254
GEORGE G. LATIES
sections being hollow cylinders, the oxygen diffusion path is shorter than
that in Pisum. Thus the relatively greater sensitivity of Avena growth
to reduced oxygen pressure cannot apparently be readily explained in
terms of a more severe depletion of oxygen in Avena than in pea. However, since the actual diffusion coefficients of oxygen in these tissues are
not known, it remains possible that the diminished growth in Avena
at concentrations of oxygen below 10% is the consequence of a limit upon
diffusion. In fact, the more effective inhibition of the growth of Avena
than of the pea by low concentrations (5:1) of CO suggests that the
oxygen tension within Avena tissue is lower than that in pea. The incomplete reversibility by light of the inhibition of growth in Avena at
high CO/0 2 (20:1) may be a consequence of insufficient illumination.
Experiments using higher light intensities will tell.
Carbon monoxide inhibition studies utilizing the foregoing technique
have been particularly useful in the case of potato tissue, within which
at least two, and possibly three, terminal oxidases are known to function
(Schade and Levy, 1949; Thimann et al, 1954). The auxin-induced
water uptake of potato disks was found to be reduced at oxygen tensions
below 15%. Growth proved extraordinarily sensitive to the presence of
carbon monoxide. Thus at C0/0 2 ratios of 10:1 (2 atmospheres CO + 1
atmosphere air) water uptake was reduced 80% while at 5:1 reduction
was 48% and at 1:1 17%. This carbon monoxide inhibition of auxininduced water absorption can be reversed simply by removing the disks
from the CO atmosphere, or by illuminating them even in the presence
of CO. The reversal by light was not complete (70%), a fact for which
the authors offer in explanation the possibility that the disks were not
uniformly illuminated throughout their depth. Again, as in the case of
Avena sections, higher intensities of illumination may settle the point.
However, a prolonged illumination of potato disks in the presence of CO
results in a subsequent loss of CO sensitivity of the respiration in the
dark (Thimann et al, 1954; Levy and Schade, 1948). The gradual disappearance of part of the CO-sensitive activity may be related to the
apparent failure to achieve complete reversal of CO inhibition by light.
However, alternative suggestions are offered below.
In the potato, as in the pea and Avena, cytochrome oxidase seems to be
the active oxidase which mediates that part of the respiration which is
related to metabolic water absorption. Unfortunately, direct measurements of the effect of reduced oxygen tension and of carbon monoxide
upon the respiration of potato disks raise some questions in regard to the
interpretation of the water uptake experiments. In the work of Thimann
and co-workers (1954) which has already been described, the enzyme
assumed to be cytochrome oxidase remained fully active in fresh tissue
GEORGE G. LATIES
sections being hollow cylinders, the oxygen diffusion path is shorter than
that in Pisum. Thus the relatively greater sensitivity of Avena growth
to reduced oxygen pressure cannot apparently be readily explained in
terms of a more severe depletion of oxygen in Avena than in pea. However, since the actual diffusion coefficients of oxygen in these tissues are
not known, it remains possible that the diminished growth in Avena
at concentrations of oxygen below 10% is the consequence of a limit upon
diffusion. In fact, the more effective inhibition of the growth of Avena
than of the pea by low concentrations (5:1) of CO suggests that the
oxygen tension within Avena tissue is lower than that in pea. The incomplete reversibility by light of the inhibition of growth in Avena at
high CO/0 2 (20:1) may be a consequence of insufficient illumination.
Experiments using higher light intensities will tell.
Carbon monoxide inhibition studies utilizing the foregoing technique
have been particularly useful in the case of potato tissue, within which
at least two, and possibly three, terminal oxidases are known to function
(Schade and Levy, 1949; Thimann et al, 1954). The auxin-induced
water uptake of potato disks was found to be reduced at oxygen tensions
below 15%. Growth proved extraordinarily sensitive to the presence of
carbon monoxide. Thus at C0/0 2 ratios of 10:1 (2 atmospheres CO + 1
atmosphere air) water uptake was reduced 80% while at 5:1 reduction
was 48% and at 1:1 17%. This carbon monoxide inhibition of auxininduced water absorption can be reversed simply by removing the disks
from the CO atmosphere, or by illuminating them even in the presence
of CO. The reversal by light was not complete (70%), a fact for which
the authors offer in explanation the possibility that the disks were not
uniformly illuminated throughout their depth. Again, as in the case of
Avena sections, higher intensities of illumination may settle the point.
However, a prolonged illumination of potato disks in the presence of CO
results in a subsequent loss of CO sensitivity of the respiration in the
dark (Thimann et al, 1954; Levy and Schade, 1948). The gradual disappearance of part of the CO-sensitive activity may be related to the
apparent failure to achieve complete reversal of CO inhibition by light.
However, alternative suggestions are offered below.
In the potato, as in the pea and Avena, cytochrome oxidase seems to be
the active oxidase which mediates that part of the respiration which is
related to metabolic water absorption. Unfortunately, direct measurements of the effect of reduced oxygen tension and of carbon monoxide
upon the respiration of potato disks raise some questions in regard to the
interpretation of the water uptake experiments. In the work of Thimann
and co-workers (1954) which has already been described, the enzyme
assumed to be cytochrome oxidase remained fully active in fresh tissue