RESPIRATION RATE IN PLANTS
255
at concentrations of oxygen down to a fraction of a per cent. In addition,
CO produced no more than a 45% inhibition of respiration at a ratio of
CO/0 2 of 9:1, and approximately a 70% inhibition at 19:1. Finally, it
was found that in as short a time as 24 hours at room temperature, the
CO-sensitive oxidase of high oxygen affinity mediating the respiration
gave way to a CO-insensitive oxidase of equally high oxygen affinity, in
spite of the fact that growth remained sensitive to CO. The experiments
involving water uptake, which take from 4 to 6 days to complete, must
be interpreted in conjunction with these known respiratory changes.
The effectiveness of CO in depressing metabolic water uptake at concentrations lower than those which inhibit respiration to the same extent
may perhaps be related to the action of CO in eliciting aerobic fermentation in green leaves having a CO-insensitive respiration (Ducet and
Rosenberg, 1953). Daly (1954) has reported even a stimulation of
respiration by CO in the plum leaf. If CO should prove able to uncouple
phosphorylation from respiration, it would not be surprising to observe
profound effects on water absorption produced even by low concentrations of CO.
The effect of a lowered partial pressure of oxygen on water uptake
is more enigmatic. It is possible that, although cytochrome oxidase
activity is intimately related to the metabolic water absorption, additional
oxidative components might also be concerned. For example, potato
homogenates can oxidize ascorbate with an activity which increases with
age; and ascorbic oxidase has been shown to require a relatively high
concentration of oxygen for maximal activity (Thimann, Yocum, and
Hackett). Newcomb (1954) has shown that ascorbic acid oxidase in
particular increases in concentration dramatically in tobacco pith cells
in conjunction with auxin-induced growth. It is not ruled out that the
greater sensitivity of growth to CO as compared to the respiration may
reflect the effect of CO on a heavy-metal-containing enzyme involved
in growth through means other than the respiration. This suggestion
may perhaps be considered with those previously offered in explanation
of the incomplete reversal of CO inhibition by light in some experiments. Yet another explanation may be offered. In growth studies the
respiration rate of 4 to 6-day-old potato disks is several times the respiration rate of the fresh disks which have perforce been used to determine
the oxygen-affinity characteristics of the CO-sensitive terminal oxidase.
Thus the oxygen concentration in aged disks is undoubtedly lower than
in fresh disks, and consequently the effectiveness of a given concentration of CO in causing inhibition would be greater in the older tissue.
255
at concentrations of oxygen down to a fraction of a per cent. In addition,
CO produced no more than a 45% inhibition of respiration at a ratio of
CO/0 2 of 9:1, and approximately a 70% inhibition at 19:1. Finally, it
was found that in as short a time as 24 hours at room temperature, the
CO-sensitive oxidase of high oxygen affinity mediating the respiration
gave way to a CO-insensitive oxidase of equally high oxygen affinity, in
spite of the fact that growth remained sensitive to CO. The experiments
involving water uptake, which take from 4 to 6 days to complete, must
be interpreted in conjunction with these known respiratory changes.
The effectiveness of CO in depressing metabolic water uptake at concentrations lower than those which inhibit respiration to the same extent
may perhaps be related to the action of CO in eliciting aerobic fermentation in green leaves having a CO-insensitive respiration (Ducet and
Rosenberg, 1953). Daly (1954) has reported even a stimulation of
respiration by CO in the plum leaf. If CO should prove able to uncouple
phosphorylation from respiration, it would not be surprising to observe
profound effects on water absorption produced even by low concentrations of CO.
The effect of a lowered partial pressure of oxygen on water uptake
is more enigmatic. It is possible that, although cytochrome oxidase
activity is intimately related to the metabolic water absorption, additional
oxidative components might also be concerned. For example, potato
homogenates can oxidize ascorbate with an activity which increases with
age; and ascorbic oxidase has been shown to require a relatively high
concentration of oxygen for maximal activity (Thimann, Yocum, and
Hackett). Newcomb (1954) has shown that ascorbic acid oxidase in
particular increases in concentration dramatically in tobacco pith cells
in conjunction with auxin-induced growth. It is not ruled out that the
greater sensitivity of growth to CO as compared to the respiration may
reflect the effect of CO on a heavy-metal-containing enzyme involved
in growth through means other than the respiration. This suggestion
may perhaps be considered with those previously offered in explanation
of the incomplete reversal of CO inhibition by light in some experiments. Yet another explanation may be offered. In growth studies the
respiration rate of 4 to 6-day-old potato disks is several times the respiration rate of the fresh disks which have perforce been used to determine
the oxygen-affinity characteristics of the CO-sensitive terminal oxidase.
Thus the oxygen concentration in aged disks is undoubtedly lower than
in fresh disks, and consequently the effectiveness of a given concentration of CO in causing inhibition would be greater in the older tissue.