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 C-13-enriched in both ambient and elevated CO2-grown plants, reflecting Rubisco's fractionation (this plant enzyme has been shown to discriminate by 29 parts per thousand, in vitro, against (CO2)-C-13). Other enzyme fractionations involved in C metabolism in plants, such as carbonic anhydrase, could also be playing an important role in the diurnal delta C-13 enrichment of the Ci. During the night, when stomata are open, Ci concentrations were higher in elevated (and the corresponding delta C-13 values were more C-13-depleted) than in ambient CO2-grown plants. Copyright (C) 2008 John Wiley & Sons, Ltd.