242
GEORGE G. LATIES
and Denny (in Goddard and Meeuse, 1950) have suggested carbon dioxide as the agent responsible for the suppression of respiration in intact
tubers and fleshy roots. Richards, as early as 1896, was of the same
opinion. However, other possibilities will be broached in later pages.
The respiration of a variety of intact organs (leaves, fruits, tubers) is
found to diminish as the partial pressure of oxygen is lowered below that
in air. With intact organs the relation of respiration rate to oxygen
tension may frequently be depicted as one arm of a parabola, with the
rate in air representing an apparent near maximum (see James, 1953a).
The parabolic relationship is considered to reflect the combination of an
enzyme (a terminal oxidase) with a substrate (molecular oxygen), in
the manner described by Michaelis and Menten (see Wilson, 1949).
With respect to those organs thought to contain a terminal oxidase with
a high affinity for oxygen (i.e., an oxidase which is saturated with oxygen
at low po 2 ) the question arises why respiration is diminished by oxygen
decrements at relatively high oxygen tensions. James (1953a) has suggested that the relative impermeability to oxygen of the tuber periderm,
fruit skin, or seedling epidermis results in an oxygen tension within the
organ which, although proportional to the external oxygen pressure, is
yet considerably lower. Burton (1950) has shown that the diffusion
resistance of a potato tuber to oxygen is almost entirely attributable to
the skin.
In spite of the diffusion barrier presented by the peripheral tissue
layers, an analysis of the gaseous atmosphere in potato (Burton, 1950),
apple (Hulme, 1951), and beet (Scott, in Goddard and Meeuse, 1950),
has shown the oxygen concentration in these organs to be at least 10%
in most instances. Cytochrome oxidase, a terminal oxidase saturated at
partial pressures of oxygen below 0.3% (Winzler, 1941), is of almost
ubiquitous occurrence in plant tissues (Webster, 1952, 1954). It is evident that the oxygen concentration in most fleshy organs is far in excess
of that necessary for the full activity of cytochrome oxidase. Thus, if
the respiration of an organ is known to decrease when the oxygen concentration is lowered below that in air, while at the same time the po 2
within the tissue can be demonstrated to be well above that required for
full cytochrome oxidase activity, it must follow that one or more oxidases
more sensitive than cytochrome oxidase to low po 2 mediate a portion of
the respiration. Apple fruit respiration declines with a drop in p 02 below
that in air (see James, 1953a) while potato tuber is relatively indifferent
to changes in p 0o down to 6% (Denny, 1947; Choudhury, 1939). It follows from what has already been said that, when the respiration rate
of fresh slices is found to be 3 to 10 times that of the intact tubers, and
the oxygen tension within slices is at best but double that in the intact
GEORGE G. LATIES
and Denny (in Goddard and Meeuse, 1950) have suggested carbon dioxide as the agent responsible for the suppression of respiration in intact
tubers and fleshy roots. Richards, as early as 1896, was of the same
opinion. However, other possibilities will be broached in later pages.
The respiration of a variety of intact organs (leaves, fruits, tubers) is
found to diminish as the partial pressure of oxygen is lowered below that
in air. With intact organs the relation of respiration rate to oxygen
tension may frequently be depicted as one arm of a parabola, with the
rate in air representing an apparent near maximum (see James, 1953a).
The parabolic relationship is considered to reflect the combination of an
enzyme (a terminal oxidase) with a substrate (molecular oxygen), in
the manner described by Michaelis and Menten (see Wilson, 1949).
With respect to those organs thought to contain a terminal oxidase with
a high affinity for oxygen (i.e., an oxidase which is saturated with oxygen
at low po 2 ) the question arises why respiration is diminished by oxygen
decrements at relatively high oxygen tensions. James (1953a) has suggested that the relative impermeability to oxygen of the tuber periderm,
fruit skin, or seedling epidermis results in an oxygen tension within the
organ which, although proportional to the external oxygen pressure, is
yet considerably lower. Burton (1950) has shown that the diffusion
resistance of a potato tuber to oxygen is almost entirely attributable to
the skin.
In spite of the diffusion barrier presented by the peripheral tissue
layers, an analysis of the gaseous atmosphere in potato (Burton, 1950),
apple (Hulme, 1951), and beet (Scott, in Goddard and Meeuse, 1950),
has shown the oxygen concentration in these organs to be at least 10%
in most instances. Cytochrome oxidase, a terminal oxidase saturated at
partial pressures of oxygen below 0.3% (Winzler, 1941), is of almost
ubiquitous occurrence in plant tissues (Webster, 1952, 1954). It is evident that the oxygen concentration in most fleshy organs is far in excess
of that necessary for the full activity of cytochrome oxidase. Thus, if
the respiration of an organ is known to decrease when the oxygen concentration is lowered below that in air, while at the same time the po 2
within the tissue can be demonstrated to be well above that required for
full cytochrome oxidase activity, it must follow that one or more oxidases
more sensitive than cytochrome oxidase to low po 2 mediate a portion of
the respiration. Apple fruit respiration declines with a drop in p 02 below
that in air (see James, 1953a) while potato tuber is relatively indifferent
to changes in p 0o down to 6% (Denny, 1947; Choudhury, 1939). It follows from what has already been said that, when the respiration rate
of fresh slices is found to be 3 to 10 times that of the intact tubers, and
the oxygen tension within slices is at best but double that in the intact