Effects of the Herbicide MCPA, and the Heavy Metals, Cadmium and Nickel on the Lipid Composition, Mg2+-ATPase Activity and Fluidity of Plasma Membranes from Rice, Oryza sativa (cv. Bahia) Shoots

Application of the herbicide 4-chloro-2-methylphenoxyaceticacid (MCPA), cadmium or nickel to the roots of rice, Oryza sativa(cv. Bahia) grown in solution culture resulted in changes tothe lipid composition and increases in the Mg²⁺-ATPase activityof the shoot plasma membranes. These increases in Mg²⁺-ATPaseactivity were associated with reductions in the campesterol:sitosterol ratio. There was no significant effect on membranefluidity (measured by steady-state fluorescence polarization)as a result of the treatments. The results suggested that therewas no consistent relationship between fluidity and Mg²⁺-ATPaseactivity. A possible involvement of sterols in the regulationof plasma membrane-bound ATPase activity is proposed. Key words: MCPA, heavy-metals, sterols, ATPase activity, plasma membranes