Isotope Ratios of Cellulose from Plants Having Different Photosynthetic Pathways

Abstract

H and C isotope ratios of cellulose nitrate and O isotope ratios of cellulose from C3, C4 and Crassulacean acid metabolism (CAM) plants were determined for plants growing within a small area in Val Verde County, Texas [USA]. Plants having CAM had distinctly higher deuterium/hydrogen (D/H) ratios than plants having C3 and C4 metabolism. When H isotope ratios are plotted against C isotope ratios, each photosynthetic mode separates into a distinct cluster of points. C4 plants had many D/H ratios similar to those of C3 plants, so that H isotope ratios cannot be used to distinguish between these 2 photosynthetic modes. Portulaca mundula, which may have a modified photosynthetic mode between C4 and CAM, had a H isotope ratio between those of the C4 and CAM plants. When O isotope ratios are plotted against carbon isotope ratios, no distinct clustering of the C4 and CAM plants occurs. O isotope ratios are not useful in distinguishing between these metabolic modes. A plot of H isotope ratios vs. O isotope ratios for this sample set shows considerable overlap between O isotope ratios of the different photosynthetic modes without a concomitant overlap in the H isotope ratios of CAM and the other 2 photosynthetic modes. This observation is consistent with the hypothesis that higher D/H ratios in CAM plants relative to C3 and C4 plants are due to isotopic fractionations occurring during biochemical reactions.