MEMBRANE RESPONSE TO DICLOFOP ACID IS PH DEPENDENT AND IS REGULATED BY THE PROTONATED FORM OF THE HERBICIDE IN ROOTS OF PEA AND RESISTANT AND SUSCEPTIBLE RIGID RYEGRASS

Electrophysiological studies in roots of pea (Pisum sativum L.) and ri gid ryegrass (Lolium rigidum Gaud.) seedlings were conducted to elucid ate the mechanism involved in the membrane response to the herbicide d iclofop. In pea, a dicotyledonous plant insensitive to diclofop, membr ane depolarization at varying pH values and herbicide concentrations i ncreased at higher concentrations of the protonated form of diclofop a cid (pK(a) 3.57). In unbuffered nutrient solution (pH 5.7), diclofop a cid (50 muM) depolarized the membrane potential (E(m)) in roots of bot h resistant and susceptible biotypes of rigid ryegrass, whereas recove ry of E(m) occurred only in the resistant biotype following removal of the herbicide. This differential response was correlated with an incr ease (450%) in the rate of acidification of the external solution by t he susceptible biotype, and the E(m) differences between biotypes were eliminated in solutions buffered at pH 5.0 or 6.0. In addition, p-chl oromercuribenzene-sulfonic acid did not prevent the depolarization of E(m) by 50 mum diclofop acid. It is concluded that the differential me mbrane response to diclofop acid in herbicide-resistant and -susceptib le biotypes of rigid ryegrass is due to pH differences at the cell wal l/plasmalemma interface. Although the membrane response is probably no t involved in the primary inhibitory effect of diclofop on plant growt h, it could reduce the concentration of the permeant protonated form o f the herbicide and possibly could contribute to increased tolerance t o diclofop and other weak acid herbicides.