Oxygen and Hydrogen Isotope Ratios of Water from Photosynthetic Tissues of CAM and C(3) Plants

Water samples from photosynthetic tissues of C₃ and Crassulacean acid metabolism (CAM) plants that grew together in the field were extracted and the stable oxygen and hydrogen isotope ratios determined. During the day, ¹⁸O/¹⁶O and deuterium/hydrogen (D/H) ratios of water from CAM plants were lower than those observed in water from C₃ plants. The patterns of diurnal variation (or lack thereof) in isotope ratios of plant water are consistent with the gross anatomical and physiological characteristics of the plants studied here. Our observations support the previously advanced hypothesis that high D/H ratios in cellulose nitrate prepared from CAM plants relative to those for C₃ plants are not caused by greater deuterium enrichment in the water in CAM plants, but rather by isotopic fractionations associated with different biochemical reactions in the two types of plants.