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 C-4 plants. An online oxygen isotope technique was developed to allow quantification of mesophyll conductance in C-4 plants and to provide an alternative estimate in C-3 plants. The technique is compared to an established carbon isotope method in three C-3 species. Mesophyll conductance of C-4 species was similar to that in the C-3 species measured, and declined in both C-4 and C-3 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 C-4 plants and, when combined with well-established carbon isotope techniques, may provide additional information on mesophyll conductance in C-3 plants.