RESPIRATION RATE IN PLANTS
269
al, 1947), a fraction of the respiration in water is cyanide-sensitive, just
like the entire respiratory increment induced by salt, although a cyanideresistant residue is always evident. To test the validity of certain suggested mechanisms of salt accumulation (see below) much attention
has been given the ratio "moles of ion absorbed/moles of oxygen consumed in the salt respiration." In this regard it might be expected that
the salt respiration would be most pronounced in tissues where the
"increment type" of respiration is completely lacking until salt is provided.
An intimate relationship between the salt accumulation process and
the activity of the cytochrome system has been found in carrot disks
(Weeks and Robertson, 1950) and in wheat roots (Lundegárdh, 1953a,
1954). It consequently appears likely that in these tissues the electron
transfers of the phosphorylative oxidative systems proceed through the
cytochrome system, whereas those of the basal respiration may not. It
does not follow that all electron transport through the cytochrome oxidase system represents phosphorylative respiration; nor can it be said
at this time whether oxidative phosphorylation may occur in respiratory
systems mediated by some terminal oxidase other than cytochrome oxidase.
In barley root tips there appears to be an ascendency with age of
respiration mediated by ascorbic acid oxidase (James and Boulter, 1955).
Although cytochrome oxidase is functional in barley root tips for the first
3 or 4 days, the respiration is entirely resistant to carbon monoxide by
8 days of age, at which time ascorbic oxidase is presumably the functional terminal oxidase. However, the respiration remains cyanideinhibitable through this period. The same type of switch-over has been
reported to occur in wheat seedlings, in which cytochrome oxidase
reputedly disappears after some 70-80 hours from germination, to be
superseded by an ascorbic oxidase system (Waygood, 1950). Evidence
by Lundegárdh (1951a, 1951b) that the cytochrome system is active
during salt absorption in 2- to 3-week-old corn and cereal roots is
apparently at variance with the observation of James and Boulter and
of Waygood.
Fritz and Beevers (1955) have largely clarified the question by investigating the terminal oxidases in wheat, barley, corn, and pea seedlings as
a function of age, while distinguishing between the ontogenetic changes
in the root and shoot. The disappearance of cytochrome oxidase with
age was found to be more obvious in roots of barley and wheat than in
the shoot. However, at a time when CO-inhibition experiments would
indicate cytochrome oxidase in root tips to be lacking or nonfunctional
(James and Boulter, 1955), it could still be demonstrated in root homog-
269
al, 1947), a fraction of the respiration in water is cyanide-sensitive, just
like the entire respiratory increment induced by salt, although a cyanideresistant residue is always evident. To test the validity of certain suggested mechanisms of salt accumulation (see below) much attention
has been given the ratio "moles of ion absorbed/moles of oxygen consumed in the salt respiration." In this regard it might be expected that
the salt respiration would be most pronounced in tissues where the
"increment type" of respiration is completely lacking until salt is provided.
An intimate relationship between the salt accumulation process and
the activity of the cytochrome system has been found in carrot disks
(Weeks and Robertson, 1950) and in wheat roots (Lundegárdh, 1953a,
1954). It consequently appears likely that in these tissues the electron
transfers of the phosphorylative oxidative systems proceed through the
cytochrome system, whereas those of the basal respiration may not. It
does not follow that all electron transport through the cytochrome oxidase system represents phosphorylative respiration; nor can it be said
at this time whether oxidative phosphorylation may occur in respiratory
systems mediated by some terminal oxidase other than cytochrome oxidase.
In barley root tips there appears to be an ascendency with age of
respiration mediated by ascorbic acid oxidase (James and Boulter, 1955).
Although cytochrome oxidase is functional in barley root tips for the first
3 or 4 days, the respiration is entirely resistant to carbon monoxide by
8 days of age, at which time ascorbic oxidase is presumably the functional terminal oxidase. However, the respiration remains cyanideinhibitable through this period. The same type of switch-over has been
reported to occur in wheat seedlings, in which cytochrome oxidase
reputedly disappears after some 70-80 hours from germination, to be
superseded by an ascorbic oxidase system (Waygood, 1950). Evidence
by Lundegárdh (1951a, 1951b) that the cytochrome system is active
during salt absorption in 2- to 3-week-old corn and cereal roots is
apparently at variance with the observation of James and Boulter and
of Waygood.
Fritz and Beevers (1955) have largely clarified the question by investigating the terminal oxidases in wheat, barley, corn, and pea seedlings as
a function of age, while distinguishing between the ontogenetic changes
in the root and shoot. The disappearance of cytochrome oxidase with
age was found to be more obvious in roots of barley and wheat than in
the shoot. However, at a time when CO-inhibition experiments would
indicate cytochrome oxidase in root tips to be lacking or nonfunctional
(James and Boulter, 1955), it could still be demonstrated in root homog-