Leaf Conductance in Relation to Rate of CO(2) Assimilation: III. Influences of Water Stress and Photoinhibition.

Rates of CO(2) assimilation and leaf conductances to CO(2) transfer were measured in plants of Zea mays during a period of 14 days in which the plants were not rewatered, and leaf water potential decreased from -0.5 to -8.0 bar. At any given ambient partial pressure of CO(2), water stress reduced rate of assimilation and leaf conductance similarly, so that intercellular partial pressure of CO(2) remained almost constant. At normal ambient partial pressure of CO(2), the intercellular partial pressure of CO(2) was estimated to be 95 microbars. This is the same as had been estimated in plants of Zea mays grown with various levels of nitrogen supply, phosphate supply and irradiance, and in plants of Zea mays examined at different irradiances.After leaves of Phaseolus vulgaris L. and Eucalyptus pauciflora Sieb. ex Spreng had been exposed to high irradiance in an atmosphere of CO(2)-free N(2) with 10 millibars O(2), rates of assimilation and leaf conductances measured in standard conditions had decreased in similar proportions, so that intercellular partial pressure of CO(2) remained almost unchanged. As the conductance of each epidermis that had not been directly irradiated had declined as much as that in the opposite, irradiated surface it was hypothesized that conductance may have been influenced by photoinhibition within the mesophyll tissue.