Growth and water relationships of red spruce seedlings exposed to atmospheric deposition and drought

Two-year-old containerized seedlings of red spruce (Picea rubens Sarg.) were subjected to ozone (O3) fumigation (0.25 ppm), simulated acid rain (pH 4.2 or 3.0), and drought prior to measurement of changes in growth and plant water status. Drought caused a significant decline in terminal height growth and new-shoot dry weight, but old-shoot dry weight, root dry weight, and root/shoot ratio were not appreciably affected. Deposition treatment (O3 and (or) acid rain) influenced both shoot and root dry weight but did not significantly affect height or root/shoot ratio. Treatment with either 0.25 PPM O3 alone or pH 3.0 rainfall alone caused the greatest reduction in growth, while treatment with 0.25 PPM O3 + pH 4.2 rain had the least effect. The influence of deposition treatment generally was more pronounced in seedlings subjected to drought than in well-watered plants. Seedlings subjected to drought had lower (more negative) water potentials than well-watered plants regardless of deposition treatment. While there were no significant differences in the water status of well-watered seedlings exposed to various deposition treatments, xylem water potential and osmotic potential of drought-stressed seedlings treated with O3 alone were consistently lower than these characteristics in seedlings treated with O3 + acid rain together. Except for one deposition treatment (0.25 PPM O3 + pH 3.0 rain) there was no evidence for osmotic adjustment of red spruce seedlings in response to stress factors imposed in this study.