Insight into the reasons of leaf wax δD-alkane values between grasses and woods

Nuclear phylogenomics of Asteraceae with increased sampling provides new insights into convergent morphological and molecular evolution

Convergent morphological evolution is widespread in flowering plants, and understanding this phenomenon relies on well-resolved phylogenies. Nuclear phylogenetic reconstruction using transcriptome datasets has been successful in various angiosperm groups, but it is limited to taxa with available fresh materials. Asteraceae, which are one of the two largest angiosperm families and are important for both ecosystems and human livelihood, show multiple examples of convergent evolution. Nuclear Asteraceae phylogenies have resolved relationships among most subfamilies and many tribes, but many phylogenetic and evolutionary questions regarding subtribes and genera remain, owing to limited sampling. Here, we increased the sampling for Asteraceae phylogenetic reconstruction using transcriptomes and genome-skimming datasets and produced nuclear phylogenetic trees with 706 species representing two-thirds of recognized subtribes. Ancestral character reconstruction supports multiple convergent evolutionary events in Asteraceae, with gains and losses of bilateral floral symmetry correlated with diversification of some subfamilies and smaller groups, respectively. Presence of the calyx-related pappus may have been especially important for the success of some subtribes and genera. Molecular evolutionary analyses support the likely contribution of duplications of MADS-box and TCP floral regulatory genes to innovations in floral morphology, including capitulum inflorescences and bilaterally symmetric flowers, potentially promoting the diversification of Asteraceae. Subsequent divergences and reductions in CYC2 gene expression are related to the gain and loss of zygomorphic flowers. This phylogenomic work with greater taxon sampling through inclusion of genome-skimming datasets reveals the feasibility of expanded evolutionary analyses using DNA samples for understanding convergent evolution.

Intra-leaf heterogeneities of hydrogen isotope compositions in leaf water and leaf wax of monocots and dicots

Several recent studies showed that leaf wax n-alkane δ²H values (δ²H_wax) within a leaf were heterogeneous in a small number of species. It still remains unclear whether the heterogeneity of intra-leaf δ²H_wax values is general for various species, how δ²H_wax values vary spatially and temporally, and whether there is a common explanation for the intra-leaf δ²H_wax heterogeneity in higher plants. Here we compared the hydrogen isotope compositions of leaf wax and corresponding leaf water (δ²H_lw) across leaf sections among a variety of monocot and dicot plant species. There is significant and consistent heterogeneity for both δ²H_wax and δ²H_lw, i.e., base-to-tip ²H-enrichment for monocots (except Hemerocallis citrina, and Dactylis glomerata) whereas base-to-tip and center-to-edge increases in δ²H_wax and δ²H_lw for dicots. The consistent occurrence of variations of δ²H_lw and δ²H_wax values within a leaf imply that δ²H_wax values probably inherit point-to-pint from in-situ δ²H_lw values, and thus the intra-leaf δ²H_wax heterogeneity mainly results from the spatial pattern of intra-leaf δ²H_lw values associated with veinal structures between dicots and monocots. The general heterogeneity of intra-leaf δ²H_wax values further intensifies that it is necessarily needed for in-depth understanding leaf wax biomarker.

Hydrogen isotope fractionation variations of n-alkanes and fatty acids in algae and submerged plants from Tibetan Plateau lakes: Implications for palaeoclimatic reconstruction

