Effect of varying CO2 [carbon dioxide] partial pressure on photosynthesis and on carbon isotope composition of carbon-4 of malate from the Crassulacean acid metabolism plant Kalanchoe daigremontiana Hamet et Perr

Intact leaves of Kalanchoë daigremontiana were exposed to CO2 partial pressures of 100, 300, and 1000 microbars. Malic acid was extracted, purified, and degraded in order to obtain isotopic composition of carbon-1 and carbon-4. From these data, it is possible to calculate the carbon isotope composition of newly fixed carbon in malate. In all three treatments, the isotopic composition of newly introduced carbon is the same as that of the CO2 source and is independent of CO2 partial pressures over the range tested. Comparison with numerical models described previously (O'Leary 1981 Phytochemistry 20: 553-567) indicates that we would expect carbon 4 of malate to be 4‰ more negative than source CO2 if diffusion is totally limiting or 7‰ more positive than source CO2 if carboxylation is totally limiting. Our results demonstrate that stomatal aperture adjusts to changing CO2 partial pressures and maintains the ratio of diffusion resistance to carboxylation resistance approximately constant. In this study, carboxylation and diffusion resistances balance so that essentially no fractionation occurs during malate synthesis. Gas exchange studies of the same leaves from which malate was extracted show that the extent of malate synthesis over the whole night is nearly independent of CO2 partial pressure, although there are small variations in CO2 uptake rate. Both the gas exchange and the isotope studies indicate that the ratio of external to internal CO2 partial pressure is the same in all three treatments. Inasmuch as a constant ratio will result in constant isotope fractionation, this observation may explain why plants in general have fairly invariable 13C contents, despite growing under a variety of environmental conditions.