Hydrogen isotopic differences between C3 and C4 land plant lipids: consequences of compartmentation in C4 photosynthetic chemistry and C3 photorespiration

Time-integrated δ2H in n-alkanes and carbohydrates from boreal needles reveal intra-annual physiological and environmental signals

Reliable insights from key δ2H bioindicators, n-alkanes and carbohydrates, are hindered by our limited understanding of isotope fractionation processes related to leaf water and primary assimilates. We addressed this with the first study to investigate time-integrated intra-annual δ2H signals of n-alkanes and carbohydrates in a natural forest. We sampled 1-yr-old needles (1N) and current-year needles (0N) from five Scots Pine trees in a coniferous forest in Hyytiälä, Finland, biweekly during 2019. The δ2H of their n-alkanes (δ2Halkane), water-soluble carbohydrates (δ2HWSC) and starch (δ2Hstarch) were evaluated for time-integrated physiological (gas-exchange) and environmental (hydrological) signals. Time integration was critical for interpreting δ2HWSC and δ2Halkane. Time-integrated net assimilation rate (T:An), the strongest signal in δ2HWSC, correlated negatively with 1N δ2HWSC (early season) and positively with 0N δ2HWSC (late season). Unexpectedly, δ2Halkane exhibited stronger environmental signals in 1N than in 0N, with the most pronounced being physiologically mediated hydrological signals. T:An is a major signal in intra-annual δ2HWSC in a boreal forest, subject to seasonal interactions with needle age and δ2Hstarch. There could be enough de novo n-alkane synthesis in situ to enhance the reliability of δ2Halkane as a hydrological indicator, bringing promise to interpretations of the n-alkane palaeohydrological record.

Phylogenetic and biochemical drivers of plant species variation in organic compound hydrogen stable isotopes: novel mechanistic constraints

Significant variation in plant organic compound hydrogen stable isotope (δ2H) values among species from a single location suggests species biochemistry diversity as a key driver. However, the biochemical mechanisms and the biological relevance behind this species-specific δ2H variation remain unclear. We analyzed δ2H values of cellulose and n-alkanes across 179 eudicot species in a botanical garden sampled in 2019, and cellulose, n-alkanes, fatty acids and phytol δ2H values from 56 eudicot species sampled in 2020. We utilized the observed species variation in δ2H values to determine phylogenetic structure and mechanistic constraints for biochemical 2H-fractionation. A strong phylogenetic signal in lipid compound δ2H values implies that the drivers of species variation in lipid δ2H values are evolutionarily conserved. By contrast, species variation in cellulose δ2H values was not strongly linked to phylogeny. Generally low-explanatory power of relationships between δ2H values of different compounds (R2 < 0.26) implies nonubiquitous drivers of species variation in plant organic compound δ2H values. Historically, variable biochemical 2H-fractionation was often attributed to δ2H values of H incorporated from NADPH. Instead, the results from this study suggest that species variation in biochemical 2H-fractionation largely occurs independently within biosynthetic pathways. For lipids, these mechanisms appear strongly linked to evolutionary history.

Hydrogen isotope fractionation in plants with C3, C4, and CAM CO2 fixation

Measurements of stable isotope ratios in organic compounds are widely used tools for plant ecophysiological studies. However, the complexity of the processes involved in shaping hydrogen isotope values (δ2H) in plant carbohydrates has limited its broader application. To investigate the underlying biochemical processes responsible for 2H fractionation among water, sugars, and cellulose in leaves, we studied the three main CO2 fixation pathways (C3, C4, and CAM) and their response to changes in temperature and vapor pressure deficit (VPD). We show significant differences in autotrophic 2H fractionation (εA) from water to sugar among the pathways and their response to changes in air temperature and VPD. The strong 2H depleting εA in C3 plants is likely driven by the photosynthetic H+ production within the thylakoids, a reaction that is spatially separated in C4 and strongly reduced in CAM plants, leading to the absence of 2H depletion in the latter two types. By contrast, we found that the heterotrophic 2H-fractionation (εH) from sugar to cellulose was very similar among the three pathways and is likely driven by the plant's metabolism, rather than by isotopic exchange with leaf water. Our study offers new insights into the biochemical drivers of 2H fractionation in plant carbohydrates.

