Ripening and Abscission in Fruit of the Oil Palm (Elaeis guineensis, Jacq.): A Biochemical Investigation

Henderson, Janice (1998). Ripening and Abscission in Fruit of the Oil Palm (Elaeis guineensis, Jacq.): A Biochemical Investigation. PhD thesis The Open University.



Palm oil and palm kernel oil are two of the seventeen major oils and fats produced and traded in the world. The maximum harvestable yield of this plantation crop is, therefore, of major economic importance. Relevant aspects of the physiology and biochemistry of fruit ripening and the subsequent abscission of the ripe fruit has been the subject of this investigation.

The following major findings have shown that:-

The plant hormone ethylene is undetectable during the ripening period of about 25-40 days and the ripening of the fruit is not inducible by ethylene. The fruit does not soften and the pectic enzymes normally associated with cell wall changes in the ripening of other fruit are not induced. A cellulase (endo-β-1,4-glucanhydrolase) is newly expressed in the fruit mesocarp and its activity increases in concert with ethylene production when at full ripeness but other pectolytic and cellulytic activities remain low.

The fruit abscission zone is a specialised tissue differentiated even at the pre-anthesis stage and is functional throughout development. It is rich in pectin, much of which appears to be polygalacturonate. The vascular tissue which passes through the zone is highly attenuated. Just prior to abscission, the barely detectable levels of ethylene rise manyfold, whereupon the fully ripe fruit is shed in less than 24 hours. A specific zone cellulase and exo- and endo-polygalacturonase isoenzymes are induced at this time with a substantial increase in the activity of other cell wall hydrolases such as β-galactosidase and pectin methylesterase.

Non-abscinding mutant palms were discovered and the ethylene production and cell wall hydrolase activities in these were compared with normal fruit. Whilst ethylene production and cellulase activity in the fruit mesocarp occurred as in normal fruit, these activities do not lead to immediate abscission. After harvesting, cell separation at the abscission zone does eventually occur in bunches or spikelets of mutant fruit but only after a delay of several days. The cellulase induced in the zone is different from that in the normal and possesses many of the characteristics of mesocarp cellulase. In contrast, polygalacturonase isoenzymes show no differences from normal separated fruit and reach similar levels of activity whilst the activity of pectin methylesterase is substantially lower. Pectin metabolism in the mutant zone appears to be different from that of normal zones; there is preferential adsorption of the pI 6.2 polygalacturonase isoenzyme to the pectins and 13C CP-MAS NMR spectroscopy suggested that the pectin is more methylated in the mutant. These results indicate that the zone-specific cellulase of the normal fruit is critical to the subsequent cell separation events and that zones of mutant fruit that do not produce this cellulase either rot on the palm or, as a post-harvest event, undergo cell separation and abscission by a variant cohort of enzymes.

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