Leaf Conductance as Related to Xylem Water Potential and Carbon Dioxide Concentration in Sitka Spruce

Abstract

AbstractCurrent year shoots of Sitka spruce [Picea sitchensis Bong. (Carr.)] from the forest canopy were equilibrated in a leaf chamber. The shoots were excised in air, and removed at differing times in order to establish a relationship between stomatal conductance and xylem water potential. The experiment was repeated at five ambient CO2 concentrations. A second set of excised forest shoots, and shoots excised from 2‐year‐ old nursery seedlings were allowed to evaporate freely in a controlled environment wind tunnel until a constant rate of transpiration was measured, to establish a relationship between cuticular conductance and xylem water potential.Cuticular conductance was estimated to be 0.012 cm s‐1 at high water potential and declined linearly to 0.007 cm s‐1 at −3.5 MPa. The implication of this decline in the subsequent calculation of stomatal and mesophyll conductance is considered.Stomatal conductance remained constant at water potentials above −1.4 MPa and was not affected by ambient carbon dioxide concentrations between 20 and 600 cm‐3. At lower water potentials, stomatal conductance declined and approached zero at −2.5 to −2.6 MPa. The results suggest that stomatal aperture is not controlled by either ambient or intercellular space carbon dioxide concentration, and that stomatal closure at low water potential is unlikely to be mediated by carbon dioxide.