INHIBITION OF CORN ROOT MEMBRANE ATPASE ACTIVITIES BG ORYZALIN

The effects of oryzalin, a herbicide, on the H+-ATPase activities of m icrosomal membranes from corn roots were investigated. The presence of the herbicide at micromolar concentrations did not significantly affe ct the hydrolytic activity of both tonoplast and plasma membrane H+-AT Pases. However, the coupled proton pumping of the H+-ATPases in crude microsomes could be inhibited by the chemical. Similarly, the presence of micromolar levels of oryzalin also caused a steady decrease in ATP -driven proton pumping in purified tonoplast and plasma membrane vesic les. Kinetic analysis indicated that oryzalin decreased the initial pr oton pumping rate without significant effects on the proton leakage of both energized and de-energized membranes. In contrast, the presence of sub-optimal concentrations of CCCP, a conventional protonophore, in hibited the apparent proton pumping by increasing the proton leakage o f the membrane. The results indicate that oryzalin may upset the gener ation and the utilization of ATP in corn root cells. The observed pref erential inhibition on the proton pumping is consistent with a previou sly proposed indirect coupling mechanism for the root membrane H+-ATPa ses.