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 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 per thousand, 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 delta(13)C enrichment of the Ci. During the night, when stomata are open, Ci concentrations were higher in elevated (and the corresponding delta(13)C values were more (13)C-depleted) than in ambient CO(2)-grown plants.