Isotope ratios of cellulose from plants having different photosynthetic pathways.

Hydrogen and carbon isotope ratios of cellulose nitrate and oxygen isotope ratios of cellulose from C(3), C(4), and Crassulacean acid metabolism (CAM) plants were determined for plants growing within a small area in Val Verde County, Texas. Plants having CAM had distinctly higher deuterium/hydrogen (D/H) ratios than plants having C(3) and C(4) metabolism. When hydrogen isotope ratios are plotted against carbon isotope ratios, each photosynthetic mode separates into a distinct cluster of points. C(4) plants had many D/H ratios similar to those of C(3) plants, so that hydrogen isotope ratios cannot be used to distinguish between these two photosynthetic modes. Portulaca mundula, which may have a modified photosynthetic mode between C(4) and CAM, had a hydrogen isotope ratio between those of the C(4) and CAM plants. When oxygen isotope ratios are plotted against carbon isotope ratios, no distinct clustering of the C(4) and CAM plants occurs. Thus, oxygen isotope ratios are not useful in distinguishing between these metabolic modes. A plot of hydrogen isotope ratios versus oxygen isotope ratios for this sample set shows considerable overlap between oxygen isotope ratios of the different photosynthetic modes without a concomitant overlap in the hydrogen isotope ratios of CAM and the other two photosynthetic modes. This observation is consistent with the hypothesis that higher D/H ratios in CAM plants relative to C(3) and C(4) plants are due to isotopic fractionations occurring during biochemical reactions.