T, J. 1990. Effects of preconditioning on electrolyte leakage and lipid composition in black spruce {Picea mariana) stressed with polyethylene glycol. -Physiol. Plant. 79: 71-77.Black spruce {Picea mariana Mill. B. S. P.) rooted cuttings were grown in solution culture and preconditioned by osmotically stressing plants with polyethylene glycol. After relief from preconditioning stress, water relations, membrane leakiness, and the composition of lipids and fatty acids were compared in preconditioned and control, unconditioned plants. Both groups of plants were subsequently subjected to a severe osmotic stress with polyethylene glycol and examined again. Osmotic stress decreased shoot water potentials and increased the leakage of electrolytes from shoots of stressed, compared with unstressed, plants. However, both unstressed and stressed preconditioned plants leaked less electrolytes compared with unconditioned plants. Changes in steroi, phospholipid and glycolipid composition were observed in preconditioned unstressed and stressed plants. Steroi and phospholipid levels declined as a result of stress, and preconditioning resulted in a decline in steroi: phospholipid ratios in plants.