Different responses of oxygen and hydrogen isotopes in leaf and tree-ring organic matter to lethal soil drought

The oxygen and hydrogen isotopic composition (δ18O, δ2H) of plant tissues are key tools for the reconstruction of hydrological and plant physiological processes and may therefore be used to disentangle the reasons for tree mortality. However, how both elements respond to soil drought conditions before death has rarely been investigated. To test this, we performed a greenhouse study and determined predisposing fertilization and lethal soil drought effects on δ18O and δ2H values of organic matter in leaves and tree rings of living and dead saplings of five European tree species. For mechanistic insights, we additionally measured isotopic (i.e. δ18O and δ2H values of leaf and twig water), physiological (i.e. leaf water potential and gas-exchange) and metabolic traits (i.e. leaf and stem non-structural carbohydrate concentration, carbon-to-nitrogen ratios). Across all species, lethal soil drought generally caused a homogenous 2H-enrichment in leaf and tree-ring organic matter, but a low and heterogenous δ18O response in the same tissues. Unlike δ18O values, δ2H values of tree-ring organic matter were correlated with those of leaf and twig water and with plant physiological traits across treatments and species. The 2H-enrichment in plant organic matter also went along with a decrease in stem starch concentrations under soil drought compared with well-watered conditions. In contrast, the predisposing fertilization had generally no significant effect on any tested isotopic, physiological and metabolic traits. We propose that the 2H-enrichment in the dead trees is related to (i) the plant water isotopic composition, (ii) metabolic processes shaping leaf non-structural carbohydrates, (iii) the use of carbon reserves for growth and (iv) species-specific physiological adjustments. The homogenous stress imprint on δ2H but not on δ18O suggests that the former could be used as a proxy to reconstruct soil droughts and underlying processes of tree mortality.

Stable Carbon Isotope Ratios (δ13C Values [‰]) of Individual Sterols in the Oils of C3, C4, and CAM Plants

The compound-specific determination of δ13C values [‰] by gas chromatography interfaced with isotope ratio mass spectrometry (GC-IRMS) is a powerful analytical method to indicate minute but relevant variations in the 13C/12C ratio of sample compounds. In this study, the δ13C values [‰] of individual sterols were measured in eleven different oils of C3, C4, and CAM plants (n = 33) by GC-IRMS. For this purpose, a suitable acetylation method was developed for sterols. Nine of the eleven phytosterols identified by GC with mass spectrometry (GC/MS) could be measured by GC-IRMS. The δ13C values [‰] of individual sterols and squalene of C3 plant oils were between 3‰ and >16‰ more negative (lighter in carbon) than in C4 and CAM oils. We also showed that the blending of C4 oils into C3 oils (exemplarily conducted with one olive and one corn oil) would be precisely determined by means of the δ13C value [‰] of β-sitosterol.

n-Alkane 13C/12C indicates differential metabolic controls of fatty lipid chain extension in C3 and C4 grasses

Fundamental differences in metabolic control of fatty acids chain extension are reflected in the contrasting carbon isotopic composition profiles of C3 and C4 grasses.

Metabolic exchange between pathways for isoprenoid synthesis and implications for biosynthetic hydrogen isotope fractionation

Hydrogen isotope ratios of plant lipids are used for paleoclimate reconstruction, but are influenced by both source water and biosynthetic processes. Measuring ² H : ¹ H ratios of multiple compounds produced by different pathways could allow these effects to be separated, but hydrogen isotope fractionations during isoprenoid biosynthesis remain poorly constrained. To investigate how hydrogen isotope fractionation during isoprenoid biosynthesis is influenced by molecular exchange between the cytosolic and plastidial production pathways, we paired position-specific ¹³ C-pyruvate labeling with hydrogen isotope measurements of lipids in Pachira aquatica saplings. We find that acetogenic compounds primarily incorporated carbon from ¹³ C2-pyruvate, whereas isoprenoids incorporated ¹³ C1- and ¹³ C2-pyruvate equally. This indicates that cytosolic pyruvate is primarily introduced into plastidial isoprenoids via glyceraldehyde 3-phosphate and that plastidial isoprenoid intermediates are incorporated into cytosolic isoprenoids. Probably as a result of the large differences in hydrogen isotope fractionation between plastidial and cytosolic isoprenoid pathways, sterols from P. aquatica are at least 50‰ less ² H-enriched relative to phytol than sterols in other plants. These results provide the first experimental evidence that incorporation of plastidial intermediates reduces ² H : ¹ H ratios of sterols. This suggests that relative offsets between the ² H : ¹ H ratios of sterols and phytol can trace exchange between the two isoprenoid synthesis pathways.

