Effects of the physical properties ofsphagnumpeat on water stress in containerizedpicea marianaseedlings under simulated field conditions

Containerized black spruce seedlings, grown in Sphagnum peat at two degrees of humification (light and moderate) and two artificially created textural grades (with or without particles <1.3 mm), were subjected to controlled degrees of water stress using polyethylene glycol (PEG) at 0, 20, 75 and 175 g PEG kg−1 H2O, with and without dialysis membranes. After a 4‐day exposure, shoot water potential and water potential at the soil‐root interface were strongly affected by the humification of the peat at the highest PEG concentration. Differences in fine particle content of the peats had little effect on the physiological variables measured. Needle conductance was affected greatly by the presence of the dialysis membrane itself. The results suggest that high aeration, one of the most desirable physical properties of a growing medium for nursery production, may become undesirable once the seedlings are outplanted.