RESPIRATION RATE IN PLANTS
267
rather than presuming that the activity of a certain respiratory system
increases inordinately through the zone of differentiation in the root, it
is considered that two respiration systems coexist in the root tip, and
that the prevalence of one system is sharply decreased within the first
10 mm. or so from the apex, then the extraordinary increase in specific
respiration rate becomes simply an arithmetic artifact. In this view a
respiratory system, most prevalent in the apical cells of the root, and
depending upon, or being proportional to, a major component of the
cold TCA-insoluble phosphate, exists together with a basal respiratory
system the activity or quantity of which bears no relationship to the
cold TCA-insoluble phosphate. Honda's data, interpreted in this way,
reaffirm the contention that more than one respiration system exists in
the roots of the cereal plants (Lundegárdh, 1954).
At least two additional observations make the previous interpretation
attractive. Honda found that the respiratory increment evoked by dinitrophenol, when expressed per unit of insoluble phosphate, was constant throughout the root. It thus appears that the type of respiration
which is correlated to the concentration of cold TCA-insoluble phosphate is also that which responds to dinitrophenol. Furthermore, since
the salt-induced and the dinitrophenol-induced respiration appear to
have the same characteristics as compared to the basal respiration (Robertson et al., 1951), it seems likely that the respiration system stimulated
by salt is the same as that which responds to dinitrophenol. Finally,
Lund et al. (1956) have made the provocative and pertinent observation that in corn roots the number of microsomes decreases while the
number of mitochondria increases as one proceeds distally from the tip.
The microsomes in all probability contain the bulk of the cold TCAinsoluble phosphate in plant tissues (Ts'o et al., 1956).
In summary, salt effects may conceivably be of two kinds, not always
readily distinguishable. There is no question that salts may stimulate
respiration independently of the absorption process (Honda, 1956;
Handley and Overstreet, 1955). That the effect of salt is most frequently
independent of the absorption process is suggested by the following observations: (1) The salt-stimulated respiration persists in the absence of
salt (Milthorpe and Robertson, 1948; Handley and Overstreet, 1955).
(2) Maximum salt respiration is educed at one-tenth or less of the salt
concentration required for maximum salt absorption (Robertson and
Wilkins, 1948). In this regard it should be noted that early experiments
linking salt absorption to salt respiration (Lundgárdh, 1937) were carried out in the low range of salt concentrations where salt respiration
and absorption are approximately proportional. (3) Stimulation of the
respiration by salt occurs in the presence of dinitrophenol, which abol-
267
rather than presuming that the activity of a certain respiratory system
increases inordinately through the zone of differentiation in the root, it
is considered that two respiration systems coexist in the root tip, and
that the prevalence of one system is sharply decreased within the first
10 mm. or so from the apex, then the extraordinary increase in specific
respiration rate becomes simply an arithmetic artifact. In this view a
respiratory system, most prevalent in the apical cells of the root, and
depending upon, or being proportional to, a major component of the
cold TCA-insoluble phosphate, exists together with a basal respiratory
system the activity or quantity of which bears no relationship to the
cold TCA-insoluble phosphate. Honda's data, interpreted in this way,
reaffirm the contention that more than one respiration system exists in
the roots of the cereal plants (Lundegárdh, 1954).
At least two additional observations make the previous interpretation
attractive. Honda found that the respiratory increment evoked by dinitrophenol, when expressed per unit of insoluble phosphate, was constant throughout the root. It thus appears that the type of respiration
which is correlated to the concentration of cold TCA-insoluble phosphate is also that which responds to dinitrophenol. Furthermore, since
the salt-induced and the dinitrophenol-induced respiration appear to
have the same characteristics as compared to the basal respiration (Robertson et al., 1951), it seems likely that the respiration system stimulated
by salt is the same as that which responds to dinitrophenol. Finally,
Lund et al. (1956) have made the provocative and pertinent observation that in corn roots the number of microsomes decreases while the
number of mitochondria increases as one proceeds distally from the tip.
The microsomes in all probability contain the bulk of the cold TCAinsoluble phosphate in plant tissues (Ts'o et al., 1956).
In summary, salt effects may conceivably be of two kinds, not always
readily distinguishable. There is no question that salts may stimulate
respiration independently of the absorption process (Honda, 1956;
Handley and Overstreet, 1955). That the effect of salt is most frequently
independent of the absorption process is suggested by the following observations: (1) The salt-stimulated respiration persists in the absence of
salt (Milthorpe and Robertson, 1948; Handley and Overstreet, 1955).
(2) Maximum salt respiration is educed at one-tenth or less of the salt
concentration required for maximum salt absorption (Robertson and
Wilkins, 1948). In this regard it should be noted that early experiments
linking salt absorption to salt respiration (Lundgárdh, 1937) were carried out in the low range of salt concentrations where salt respiration
and absorption are approximately proportional. (3) Stimulation of the
respiration by salt occurs in the presence of dinitrophenol, which abol-