Carbon Isotope Composition of CAM Species

Like Rubisco from C3 and C4 plants, the enzyme from CAM plants discriminate against 13CO2, but, the fractionation at the leaf level is considerably less than that of C3 plants and similar to that of C4 species (Fig. 44). This is expected, as the stomata are closed during malate decarboxylation and fixation of CO2 by Rubisco. Hence, only a small amount of CO2 diffuses back from the leaves to the atmosphere, and Rubisco processes the accumulated 13CO2 (Sects. 9.3 and 9.4).

Upon a shift from C3 to CAM photosynthesis in facultative CAM plants, the stomata are closed during most of the day and open at night, and the carbon-isotope fractionation decreases (Fig. 49). Hence, the carbon-isotope composition of CAM plants can be used as an estimate of the employment of the CAM pathway during past growth.

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