Photosynthesis in Sitka spruce (Picea sitchensis (Bong.) Carr.) VIII. Measurements of stomatal conductance and 14CO2 uptake in controlled environments.
[See FA 38, 1151]. Stomatal conductance (ks), photosynthesis (Fc) and shoot water potential ( psi ) were measured for seedlings of P. sitchensis grown under various soil water regimes and vapour pressure deficits (VPD). Fc and ks were relatively constant as psi decreased from -6 to -16 bar, but fell sharply between -16 and -19 bar. Increase in VPD caused a linear decrease in ks from 0.5 to 10 mbar but the decrease was less rapid at larger deficits. The transpiration rate increased with increase in VPD up to 15 mbar; at this deficit, the tendency to increase was exactly counteracted by the concurrent decline in ks. Fc increased asymptotically with increasing ks. A constant mesophyll conductance of 0.092 cm/sec was derived from the rectangular hyperbola of best fit. This value followed the experimental data closely at high values of ks, but overestimated Fc at low ks. From authors' summary.