Herbicidal disruption of proton gradient development and maintenance by plasmalemma and tonoplast vesicles from oat root

Abstract

Quinacrine fluorescence quenching was used to evaluate the effects of 23 herbicides (100 μ M) on ATP-dependent proton gradient development and maintenance by low-density (tonoplast-enriched) and high-density (plasmalemma-enriched) vesicles from oat ( Avena sativa L.) roots. Naptalam stimulated the rate of proton gradient formation but did not stimulate total ATPase activity of high-density vesicles. Naptalam action was specific for this vesicle type. (2-Methyl-4-chlorophenoxy)acetic acid, dinitramine, chlorpropham, and S-ethyl dipropylthiocarbamate inhibited the initial rate of proton gradient formation and decreased the magnitude of the equilibrium proton gradient formed by low-density vesicles. Dinitramine and chlorpropham were also effective disruptors of proton gradient formation by high-density vesicles. Diclofop-methyl, fluazifop-butyl, and flamprop-isopropyl altered proton gradient generation and/or maintenance across both vesicle types. The free acid forms of these compounds were less effective disruptors than the ester forms. ATPase activity of both high- and low-density vesicles was either unaffected or slightly stimulated by these compounds. Both acifluorfen and alachlor accelerated the rate of decay of proton gradients formed across low-density but not high-density vesicles and stimulated the KCl, Mg 2+-ATPase activity of low-density but not high-density vesicles. Atrazine, metribuzin, dalapon, glyphosate, amitrole, pyrazon, picloram, and bentazon did not affect proton-gradient formation or maintenance by either vesicle type. These results support the hypothesis that some herbicides can have rapid, direct effects on proton gradient maintenance by the plasmalemma and the tonoplast in plant cells.