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

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