RESPIRATION RATE IN PLANTS
241
constituting a mere 1% or less of the entire tuber volume. One must
thus rule out the possibility that, although the concentration of oxygen
in the gas phase of the potato is frequently little lower than that of air,
the rate of supply of oxygen to the cells may be limiting.
The data of Burton emphasize the enormous effect upon tissue aeration of what appears to be a comparatively trivial gas space in a fleshy
root or tuber. Thus on the basis of a minimal estimate of the gas space
in potato, namely, 0.62% of the total volume, the oxygen pressure in the
center of a tuber of 5 cm. radius, respiring at a rate of 5.0 /xl./g./hour,
would be but 2.5% less than that occurring immediately below the skin
—whereas in the absence of such a gas space, the tissue might be expected to be devoid of oxygen at a depth of 2 mm. These calculations
of Burton s should emphasize the danger of promiscuously substituting
the diffusion coefficient of oxygen in water, or in watery solutions, into
the expressions derived by Warburg and others (see Goddard, 1946)
for the oxygen tension to be expected within tissues of different sizes,
shapes, and respiratory intensity. It must also be reemphasized that
expressions such as that derived by Warburg are valid only when it is
known that the respiratory rate of the tissue is constant throughout its
mass. It is clearly not permissible to measure the respiratory rate of a
whole tuber, or fleshy root, and to insert the specific respiratory value
thus obtained back into the expression designed to determine the partial
pressure of oxygen at any given depth within the tissue, unless the stated
criteria are first known to be met.
Burton has demonstrated that the respiratory activity of intact potatoes
remains unaffected when the partial pressure of oxygen is increased
above that in air. What is more, the respiration of intact potato tubers in
air can be increased fivefold almost at once by gamma irradiation (Sussman, 1953). Similarly, Hackney (1944) has shown that the respiration
of apple fruits in certain stages of development is the same whether the
fruits are in air or in pure oxygen. Since fresh slices from the above
organs in each case respired at a greater rate than a comparable mass
of tissue in bulk (see also Pearson and Robertson, 1954), it follows that
the increase in respiratory rate in slices is not attributable to an increase
in oxygen tension. In further affirmation of this contention is the observation that r during the climacteric rise in respiration of apple fruit (see
below), the specific respiration rate of the fruit approaches that of
apple slices (Pearson and Robertson, 1952), albeit the skin is less permeable to oxygen than previously (Hackney, 1944). Goddard and Meeuse
(1950) have reviewed the numerous examples wherein the respiration
of fleshy organs has been stimulated by first sweeping out the internal
gases by passing an inert gas through borings made in the organ. Scott,
241
constituting a mere 1% or less of the entire tuber volume. One must
thus rule out the possibility that, although the concentration of oxygen
in the gas phase of the potato is frequently little lower than that of air,
the rate of supply of oxygen to the cells may be limiting.
The data of Burton emphasize the enormous effect upon tissue aeration of what appears to be a comparatively trivial gas space in a fleshy
root or tuber. Thus on the basis of a minimal estimate of the gas space
in potato, namely, 0.62% of the total volume, the oxygen pressure in the
center of a tuber of 5 cm. radius, respiring at a rate of 5.0 /xl./g./hour,
would be but 2.5% less than that occurring immediately below the skin
—whereas in the absence of such a gas space, the tissue might be expected to be devoid of oxygen at a depth of 2 mm. These calculations
of Burton s should emphasize the danger of promiscuously substituting
the diffusion coefficient of oxygen in water, or in watery solutions, into
the expressions derived by Warburg and others (see Goddard, 1946)
for the oxygen tension to be expected within tissues of different sizes,
shapes, and respiratory intensity. It must also be reemphasized that
expressions such as that derived by Warburg are valid only when it is
known that the respiratory rate of the tissue is constant throughout its
mass. It is clearly not permissible to measure the respiratory rate of a
whole tuber, or fleshy root, and to insert the specific respiratory value
thus obtained back into the expression designed to determine the partial
pressure of oxygen at any given depth within the tissue, unless the stated
criteria are first known to be met.
Burton has demonstrated that the respiratory activity of intact potatoes
remains unaffected when the partial pressure of oxygen is increased
above that in air. What is more, the respiration of intact potato tubers in
air can be increased fivefold almost at once by gamma irradiation (Sussman, 1953). Similarly, Hackney (1944) has shown that the respiration
of apple fruits in certain stages of development is the same whether the
fruits are in air or in pure oxygen. Since fresh slices from the above
organs in each case respired at a greater rate than a comparable mass
of tissue in bulk (see also Pearson and Robertson, 1954), it follows that
the increase in respiratory rate in slices is not attributable to an increase
in oxygen tension. In further affirmation of this contention is the observation that r during the climacteric rise in respiration of apple fruit (see
below), the specific respiration rate of the fruit approaches that of
apple slices (Pearson and Robertson, 1952), albeit the skin is less permeable to oxygen than previously (Hackney, 1944). Goddard and Meeuse
(1950) have reviewed the numerous examples wherein the respiration
of fleshy organs has been stimulated by first sweeping out the internal
gases by passing an inert gas through borings made in the organ. Scott,