Photosynthesis in Sitka Spruce (Picea sitchensis (Bong.) Carr.). VIII. Measurements of Stomatal Conductance and 14 CO 2 Uptake in Controlled Environments

(1) Seedlings of Sitka spruce (Picea sitchensis (Bong.) Carr.) were grown in controlled environments at various soil water regimes and vapour pressure deficits. Stomatal conductance (k$_s$) was measured by null-balance diffusion porometry, photosynthesis (F$_c$) by $^{14}$CO$_2$ uptake and shoot water potential ($\psi$) by pressure chamber. (2) On changing from light to dark and vice versa, most of the changes in k$_s$, F$_c$ and $\psi$ occurred within 3 h and equilibrium values were reached after about 6 h. (3) There was little change in k$_s$ and F$_c$ as $\psi$ fell from -6 to -16 bar, but large reductions occurred between -16 to -19 b. (4) k$_s$ declined linearly with increasing vapour pressure deficit between 0.5 and 10 mb but was less sensitive at larger deficits. (5) The response of k$_s$ to a change in vapour pressure deficit was evident after 4-5 mm and a new steady value was reached in about 40 min. (6) The rate of transpiration tended to increase in response to increases in vapour pressure deficit up to c. 15 mb, at which deficit the tendency to increase was exactly counteracted by the concurrent decline in k$_s$. (7) F$_c$ mcreased asymptotically with increasing k$_s$, becoming largely independent of j$_s$ over c. 0.3 cm s$^{-1}$. A constant mesophyll conductance of 0.092 cm s$^{-1}$ was derived from the rectangular hyperbola of best fit. This followed the data closely at high values of k$_s$ but somewhat overestimated F$_c$ at low k$_s$.