Different hydrogen isotope fractionations during lipid formation in higher plants: Implications for paleohydrology reconstruction at a global scale

Abstract

Leaf wax δDn-alkane values have shown to differ significantly among plant life forms (e.g., among grasses, shrubs, and trees) in higher plants. However, the underlying causes for the differences in leaf wax δDn-alkane values among different plant life forms remain poorly understood. In this study, we observed that leaf wax δDn-alkane values between major high plant lineages (eudicots versus monocots) differed significantly under the same environmental conditions. Such a difference primarily inherited from different hydrogen biosynthetic fractionations (εwax-lw). Based upon a reanalysis of the available leaf wax δDn-alkane dataset from modern plants in the Northern Hemisphere, we discovered that the apparent hydrogen fractionation factor (εwax-p) between leaf wax δDn-alkane values of major angiosperm lineages and precipitation δD values exhibited distinguishable distribution patterns at a global scale, with an average of -140‰ for monocotyledonous species, -107‰ for dicotyledonous species. Additionally, variations of leaf wax δDn-alkane values and the εwax-p values in gymnosperms are similar to those of dicotyledonous species. Therefore, the data let us believe that biological factors inherited from plant taxonomies have a significant effect on controlling leaf wax δDn-alkane values in higher plants.

Figure 1. Whole leaf hydrogen isotope compositions from various sites in Northwestern China showing distinct difference in leaf wax δD_n-alkane values between dicotyledonous species and monocotyledonous species.

Samples were from Lantian (site 1) and Xi’an (site 2) and from the Heshui County (b).

Figure 2. Variations in leaf water and corresponding leaf wax δD_n-alkane values.

Variations of leaf water and corresponding leaf wax δD_n-alkane values on whole leaves from Xi’an, Lantian, and Binxian (a). Treatment on segmented leaf sections conducted from base to tip in monocotyledonous leaves and in base-to-tip and center-to-margin directions in dicotyledonous leaves, resulting difference in ε_wax-lw between dicotyledonous and monocotyledonous species. The difference in ε_wax-lw values derived from the quality-weighted mean ε_wax-lw values from segmented leaves was responsible for the difference in leaf wax δD_n-alkane values of whole leaf (b). Conceptual diagram illustrating a model of hydrogen isotope transformation in terrestrial plants. The transformation comprises two processes: D-enrichment by evapotranspiration and D-depletion by biosynthetic processes. The veinal structures inside the leaves (veinal water pathways, green box) control the isotopic gradients of leaf water and corresponding leaf wax (c).

Figure 3. Leaf wax δD_n-alkane values in living angiosperms along a latitudinal gradient in the North Hemisphere.

Comparisons of leaf wax δD_n-alkane values (n = 233) between herbaceous plants and woody plants, and the precipitation average δD were presented as δD = −0.037 × (latitude)² + 1.1674 × (latitude) −35.423 from Liu and Yang (2008) (a). Comparisons between monocotyledonous and dicotyledonous species based on our expanded database (n = 503). Notice, the observed higher δD_n-alkane values in Stiffkey saltmarsh (Eley et al., 2014) may be due to the fact that samples were obtained from a salt marsh (b). The parallel ε_wax-p values in dicotyledonous and monocotyledonous species are insensitive to variations of precipitation δD values, suggesting different biosynthetic processes between eudicots and monocots (c).

Figure 4. Geographic map showing sample sites in Heshui, Lantian and Xi’an and the distribution of compiled modern leaf wax δD_n-alkane values of higher plants across the North hemisphere.

Black rectangles are data based upon Liu and Yang, (2008); red dots are additional data points. (Fig. 4 was created by CorelDRAW 12; My co-authors and I grant NPG to publish the image under an Open Acess license; We grant NPG to publish the image in all formats i.e. print and digital).