Interactions of indoleacetic Acid and gibberellic Acid in leaf abscission control.

Debladed midribs of citrus leaves showed the typical delay of abscission in response to indoleacetic acid (IAA), and the typical acceleration of abscission in response to gibberellic acid (GA). Interaction experiments with these 2 hormones indicated that the balance of the 2 hormones may be more important in regulating abscission than the quantity of either. The often reported acceleration of abscission with low quantities of IAA did not seem to exist in citrus. IAA did accelerate abscission in this tissue when its application was delayed for at least 24 hours after deblading, which suggests the 2-stage effect is also present in citrus.When abscission was first delayed with IAA and then allowed to continue, the rate of abscission proceeded at a slower rate than was typical for this tissue. This slower rate was also typical of the effect observed when GA overcame the abscission retarding effect of IAA. The phenylurethane, Barban, blocked the GA acceleration of abscission, but it did not affect the rate of abscission of control or IAA treated midribs.