Changes in xylem pressure potential of red spruce seedlings treated with ozone and simulated acid rain

Two-year-old containerized seedlings of red spruce (Picearubens Sarg.) were exposed for 8 weeks to charcoal-filtered air (control) or to 7 and 15 parts per hundred million (pphm) ozone (O 3 ) alone and 15 pphm O 3 in combination with simulated acid rainfall at pH 4.2 and 3.0 prior to measuring shoot xylem pressure potential. Pressure potential determinations for seedlings treated with O 3 alone or O 3 + acid rain were always higher (less negative) than similar determinations made on comparable seedlings grown in charcoal-filtered air. These results suggest that exposure to atmospheric deposition may affect xylem pressure potential indirectly by influencing transpirational water loss via stomatal closure and (or) reductions in leaf area expansion. Calculations of osmotic potential and turgor pressure suggest the possibility of some osmotic adjustment in response to deposition stress in this species.