PHYSIOLOGICAL ASPECTS OF ABSCISSION IN CITRUS*
Degradation of cell-wall does occur in non-abscising shoot/peduncle AZ of mature fruits, but cell separation is limited to "pockets" which accounts for the absence of abscission inspite of the increased activity of cellulase and polygalacturonase (PG).
Abscisic acid (ABA) induces cell division and the proliferation of callus at the AZ in explants grown in vitro. The ABA effect is mediated via ethylene formation.
Exogenously applied ethylene enhances and auxin delays the synthesis and activity of the hydrolytic enzymes and abscission. Cellulase and PG, both exo- and endo-cellular, are the main enzymes responsible for abscission, but only the exo-cellular are actually involved in the process. Nine isoenzymes have been identified in the AZs, and the activity of most is enhanced by ethylene and delayed by auxin.
Gibberellin probably plays a secondary role in the control of abscission and the enzymic activity, while ABA-induced abscission as well as cellulase and PG activity are indirectly mediated via ethylene formation. Abscising-inducing chemicals cause abscission by means of wound ethylene (except for ethephon which is converted to ethylene in the tissue) which increases the activity of hydrolases.
Bioassay studies revealed an increase in auxin content at the AZ of leaves a few hours after excision followed by a marked decrease later on which is not due to in situ conjugation. Although ethylene stimulates some auxin degradation by peroxidase, its main role in decreasing auxin levels is by the inhibition of auxin transport from the leaf blade resulting from increased auxin metabolization which reduces the level of free auxin in the transport system.
It is suggested that the ethylene-induced decrease in auxin level in the tissues further inhibit auxin transport.
It is concluded that auxin is also required during the first stage of abscission for cell enlargement which occurs at the AZ prior to abscission, and that the following decrease in free auxin level is part of the phytogerontological events which lead to abscission. Ethylene enhances these effects by lowering the level of free auxin at the AZ.
*Results are based on both published data (see "Reference") and on works which are yet unpublished which were done in collaboration with J. Riov; M. Huberman; E. Zamski (Dept. of Agricultural Botany); Ch. Brisker and N. Dror.