The Kinetics of Mesophyll Conductance and Photorespiration During Light Induction

Mesophyll conductance to CO₂ (g_m) act as a significant limiting factor influencing the CO₂ assimilation rate (A_N) during photosynthetic induction. However, the effect of vapor pressure deficit (VPD) on g_m kinetics during light induction is not well clarified. We combined gas exchange with chlorophyll fluorescence measurements to assess the induction kinetics of g_m during light induction under contrasting vapor pressure deficit (VPD) in two tree species with different stomatal conductance (g_s) behavior, Catalpa fargesii and Pterocarya stenoptera. Our results revealed three key findings: (1) the coordination of g_m and g_s kinetics during light induction occurred in C. fargesii but not in P. stenoptera, and the model of g_s kinetics largely determines whether the coordination of g_s and g_m exist in a given species; (2) a high VPD induced simultaneous changes in g_s and g_m kinetics in C. fargesii but had separated effects on g_s and g_m kinetics in P. stenoptera, indicating that the response of g_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 g_m and photorespiration during light induction.