An in vitro bioassay for chemicals that affect Citrus abscission was used

to identify three inhibitors of stylar abscission in lemon pistil explants

Incubated on defined nutrient media. The three inhibitors (picloram, 4-

chlorophenoxyacetic acid, and 3,5,6-trlchloropyridine-2-oxyacetic acid) are

all auxins, and the most potent of them (ie. picloram) was found to be at

least 10 times more actve in the bioassay than 2,4-dichlorophenoxyacetic

acid. Picloram (2 micromolar) also was shown to be effective in inhibiting

stylar abscission in pistil explants from otber Citrs cultivars such as

mandarin, Valencia, and Washington navel oranges and grapefruit. To

study the physiology of auxins active as abscission inhibitors vers inactive

auxins in lemon pistils, the trsort and metabolism of 11-'4CI-2,4di

chlorophenoxyacetic acid was compared with that of 12-14CIindole-3-acetic

acid, which is without effect in the bioassay over the range from 0.1-100

micromolar. nsnicant quantities of blbeled indoe-3-acetic acid and/or

labeled derivates were found to reach the presumptive zone of stylar

absclssion under the test conditions. Labeled 2,4-dichlorophenoxyacetic

acid and/or labeled derivatives also were transported slowly through pistils,

but some radioactivity could be detected in the stylar abscission zone as

early as 24 hours after the start of Incubation. Extensive conversion of 12-

14Clindole_3_acetic acid to labeled compounds tentatively considered to be

glycoside and cellulosic glucan derivatives was found with the use of solvent

extraction methodology. A significandy smaller percentage of the radioactivity

In pistils incubated on I1-14Cj-2,4-dichIorophenoxyacetic acid was

found in fractions correspnding to these derivatives. Both transport and

metabolism appear to be important factors affecting the activity of auxins

as abscission inhibitors in the bioassay.