RESPIRATION RATE IN PLANTS
285
Goddard, 1951; Mapson and Moustafa, 1956). An electron transport sequence involving pyridine nucleotide, glutathione, ascorbate, and ascorbic oxidase has been described both for soluble (Mapson and Moustafa)
and mitochondrial (Young and Conn, 1956) systems.
Considerable attention has been devoted to the means whereby DPNH
or TPNH is oxidized without the participation either of glutathione or
ascorbate. In some instances electron transport has been found to
proceed through the intramitochondrial pathway which is normally associated with the oxidations of the tricarboxylic acid cycle; in other cases
atypical mitochondrial paths were traversed, while in yet other situations
oxidation was effected exclusively by nonmitochondrial systems. Conn et
al. (1952) and Humphreys (1955) have isolated TPNH oxidases from
wheat germ. Hackett (1956b) has described a DPNH oxidase in potato
mitochondria which is antimycin Α-insensitive in the presence of added
cytocrome c, and antimycin Α-sensitive in its absence. Humphreys
and Conn (1956) have reported a comparable situation in lupine mitochondria. Both Hackett (1956b) and Romberger (1956) have demonstrated a nonmitochondrial DPNH oxidase in potato and in barley roots,
respectively. Martin and Morton (1955, 1956) have studied microsomal
systems from the petioles of beet leaves which oxidize DPNH and
TPNH with the intermediation of cytochrome b 3 . One of the major
enzymatic steps leading to the reduction of TPN in plant tissues has been
investigated by Gibbs (1954), Beevers and Gibbs (1954), and Gibbs and
Beevers (1955), who found that in many mature tissues as much as 50%
of the respiration proceeds by way of the direct oxidation pathway of
hexose, i.e., by way of the TPN-dependent oxidation of glucose-6phosphate (pentose cycle, hexose monophosphate shunt; see Cohen,
1954, for an extensive review of the alternative pathways of carbohydrate metabolism).
In brief, there are many ways in which substrate oxidation may be
linked to the utilization of molecular oxygen. However, except for the
cytochrome-oxidase-mediated, phosphorylative systems already discussed, little if anything is known of the rate-regulating and energycoupling mechanisms. It will be of great interest to determine whether
oxidative phosphorylation accompanies some of the alternative systems
which have been mentioned.
Finally, there is the problem of changes in metabolic patterns during
ontogeny. Several instances have been cited in which a qualitative as
well as a quantitative change occurs as tissues or organs develop. James
(1953b) discusses these matters at length. Early reports by Marsh and
Goddard (1939) indicated that in developing carrot leaves, a cyanidesensitive respiratory system is replaced by a cyanide-resistant system
285
Goddard, 1951; Mapson and Moustafa, 1956). An electron transport sequence involving pyridine nucleotide, glutathione, ascorbate, and ascorbic oxidase has been described both for soluble (Mapson and Moustafa)
and mitochondrial (Young and Conn, 1956) systems.
Considerable attention has been devoted to the means whereby DPNH
or TPNH is oxidized without the participation either of glutathione or
ascorbate. In some instances electron transport has been found to
proceed through the intramitochondrial pathway which is normally associated with the oxidations of the tricarboxylic acid cycle; in other cases
atypical mitochondrial paths were traversed, while in yet other situations
oxidation was effected exclusively by nonmitochondrial systems. Conn et
al. (1952) and Humphreys (1955) have isolated TPNH oxidases from
wheat germ. Hackett (1956b) has described a DPNH oxidase in potato
mitochondria which is antimycin Α-insensitive in the presence of added
cytocrome c, and antimycin Α-sensitive in its absence. Humphreys
and Conn (1956) have reported a comparable situation in lupine mitochondria. Both Hackett (1956b) and Romberger (1956) have demonstrated a nonmitochondrial DPNH oxidase in potato and in barley roots,
respectively. Martin and Morton (1955, 1956) have studied microsomal
systems from the petioles of beet leaves which oxidize DPNH and
TPNH with the intermediation of cytochrome b 3 . One of the major
enzymatic steps leading to the reduction of TPN in plant tissues has been
investigated by Gibbs (1954), Beevers and Gibbs (1954), and Gibbs and
Beevers (1955), who found that in many mature tissues as much as 50%
of the respiration proceeds by way of the direct oxidation pathway of
hexose, i.e., by way of the TPN-dependent oxidation of glucose-6phosphate (pentose cycle, hexose monophosphate shunt; see Cohen,
1954, for an extensive review of the alternative pathways of carbohydrate metabolism).
In brief, there are many ways in which substrate oxidation may be
linked to the utilization of molecular oxygen. However, except for the
cytochrome-oxidase-mediated, phosphorylative systems already discussed, little if anything is known of the rate-regulating and energycoupling mechanisms. It will be of great interest to determine whether
oxidative phosphorylation accompanies some of the alternative systems
which have been mentioned.
Finally, there is the problem of changes in metabolic patterns during
ontogeny. Several instances have been cited in which a qualitative as
well as a quantitative change occurs as tissues or organs develop. James
(1953b) discusses these matters at length. Early reports by Marsh and
Goddard (1939) indicated that in developing carrot leaves, a cyanidesensitive respiratory system is replaced by a cyanide-resistant system