Postharvest Variation in Cellulase, Polygalacturonase, and Pectinmethylesterase in Avocado (Persea americana Mill, cv. Fuerte) Fruits in Relation to Respiration and Ethylene Production

Abstract

Cellulase, polygalacturonase (PG), pectinmethylesterase (PME), respiration, and ethylene production were determined in single "Fuerte" avocado fruits from the day of harvest through the start of fruit breakdown. PME declined from its maximum value at the time of picking to a low level early in the climacteric. PG activity was not detectable in the preclimacteric stage, increased during the climacteric, and continued to increase during the postclimacteric phase to a level three times greater than when the fruit reached the edible soft stage. Celulase activity was low in the preclimacteric fruit, started to increase just as respiration increased, and reached a level two times greater than at the edible soft stage. Cellulase activity started to increase 3 days before PG activity could be detected. Increased production of ethylene foUlowed the increase in respiration and ceUulase activity by about 1.5 days. These results indicate that a close relation exists between the rapid increase in the ceOl waUl-depolymerizing enzymes and the rise in respiration and ethylene production and refocused attention on the role of the cell wail and the associated plasma membrane in the early events of fruit ripening.The avocado fruit starts to ripen only after being detached from the tree. After harvest, the most obvious ripening change is the rapid transition of the mesocarp from a hard to a soft, butter-like consistency with an apparent total loss of structural integrity.Several researchers have studied the relation between softening of the avocado fruit and the activity of cell wall-degrading enzymes. Lewis et al. (17) were the first to note that the hard avocado mesocarp presented barely detectable levels of cellulase activity whereas the activity in soft fruit was highest ever observed in plant tissues. Pesis et al. (20) found a direct correlation between cellulase activity, softening, respiration, and ethylene production. Awad (2) also found a close relation between the rapid increase in cellulase content after harvest, the climacteric rise in respiration, and softening of the fruit. He determined that edible softness occurred before maximum cellulase levels were reached.Raymond and Phaff (21) first showed a positive relationship between PG activity and softening of the avocado fruit. Later, Barash and Khazzam (3) and Zauberman and Schiffmann-Nadel (24) found that PG3 activity increased rapidly after harvest.The postharvest decrease in PME activity in the avocado fruit 'Research Fellow, CNPq, Brazil. (9), and Barmore and Rouse (4). We lack, however, precise information on the simultaneous variaton of these three enzymes as well as their relation to respiration, ethylene production, and softening of the fruit. Since there is considerable variation in the time of ripening of individual fruit, we show here the changes in three enzymes in relation to respiration and ethylene production in single fruit. MATERIALS AND METHODSMeasurement of Respiration and Ethylene Production. "Fuerte" avocado fruits were placed in glass ja...

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