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Effect of quercetin, a kind of natural plant flavonoids, on auxin-induced ethylene production in barley coleoptiles was studied. Auxin-induced ethylene production was apparently stimulated by quercetin. This stimulatory effect of quercetin appeared after 4 h of incubation period. Ethylene production was stimulated 200% over the control after 8 h of incubation by 3 × 10 exp (-5) M quercetin. The quercetin effect was most prominent at 10^-4 M of IAA. Ethylene production induced by the synthetic auxin, 2,4-D and NAA, was not significantly affected by quercetin. Also ACC-based ethylene production was unaffected by the flavonoid. In an effort to elucidate mechanisms of quercetin action on auxin-induced ethylene production, the effect of quercetin on IAA metabolism was studied. Data obtained from these experiments indicate that quercetin treatment resulted in about 90% inhibition of IAA oxidase activity. IAA ( 3×10 exp (-5) M) conjugation was found to be not affected by quercetin. This results suggest that the stimulatory effect of quercetin on auxin-induced ethylene production may be due to the fact that quercetin inhibits IAA oxidase activity, thus increasing the free IAA level.
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