Leaf water potential and control of water loss in droughted Sitka spruce seedlings

Sitka spruce seedlings were subjected to drought in experiments in a growthroom, a greenhouse, and out of doors. The plants were grown in a double chamber with the bulk of the roots in the upper part where they dried out the soil when water was withheld. A few new roots penetrated into the lower part in which the soil remained moist. The double chamber system enabled the plant to attain a high water psotential by night and the shoot was only periodically under mild water stress. Measurements were made on soil water potential (ψsolt), leaf water potential (ψ1), transpiration (E), and stomatal conductance (ks). As ψ soil decreased over a period of 4.5 d, E and ks decreased progressively. The decline in E and ks which indicated stomatal closure, occurred at a higher ψ1 than has been reported for Sitka spruce. The behaviour of the stomata appeared to be modified by conditions at the root, and it is proposed that differences in the response toψ1, depend on Whether the latter is reduced by resistances in the xylem between root and leaf, as is known to occur in large trees in moist soil, or by stresses at the root itself.