RESPIRATION RATE IN PLANTS
243
tuber, it is clear, in view of the parabolic relation between respiration
rate and oxygen concentration, that the increased oxygen tension cannot
really be the cause of the respiratory increase.
Experiments designed to elucidate the nature of the terminal oxidases
participating in the respiration of bulky tissues have been carried out
primarily with thin slices in order to minimize the uncertainty as to the
composition of the gas phase in the intact material. It has been no easy
matter to ascertain the relative contribution of a particular oxidase to the
total respiration in tissues such as apple (Hackney, 1949a, b,) or potato
(Schade and Levy, 1949; Thimann et al., 1954), which are known to
contain at least three different terminal oxidases. Clearly, to demonstrate
that a given oxidase is active in a homogenate of a certain tissue fails
to indicate whether that oxidase functions at all in vivo, let alone to what
extent it may function.
Schade and Levy (1949), Thimann et al. (1954), and Hackett (1956b)
have investigated the nature of the functional terminal oxidase in potato
slices. Hackett (1956a), and W. D. Bonner and Yocum (1956), have
examined terminal electron pathways in the spadix of skunk cabbage,
while W. D. Bonner (1956) has done the same for mung bean seedlings.
In potato slices the oxidase characteristics change dramatically with time
from cutting. The change is temperature-dependent and cyanide-sensitive. Surprisingly, it occurs in the presence of dinitrophenol (Hackett,
1956b). Although at first the respiration is predominantly sensitive to
carbon monoxide and to cyanide, after slices have been incubated for
24 hours the respiration exhibits little or no carbon monoxide sensitivity,
and considerably diminished cyanide sensitivity (Levy and Schade, 1948;
Thimann et al., 1954). The experiments by Thimann and co-workers
designed to test CO sensitivity were elegantly carried out under conditions where the high ratios of CO to 0 2 necessary for the demonstration
of CO inhibition were achieved by introducing CO under pressure, instead of by mixing CO with air at atmospheric pressure and thereby
lowering the partial pressure of oxygen. Thus the effect of CO was
clearly separated from the effect of reduced oxygen pressure. In all cases
inhibition by CO was found to be light-reversible.
Regarding the effect of oxygen concentration on the respiration of potato slices, Thimann, Yocum, and Hackett observed that the oxygen
tension in the environment could be sharply reduced without an appreciable effect upon the respiration. With fresh slices, lowering the
p 0 o to 6.5% failed to affect the respiration, while lowering the oxygen
concentration to 1% decreased respiration by but 34%. Significantly, aged
tissue displayed the same characteristics as fresh tissue with respect to
243
tuber, it is clear, in view of the parabolic relation between respiration
rate and oxygen concentration, that the increased oxygen tension cannot
really be the cause of the respiratory increase.
Experiments designed to elucidate the nature of the terminal oxidases
participating in the respiration of bulky tissues have been carried out
primarily with thin slices in order to minimize the uncertainty as to the
composition of the gas phase in the intact material. It has been no easy
matter to ascertain the relative contribution of a particular oxidase to the
total respiration in tissues such as apple (Hackney, 1949a, b,) or potato
(Schade and Levy, 1949; Thimann et al., 1954), which are known to
contain at least three different terminal oxidases. Clearly, to demonstrate
that a given oxidase is active in a homogenate of a certain tissue fails
to indicate whether that oxidase functions at all in vivo, let alone to what
extent it may function.
Schade and Levy (1949), Thimann et al. (1954), and Hackett (1956b)
have investigated the nature of the functional terminal oxidase in potato
slices. Hackett (1956a), and W. D. Bonner and Yocum (1956), have
examined terminal electron pathways in the spadix of skunk cabbage,
while W. D. Bonner (1956) has done the same for mung bean seedlings.
In potato slices the oxidase characteristics change dramatically with time
from cutting. The change is temperature-dependent and cyanide-sensitive. Surprisingly, it occurs in the presence of dinitrophenol (Hackett,
1956b). Although at first the respiration is predominantly sensitive to
carbon monoxide and to cyanide, after slices have been incubated for
24 hours the respiration exhibits little or no carbon monoxide sensitivity,
and considerably diminished cyanide sensitivity (Levy and Schade, 1948;
Thimann et al., 1954). The experiments by Thimann and co-workers
designed to test CO sensitivity were elegantly carried out under conditions where the high ratios of CO to 0 2 necessary for the demonstration
of CO inhibition were achieved by introducing CO under pressure, instead of by mixing CO with air at atmospheric pressure and thereby
lowering the partial pressure of oxygen. Thus the effect of CO was
clearly separated from the effect of reduced oxygen pressure. In all cases
inhibition by CO was found to be light-reversible.
Regarding the effect of oxygen concentration on the respiration of potato slices, Thimann, Yocum, and Hackett observed that the oxygen
tension in the environment could be sharply reduced without an appreciable effect upon the respiration. With fresh slices, lowering the
p 0 o to 6.5% failed to affect the respiration, while lowering the oxygen
concentration to 1% decreased respiration by but 34%. Significantly, aged
tissue displayed the same characteristics as fresh tissue with respect to