Effect of leaf water deficit on stomatal and nonstomatal regulation of net carbon dioxide assimilation.

The effect of leaf water deficit on net CO(2) assimilation was studied under two conditions: in one, the stomata were allowed to contribute to the regulation of CO(2) assimilation; in the other, air was forced through the leaf at a constant rate to overcome the effects of change in stomatal resistance accompanying changes in leaf water deficit. When the stomata were allowed to regulate the gaseous diffusive resistance of the leaf, CO(2) assimilation decreased with increasing leaf water deficit. However, when air was forced through the leaf, the rate of assimilation was not inhibited by increasing leaf water deficit. The results indicate that the inhibition of net CO(2) assimilation with increasing leaf water deficit is a consequence of an increase in the diffusive resistance to gas exchange and not of a change in apparent mesophyll resistance.