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

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.

Reconciling water-use efficiency estimates from carbon isotope discrimination of leaf biomass and tree rings: nonphotosynthetic fractionation matters

Carbon isotope discrimination (∆) in leaf biomass (∆BL) and tree rings (∆TR) provides important proxies for plant responses to climate change, specifically in terms of intrinsic water-use efficiency (iWUE). However, the nonphotosynthetic 12C/13C fractionation in plant tissues has rarely been quantified and its influence on iWUE estimation remains uncertain. We derived a comprehensive, ∆ based iWUE model (iWUEcom) which includes nonphotosynthetic fractionations (d) and characterized tissue-specific d-values based on global compilations of data of ∆BL, ∆TR and real-time ∆ in leaf photosynthesis (∆online). iWUEcom was further validated with independent datasets. ∆BL was larger than ∆online by 2.53‰, while ∆BL and ∆TR showed a mean offset of 2.76‰, indicating that ∆TR is quantitatively very similar to ∆online. Applying the tissue-specific d-values (dBL = 2.5‰, dTR = 0‰), iWUE estimated from ∆BL aligned well with those estimated from ∆TR or gas exchange. ∆BL and ∆TR showed a consistent iWUE trend with an average CO2 sensitivity of 0.15 ppm ppm-1 during 1975-2015. Accounting for nonphotosynthetic fractionations improves the estimation of iWUE based on isotope records in leaf biomass and tree rings, which is ultimate for inferring changes in carbon and water cycles under historical and future climate.

Biochemical and biophysical drivers of the hydrogen isotopic composition of carbohydrates and acetogenic lipids

The hydrogen isotopic composition (δ2H) of plant compounds is increasingly used as a hydroclimatic proxy; however, the interpretation of δ2H values is hampered by potential coeffecting biochemical and biophysical processes. Here, we studied δ2H values of water and carbohydrates in leaves and roots, and of leaf n-alkanes, in two distinct tobacco (Nicotiana sylvestris) experiments. Large differences in plant performance and biochemistry resulted from (a) soil fertilization with varying nitrogen (N) species ratios and (b) knockout-induced starch deficiency. We observed a strong 2H-enrichment in sugars and starch with a decreasing performance induced by increasing NO3-/NH4+ ratios and starch deficiency, as well as from leaves to roots. However, δ2H values of cellulose and n-alkanes were less affected. We show that relative concentrations of sugars and starch, interlinked with leaf gas exchange, shape δ2H values of carbohydrates. We thus provide insights into drivers of hydrogen isotopic composition of plant compounds and into the mechanistic modeling of plant cellulose δ2H values.

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.

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.

δ13C of bulk organic matter and cellulose reveal post-photosynthetic fractionation during ontogeny in C4 grass leaves

The 13C isotope composition (δ13C) of leaf dry matter is a useful tool for physiological and ecological studies. However, how post-photosynthetic fractionation associated with respiration and carbon export influences δ13C remains uncertain. We investigated the effects of post-photosynthetic fractionation on δ13C of mature leaves of Cleistogenes squarrosa, a perennial C4 grass, in controlled experiments with different levels of vapour pressure deficit and nitrogen supply. With increasing leaf age class, the 12C/13C fractionation of leaf organic matter relative to the δ13C of atmosphere CO2 (ΔDM) increased while that of cellulose (Δcel) was almost constant. The divergence between ΔDM and Δcel increased with leaf age class, with a maximum value of 1.6‰, indicating the accumulation of post-photosynthetic fractionation. Applying a new mass balance model that accounts for respiration and export of photosynthates, we found an apparent 12C/13C fractionation associated with carbon export of -0.5‰ to -1.0‰. Different ΔDM among leaves, pseudostems, daughter tillers, and roots indicate that post-photosynthetic fractionation happens at the whole-plant level. Compared with ΔDM of old leaves, ΔDM of young leaves and Δcel are more reliable proxies for predicting physiological parameters due to the lower sensitivity to post-photosynthetic fractionation and the similar sensitivity in responses to environmental changes.

