Leaf discs of the wildtype poplar hybrid Populus canescens and its two transgenic lines overexpressing gamma-glutamylcysteine synthetase in the cytosol or in the chloroplasts were exposed to the herbicide paraquat (4 x 10(-9) to 4 x 10(-6) M). Leaf discs were incubated on tissue culture media containing 0.2, 1.0 or 2.0% sucrose. Exposure to paraquat led to a concentration-dependent decrease of GST activities at 0.2% sucrose. However, at higher external sucrose supplies, paraquat brought about marked inductions of GST activities in all poplar lines. The GST induction was light-dependent. Fluorescence measurements showed that the efficiency of the photosynthetic apparatus was inhibited by high paraquat concentrations (4 x 10(-7) - 4 x 10(-6) M) in all poplar clones. The photosynthetic efficiency (F-v/F-m) decreased also in leaf discs incubated in continuous darkness in the presence of paraquat.