The Kinetics of Mesophyll Conductance and Photorespiration During Light Induction

Mesophyll conductance to CO₂ (<i>g</i>_m) act as a significant limiting factor influencing the CO₂ assimilation rate (<i>A</i>_N) during photosynthetic induction. However, the effect of vapor pressure deficit (VPD) on <i>g</i>_m kinetics during light induction is not well clarified. We combined gas exchange with chlorophyll fluorescence measurements to assess the induction kinetics of <i>g</i>_m during light induction under contrasting vapor pressure deficit (VPD) in two tree species with different stomatal conductance (<i>g</i>_s) behavior, <i>Catalpa fargesii</i> and <i>Pterocarya stenoptera</i>. Our results revealed three key findings: (1) the coordination of <i>g</i>_m and <i>g</i>_s kinetics during light induction occurred in <i>C. fargesii</i> but not in <i>P. stenoptera</i>, and the model of <i>g</i>_s kinetics largely determines whether the coordination of <i>g</i>_s and <i>g</i>_m exist in a given species; (2) a high VPD induced simultaneous changes in <i>g</i>_s and <i>g</i>_m kinetics in <i>C. fargesii</i> but had separated effects on <i>g</i>_s and <i>g</i>_m kinetics in <i>P. stenoptera</i>, indicating that the response of <i>g</i>_m kinetics during light induction to VPD differs between species; and (3) the relative contribution of photorespiration to total electron flow was flexible in response to the change in relative diffusional and biochemical limitations, pointing out that photorespiration has a significant role in the regulation of photosynthetic electron flow during light induction. These results provide new sight into the species-dependent kinetics of <i>g</i>_m and photorespiration during light induction.