Effect of Varying CO2 Partial Pressure on Photosynthesis and on Carbon Isotope Composition of Carbon-4 of Malate from the Crassulacean Acid Metabolism Plant Kalanchoë daigremontiana Hamet et Perr.

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Abstract
Intact leaves of K. daigremontiana were exposed to CO2 partial pressures of 100, 300 and 1000 .mu.bars. Malic acid was extracted, purified and degraded to obtain isotopic composition of C-1 and C-4. The C isotope composition of newly fixed C in malate can be calculated. In all 3 treatments, the isotopic composition of newly introduced C is the same as that of the CO2 source and is independent of CO2 partial pressures over the range tested. Comparison with numerical models descibed previously (O''Leary, 1981) indicates that C 4 of malate should be 4.permill. more negative than source CO2 if diffusion is totally limiting or 7.permill. more positive than source Co2 if carboxylation is totally limiting. Stomatal aperture adjusts to changing CO2 partial pressures and maintains the ratio of diffusion resistance to carboxylation resistance approximately constant. 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. The gas exchange and the isotope studies indicate that the ratio of external to internal CO2 partial pressure is the same in all 3 treatments. Inasmuch as a constant ratio will result in constant isotope fractionation, this may explain why plants in general have fairly invariable 13C contents, despite growing under a variety of environmental conditions.