Effect of Ethylene on [14C]Indole-3-Acetic Acid Metabolism in Leaf Tissues of Woody Plants

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Abstract
The effect of ethylene on [14C] IAA metabolism was investigated in defoliation sensitive leaf tissues of citrus (Citrus sinensis) and resistant leaf tissues of eucalyptus (Eucalyptus camaldulensis). IAA metabolites were fractionated into 80% ethanol-soluble, H2O-soluble, NaOH-soluble and insoluble components. In citrus, pretreatment with 25 .mu.l/l ethylene for 24 h significantly increased the amount of ethanol- and H2O-extractable conjugates during the first hour of incubation in [14C]IAA and increased 3- to 4-fold the formation of NaOH-extractable conjugates during the entire 6-h incubation period. Induction of the IAA-aspartate conjugation system was inhibited by ethylene. In eucalyptus, ethylene pretreatment only slightly stimulated the formation of IAA metabolites. Increased formation of ethanol-extractable conjugates in ethylene-pretreated eucalyptus tissues occurred only after 6 h of incubation. Chromatographic analysis indicated that the ethanol and H2O extracts of both species contained various low MW conjugates, whereas in citrus leaf tissues high MW conjugates accounted for most of the greater radioactivity detected in the NaOH extracts as a result of ethylene-pretreatment. Apparently, ethylene may reduce the level of endogenous IAA in citrus leaf tissues by stimulating IAA conjugation.
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