CHARACTERIZATION OF PARAQUAT TRANSPORT IN PROTOPLASTS FROM MAIZE (ZEA-MAYS L) SUSPENSION CELLS

Protoplasts isolated from maize (Zea mays L.) suspension cells were us ed to study transport of paraquat. [C-14]Paraquat uptake was measured in 400-mu L centrifuge tubes using silicon oil centrifugation techniqu es. Approximately 50% of accumulation from a 100 mu M paraquat solutio n occurred in the first 10 s, and net accumulation reached a maximum a fter about 10 min. Membrane binding accounted for about 30% of apparen t accumulation. Concentration-dependent uptake kinetics were character ized by a nonsaturating curve, which was resolved into a linear and a saturable component. The K-m of the saturable component was 132 mu M, and the V-max was 0.512 nmol mu L of protoplasts(-1) min(-1). In the a bsence of sucrose, the V-max of the saturable component was reduced by 52%, suggesting that paraquat uptake across the plasmalemma is energy dependent. Measurement of concentration-dependent binding of paraquat to burst protoplasts showed a linear response. This suggests that the linear component from intact protoplast concentration kinetics repres ented paraquat binding to the plasmalemma surface. Calcium inhibited t he saturable component, and this inhibition was shown by Lineweaver-Bu rk analysis to be noncompetitive. Putrescine, a divalent cationic poly amine with a charge distribution similar to that of paraquat, competit ively inhibited paraquat uptake. These results show that paraquat tran sport characteristics at the plasmalemma of maize protoplasts are simi lar to those reported earlier for paraquat transport in roots of intac t maize seedlings.