Photosynthesis as related to xylem water potential and carbon dioxide concentration in Sitka spruce

Current-year shoots of Sitka spruce ( Picea sitchensis (Bong.) Carr.) were removed from the forest canopy. After steady-state rates of net photosynthesis were obtained in a leaf chamber, the shoots were excised in air and removed at different times to establish a relationship between net photosynthesis and xylem water potential. The experiment was repeated at five ambient carbon dioxide concentrations.
Net photosynthesis remained constant over a wide range of xylem water potential and increased linearly with ambient carbon dioxide concentration between 20 and 300 cm³ m^?3. At low water potential net photosynthesis declined at each ambient carbon dioxide concentration and there was little difference in the potential (±0.05 MPa) at which zero photosynthesis was observed.
There was a small increase in the CO₂ compensation concentration at low xylem water potentials, but calculated mesophyll conductance still declined at low water potential after correction for this change in compensation concentration. Mesophyll conductance reached zero within the same range of water potential as net photosynthesis. The results suggested that the non-stomatal contribution to the decline of photosynthesis was approximately 30% until almost complete stomatal closure occurred.