Effects of Drought Stress On Water Use Efficiency and Component Limitations to Photosynthesis in Soybean.

It is generally agreed that water stress reduces photosynthesis in soybean (Glycine max L. Merr.) and other species by inducing stomatal closure and thus reducing leaf internal CO₂ concentrations.  However, less is known about the magnitude and physiological basis of residual limitations to photosynthesis that may persist following relief of water stress.  We conducted a controlled environment experiment to compare six Ontario-adapted commercial soybean cultivars for their ability to recover photosynthetic capacity following a simulated water stress.  Plants were exposed to two cycles of controlled soil dry down over a period of two weeks in a greenhouse.  The water stress treatment reduced both shoot dry matter and total plant water use by 50%, with no effect on whole plant water use efficiency.  Combined leaf gas exchange and chlorophyll fluorescence analysis was used to quantify component limitations to leaf photosynthesis of fully expanded leaves 24 h after rewatering.  No treatment x cultivar interactions were found for leaf-level measurements, so analysis was combined across cultivars.  Compared to control plants, plants that had been exposed to water stress had reduced stomatal conductance and leaf net CO₂ assimilation rates.   However, gas phase leaf internal CO₂ concentrations were only slightly reduced.  In contrast, chloroplast CO₂ concentrations were strongly reduced, as was mesophyll conductance to CO₂.  We conclude that increased resistance to CO₂ diffusion between the substomatal cavity and chloroplasts constitutes a major component of the persistent limitation to photosynthesis in soybean following recovery from water stress.