The Effect of Diclofop-Methyl and Its Antagonist, Vitamin E, on Membrane Lipids in Oat (Avena sativa L.) and Leafy Spurge (Euphorbia esula L.)

Abstract

Diclofop-methyl (DM) increased the total and unsaturated free fatty acid content (FFA) in leaves of oat (Avena sativa L.), a DM-susceptible monocotyledon, and in mature leaves of leafy spurge (Euphorbia esula L.), a dicotyledonous 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, respectively. In leafy spurge these fractions increased 5.7- and 7.2-fold over control levels, respectively. Linolenic acid, the major component of the free polyunsaturated fatty acid fraction, increased dramatically in both oat and leafy spurge. The DM-induced increase in the FFA fraction was accompanied by decreases in both the total and unsaturated polar lipid fatty acid (PL-FA) fractions to approximately 25 and 60% of control in oat and leafy spurge, respectively. The PL fraction contains phospholipids and glycolipids and is likely the source of the the FFAs. DM treatment increased the total and unsaturated neutral lipid fatty acid fraction 1.9- and 2.2-fold over controls, respectively, in oat. The effect on leafy spurge was negligible. A protective effect of vitamin E, a free radical scavenger, against membrane deterioration (as indicated by changes in lipid profiles) caused by DM was apparent 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 fractions 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 unsaturated PL-FA fraction of oat increased from approximately 25% with DM alone to 47% with the vitamin E+DM combination. The constitutive enzyme phospholipase 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, with little activity above pH 7.0. Lipoxygenase activity was unaffected by either 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 etiolated oat leaves in vitro. Taken collectively, the lipid analysis and enzymatic data support the hypothesis that the primary lethal mechanism of DM is accelerated senescence via membrane disassembly, membrane lipid catabolism, and oxidative stress that is mediated by metabolically induced active oxygen species.