). An interesting exception to this generalization is offered by
Newcomb (1954), who has shown that auxin may bring about a pronounced increase in the respiration of tobacco pith under selected conditions without causing enlargement. In Newcomb's view, a variety of
metabolic transformations, which precede and are responsible for overt
growth, bring about an increase in the respiration. The means whereby
this is accomplished is presumably by the utilization of ATP or its
equivalent in reactions leading to growth. In this way the growth process
results in the formation of the phosphate acceptor, ADP, which in turn
stimulates the respiration. Thus visible growth is in a sense a secondary
phenomenon which follows the primary events just alluded to.