EFFECT OF DROUGHT STRESS, ABSCISIC-ACID, AND ABSCISIC-ACID ANALOGS ON THE EFFICACY OF DICLOFOP-METHYL AND TRALKOXYDIM

The effects of drought stress, abscisic acid (ABA), and four ABA analogues on diclofop-methyl and tralkoxydim efficacy were investigated in oat (Avena sativa). Drought stress conditions (6% soil moisture content) reduced the efficacy of diclofop-methyl at 350 g ha(-1), but not at 700 g ha(-1). Similarly, tralkoxydim efficacy was reduced by drought stress at 62.5 and 125 g ha(-1), but not at 250 g ha(-1). ABA (100 mu M), applied as a root drench 2 days before the herbicide, protected oat plants against all rates of diclofop-methyl and against low rates of tralkoxydim. Two ABA analogues protected oat plants from diclofop-methyl injury, whereas two others had no effect. Foliage applications of ABA were much less effective than root applications in protecting against herbicide injury. Protection by ABA and the two active analogues was dependent on the relative time of application with respect to the herbicides. Optimal protection by ABA and analogue I was obtained when they were applied between 2 days before and 1 day after diclofop-methyl application. Analogue IV protected plants when applied between 3 days before and 1 day after diclofop-methyl application. Partial protection against tralkoxydim activity by ABA was observed when it was applied between 1 day before and 1 day after herbicide application. Analogue I did not afford any protection against tralkoxydim, and analogue IV afforded partial protection when applied the same day or 1 day after tralkoxydim. The results indicate that protection against these postemergence herbicides, similar to that conferred by water stress, can be induced by ABA and structural analogues that apparently mimic the action of ABA.