EFFECTS OF THE PHYSICAL-PROPERTIES OF SPHAGNUM PEAT ON WATER-STRESS IN CONTAINERIZED PICEA-MARIANA SEEDLINGS UNDER SIMULATED FIELD CONDITIONS

Containerized black spruce seedlings, grown in Sphagnum peat at two de grees of humification (light and moderate) and two artificially create d 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 membra nes. 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 par ticle content of the peats had little effect on the physiological vari ables measured. Needle conductance was affected greatly by the presenc e of the dialysis membrane itself. The results suggest that high aerat ion, one of the most desirable physical properties of a growing medium for nursery production, may become undesirable once the seedlings are outplanted.