Assessing the stable carbon isotopic composition of intercellular CO2 in a CAM plant using gas chromatography-combustion-isotope ratio mass spectrometry

Most of the literature focused on internal CO2 (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 CO2 concentrations combined with stable carbon isotopic composition in O. ficus‐indica plants. Plants were grown at both ambient and elevated CO2 concentration. During the day period, when the stomata are closed, the Ci was high and was very 13C‐enriched in both ambient and elevated CO2‐grown plants, reflecting Rubisco's fractionation (this plant enzyme has been shown to discriminate by 29‰, in vitro, against 13CO2). Other enzyme fractionations involved in C metabolism in plants, such as carbonic anhydrase, could also be playing an important role in the diurnal δ13C enrichment of the Ci. During the night, when stomata are open, Ci concentrations were higher in elevated (and the corresponding δ13C values were more 13C‐depleted) than in ambient CO2‐grown plants. Copyright © 2008 John Wiley & Sons, Ltd.