Theories concerning the immediate origin of the increased rate of respiration at the climacteric in apples have been largely centred round an increase in mitochondrial activity (Hatch, Pearson, Millerd & Robertson 1959) probably because an increase in energy is required for synthetic processes occurring at this time (Pearson & Robertson 1954; Hulme 1954). Alternatively, it has been suggested (Millerd, Bonner & Biale 1953) that the close coupling between oxidation and phosphorylation in the mitochondrial system is destroyed during the climacteric period so that the respiration becomes uncontrolled. An improved method is used here for the isolation of mitochondria and soluble enzymes from apple tissue which obviates the inhibition of enzyme action by polyphenolic com pounds present in the tissue. The potential activity of isolated mitochondria increases over the climacteric, especially in the peel tissue, and the increase commences some days before any rise in CO
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production of whole fruit is observed. In fruit detached from the tree before the climacteric has commenced, the climacteric is accompanied by a steep rise in the activity of malic enzyme and pyruvic carboxylase, particularly in the peel of fruit. After the climacteric peak, respiration rate and activity of malic enzyme and carboxylase fall, the changes running parallel particularly in the peel. The origin of the climacteric in fruit both ‘on’ and ‘off’ the tree, it is suggested, is due to an increase in activity — a synthesis, in fact— of malic enzyme and carboxylase, the source of energy for this synthesis being mitochondrial activity. This would also account for the increase in the respiratory quotient over the climacteric. The higher respiration rate of fruit at the climacteric peak on the tree (one-third more Co
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production than in detached fruit) which is associated with a peculiar wateriness of the pulp, is due, it is claimed, to an increased permeability of the pulp tissue which allows a more rapid union between enzymes and substrates. The fall in respiration after the climacteric peak in storage (it has not been possible to follow post-climacteric changes in fruit on the tree) is attributed to a decrease in the activity of malic enzyme, carboxylase and the mitochondrial system operating the Krebs cycle; finally acid substrate becomes limiting. There is no evidence of an uncoupling, in the mitochondria, of oxidation and phosphorylation over the climacteric period.
Contribution of Malic Enzyme, Pyruvate Kinase, Phosphoenolpyruvate Carboxylase, and the Krebs Cycle to Respiration and Biosynthesis and to Intracellular pH Regulation during Hypoxia in Maize Root Tips Observed by Nuclear Magnetic Resonance Imaging and Gas Chromatography-Mass Spectrometry
