The response of mesophyll conductance to short-term CO 2 variation is related to stomatal conductance.

The response of mesophyll conductance (g m ) to CO 2 plays a key role in photosynthesis and ecosystem carbon cycles under climate change. Despite numerous studies, there is still debate about how g m responds to short-term CO 2 variations. Here we used multiple methods and looked at the relationship between stomatal conductance to CO 2 (g sc ) and g m to address this aspect. We measured chlorophyll fluorescence parameters and online carbon isotope discrimination (Δ) at different CO 2 mole fractions in sunflower (Helianthus annuus L.), cowpea (Vigna unguiculata L.), and wheat (Triticum aestivum L.) leaves. The variable J and Δ based methods showed that g m decreased with an increase in CO 2 mole fraction, and so did stomatal conductance. There were linear relationships between g m and g sc across CO 2 mole fractions. g m obtained from A-C i curve fitting method was higher than that from the variable J method and was not representative of g m under the growth CO 2 concentration. g m could be estimated by empirical models analogous to the Ball-Berry model and the USO model for stomatal conductance. Our results suggest that g m and g sc respond in a coordinated manner to short-term variations in CO 2 , providing new insight into the role of g m in photosynthesis modelling.