EFFECTS OF 4-CHLORO-2-METHYLPHENOXYPROPIONATE (AN AUXIN ANALOG) ON PLASMA-MEMBRANE ATPASE ACTIVITY IN HERBICIDE-RESISTANT AND HERBICIDE-SUSCEPTIBLE BIOTYPES OF STELLARIA-MEDIA L

ATPase activity was examined in plasma membrane (PM) fractions prepared from mecoprop-resistant and -susceptible biotypes of Stellaria media L. (chickweed). Treatment with the herbicide caused an 18% increase in ATP hydrolysis, but this was not significantly different from control plants and was similar for both biotypes. However, there was an overall significant biotype effect, herbicide-resistant plants having greater enzyme activity than susceptible ones. Proton-pumping was readily demonstrated in PM fractions obtained from both biotypes using the fluorescent probe amino-chloro-methoxyacridine (ACMA), indicating a relatively large proportion of 'inside-out' vesicles. Proton-pumping was significantly greater in PM preparations obtained from the resistant compared with susceptible plants. The differences in ATPase activity between the two biotypes could not be attributed to differences in the main sterol or phospholipid components of the PM. There were no effects of the herbicide on ATP hydrolysis in vitro, but proton-pumping was affected in a herbicide concentration-dependent manner. At 1.0 mol m-6 mecoprop caused an increase in the rate of proton-pumping, whereas at 10 and 100 mol m-6, an inhibition in this rate was observed. Both biotypes behaved similarly, irrespective of mecoprop concentration. These data indicate that mecoprop resistance in chickweed is unlikely to be due to a direct effect of the herbicide on PM H+-ATPase activity.