Abscission of Citrus Leaf Explants

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Abstract
The question whether abscisic acid (ABA) induces cellulase and polygalacturonase activity and, hence, abscission directly or whether its action is mediated by C2H4 was studied in citrus (Osbeck var. Shamouti) leaf explants using aminoethoxyvinyl glycine (AVG), an inhibitor of C2H4 biosynthesis. ABA in concentrations of 10 micromolar and higher induced C2H4 production and accelerated abscission. AVG inhibited C2H4 formation, activity of cellulase and polygalacturonase, and abscission in ABA-treated explants. AVG did not inhibit the increase in the activity of the cell-wall degrading enzymes or abscission in a saturating level of externally supplied C2H4. This indicates that the effect of AVG resulted from inhibition of the formation of endogenous ethylene. The data indicate that in citrus leaf explants the induction of the activity of cellulase and polygalacturonase and abscission by ABA is mediated by C2H4.