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.

Leaf transition from heterotrophy to autotrophy is recorded in the intraleaf C, H and O isotope patterns of leaf organic matter

RATIONALE

Quantitatively relating ¹³ C/¹² C, ² H/¹ H and ¹⁸ O/¹⁶ O ratios of plant α-cellulose and ² H/¹ H of n-alkanes to environmental conditions and metabolic status should ideally be based on the leaf, the plant organ most sensitive to environmental change. The fact that leaf organic matter is composed of isotopically different heterotrophic and autotrophic components means that it is imperative that one be able to disentangle the relative heterotrophic and autotrophic contributions to leaf organic matter.

METHODS

We tackled this issue by two-dimensional sampling of leaf water and α-cellulose, and specific n-alkanes from greenhouse-grown immature and mature and field-grown mature banana leaves, taking advantage of their large areas and thick waxy layers. Leaf water, α-cellulose and n-alkane isotope ratios were then characterized using elemental analysis isotope ratio mass spectrometry (IRMS) or gas chromatography IRMS. A three-member (heterotrophy, autotrophy and photoheterotrophy) conceptual linear mixing model was then proposed for disentangling the relative contributions of the three trophic modes.

RESULTS

We discovered distinct spatial leaf water, α-cellulose and n-alkane isotope ratio patterns that varied with leaf developmental stages. We inferred from the conceptual model that, averaged over the leaf blade, only 20% of α-cellulose in banana leaf is autotrophically laid down in both greenhouse-grown and field-grown banana leaves, while approximately 60% and 100% of n-alkanes are produced autotrophically in greenhouse-grown and field-grown banana leaves, respectively. There exist distinct lateral (edge to midrib) gradients in autotrophic contributions of α-cellulose and n-alkanes.

CONCLUSIONS

Efforts to establish quantitative isotope-environment relationships should take into account the fact that the evaporative leaf water ¹⁸ O and ² H enrichment signal recorded in autotrophically laid down α-cellulose is significantly diluted by the heterotrophically formed α-cellulose. The δ² H value of field-grown mature banana leaf n-alkanes is much more sensitive than α-cellulose as a recorder of the growth environment. Quantitative isotope-environment relationship based on greenhouse-grown n-alkane δ² H values may not be reliable.

The effect of processing medium on the 2 H/1 H of carbon-bound hydrogen in α-cellulose extracted from higher plants

RATIONALE

Although the ² H/¹ H ratio of the carbon-bound hydrogens (C-Hs) in α-cellulose extracted from higher plants has long been used successfully for climate, environmental and metabolic studies, the assumption that bleaching with acidified NaClO₂ to remove lignin before pure α-cellulose can be obtained does not alter the ² H/¹ H ratio of α-cellulose C-Hs has nonetheless not been tested.

METHODS

For reliable application of the ² H/¹ H ratio of α-cellulose C-H, we processed plant materials representing different phytochemistries and photosynthetic carbon assimilation modes in isotopically contrasting bleaching media (with an isotopic difference of 273 mUr). All the isotope ratios were measured by elemental analyzer/isotope ratio mass spectrometry (EA/IRMS).

RESULTS

Our results show that H from the bleaching medium does appear in the final pure α-cellulose product, although the isotopic alteration to the C-H in α-cellulose due to the incorporation of processing H from the medium is small if isotopically "natural" water is used to prepare the processing medium. However, under prolonged bleaching such an isotope effect can be significant, implying that standardizing the bleaching process is necessary for reliable ² H/¹ H measurement.

CONCLUSIONS

The currently adopted method for removing lignin for α-cellulose extraction from higher plant materials with acidified NaClO₂ bleaching is considered acceptable in terms of preserving the isotopic fidelity if isotopically "natural" water is used to prepare the bleaching solution.

