ROOT-GROWTH AND PHYSIOLOGY OF POTTED AND FIELD-GROWN TREMBLING ASPEN EXPOSED TO TROPOSPHERIC OZONE

We studied root growth and respiration of potted plants and field-grow n aspen trees (Populus tremuloides Michx.) exposed to ambient or twice -ambient ozone. Root dry weight of potted plants decreased up to 45% a fter 12 weeks of ozone treatment, and root system respiration decrease d by 27%. The ozone-induced decrease in root system respiration of pot ted plants was more closely correlated with decreased root dry weight than with specific root respiration, suggesting that aspen root metabo lism was less affected by ozone than root growth. We used minirhizotro ns to study the appearance and disappearance of roots in the field. Le ngth 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 twice-ambien t ozone treatment, live root lengths were 17% less than those of contr ols, but the effect was not statistically significant. Seasonal soil C O2 efflux of field-grown trees decreased significantly in the ozone tr eatments, but because differences in live root length were not signifi cant and root dry weights were not available, the effect on CO2 efflux could not be attributed directly to decreased root growth.