RESPIRATION RATE IN PLANTS
279
from that in the presence of added substrate. The possibility remains,
therefore, that the specific activity of malic and citric oxidases and succinoxidase goes up at the time of the climacteric. To explain the increase,
the authors suggest a decrease in the intramitochondrial ATP/ADP
ratio, presumably effected by an increased utilization of ATP. An alternative possibility offered by Millerd et al. (1953) is discussed below.
2. The Climacteric in the Avocado
In distinction to the apple, the avocado fruit fails to undergo a climacteric rise unless removed from the tree (Biale, 1950; Millerd et al, 1953).
Following harvest, its respiration drops to a preclimactic minimum in 4
days, and then rises to a climacteric maximum 8 days after harvest
(Millerd, Bonner, and Biale, 1953). Also, in contrast to the apple, slices
from the avocado fruit fail to show a climacteric rise even though the
rise in the intact fruit is pronounced. Nevertheless, a profound change
takes place in slice respiration.
When dinitrophenol is added to avocado slices, the respiration is stimulated 122% in fruit taken fresh from the tree, 100% in fruit at the preclimacteric minimum, 16% during the climacteric rise, and not at all at
the climacteric peak. The avocado thus resembles the apple in so far
as the fruit respiration appears to be limited by the level of phosphate
acceptor before the climacteric, and that it is independent of it (or nearly
so, in apple) at the climacteric peak.
Millerd et al. have shown that mitochondrial preparations from
avocado fruit oxidize the acids of the tricarboxylic acid cycle. Adenylate
is a requirement for these oxidations. Dinitrophenol was in part able
to replace adenylate in the oxidation of α-ketoglutarate by particles from
preclimacteric fruit. In a separate test in which the oxidation of α-ketoglutarate by mung mean mitochondria was studied, the supernatant from
a homogenate of avocado in the climacteric phase was found to replace
the adenylate requirement, whereas the supernatant from preclimacteric
material did not. Concomitant with the respiratory stimulation by this
supernatant from climacteric fruit, the P/O ratio was lowered almost to
zero. The authors deduced that the respiratory increase in ripening
avocado fruit was a consequence of the development of an endogenous
uncoupling agent, which was assumed to be enzymatic in nature on the
basis of dialysis and heating experiments. Apart from the basis which
the suggested uncoupling phenomenon offers for the respiratory rise during the development of the climacteric, the hypothesis is attractive as
a general explanation of the ripening process. That is to say, in a fruit
in which uncoupling occurs, cessation of anabolic processes results in
the ascendency of the great variety of degradative transformations which
279
from that in the presence of added substrate. The possibility remains,
therefore, that the specific activity of malic and citric oxidases and succinoxidase goes up at the time of the climacteric. To explain the increase,
the authors suggest a decrease in the intramitochondrial ATP/ADP
ratio, presumably effected by an increased utilization of ATP. An alternative possibility offered by Millerd et al. (1953) is discussed below.
2. The Climacteric in the Avocado
In distinction to the apple, the avocado fruit fails to undergo a climacteric rise unless removed from the tree (Biale, 1950; Millerd et al, 1953).
Following harvest, its respiration drops to a preclimactic minimum in 4
days, and then rises to a climacteric maximum 8 days after harvest
(Millerd, Bonner, and Biale, 1953). Also, in contrast to the apple, slices
from the avocado fruit fail to show a climacteric rise even though the
rise in the intact fruit is pronounced. Nevertheless, a profound change
takes place in slice respiration.
When dinitrophenol is added to avocado slices, the respiration is stimulated 122% in fruit taken fresh from the tree, 100% in fruit at the preclimacteric minimum, 16% during the climacteric rise, and not at all at
the climacteric peak. The avocado thus resembles the apple in so far
as the fruit respiration appears to be limited by the level of phosphate
acceptor before the climacteric, and that it is independent of it (or nearly
so, in apple) at the climacteric peak.
Millerd et al. have shown that mitochondrial preparations from
avocado fruit oxidize the acids of the tricarboxylic acid cycle. Adenylate
is a requirement for these oxidations. Dinitrophenol was in part able
to replace adenylate in the oxidation of α-ketoglutarate by particles from
preclimacteric fruit. In a separate test in which the oxidation of α-ketoglutarate by mung mean mitochondria was studied, the supernatant from
a homogenate of avocado in the climacteric phase was found to replace
the adenylate requirement, whereas the supernatant from preclimacteric
material did not. Concomitant with the respiratory stimulation by this
supernatant from climacteric fruit, the P/O ratio was lowered almost to
zero. The authors deduced that the respiratory increase in ripening
avocado fruit was a consequence of the development of an endogenous
uncoupling agent, which was assumed to be enzymatic in nature on the
basis of dialysis and heating experiments. Apart from the basis which
the suggested uncoupling phenomenon offers for the respiratory rise during the development of the climacteric, the hypothesis is attractive as
a general explanation of the ripening process. That is to say, in a fruit
in which uncoupling occurs, cessation of anabolic processes results in
the ascendency of the great variety of degradative transformations which