Effect of fusicoccin on abscission, cellulase activity and ethylene production in citrus leaf explants

Abstract

The effect of fusicoccin (a phytotoxic carbotricyclic diterpene glucoside produced by Fusicoccum amygdali Del.) on abscission, ethylene production and cellulase activity was determined in citrus leaf explants. Activity of fusicoccin was compared with eight abscission accelerators and inhibitors. Abscission was retarded for 65 to 72 h with fusicoccin at 0·1 μg/explant. Fusicoccin was more effective in inhibiting abscission than indoleacetic acid, napththalene-acetic acid and kinetin. Indoleacetic acid often accelerated abscission in explants from 2-month-old leaves. Fusicoccin was a less effective inhibitor of a abscission than either cycloheximide, an inhibitor of protein synthesis, or 2,4-dichlorophenoxyacetic acid both of which inhibited abscission for 120 h at 0·025 μg/explant. Treatment of 0·4 μg/explant with either fusicoccin or 2-chloroethylphosphonic acid, a compound that readily releases ethylene in plant tissues, increased ethylene evolution six- to sevenfold within 24 h and sustained high ethylene production with a concomitant increase in cellulase activity for 72 h. Ethylene evolution was rapidly increased by 2,4-dichlorpphenoxyacetic acid and less rapidly by cycloheximide, but cellulase activity and abscission were retarded. An increase in abscission was associated with an increase in cellulase activity, except in fusicoccin-treated explants. Inhibition of abscission by fusicoccin appears to be due to several interacting factors, including toxicity to, and the impairment of, the sensitivity of the tissues, in the separation zone, to the abscission-mediating action of ethylene.