ELEVATED CO2 AND DROUGHT ALTER TISSUE WATER RELATIONS OF BIRCH (BETULA-POPULIFOLIA MARSH) SEEDLINGS

The effect of increasing atmospheric CO2 concentrations on tissue wate r relations was examined in Betula populifolia, a common pioneer tree species of the northeastern U.S. deciduous forests. Components of tiss ue water relations were estimated from pressure volume curves of tree seedlings grown in either ambient (350 mul l-1) or elevated CO2 (700 m ul l-1), and both mesic and xeric water regimes. Both CO2 and water tr eatment had significant effects on osmotic potential at full hydration , apoplasmic fractions, and tissue elastic moduli. Under xeric conditi ons and ambient CO2 concentrations, plants showed a decrease in osmoti c potentials of 0.15 MPa and an increase in tissue elastic moduli at f ull hydration of 1.5 MPa. The decrease in elasticity may enable plants to improve the soil-plant water potential gradient given a small chan ge in water content, while lower osmotic potentials shift the zero tur gor loss point to lower water potentials. Under elevated CO2, Plants i n xeric conditions had osmotic potentials 0.2 MPa lower than mesic pla nts and decreased elastic moduli at full hydration. The increase in ti ssue elasticity at elevated CO2 enabled the xeric plants to maintain p ositive turgor pressures at lower water potentials and tissue water co ntents. Surprisingly, the elevated CO2 plants under mesic conditions h ad the most inelastic tissues. We propose that this inelasticity may e nable plants to generate a favorable water potential gradient from the soil to the plant despite the low stomatal conductances observed unde r elevated CO2 conditions.