RESPIRATION RATE IN PLANTS
237
was manifested. In such a case it may be assumed that the aerobic
respiratory system can accommodate the additional product of enhanced
glycolysis, whereas in those cases where aerobic fermentation occurs
immediately upon the stimulation of glycolysis, it cannot. It is clear that
the stimulation of glycolysis in air need not be accompanied by alcohol
formation.
The absence of aerobic fermentation in most higher plant tissues appears to be due, in the first place, to their limited concentrations of glycolytic enzymes (compared to yeast, for example), and in the second place,
to the effectiveness of the Pasteur mechanism in limiting glycolysis to a
level where the products are readily consumed in the aerobic system. It
is unreasonable to suppose that alcohol formation in air is precluded
merely by the preferential oxidation of DPNH by molecular oxygen. If
that were the case, it would follow that no constraint was imposed upon
the oxidative step of glycolysis, and hence no Pasteur effect would arise.
Treatments which cause pyruvate to accumulate may result in aerobic
fermentation. Thus Beevers (1952) has induced aerobic fermentation in
corn roots by malonate inhibition of the tricarboxylic acid cycle, while
Simon (1953c) and many others have elicited aerobic fermentation by
the inhibition of respiration with cyanide. Inhibitions of the sort mentioned not only interfere with the metabolism of pyruvate, but at the
same time prevent the saturation of the phosphate acceptors by the aerobic systems, and thereby diminish the Pasteur effect.
James (1953a), in discussing the effect of oxygen tension on the
respiration of intact organs such as leaves and fruits, has presented a
variety of data to show that aerobic fermentation does not begin immediately upon the diminution of respiration as a result of a lowering of
the oxygen tension. Since measurements of anaerobic fermentation by
these same tissues indicate that a Pasteur effect does exist, it might
be expected that any diminution in the aerobic respiration should increase the fraction of the total phosphate acceptor available to the
glycolytic system, and should thereby cause immediate aerobic fermentation. The fact that in many organs more than one type of respiratory
system operates may suggest a basis for an explanation of the foregoing
inconsistency. It seems entirely possible that that part of the respiration
which is most readily affected by a drop in the p 0o is either nonphosphorylative in nature, not concerned with pyruvate oxidation, or
both. The existence of more than one oxidative system in a variety of
tissues is suggested by considerable evidence (see below). The respiration of tissues known to contain cytochrome oxidase is frequently reduced
by lowering the external oxygen tension, although the oxygen concentration within such tissues would still be expected to support maximal
237
was manifested. In such a case it may be assumed that the aerobic
respiratory system can accommodate the additional product of enhanced
glycolysis, whereas in those cases where aerobic fermentation occurs
immediately upon the stimulation of glycolysis, it cannot. It is clear that
the stimulation of glycolysis in air need not be accompanied by alcohol
formation.
The absence of aerobic fermentation in most higher plant tissues appears to be due, in the first place, to their limited concentrations of glycolytic enzymes (compared to yeast, for example), and in the second place,
to the effectiveness of the Pasteur mechanism in limiting glycolysis to a
level where the products are readily consumed in the aerobic system. It
is unreasonable to suppose that alcohol formation in air is precluded
merely by the preferential oxidation of DPNH by molecular oxygen. If
that were the case, it would follow that no constraint was imposed upon
the oxidative step of glycolysis, and hence no Pasteur effect would arise.
Treatments which cause pyruvate to accumulate may result in aerobic
fermentation. Thus Beevers (1952) has induced aerobic fermentation in
corn roots by malonate inhibition of the tricarboxylic acid cycle, while
Simon (1953c) and many others have elicited aerobic fermentation by
the inhibition of respiration with cyanide. Inhibitions of the sort mentioned not only interfere with the metabolism of pyruvate, but at the
same time prevent the saturation of the phosphate acceptors by the aerobic systems, and thereby diminish the Pasteur effect.
James (1953a), in discussing the effect of oxygen tension on the
respiration of intact organs such as leaves and fruits, has presented a
variety of data to show that aerobic fermentation does not begin immediately upon the diminution of respiration as a result of a lowering of
the oxygen tension. Since measurements of anaerobic fermentation by
these same tissues indicate that a Pasteur effect does exist, it might
be expected that any diminution in the aerobic respiration should increase the fraction of the total phosphate acceptor available to the
glycolytic system, and should thereby cause immediate aerobic fermentation. The fact that in many organs more than one type of respiratory
system operates may suggest a basis for an explanation of the foregoing
inconsistency. It seems entirely possible that that part of the respiration
which is most readily affected by a drop in the p 0o is either nonphosphorylative in nature, not concerned with pyruvate oxidation, or
both. The existence of more than one oxidative system in a variety of
tissues is suggested by considerable evidence (see below). The respiration of tissues known to contain cytochrome oxidase is frequently reduced
by lowering the external oxygen tension, although the oxygen concentration within such tissues would still be expected to support maximal