RESPIRATION RATE IN PLANTS
253
to 1) which would bring about an effective inhibition of the enzyme.
As long as the determination of CO sensitivity at the same time involved
a diminution of the partial pressure of oxygen, a persistent ambiguity
remained with respect to the nature of the existing oxidases in the
tissue, and as to the extent of their relative contribution to the over-all
respiration. Although cytochrome oxidase in vitro functions optimally
at oxygen pressures well below that of air, it is frequently difficult to
establish unequivocally that the oxygen concentration throughout a
tissue is in fact sufficient for maximal cytochrome oxidase activity.
Whether the internal partial pressure of oxygen in tissue exposed to air
is sufficient for maximal cytochrome oxidase activity or not, it is obvious
that the CO sensitivity (95% CO, 5% 0 2 ) of that part of the respiration
which is largely abolished by lowering the oxygen concentration to 5%
cannot be determined.
Hackett and Schneiderman (1953), and Hackett et al. (1953), have
neatly resolved the above difficulty in a series of experiments wherein
CO inhibition was affected under high pressure, in a gas mixture comprised of 1 atmosphere of air and from 2 to 4 atmospheres of CO. Thus
CO/0 2 ratios of 20:1 were achieved without lowering the partial pressure of oxygen below that in air. To test the reversibility by light of
the inhibition, illumination was provided directly through the transparent lucite walls of the pressure chamber.
Growth of both Avena coleoptile sections and of Pisum epicotyl sections was shown to be inhibited by some 90% at CO/0 2 ratios of 20:1,
and by about 75% at CO/0 2 ratios of 10:1. An appreciable growth inhibition occurred even at ratios as low as 5:1. In peas, growth inhibition
was completely reversed by light, whereas in Avena sections complete
reversal by light was obtained at ratios of 10:1, and about a 60% reversal
at 20:1. Growth in Avena was inhibited 70% by 4.6 Χ ΙΟ"
4 Μ cyanide.
Such philo-copper inhibitors as thiourea and thiouracil caused little or no
inhibition.
The growth of Pisum proved to be independent of oxygen pressure at
oxygen concentrations even as low as 5%, whereas the growth of Avena
sections was affected by lowering the concentration of oxygen below 10%.
Growth of Avena was reduced 60% at 0.05 atmosphere oxygen. In mixtures of air plus 2 atmospheres of oxygen, neither pea nor Avena showed
any additional growth over the amount taking place in air.
The evidence is compelling that the respiratory metabolism responsible for growth in pea epicotyls is mediated by cytochrome oxidase. The
likelihood is great that in Avena sections the same situation obtains, but
the experimental evidence is somewhat less certain. The specific respiration rate of Avena is, if anything, less than that of Pisum and, coleoptile
253
to 1) which would bring about an effective inhibition of the enzyme.
As long as the determination of CO sensitivity at the same time involved
a diminution of the partial pressure of oxygen, a persistent ambiguity
remained with respect to the nature of the existing oxidases in the
tissue, and as to the extent of their relative contribution to the over-all
respiration. Although cytochrome oxidase in vitro functions optimally
at oxygen pressures well below that of air, it is frequently difficult to
establish unequivocally that the oxygen concentration throughout a
tissue is in fact sufficient for maximal cytochrome oxidase activity.
Whether the internal partial pressure of oxygen in tissue exposed to air
is sufficient for maximal cytochrome oxidase activity or not, it is obvious
that the CO sensitivity (95% CO, 5% 0 2 ) of that part of the respiration
which is largely abolished by lowering the oxygen concentration to 5%
cannot be determined.
Hackett and Schneiderman (1953), and Hackett et al. (1953), have
neatly resolved the above difficulty in a series of experiments wherein
CO inhibition was affected under high pressure, in a gas mixture comprised of 1 atmosphere of air and from 2 to 4 atmospheres of CO. Thus
CO/0 2 ratios of 20:1 were achieved without lowering the partial pressure of oxygen below that in air. To test the reversibility by light of
the inhibition, illumination was provided directly through the transparent lucite walls of the pressure chamber.
Growth of both Avena coleoptile sections and of Pisum epicotyl sections was shown to be inhibited by some 90% at CO/0 2 ratios of 20:1,
and by about 75% at CO/0 2 ratios of 10:1. An appreciable growth inhibition occurred even at ratios as low as 5:1. In peas, growth inhibition
was completely reversed by light, whereas in Avena sections complete
reversal by light was obtained at ratios of 10:1, and about a 60% reversal
at 20:1. Growth in Avena was inhibited 70% by 4.6 Χ ΙΟ"
4 Μ cyanide.
Such philo-copper inhibitors as thiourea and thiouracil caused little or no
inhibition.
The growth of Pisum proved to be independent of oxygen pressure at
oxygen concentrations even as low as 5%, whereas the growth of Avena
sections was affected by lowering the concentration of oxygen below 10%.
Growth of Avena was reduced 60% at 0.05 atmosphere oxygen. In mixtures of air plus 2 atmospheres of oxygen, neither pea nor Avena showed
any additional growth over the amount taking place in air.
The evidence is compelling that the respiratory metabolism responsible for growth in pea epicotyls is mediated by cytochrome oxidase. The
likelihood is great that in Avena sections the same situation obtains, but
the experimental evidence is somewhat less certain. The specific respiration rate of Avena is, if anything, less than that of Pisum and, coleoptile