The effect of diclofop-methyl and its antagonist, vitamin E, on membrane lipids in oat (Avena sativa L.) and leafy spurge (Euphorbia esula L.)

Diclofop-methyl (DM) increased the total and unsaturated free fatty acid co ntent (FFA) in leaves of oat (Avena sativa L.), a DM-susceptible monocotyle don, and in mature leaves of leafy spurge (Euphorbia esula L.), a dicotyled onous perennial weed whose mature leaves are killed by DM. In oat the total and unsaturated FFAs increased 9.0- and 17.4-fold over control levels, res pectively In leafy spurge these fractions increased 5.7- and 7.2-fold over control levels, respectively. Linolenic acid, the major component of the fr ee polyunsaturated fatty acid fraction, increased dramatically in both oat and leafy spurge. The DM-induced increase in the FFA fraction was accompani ed by decreases in both the total and unsaturated polar lipid fatty acid (P L-FA) fractions to approximately 25 and 60% of control in oat and leafy spu rge, respectively. The PL fraction contains phospholipids and glycolipids a nd is likely the source of the the FFAs. DM treatment increased the total a nd unsaturated neutral lipid fatty acid fraction 1.9- and 2.2-fold over con trols, respectively, in oat. The effect on leafy spurge was negligible. A p rotective effect of vitamin E, a free radical scavenger, against membrane d eterioration (as indicated by changes in lipid profiles) caused by DM was a pparent in both oat and leafy spurge. The total and unsaturated FFA levels of oat leaves treated with a vitamin E+DM combination increased 4.5- and 8. 4-fold over those of controls, respectively. In leafy spurge these fraction s increased 4.2- and 5.1-fold over those of controls. These increases were significantly lower than the levels reported for DM alone. The total and un saturated PL-FA fraction of oat increased from approximately 25% with DM al one to 47% with the vitamin E+DM combination. The constitutive enzyme phosp holipase D (PLD), involved with membrane disassembly, and lipoxygenase (LOX ), involved in the generation of reactive oxygen species, were assayed. The activity of PLD was high in both controls and DM-treated oat at pH 6.0, wi th little activity above pH 7.0. Lipoxygenase activity was unaffected by ei ther DM or vitamin E. However, 2,4-D (shown by others to protect against DM treatment in the field) decreased LOX activity approximately 50% in etiola ted oat leaves in vitro. Taken collectively, the lipid analysis and enzymat ic data support the hypothesis that the primary lethal mechanism of DM is a ccelerated senescence via membrane disassembly, membrane lipid catabolism, and oxidative stress that is mediated by metabolically induced active oxyge n species.