Stress response capacity (Fv/Fm at 690 nm and F690/F735 at Fmax) of untransformed hybrid poplar, Populus x canescens (P tremula x P alba), and two transgenic lines overexpressing gamma-ECS (gamma-glutamylcysteine synthetase) either in the cytosol (cyt-ECS) or in the chloroplast (chl-ECS) was studied in response to the herbicide paraquat (4.0 x 10(-9) to 4.0 x 10(-6) M) for 21 days. Significant differences at sublethal (4.0 x 10(-7) M) and bleaching (4.0 x 10(-6) M) concentrations of paraquat were observed with about a two-fold and eight-fold decrease in the photosynthetic activity (Fv/Fm at 690 nm and F690/F735 at Fmax), respectively. None of the gshI transgenic lines (cyt-ECS, chl-ECS) with elevated GSH content exhibited significant tolerance to paraquat. Semiquantitative RT-PCR of the cyt-ECS clone was used for gene expression analysis of the nuclear encoded rbcS gene and the stress responsive gst gene. Expression of the constitutively expressed 26SrRNA ribosomal gene was probed as a control for all RT-PCR reactions. The relative intensities of gene expressions normalized to the level of 26SrRNA intensity showed a 50% decrease in the nuclear encoded rbcS expression and a 120% increase in the stress responsive gst gene expression of the paraquat treated (4.0 x 10(-7) M) samples of the transgenic poplar line (cyt-ECS).