Assessing the stable carbon isotopic composition of intercellular CO 2 in a CAM plant using gas chromatography‐combustion‐isotope ratio mass spectrometry

Most of the literature focused on internal CO 2 (Ci) determinations in plants has used indirect methods based on gas‐exchange estimations. We have developed a new method based on the capture of internal air gas samples and their analysis by gas chromatography‐combustion‐isotope ratio mass spectrometry (GC‐C‐IRMS). This method provided a direct measure of intercellular CO 2 concentrations combined with stable carbon isotopic composition in O. ficus‐indica plants. Plants were grown at both ambient and elevated CO 2 concentration. During the day period, when the stomata are closed, the Ci was high and was very 13 C‐enriched in both ambient and elevated CO 2 ‐grown plants, reflecting Rubisco's fractionation (this plant enzyme has been shown to discriminate by 29‰, in vitro , against 13 CO 2 ). Other enzyme fractionations involved in C metabolism in plants, such as carbonic anhydrase, could also be playing an important role in the diurnal δ 13 C enrichment of the Ci. During the night, when stomata are open, Ci concentrations were higher in elevated (and the corresponding δ 13 C values were more 13 C‐depleted) than in ambient CO 2 ‐grown plants. Copyright © 2008 John Wiley & Sons, Ltd.