Abscission of Citrus Leaf Explants: INTERRELATIONSHIPS OF ABSCISIC ACID, ETHYLENE, AND HYDROLYTIC ENZYMES.

The question whether abscisic acid (ABA) induces cellulase and polygalacturonase activity and, hence, abscission directly or whether its action is mediated by C(2)H(4) was studied in citrus (Osbeck var. Shamouti) leaf explants using aminoethoxyvinyl glycine (AVG), an inhibitor of C(2)H(4) biosynthesis. ABA in concentrations of 10 micromolar and higher induced C(2)H(4) production and accelerated abscission. AVG inhibited C(2)H(4) 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 C(2)H(4). 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 C(2)H(4).