Root growth and physiology of potted and field-grown trembling

plants and field-grown aspen trees (Populus tremuloides Michx.) exposed to ambient or twice-ambient ozone. Root dry weight of potted plants decreased up to 45% after 12 weeks of ozone treatment, and root system respiration decreased by 27%. The ozone-induced decrease in root system respiration of potted plants was more closely correlated with decreased root dry weight than with specific root respiration, suggesting that aspen root metabolism was less affected by ozone than root growth. We used minirhizotrons to study the appearance and disappearance of roots in the field. Length of live roots of field-grown trees increased rapidly early in the season and peaked by midseason in association with a decrease in root production and an increase in root disappearance. In the twiceambient ozone treatment, live root lengths were 17% less than those of controls, but the effect was not statistically significant. Seasonal soil CO2 efflux of field-grown trees decreased significantly in the ozone treatments, but because differences in live root length were not significant and root dry weights were not available, the effect on CO2 efflux could not be attributed directly to decreased root growth.