Induction of ethylene as an indicator of senescence in the mode of action of diclofop-methyl

Diclofop-methyl (DM) is an effective inducer of ethylene in leafy spurge (Euphorbia esula), a dicotyledonous weed, and oat (Avena sativa), a DM-susceptible monocotyledon. Ethylene was not induced in resistant wheat (Triticum aestivum) which metabolizes and detoxifies DM very rapidly. Immature leafy spurge leaves were resistant to DM, whereas mature leaves were highly susceptible with death occurring within 6-7 days following application of 4.5 mM DM, contrary to the premise of the ACCase hypothesis. DM induced significant ethylene production in immature and mature leaves (278 and 616% of control, respectively) within 7 hr after DM application. Metabolism and detoxification of DM did not differ between immature and mature leaves and a putative ACCase was resistant to inhibition by DM in both tissues. The most likely mechanism of action of DM is the induction of accelerated senescence with lipid peroxidation and free radical oxidation as the lethal process. Ethylene production in normal or accelerated (hypersensitive response reaction) senescence appears to be a consequence of lipid peroxidation and reactive oxygen (free radical) formation. DM (4.5 mM) and 2,4-D (2.0, 3.0 and 4.5 mM) applied separately induced significant production of ethylene in oat (232 and ca. 300% of control, respectively) after 7 hr of treatment. All applied concentrations of 2,4-D, except for 0.75 mM. reversed DM phytotoxicity in oat when applied as combinations. The reversal of DM phytotoxicity by 2,4-D was accompanied by the absence of DM-induced ethylene. Therefore, 2,4-D appears to reverse DM phytotoxicity by preventing lipid peroxidation and reactive oxygen formation as indicated by the absence of DM-induced ethylene. When DM phytotoxicity was not reversed (4.5 mM DM plus 0.75 mM 2,4-D) ethylene production in oat was approximately equal to the sum of ethylene induced by DM and 2,4-D applied separately. Therefore, induction of accelerated senescence appears to be a major mechanism of lethality by DM that is correlated to its action on the membrane to initiate a series of multiple responses.