Using New Gas Exchange Methods to Estimate Mesophyll Conductance and Non-Stomatal Inhibition of Photosynthesis Caused by Water Deficits

Soil water deficits remain one of the most important factors reducing the yield of crop plants, and may become even more limiting with changes in the global climate and competition for fresh water resources.  Soil water deficits reduce plant growth partly by reducing photosynthesis.  However, it remains unclear how important non-stomatal factors are in limiting photosynthesis under moderate water stress, and whether rising atmospheric carbon dioxide may alter which processes limit photosynthesis under water stress.  The conductance to CO₂ from the sub-stomatal air space to the site of carboxylation inside chloroplasts in C₃ plants is now termed mesophyll conductance. Because of the competition between CO₂ and O₂ for RuBisco, the carbon dioxide concentration at the chloroplast can be estimated from the O₂ sensitivity of photosynthesis, providing a new method of estimating mesophyll conductance.  It has also recently been realized that partial stomatal closure due to water stress can often be reversed by exposing leaves to low CO₂.  This provides a new means of assessing the non-stomatal component of the inhibition of photosynthesis by water stress.  These methods were applied to four C₃ species, and revealed that mesophyll conductance decreased substantially with water stress in two of the four species, and that re-opening of stomata did not eliminate the reduction in photosynthesis caused by moderate water stress at either the current ambient or elevated CO₂ concentrations.