The influence of plant water stress on stomatal control of gas exchange at different levels of atmospheric humidity.

Leaves of well-watered and mildly water-stressed seedlings of Betula pendula Roth. and Gmelina aroborea L. were subjected to a range of vapour pressure deficits (VPD) between 10 and 24 kPa. The stomatal conductance of birch seedlings decreased as VPD was increased and at least in mildly-stressed seedlings this response seemed to be closely linked to the water status of the air rather than to the bulk water status of the plant. Mild water stressing enhanced the degree of the stomatal humidity-response and resulted in a significant increase in the efficiency of water use at high VPD. Stomata of Gmelina were apparently insensitive to variation in VPD, but were more sensitive to a decrease in bulk leaf water status than were stomata of birch. Water use efficiency of Gmelina seedlings was comparatively high, even when VPD was high and the stomata were fully open.