EFFECT OF ELEVATED CARBON-DIOXIDE CONCENTRATION AND ROOT RESTRICTION ON NET PHOTOSYNTHESIS, WATER RELATIONS AND FOLIAR CARBOHYDRATE STATUS OF LOBLOLLY-PINE SEEDLINGS

To determine the effects of CO2-enriched air and root restriction on p hotosynthetic 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 mu mol mol(-1)) or 2x ambient CO2 (720 mu mol mol(-1)) c oncentration for 23 weeks. We also measured needle carbohydrate concen tration and water relations to determine whether feedback inhibition o r water stress was responsible for any decreases in net photosynthesis . Across all treatments, carbon dioxide enrichment increased net photo synthesis by approximately 60 to 70%. Net photosynthetic rates of seed lings in the smallest pots decreased over time with the reduction occu rring first in the ambient CO2 treatment and then in the 2x ambient CO 2 treatment. Needle starch concentrations of seedlings grown in the sm allest pots were two to three times greater in the 2x ambient CO2 trea tment than in the ambient CO2 treatment, but decreased net photosynthe sis was not associated with increased starch or sugar concentrations. The reduction in net photosynthesis of seedlings in small pots was cor related with decreased needle water potentials, indicating that seedli ngs in the small pots had restricted root systems and were unable to s upply sufficient water to the shoots. We conclude that the decrease in net photosynthesis of seedlings in small pots was not the result of C O2 enrichment or an accumulation of carbohydrates causing feedback inh ibition, but was caused by water stress.