Covariance of Oxygen and Hydrogen Isotopic Compositions in Plant Water: Species Effects

Abstract

Leaf water becomes enriched in the heavy isotopes of oxygen and hydrogen during evapotranspiration. The magnitude of the enrichment has been shown to be influenced by temperature and humidity, but the effects of species—specific factors on leaf water enrichment of D and 18O have not been studied for different plants growing together. Accordingly, to learn whether leaf water enrichment patterns and processes for D and 18O are different for individual species growing under the same environmental conditions we tested the proposal that leaf waters in plants with crassulacean acid metabolism (CAM) show higher slopes (m in the leaf water equation °D = m °18O + b) than in C3 plants. We determined the relationships between the stable hydrogen (°D) and oxygen (°18O) isotope ratios of leaf waters collected during the diurnal cycle of evapotranspiration for Yucca schidigera, Ephedra aspera, Agave deserti, Prunus ilicifolia, Yucca whipplei, Heteromeles arbutifolia, Dyckia fosteriana, Simmondsia chinensis, and Encelia farinosa growing at two sites in southern California. Slopes (m in the above leaf water equation) ranged from 1.50 to 3.21, compared to °8 for meteoric water, but differences in slope could not be attributed to carboxylation pathway (CAM vs. C3) nor climate (coastal California vs. Sonoran Desert). Higher slopes were correlated with greater overall ranges of leaf water enrichment of D and 18O. Water in plants with higher slopes also differed most from unaltered meteoric water. Leaf water isotope ratios in plants with lower slopes were better correlated with temperature and humidity. The findings indicate that m in the aforementioned equation is related to the overall residence time for water in the leaf and proportions of water subjected to repeated evapotranspiration enrichments of heavy isotopes.