Effects of elevated CO2 on stem growth, vessel area and hydraulic conductivity of oak and cherry seedlings

Plants of Quercus robur L. and Prunus avium L. x P. pseudocerasus Lind, were grown in either ambient (350 vpm) or elevated (700 vpm) CO2. The intention was to examine the effects of elevated CO2 on the morphological and functional development of the stem. The relationships between stem longitudinal transport capacity and development were explored in several ways: stem hydraulic function was related to stem cross-sectional area, supplied leaf area and total stem vessel lumen area. The mean total vessel number and the total vessel lumen area per stem, for both species, was determined from basal sections of the xylem. In Prunus seedlings grown in different CO2 concentrations there was no significant change in the mean vessel size or number of vessels per stem. Quercus seedlings grown at elevated CO2 showed a significant increase in both vessel number and mean vessel size. When total stem vessel area was calculated it had increased twofold for Quercus plants grown at elevated CO2. Measured stem hydraulic conductivity was shown to increase linearly with supplied leaf area, except in Quercus seedlings grown at elevated CO2. Stem hydraulic conductivity for Quercus seedlings grown at elevated CO2 did not change with the increase in supplied leaf area. This absence of an increase in the stem hydraulic conductivity appeared to relate to changes in total stem vessel area. Despite total stem vessel area being greater at elevated CO2 than that at ambient, it similarly did not increase with supplied leaf area. The implications of this change in the relationship between leaf area and stem hydraulic conductivity are discussed with respect to the possible effects the change might have on the plant's water balance. The possible causes and significance of the changes in xylem anatomy are also considered in relation to direct effects caused by CO2 or indirect effects on changes in cambial maturity and tree growth.