REGULATING THE ABSCISSION PROCESS IN CITRUS BY GROWTH SUBSTANCES¹

The role of ethylene, auxin and abscisic acid (ABA) in the regulation of abscission was investigated in leaf and fruit explants of the orange tree (Citrus sinensis L. Osbeck). It was found that ethylene enhanced the increase in the activity of cellulase and polygalacturonase (PG) and abscission while auxin delayed them. Whenever ethylene enhanced abscission, an increase in the activity of cellulase and PG preceded separation. No differences in the response of either PG or cellulase (total or cellulase isoenzymes) to growth regulators were found when activity was studied in endo- and exo-cellular fractions of the abscission zone (AZ). Structural and ultrastructural studies revealed that when the shoot-peduncle AZ of the mature fruit (AZ-A) did not abscise, but cellulase and PG reacted as in abscising AZ, cell-wall degradation did occur at the AZ, but cell separation was limited to "pockets" of cells within the AZ. This is the reason for the absence of abscission at AZ-A of fruits two months and more after fruit setting in spite of the induction of hydrolytic enzymes. ABA-induced activity of PG and cellulase, as well as abscission, was indirectly initiated via ABA-induced ethylene formation. Uronic acid oxidase responded to ethylene and auxin, as did PG and cellulase, but its activity lacked the typical specificity at the AZ and was lowest at this site. It is therefore not yet clear whether it is directly involved in the hormonal regulation of abscission.

Bioassay studies revealed an increase in auxin content at the AZ of leaves few hours after excision, followed by a marked decrease later on which is not due to in situ conjugation. Although ethylene stimulated some auxin degradation by peroxidase, its main role in decreasing auxin level was the inhibition of auxin transport from the leaf blade resulting from increased auxin metabolization which reduced the level of free auxin in the transport system. It is suggested that the ethylene-induced decrease in auxin level in the tissues further inhibited auxin transport, as explained by the chemiosmotic theory of auxin transport. It was concluded that auxin is required during the first stage of abscission for cell enlargement, which occurred at the AZ prior to abscission, and that the following decrease in free auxin level is part of the phytogerontological events which led to abscission. Ethylene enhanced these effects by lowering the level of free auxin at the AZ and by stimulating the induction of PG and cellulase and their secretion into the cell wall.