250
GEORGE G. LATIES
pletely sensitive to malonate. However, whereas the addition of malate to
fresh tissue, either in the presence or absence of malonate, with or without
dinitrophenol, was completely without effect, the addition of malate to
malonate-inhibited, incubated tissue restored the rate to the noninhibited
level. Thus, freshly prepared disks were unable to oxidize malate, while
aged disks were able to oxidize it very well. Interestingly enough, mitochondrial preparations from both fresh and aged tissue snowed precisely
the same oxidative activity towards malate. It seems that either some
cofactor of oxidation, or some ancillary enzyme system concerned with
malate oxidation other than by electron transfer, must develop in the
presence of air, at room temperature, during aging. The attainment of a
maximum rate of respiration in potato and carrot is thus demonstrably
limited by systems having to do with phosphorylation or phosphate transfer. The maximum rate is not determined by the substrate level. However, there is evidence that, in addition to the removal of a limitation
on a rate determinant, there arises a qualitatively distinct component of
the respiration—a component presumably more intimately associated
with phosphorylative metabolism than is the initial basal respiration.
The question remains why the changes observed upon incubation of
slices fail to occur within the intact root. Subdivision of the organ is a
clear prerequisite for the change to occur, for no alteration of the
metabolic pattern occurs when whole roots are kept at room temperature.
With potato, at least, the oxygen concentration within the tuber is known
to be high enough to support maximal activity of the cytochrome oxidase
system. To explain the failure of the respiration to increase within the
tuber at room temperature, one must postulate either that a volatile
metabolic product is in fact inhibitory to the changes which must occur,
or that the changes are consequences of the activity of metabolic systems
which proceed through some terminal oxidase other than cytochrome
oxidase, some terminal oxidase whose activity is in fact limited by the
reduced partial pressure of oxygen within the tissue. The respiratory
behavior of intact potato tubers as a function of po 2 (Denny, 1947)
makes the second alternative unlikely.
2. Growth and Water Uptake
Although there is considerable diversity of opinion about the mechanism whereby higher plant cells increase in size, few would question that
this increase depends upon metabolic processes. Metabolic water absorption may or may not be "active" water absorption. The latter term, which
refers to the nonosmotic movement of water (Levitt, 1953), has a precise
thermodynamic meaning—it is the movement of water against a water
chemical potential gradient. Thus, whether water enters the cell osmo-
GEORGE G. LATIES
pletely sensitive to malonate. However, whereas the addition of malate to
fresh tissue, either in the presence or absence of malonate, with or without
dinitrophenol, was completely without effect, the addition of malate to
malonate-inhibited, incubated tissue restored the rate to the noninhibited
level. Thus, freshly prepared disks were unable to oxidize malate, while
aged disks were able to oxidize it very well. Interestingly enough, mitochondrial preparations from both fresh and aged tissue snowed precisely
the same oxidative activity towards malate. It seems that either some
cofactor of oxidation, or some ancillary enzyme system concerned with
malate oxidation other than by electron transfer, must develop in the
presence of air, at room temperature, during aging. The attainment of a
maximum rate of respiration in potato and carrot is thus demonstrably
limited by systems having to do with phosphorylation or phosphate transfer. The maximum rate is not determined by the substrate level. However, there is evidence that, in addition to the removal of a limitation
on a rate determinant, there arises a qualitatively distinct component of
the respiration—a component presumably more intimately associated
with phosphorylative metabolism than is the initial basal respiration.
The question remains why the changes observed upon incubation of
slices fail to occur within the intact root. Subdivision of the organ is a
clear prerequisite for the change to occur, for no alteration of the
metabolic pattern occurs when whole roots are kept at room temperature.
With potato, at least, the oxygen concentration within the tuber is known
to be high enough to support maximal activity of the cytochrome oxidase
system. To explain the failure of the respiration to increase within the
tuber at room temperature, one must postulate either that a volatile
metabolic product is in fact inhibitory to the changes which must occur,
or that the changes are consequences of the activity of metabolic systems
which proceed through some terminal oxidase other than cytochrome
oxidase, some terminal oxidase whose activity is in fact limited by the
reduced partial pressure of oxygen within the tissue. The respiratory
behavior of intact potato tubers as a function of po 2 (Denny, 1947)
makes the second alternative unlikely.
2. Growth and Water Uptake
Although there is considerable diversity of opinion about the mechanism whereby higher plant cells increase in size, few would question that
this increase depends upon metabolic processes. Metabolic water absorption may or may not be "active" water absorption. The latter term, which
refers to the nonosmotic movement of water (Levitt, 1953), has a precise
thermodynamic meaning—it is the movement of water against a water
chemical potential gradient. Thus, whether water enters the cell osmo-