To determine the effects of CO 2 -enriched air and root restriction on photosynthetic capacity, we measured net photosynthetic rates of 1-year-old loblolly pine seedlings grown in 0.6-, 3.8-or 18.9-liter pots in ambient (360 µmol mol −1 ) or 2× ambient CO 2 (720 µmol mol −1 ) concentration for 23 weeks. We also measured needle carbohydrate concentration and water relations to determine whether feedback inhibition or water stress was responsible for any decreases in net photosynthesis. Across

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... treatments, carbon dioxide enrichment increased net photosynthesis by approximately 60 to 70%. Net photosynthetic rates of seedlings in the smallest pots decreased over time with the reduction occurring first in the ambient CO 2 treatment and then in the 2× ambient CO 2 treatment. Needle starch concentrations of seedlings grown in the smallest pots were two to three times greater in the 2× ambient CO 2 treatment than in the ambient CO 2 treatment, but decreased net photosynthesis was not associated with increased starch or sugar concentrations. The reduction in net photosynthesis of seedlings in small pots was correlated with decreased needle water potentials, indicating that seedlings in the small pots had restricted root systems and were unable to supply sufficient water to the shoots. We conclude that the decrease in net photosynthesis of seedlings in small pots was not the result of CO 2 enrichment or an accumulation of carbohydrates causing feedback inhibition, but was caused by water stress.