Covariation between oxygen and hydrogen stable isotopes declines along the path from xylem water to wood cellulose across an aridity gradient

Oxygen and hydrogen isotopes of cellulose in plant biology are commonly used to infer environmental conditions, often from time series measurements of tree rings. However, the covariation (or the lack thereof) between δ18 O and δ2 H in plant cellulose is still poorly understood. We compared plant water, and leaf and branch cellulose from dominant tree species across an aridity gradient in Northern Australia, to examine how δ18 O and δ2 H relate to each other and to mean annual precipitation (MAP). We identified a decline in covariation from xylem to leaf water, and onwards from leaf to branch wood cellulose. Covariation in leaf water isotopic enrichment (Δ) was partially preserved in leaf cellulose but not branch wood cellulose. Furthermore, whilst δ2 H was well-correlated between leaf and branch, there was an offset in δ18 O between organs that increased with decreasing MAP. Our findings strongly suggest that postphotosynthetic isotope exchange with water is more apparent for oxygen isotopes, whereas variable kinetic and nonequilibrium isotope effects add complexity to interpreting metabolic-induced δ2 H patterns. Varying oxygen isotope exchange in wood and leaf cellulose must be accounted for when δ18 O is used to reconstruct climatic scenarios. Conversely, comparing δ2 H and δ18 O patterns may reveal environmentally induced shifts in metabolism.

13C labeling unravels carbon dynamics in banana between mother plant, sucker and corm under drought stress

Banana is a perennial crop and typically consists of a mother plant and one or more suckers that will serve as the next generation. Suckers are photosynthetically active, but also receive photo-assimilates from the mother plant. While drought stress is the most important abiotic constraint to banana cultivation, its effect on suckers or banana mats as a whole remains unknown. To investigate whether parental support to suckers is altered under drought stress and to determine the photosynthetic cost to the parental plant, we conducted a ¹³C labeling experiment. We labeled banana mother plants with ¹³CO₂ and traced the label up to two weeks after labeling. This was done under optimal and drought-stressed conditions in plants with and without suckers. We retrieved label in the phloem sap of the corm and sucker as soon as 24 hours after labeling. Overall, 3.1 ± 0.7% of label assimilated by the mother plant ended up in the sucker. Allocation to the sucker seemed to be reduced under drought stress. The absence of a sucker did not enhance the growth of the mother plant; instead, plants without suckers had higher respiratory losses. Furthermore, 5.8 ± 0.4% of the label was allocated to the corm. Sucker presence and drought stress each led to an increase in starch accumulation in the corm, but when both stress and a sucker were present, the amount was severely reduced. Furthermore, the second to fifth fully open leaves were the most important source of photo-assimilates in the plant, but the two younger developing leaves assimilated the same amount of carbon as the four active leaves combined. They exported and imported photo-assimilates simultaneously, hence acting as both source and sink. ¹³C labeling has allowed us to quantify source and sink strengths of different plant parts, as well as the carbon fluxes between them. We conclude that drought stress and sucker presence, respectively causing a reduction in supply and an increase in carbon demand, both increased the relative amount of carbon allocated to storage tissues. Their combination, however, led to insufficient availability of assimilates and hence a reduced investment in long-term storage and sucker growth.

The metabolic sensitivity of hydrogen isotope fractionation differs between plant compounds

Hydrogen stable isotope analyses (δ²H) of plant derived organic compounds are a useful tool for ecological, environmental, and palaeoclimatological research. However, during organic compound synthesis, variable biosynthetic ²H-fractionation has been suggested to occur as a result of changes in plant carbon fluxes. So far, inference has been based on examining the δ²H patterns of plant compounds along environmental gradients, among plant species, and between plant organs. In an alternative approach, we used four plant species with four different types of mutations that cause impaired starch synthesis to assess whether variability in carbon metabolism affects the biosynthetic ²H-fractionation during cellulose, phytol, and acetogenic lipid synthesis. We found that mutants with impaired starch synthesis always had higher cellulose and phytol δ²H values compared to the wild type. By contrast, ²H-fractionation during acetogenic lipid biosynthesis generally did not show strong metabolic sensitivity. We rationalise these differences by considering the biosynthetic pathway of each compound and the likely source of the variable isotope fractionation. In different organic compounds, the sensitivity of variable biosynthetic ²H-fractionation to changes in C-metabolism depends on incorporation of specific H atoms from precursor molecules. As such, we determined that the similar increase in cellulose and phytol δ²H values as an effect of impaired starch synthesis most likely originates in triose-phosphates.

Species variation in the hydrogen isotope composition of leaf cellulose is mostly driven by isotopic variation in leaf sucrose

