Online CO₂ and H₂O oxygen isotope fractionation allows estimation of mesophyll conductance in C₄ plants, and reveals that mesophyll conductance decreases as leaves age in both C₄ and C₃ plants

Mesophyll conductance significantly, and variably, limits photosynthesis but we currently have no reliable method of measurement for C₄ plants. An online oxygen isotope technique was developed to allow quantification of mesophyll conductance in C₄ plants and to provide an alternative estimate in C₃ plants. The technique is compared to an established carbon isotope method in three C₃ species. Mesophyll conductance of C₄ species was similar to that in the C₃ species measured, and declined in both C₄ and C₃ species as leaves aged from fully expanded to senescing. In cotton leaves, simultaneous measurement of carbon and oxygen isotope discrimination allowed the partitioning of total conductance to the chloroplasts into cell wall and plasma membrane versus chloroplast membrane components, if CO₂ was assumed to be isotopically equilibrated with cytosolic water, and the partitioning remained stable with leaf age. The oxygen isotope technique allowed estimation of mesophyll conductance in C₄ plants and, when combined with well-established carbon isotope techniques, may provide additional information on mesophyll conductance in C₃ plants.