The hydrogen isotope compositions (δD) of n-alkanes and fatty acids (FAs) are widely applied in palaeoclimatic reconstructions, and the determinations of their hydrogen isotope fractionation factor values (ε) are vital for quantitatively reconstructing past precipitation variations. Currently, studies on n-alkane and FA ε values focus on terrestrial plants, which, however, show large uncertainties because of the influence of evapotranspiration. Therefore, in this study, we analysed the ε values of algae and submerged plants immersed in lakes, which are not affected by evapotranspiration, to understand the hydrogen isotope fractionation of plant lipid synthesis. By investigating the δD values of lipids (n-alkanes and FAs) in algae and submerged plants and the δD values of co-existing water (including lake bottom water, surface sediment water, and leaf water of algae and submerged plants) from five Tibetan Plateau lakes, we find that the n-alkane ε values of algae and submerged plants show narrow changes, ranging from -176 to -159‰ and -167 to -142‰, respectively. The FA ε values of algae and submerged plants also show small variations, ranging from -160 to -121‰ (except Chara) and -161 to -138‰, respectively. Therefore, the average biosynthetic hydrogen isotope fractionation of these plants is -162‰ for n-alkanes and -145‰ for FAs, and the small ε differences between FAs and n-alkanes can be related to the different magnitudes of FA utilization in n-alkane synthesis. Finally, we find that the biosynthetic hydrogen isotope fractionation factors of aquatic plants are close to those of terrestrial grasses but slightly more negative than those of terrestrial woody plants. Thus, our results are helpful for understanding the hydrogen isotope fractionation variations in terrestrial plant lipids, which is beneficial for palaeohydrological reconstructions.

Variations in hydrogen isotopic fractionation in higher plants and sediments across different latitudes: Implications for paleohydrological reconstruction

Sedimentary δD_n-alkane value is widely utilized as a reliable proxy for paleo-hydrological reconstruction. Applications of this proxy must be based upon a globally clear understanding of the relationship between leaf wax δD_n-alkane values and precipitation δD (δD_p), defined as apparent fractionation (ε_app). However, there is a critical concern about whether relatively constant ε_app values exist across different latitudes. In this study, we systematically analyzed the variations of available ε_app with latitudes based upon two compiled-new databases of higher plants and sediments over the world. We found that the total average ε_app was relatively constant, i.e., -116 ± 5‰ (n = 941), in higher plants across different latitudes without consideration of plant types (e.g., dicots, monocots, gymnosperms), and was still constant but slightly lower average ε_app, i.e., -125 ± 6‰ (n = 460), in sediments across the latitudes. The slightly lower average ε_app in sediments relative to higher plants probably derived from the contribution of aquatic plants with isotopically D-depleted ε_app in lake sediments. Interestingly, with consideration of plant types, average ε_app increased in dicots but decreased in monocots slightly from low to high latitudes. The counteraction of these competing trends generates relatively constant average ε_app values in higher plants, and resultantly constant average ε_app values occur in sediments at the global scale. It is important to elaborate relatively constant ε_app values from higher plants and sediments across different latitudes when sedimentary δD_n-alkane is utilized as a proxy for paleohydrological reconstruction.

Using δDn-alkane as a proxy for paleo-environmental reconstruction: A good choice to sample at the site dominated by woods

Some studies have demonstrated that leaf wax δD_n-alkane values for a single species varied significantly with seasons. However, it is still not clear that the seasonality patterns of leaf wax δD_n-alkane values in higher plants. Meanwhile, few efforts have been pursued to assess the effect of the light slopes (sunny vs. cloudy) on leaf wax δD_n-alkane values. In this study, we systematically investigated plant wax δD_n-alkane values and soil n-alkane δD values along different light slopes in different seasons (spring vs. autumn), as well as the relationship of n-alkane δD values between plant leaves and soil. We found that plant wax δD_n-alkane values were D-enriched by ca. 20‰ in spring relative to autumn, and ca. 10‰ in the sunny slope than in the cloudy slope. Moreover, surface soil n-alkane δD values varied consistently with plant wax δD_n-alkane values for different seasons and light slopes. More importantly, plant wax δD_n-alkane values showed clear seasonal variations, but varied slightly with light slopes. The variations of plant wax δD_n-alkane values can be recorded in soil n-alkane δD_n-alkane values. In addition, we found that leaf wax δD_n-alkane values in a majority of species differed significantly among woods, non-woods and grasses at a site. Therefore, we suggested a good choice to sample at the site dominated by woods when leaf wax δD_n-alkane values are utilized as a proxy for the reconstruction of the paleoenvironment.

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

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.