REGULATING THE ABSCISSION PROCESS IN CITRUS BY GROWTH SUBSTANCES1
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.