Mitochondrina were isolated from 'Fuerte' avocado fruit (Persea americana Mill.) at four different stages of the respiratory climacteric. PrecUmacteric fruit had the highest rate of succinate oxidation and the postdimacteric mitochondria the lowest. Subsequently, successive additions of ADP increased the respiratory control ratio.Arrhenius plots of succinate oxidation of intact mitochondra from dimacteric rise and climacteric peak fruit showed two transition temperatures, while only one was observed in predlimacteric fruit. The low tempertore phase transition was at about 9 C, while the high one was at 20 C. In postclimacteric fruit, the low temperature transition decreased to between 5 and 2 C. The state 3 rate of succinate oxidation was highest for mitochondria from preclimacteric fruit and decreased for each later stage. The state 4 rates for preclimacteric and climacteric rise were the same, while both the climacteric peak and postclmacteric rates were about 40% lower than the predimacteric 02 uptake.The results indicate continuous changes in the mitochondrial membrane of the electron transport chain throughout the climacteric cycle.The change in the membrane influencing the phosphorylation system is greatest between cimacteric rise and peak stages. Mitochondrial membranes of postdmacteric fruit are presumed to change from flexible disordered to solid ordered phase at a lower temperature than those of other climacteric stages.The changes occurring in ripening which govern the initiation of the respiratory climacteric have not been identified. A number of proposed mechanisms have been reviewed (21). Mitochondria have been examined from several fruit including avocado (9), apple (5), and tomato (3) reflect a change in the lipid fraction of the mitochondrial membrane from a fluid to a solid phase. The change of lipid in the membrane from liquid to solid increases the activation energy of the enzyme complex by causing a conformational alteration of the membrane-bound enzyme.The change in chilling sensitivity suggested to us that ripening may be regulated by membrane lipids rather than by changes in the enzyme proteins themselves. We report here studies of Arrhenius plots of succinoxidase activity for avocado mitochondria at four stages of the respiratory climacteric as evidence for changes in the lipid components of the mitochondrial membrane.
MATERIALS AND METHODSMature 'Fuerte' avocado fruit (Persea americana Mill.) of uniform size were harvested from one tree at the South Coast Field Station of the University of California at Irvine. Fruit were placed in individual 2-liter wide mouth jars at 20 C. CO2 production was monitored by Beckman model 215 IR gas analyzer as described previously (7).As soon as a fruit achieved a respiratory rate of a particular stage on the climacteric, the fruit was cooled to 2 to 4 C prior to cutting and grating. Mitochondria were isolated and immediately assayed for succinoxidase activity as described below.Isolation Procedure. The technique used throughout this study was th...
Chilling sensitivity of ‘Fuerte’ and ‘Hass’ avocados (Persea americana Mill.) is a function of the stage of the climacteric. The least sensitive stage is postclimacteric where fruit can be kept at 2°C for 6 to 7 weeks. ‘Hass’ avocados on the climacteric rise and at the climacteric peak were most sensitive to chilling and showed injury after 19 days of treatment at 2°. Postclimacteric fruit could be transferred to 2° at 36 to 48 hours after the climacteric peak. The time preclimacteric fruit could be held at 2° varied during the picking season but could be as long as 30 days.
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