Novel Position-Specific 18O/16O Measurement of Carbohydrates. II. The Complete Intramolecular 18O/16O Profile of the Glucose Unit in a Starch of C4 Origin

Information about plant photosynthetic carbon assimilation, physiology, and biochemistry is locked in the ¹⁸O/¹⁶O ratios of the individual positions of higher plants carbohydrates but is under-utilized, because of the difficulty of making these determinations. We report the extension of the wet chemistry approach we used to access the ¹⁸O/¹⁶O ratio of O-3 of glucose with a novel GC/Pyrolysis/IRMS-based method, to determine the ¹⁸O/¹⁶O ratios of O-4, O-5, and O-6. The O atoms (OH groups) at positions 1, 2, 5, and 6 of glucose were protected by acetonation (converting to 1,2;5,6-di-O-isopropylidene-glucofuranose, DAGF). The DAGF was then converted to 6-bromo-6-deoxy-1,2;3,5-di-O-isopropylidene-glucofuranose (6-bromoDAGF) with the simultaneous removal of O-6 with N-bromosuccinimide and triphenylphosphine. The DAGF was also methylated at O-3 with CH₃I under the catalysis of NaH to 3-methylDAGF, which was then deacetonated to 1,2-O-isopropylidene-3-O-methyl-glucofuranose (3-methylMAGF). O-5 and O-6 were then removed as a whole from 3-methylMAGF by I₂ oxidization under the catalysis of Ph₃P and imidazole. Isotope mass balance was then applied to calculate the ¹⁸O/¹⁶O of O-5 and O-6 as a whole and O-6, respectively. Sampling at different stages of substrate conversion to product and applying a Rayleigh-type fractionation model were employed, when quantitative conversion of substrate was unachievable to calculate the δ¹⁸O of the converted substrate. Quantitative conversion of glucose with phenylhydrazine to phenylglucosazone also allowed for the calculation of δ¹⁸O₂ by applying isotope mass balance between the two. A C4 starch-derived glucose intramolecular δ¹⁸O profile is now determined: O-3 is relatively enriched (by 12.16 mUr), O-4 is relatively depleted (by 20.40-31.11 mUr), and O-2 is marginally enriched (by 2.40 mUr) against the molecular average.

Agrometeorological models to forecast açaí (Euterpe oleracea Mart.) yield in the Eastern Amazon

BACKGROUND

The increasing demand in Brazil and the world for products derived from the açaí palm (Euterpe oleracea Mart) has generated changes in its production process, principally due to the necessity of maintaining yield in situations of seasonality and climate fluctuation. The objective of this study was to estimate açaí fruit yield in irrigated system (IRRS) and rainfed system or unirrigated (RAINF) using agrometeorological models in response to climate conditions in the eastern Amazon. Modeling was done using multiple linear regression using the 'stepwise forward' method of variable selection. Monthly air temperature (T) values, solar radiation (SR), vapor pressure deficit (VPD), precipitation + irrigation (P + I), and potential evapotranspiration (PET) in six phenological phases were correlated with yield. The thermal necessity value was calculated through the sum of accumulated degree days (ADD) up to the formation of fruit bunch, as well as the time necessary for initial leaf development, using a base temperature of 10 °C.

RESULTS

The most important meteorological variables were T, SR, and VPD for IRRS, and for RAINF water stress had the greatest effect. The accuracy of the agrometeorological models, using maximum values for mean absolute percent error (MAPE), was 0.01 in the IRRS and 1.12 in the RAINF.

CONCLUSION

Using these models yield was predicted approximately 6 to 9 months before the harvest, in April, May, November, and December in the IRRS, and January, May, June, August, September, and November for the RAINF. © 2019 Society of Chemical Industry.

2 H-fractionations during the biosynthesis of carbohydrates and lipids imprint a metabolic signal on the δ2 H values of plant organic compounds

Hydrogen (H) isotope ratio (δ² H) analyses of plant organic compounds have been applied to assess ecohydrological processes in the environment despite a large part of the δ² H variability observed in plant compounds not being fully elucidated. We present a conceptual biochemical model based on empirical H isotope data that we generated in two complementary experiments that clarifies a large part of the unexplained variability in the δ² H values of plant organic compounds. The experiments demonstrate that information recorded in the δ² H values of plant organic compounds goes beyond hydrological signals and can also contain important information on the carbon and energy metabolism of plants. Our model explains where ² H-fractionations occur in the biosynthesis of plant organic compounds and how these ² H-fractionations are tightly coupled to a plant's carbon and energy metabolism. Our model also provides a mechanistic basis to introduce H isotopes in plant organic compounds as a new metabolic proxy for the carbon and energy metabolism of plants and ecosystems. Such a new metabolic proxy has the potential to be applied in a broad range of disciplines, including plant and ecosystem physiology, biogeochemistry and palaeoecology.

