Abstract
We determined the direct effects of atmospheric CO2 concentration ([CO2]) on leaf gas exchange, phenolic and carbohydrate allocation in live oak seedlings (Quercus virginiana Mill.) grown at present (370 μmol · mol-1) or elevated (520 μmol · mol-1) [CO2] for 6 months in open-top chambers. Two soil nitrogen (N) treatments (20 and 90 μmol · mol-1 total N, low N and high N treatments, respectively) were imposed by watering the plants every 5 d with modified water soluble fertilizer. Enhanced rates of leaf-level photosynthesis were maintained in plants subjected to elevated [CO2] over the 6-month treatment period in both N treatments. A combination of increased rates of photosynthesis and decreased stomatal conductance was responsible for nearly doubling water use efficiency under elevated [CO2]. The sustained increase in photosynthetic rate was accompanied by decreased dark respiration in elevated [CO2]. Elevated [CO2] led to increased growth rates, while total non-structural carbohydrate (sugars and starch) concentrations were not significantly affected by elevated [CO2] treatment. The concentration of phenolic compounds increased significantly under elevated [CO2].