Experimental approaches to isolate drivers of variation in the carbon-bound hydrogen isotope composition (δ² H) of plant cellulose are rare and current models are limited in their application. This is in part due to a lack in understanding of how ² H-fractionations in carbohydrates differ between species. We analysed, for the first time, the δ² H of leaf sucrose along with the δ² H and δ¹⁸ O of leaf cellulose and leaf and xylem water across seven herbaceous species and a starchless mutant of tobacco. The δ² H of sucrose explained 66% of the δ² H variation in cellulose (R²  = 0.66), which was associated with species differences in the ² H enrichment of sucrose above leaf water ( ε sucrose <math altimg="urn:x-wiley:01407791:media:pce14362:pce14362-math-0001" wiley:location="equation/pce14362-math-0001.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mtext>\unicode{x003B5}</mtext><mtext>sucrose</mtext></msub></mrow></math> : -126% to -192‰) rather than by variation in leaf water δ² H itself. ε sucrose <math altimg="urn:x-wiley:01407791:media:pce14362:pce14362-math-0002" wiley:location="equation/pce14362-math-0002.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mtext>\unicode{x003B5}</mtext><mtext>sucrose</mtext></msub></mrow></math> was positively related to dark respiration (R²  = 0.27), and isotopic exchange of hydrogen in sugars was positively related to the turnover time of carbohydrates (R²  = 0.38), but only when ε sucrose <math altimg="urn:x-wiley:01407791:media:pce14362:pce14362-math-0003" wiley:location="equation/pce14362-math-0003.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><msub><mi mathvariant="normal">\unicode{x003B5}</mi><mtext>sucrose</mtext></msub></mrow></mrow></math> was fixed to the literature accepted value of - 171 <math altimg="urn:x-wiley:01407791:media:pce14362:pce14362-math-0004" wiley:location="equation/pce14362-math-0004.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><mo>\unicode{x02212}</mo><mn>171</mn></mrow></mrow></math> ‰. No relation was found between isotopic exchange of hydrogen and oxygen, suggesting large differences in the processes shaping post-photosynthetic fractionation between elements. Our results strongly advocate that for robust applications of the leaf cellulose hydrogen isotope model, parameterization utilizing δ² H of sugars is needed.

Distinguishing carbon gains from photosynthesis and heterotrophy in C3-hemiparasite-C3-host pairs

BACKGROUND AND AIMS

Previous carbon stable isotope (13C) analyses have shown for very few C3-hemiparasites utilizing C4- or CAM-hosts the use of two carbon sources, autotrophy and heterotrophy. This 13C approach, however, failed for the frequently occurring C3-C3 parasite-host pairs. Thus, we used hydrogen stable isotope (2H) natural abundances as a substitute for 13C within a C3-Orobanchaceae sequence graded by haustoria complexity and C3-Santalaceae.

METHODS

Parasitic plants and their real or potential host plants as references were collected in Central European lowland and alpine mountain meadows and forests. Parasitic plants included the xylem-feeding holoparasite Lathraea squamaria parasitizing on the same carbon nutrient source (xylem-transported organic carbon compounds) as potentially Pedicularis, Rhinanthus, Bartsia, Melampyrum and Euphrasia hemiparasites. Reference plants were used for an autotrophy-only isotope baseline. A multi-element stable isotope natural abundance approach was applied.

KEY RESULTS

Species-specific heterotrophic carbon gain ranging from 0 to 51 % was estimated by a 2H mixing-model. The sequence in heterotrophic carbon gain mostly met the morphological grading by haustoria complexity: Melampyrum- < Rhinanthus- < Pedicularis-type.

CONCLUSION

Due to higher transpiration and lower water-use efficiency, depletion in 13C, 18O and 2H compared to C3-host plants should be expected for tissues of C3-hemiparasites. However, 2H is counterbalanced by transpiration (2H-depletion) and heterotrophy (2H-enrichment). Progressive 2H-enrichment can be used as a proxy to evaluate carbon gains from hosts.

A high-temperature water vapor equilibration method to determine non-exchangeable hydrogen isotope ratios of sugar, starch and cellulose

The analysis of the non-exchangeable hydrogen isotope ratio (δ² H_ne ) in carbohydrates is mostly limited to the structural component cellulose, while simple high-throughput methods for δ² H_ne values of non-structural carbohydrates (NSC) such as sugar and starch do not yet exist. Here, we tested if the hot vapor equilibration method originally developed for cellulose is applicable for NSC, verified by comparison with the traditional nitration method. We set up a detailed analytical protocol and applied the method to plant extracts of leaves from species with different photosynthetic pathways (i.e., C₃ , C₄ and CAM). δ² H_ne of commercial sugars and starch from different classes and sources, ranging from -157.8 to +6.4‰, were reproducibly analysed with precision between 0.2‰ and 7.7‰. Mean δ² H_ne values of sugar are lowest in C₃ (-92.0‰), intermediate in C₄ (-32.5‰) and highest in CAM plants (6.0‰), with NSC being ² H-depleted compared to cellulose and sugar being generally more ² H-enriched than starch. Our results suggest that our method can be used in future studies to disentangle ² H-fractionation processes, for improving mechanistic δ² H_ne models for leaf and tree-ring cellulose and for further development of δ² H_ne in plant carbohydrates as a potential proxy for climate, hydrology, plant metabolism and physiology.