RESPIRATION RATE IN PLANTS
245
is especially true at low p 0 <» where oxygen diffusion may limit the rate
of respiration of incubated tissue while affecting the respiration rate
of fresh tissue but little. In other words, the greater relative reduction
of respiration in aged tissue by a given reduction in p Ö2 can be largely
explained by the limitations imposed by the diffusion of 0 2 into the
cells as a consequence of the increased rate of respiration.
There is seemingly a paradox between the observation by Burton
(1950) that a potato tuber 10 cm. in diameter has, at 25° C, an internal
oxygen concentration in excess of 10% throughout its bulk, and the presumption made by Steward et al. (1932) that a potato disk of thickness
much in excess of 4 mm. is essentially devoid of oxygen at its center.
Steward and co-workers have clearly shown that respiratory activity increases for unit weights of potato slices as the surface-volume ratio of
such slices is increased by cutting ever thinner disks. From experiments
such as these it was deduced that a layer of no more than 2-3 cells thick
at the surface of the disks is sufficiently aerated to support maximal
respiration and, by the same token, maximal metabolic activity. Burton
has calculated that since the diffusion coefficient of oxygen in air is
approximately 3 X 10
5 times that in water, diffusion of oxygen into
a potato tuber will be sufficiently rapid to supply the oxygen requirements of even a larger tuber if cognizance is taken of the fraction of a
per cent of intercellular air space which contributes to the diffusion
path. If in the process of making slices the minute air spaces were to
become injected with water, the diffusion coefficient for oxygen in potato
tissue would change enormously. If, as a limit, the diffusion coefficient
for oxygen were taken to be that for oxygen in water, calculations made
on the basis of Warburg's equations would indicate that the oxygen
tension in potato slices would be approximately zero not much beyond
0.3 mm. beneath the surface (Steward, 1932, 1933). However, if it is
the internal oxygen tension which primarily controls the respiration
rate of potato disks respiring in air, it would be expected that in the
range of thickness where respiration rate and thickness are inversely
related, an increase in the p 02 above that in air would increase the
respiration rate. Nevertheless, Steward (1932, 1933) observed no stimulation at oxygen concentrations in excess of 20%.
Some recent experiments (Laties, manuscript) may partly resolve the
difficulties alluded to. Laties observed that the respiration of freshly
cut potato disks was independent of thickness. The specific respiration
rate of potato cores 12 mm. in diameter and 18 mm. in length was found
to be the same as that of fresh 0.5 mm. thick disks. In fact, peeled whole
tubers rather quickly achieved a comparable respiration rate. When
relatively thin disks or large cores in water were repeatedly placed under
245
is especially true at low p 0 <» where oxygen diffusion may limit the rate
of respiration of incubated tissue while affecting the respiration rate
of fresh tissue but little. In other words, the greater relative reduction
of respiration in aged tissue by a given reduction in p Ö2 can be largely
explained by the limitations imposed by the diffusion of 0 2 into the
cells as a consequence of the increased rate of respiration.
There is seemingly a paradox between the observation by Burton
(1950) that a potato tuber 10 cm. in diameter has, at 25° C, an internal
oxygen concentration in excess of 10% throughout its bulk, and the presumption made by Steward et al. (1932) that a potato disk of thickness
much in excess of 4 mm. is essentially devoid of oxygen at its center.
Steward and co-workers have clearly shown that respiratory activity increases for unit weights of potato slices as the surface-volume ratio of
such slices is increased by cutting ever thinner disks. From experiments
such as these it was deduced that a layer of no more than 2-3 cells thick
at the surface of the disks is sufficiently aerated to support maximal
respiration and, by the same token, maximal metabolic activity. Burton
has calculated that since the diffusion coefficient of oxygen in air is
approximately 3 X 10
5 times that in water, diffusion of oxygen into
a potato tuber will be sufficiently rapid to supply the oxygen requirements of even a larger tuber if cognizance is taken of the fraction of a
per cent of intercellular air space which contributes to the diffusion
path. If in the process of making slices the minute air spaces were to
become injected with water, the diffusion coefficient for oxygen in potato
tissue would change enormously. If, as a limit, the diffusion coefficient
for oxygen were taken to be that for oxygen in water, calculations made
on the basis of Warburg's equations would indicate that the oxygen
tension in potato slices would be approximately zero not much beyond
0.3 mm. beneath the surface (Steward, 1932, 1933). However, if it is
the internal oxygen tension which primarily controls the respiration
rate of potato disks respiring in air, it would be expected that in the
range of thickness where respiration rate and thickness are inversely
related, an increase in the p 02 above that in air would increase the
respiration rate. Nevertheless, Steward (1932, 1933) observed no stimulation at oxygen concentrations in excess of 20%.
Some recent experiments (Laties, manuscript) may partly resolve the
difficulties alluded to. Laties observed that the respiration of freshly
cut potato disks was independent of thickness. The specific respiration
rate of potato cores 12 mm. in diameter and 18 mm. in length was found
to be the same as that of fresh 0.5 mm. thick disks. In fact, peeled whole
tubers rather quickly achieved a comparable respiration rate. When
relatively thin disks or large cores in water were repeatedly placed under