RESPIRATION RATE IN PLANTS
281
uncoupling action of dinitrophenol. If metabolically generated ATP
were to be channelled by hexokinase into glucose phosphate formation,
for example, to the exclusion of other uses, the net effect would resemble
uncoupling in many instances, since glucose phosphate is a so-called lowenergy phosphate ester which has but limited utility in promoting
anabolic reactions. The association of hexokinase with the mitochondria
has been demonstrated in a variety of plants (Saltman, 1953). However,
no hexokinase measurements were made by Romani and Biale. Mitochondrial particles from avocado are difficult to prepare free from fats,
and a simpler explanation, which cannot at present be ruled out, is that
some physical barrier, such as an adsorbed lipoid layer on the mitochondria, may reduce the effectiveness of low concentrations of dinitrophenol.
No explanation has been offered for the absence of an obvious climacteric in avocado slices. There is a strong likelihood that the avocado
fruit contains more than one oxidative system, since it has been shown
that the respiration of avocado slices, at a certain stage of development
at least, is resistant to cyanide (Biale, 1950), while at the same time the
mitochondria from avocado at all stages have a cyanide-sensitive respiration. The development of the climacteric in avocado is reminiscent of
the respiratory rise in the developing spadix of Arum (James and
Beevers, 1950). In the latter tissue, too, slices may be prepared which
exhibit cyanide-resistant respiration; while mitochondrial preparations
from the same tissue carry out the normally cyanide-sensitive oxidations
of the tricarboxylic acid cycle (Hackett and Simon, 1954).
The higher specific respiration rate in avocado slices as compared to
the intact fruit is not due to oxygen availability, since Biale (1946) has
shown that the respiration of avocado fruits throughout their development is little affected by an increase in oxygen tension above that in air.
The situation rather appears to resemble that in potato and chicory,
wherein it was deduced that in the intact organ a product of the basal
respiratory metabolism represses a part of the basal respiration itself, as
well as the development of a qualitatively different respiratory system
which subsequently arises in slices.
It is tempting to conjecture that in avocado and in other fruits which
undergo a climacteric the respiratory rise in the intact fruit corresponds
to the induced respiration which in a variety of tubers and fleshy roots
occurs only in tissue slices. If such were the case, it must follow that
the basal metabolism, which has already been implicated in the suppression of the development of the induced respiration in certain fleshy
organs, is either abated or disappears during fruit ripening. The char-
281
uncoupling action of dinitrophenol. If metabolically generated ATP
were to be channelled by hexokinase into glucose phosphate formation,
for example, to the exclusion of other uses, the net effect would resemble
uncoupling in many instances, since glucose phosphate is a so-called lowenergy phosphate ester which has but limited utility in promoting
anabolic reactions. The association of hexokinase with the mitochondria
has been demonstrated in a variety of plants (Saltman, 1953). However,
no hexokinase measurements were made by Romani and Biale. Mitochondrial particles from avocado are difficult to prepare free from fats,
and a simpler explanation, which cannot at present be ruled out, is that
some physical barrier, such as an adsorbed lipoid layer on the mitochondria, may reduce the effectiveness of low concentrations of dinitrophenol.
No explanation has been offered for the absence of an obvious climacteric in avocado slices. There is a strong likelihood that the avocado
fruit contains more than one oxidative system, since it has been shown
that the respiration of avocado slices, at a certain stage of development
at least, is resistant to cyanide (Biale, 1950), while at the same time the
mitochondria from avocado at all stages have a cyanide-sensitive respiration. The development of the climacteric in avocado is reminiscent of
the respiratory rise in the developing spadix of Arum (James and
Beevers, 1950). In the latter tissue, too, slices may be prepared which
exhibit cyanide-resistant respiration; while mitochondrial preparations
from the same tissue carry out the normally cyanide-sensitive oxidations
of the tricarboxylic acid cycle (Hackett and Simon, 1954).
The higher specific respiration rate in avocado slices as compared to
the intact fruit is not due to oxygen availability, since Biale (1946) has
shown that the respiration of avocado fruits throughout their development is little affected by an increase in oxygen tension above that in air.
The situation rather appears to resemble that in potato and chicory,
wherein it was deduced that in the intact organ a product of the basal
respiratory metabolism represses a part of the basal respiration itself, as
well as the development of a qualitatively different respiratory system
which subsequently arises in slices.
It is tempting to conjecture that in avocado and in other fruits which
undergo a climacteric the respiratory rise in the intact fruit corresponds
to the induced respiration which in a variety of tubers and fleshy roots
occurs only in tissue slices. If such were the case, it must follow that
the basal metabolism, which has already been implicated in the suppression of the development of the induced respiration in certain fleshy
organs, is either abated or disappears during fruit ripening. The char-