INVESTIGATIONS INTO THE CATION SPECIFICITY AND METABOLIC REQUIREMENTS FOR PARAQUAT TRANSPORT IN ROOTS OF INTACT MAIZE SEEDLINGS

The effect of inorganic polyvalent cations and metabolic inhibitors on the transport of paraquat into maize root epidermal and cortical cells was investigated. Increasing concentrations of Ca2+ caused a progressive inhibition of the saturable component of paraquat uptake, which represents influx across the plasmalemma, and a more modest inhibition in the slope of the linear component, which is due to binding of paraquat within the cell wall. Lineweaver-Burk transformation of the saturable component from these experiments indicated that Ca2+ inhibition of paraquat uptake is noncompetitive. Reciprocal interactions between paraquat and Ca2+ were observed, in that paraquat also inhibited Ca2+ influx into root cells. Saturable paraquat uptake was not affected when the calcium channel blocker verapamil (400 μM) was present in the uptake medium, indicating that paraquat does not move across the plasmalemma via calcium channels. Mg2+ and La3+ also inhibited the saturable component in a manner similar to that of Ca2+. Cyanide (500 μM) and carbonyl cyanide m-chlorophenylhydrazone (CCCP) (20 μM) had little effect on the saturable component in intact roots, suggesting that paraquat uptake into maize root cells is not directly dependent on cytoplasmic ATP levels or H+ gradients. When roots reloaded for 160 min with 100 μM paraquat (to increase cytoplasmic paraquat levels, and in turn, reduce the inwardly directed chemical gradient for paraquat influx) were exposed to 20 μM CCCP, a 37% reduction in the Vmax for saturable paraquat uptake was determined. These results suggest that the driving force for paraquat uptake is the gradient for electrochemical potential of paraquat across the plasmalemma. © 1993 Academic Press.