The modulation of the activity and expression of soybean lipoxygenases 1 (LOX-1) [Vliegenthart, J.F.G. and Veldink, G.A. (1982) in: Free Radicals in Biology (Pryor, W.A., Ed.) pp. 29-64, Academic Press, New York] and 2 (LOX-2) by water deficit (osmotic stress) has been investigated, by following gene expression at the transcriptional and translational levels. Osmotic stress enhanced the transcription of the genes of both isoenzymes, leading to increased levels of the corresponding mRNAs, protein contents and specific activities. Abscisic acid (ABA) did not mediate enhancement of LOX expression, but caused a decrease of LOX-2 activity and was ineffective on LOX-1. Water deficit also caused oxidative modification of soybean membrane pool lipids [Schmidt, W.E. and Ebel, J. (1987) Proc. Natl. Acad. Sci. USA 84, 4117-4121], attributable to the increase of conjugated hydroperoxides in the esterified fatty acids of the lipid bilayer.