Although relatively little is known about the biochemistry of the cli- 
macteric, some recent advances provide a partial understanding of its operation. 
Hulme (25, 28) and Turner (76) have shown that there is a net increase in pro- 
tein nitrogen in the apple during the climacteric. It has also been observed 
frequently that the respiratory quotient exceeds unity during the climacteric, 
i.e., carbon dioxide evolved is greater than the oxygen consumed. Okamoto (53), 
Dilley (12), and Hulme and Wooltorton (32) have shown that a malic enzyme is 
operative in apple tissue and exhibits a steep rise in activity during the 
climacteric. Hulme, Jones, and Wooltorton (30) suggest that the origin of the 
climacteric is due to an increase in activity and a synthesis of malic enzyme 
and carboxylase, the energy for the synthesis being supplied by mitochondrial 
activity. They also indicated that a loss in malic acid content is rapid during 
this period. Malic acid may serve as a substrate for carbon dioxide production 
without utilizing oxygen. The synthesis of protein during the climacteric and 
the failure of others (30) to find any large scale uncoupling between oxidation 
and phosphorylation makes the uncoupling theory for the climacteric in the avo- 
cado as proposed by Millerd, Bonner, and Biale (51) untenable for apples. 
Hartman (19) has also found maximum aldolase activity in apple tissue 
during the climacteric. This agrees with an earlier report by Tager and Biale 
(73) who found an increase in activity of aldolase and carboxylase during ripen- 
ing of bananas, 
Many of the enzymes involved in respiration are associated with sub- 
cellular cytoplasmic particles or mitochondria. Considerable difficulty has 
been encountered in isolating active particles from apple tissue because of the 
high acidity and the presence of phenolic compounds. However, with special 
techniques, a number of workers (20, 35, 43, 44, 52, 54, 72) have obtained pre- 
parations that oxidize various acids in the Krebs cycle and exhibit cytochrome 
oxidase activity. The possibility now exists that more of the basic mechanisms 
concerned with respiration and the climacteric may be understood by studying 
the activities of these particles or the sonicated particles at different stages 
of disintegration. Hulme and Wooltorton (32) have shown that low sonication 
times release more of the malic enzyme and carboxylase from the particles, 
whereas higher sonication times release more of the Krebs cycle enzymes. This 
suggests that some enzymes are more tightly bound than others. 
Smock (63) has reviewed some of the earlier enzyme studies which dealt 
primarily with oxidase and catalase. A later more detailed review of the bio- 
chemistry of the climacteric is given by Hulme (28). 
Size of apple is another factor that affects respiration. Sullivan and 
Enzie (71) noted that large apples respire more rapidly than small apples 
immediately after harvest and also after storage at 35° F, Caldwell (10) found 
an increase in partial pressure of oxygen was toxic to apple tissue at high 
pressures and that actual pressure itself was not responsible for disorganiza- 
tion of the cells. Twenty atmospheres of 5 percent oxygen had essentially the 
same effect on carbon dioxide production as one atmosphere of 100 percent oxy- 
gen. Barker (2) in a later report agrees with Caldwell in part but asserts that 
an absolute pressure of three atmospheres with oxygen remaining at 20 percent 
of one atmosphere inhibits carbon dioxide production to some extent and that 
this can be attributed to mechanical distortion of the tissue. 
Woodruff and Crandall (82) screened 18 chemicals for their effect on 
inhibiting respiration in apple slices then tried the most promising on whole 
apples by injecting solutions into the core area. Solutions of sodium malonate, 
3-indolepropionic acid, hippuric acid, and benzimidazole were most effective 
with a maximum reduction of about 10 percent in respiration of the whole fruit. 
112 