On the contributions of photorespiration and compartmentation to the contrasting intramolecular 2H profiles of C3 and C4 plant sugars

Compartmentation of C₄ photosynthetic biochemistry into bundle sheath (BS) and mesophyll (M) cells, and photorespiration in C₃ plants is predicted to have hydrogen isotopic consequences for metabolites at both molecular and site-specific levels. Molecular-level evidence was recently reported (Zhou et al., 2016), but evidence at the site-specific level is still lacking. We propose that such evidence exists in the contrasting ²H distribution profiles of glucose samples from naturally grown C₃, C₄ and CAM plants: photorespiration contributes to the relative ²H enrichment in H⁵ and relative ²H depletion in H¹ & H⁶ (the average of the two pro-chiral Hs and in particular H^6,pro-R) in C₃ glucose, while ²H-enriched C₃ mesophyll cellular (chloroplastic) water most likely contributes to the enrichment at H⁴; export of (transferable hydrogen atoms of) NADPH from C₄ mesophyll cells to bundle sheath cells (via the malate shuttle) and incorporation of ²H-relatively unenriched BS cellular water contribute to the relative depletion of H⁴ & H⁵ respectively; shuttling of triose-phosphates (PGA: phosphoglycerate dand DHAP: dihydroacetone phosphate) between C₄ bundle sheath and mesophyll cells contributes to the relative enrichment in H¹ & H⁶ (in particular H^6,pro-R) in C₄ glucose.

Natural Isotope Abundance in Metabolites: Techniques and Kinetic Isotope Effect Measurement in Plant, Animal, and Human Tissues

The natural isotope abundance in bulk organic matter or tissues is not a sufficient base to investigate physiological properties, biosynthetic mechanisms, and nutrition sources of biological systems. In fact, isotope effects in metabolism lead to a heterogeneous distribution of ²H, ¹⁸O, ¹³C, and ¹⁵N isotopes in metabolites. Therefore, compound-specific isotopic analysis (CSIA) is crucial to biological and medical applications of stable isotopes. Here, we review methods to implement CSIA for ¹⁵N and ¹³C from plant, animal, and human samples and discuss technical solutions that have been used for the conversion to CO₂ and N₂ for IRMS analysis, derivatization and isotope effect measurements. It appears that despite the flexibility of instruments used for CSIA, there is no universal method simply because the chemical nature of metabolites of interest varies considerably. Also, CSIA methods are often limited by isotope effects in sample preparation or the addition of atoms from the derivatizing reagents, and this implies that corrections must be made to calculate a proper δ-value. Therefore, CSIA has an enormous potential for biomedical applications, but its utilization requires precautions for its successful application.

The δ18 O and δ2 H of water in the leaf growth-and-differentiation zone of grasses is close to source water in both humid and dry atmospheres

The oxygen and hydrogen isotope composition of water in the leaf growth-and-differentiation zone, LGDZ, (δ¹⁸ O_LGDZ , δ² H_LGDZ ) of grasses influences the isotopic composition of leaf cellulose (oxygen) and wax (hydrogen) - important for understanding (paleo)environmental and physiological information contained in these biological archives - but is presently unknown. This work determined δ¹⁸ O_LGDZ and δ² H_LGDZ , ¹⁸ O- and ² H-enrichment of LGDZ (∆¹⁸ O_LGDZ and ∆² H_LGDZ ), and the ¹⁸ O- and ² H-enrichment of leaf blade water (∆¹⁸ O_LW, ∆² H_LW ) in two C₃ and three C₄ grasses grown at high and low vapor pressure deficit (VPD). The proportion of unenriched water (p_x ) in the LGDZ ranged from 0.9 to 1.0 for ¹⁸ O and 1.0 to 1.2 for ² H. VPD had no effect on the proportion of ¹⁸ O- and ² H-enriched water in the LGDZ, and species effects were small or nonsignificant. Deuterium discrimination caused depletion of ² H in LGDZ water, increasing (apparent) p_x -values > 1.0 in some cases. The isotopic composition of water in the LGDZ was close to that of source water, independent of VPD and much less enriched than previously supposed, but similar to reported xylem water in trees. The well-constrained p_x will be useful in future investigations of oxygen and hydrogen isotopic fractionation during cellulose and wax synthesis, respectively.