A model of photosynthetic CO2 assimilation in C3 leaves accounting for respiration and energy recycling by the plastidial oxidative pentose phosphate pathway

Recently, we reported estimates of anaplerotic carbon flux through the oxidative pentose phosphate pathway (OPPP) in chloroplasts into the Calvin-Benson cycle. These estimates were based on intramolecular hydrogen isotope analysis of sunflower leaf starch. However, the isotope method is believed to underestimate the actual flux at low atmospheric CO₂ concentration (C_a ). Since the OPPP releases CO₂ and reduces NADP⁺ , it can be expected to affect leaf gas exchange under both rubisco- and RuBP-regeneration-limited conditions. Therefore, we expanded Farquhar-von Caemmerer-Berry models to account for OPPP metabolism. Based on model parameterisation with values from the literature, we estimated OPPP-related effects on leaf carbon and energy metabolism in the sunflowers analysed previously. We found that flux through the plastidial OPPP increases both above and below C_a  ≈ 450 ppm (the condition the plants were acclimated to). This is qualitatively consistent with our previous isotope-based estimates, yet gas-exchange-based estimates are larger at low C_a . We discuss our results in relation to regulatory properties of the plastidial and cytosolic OPPP, the proposed variability of CO₂ mesophyll conductance, and the contribution of day respiration to the A/C_i curve drop at high C_a . Furthermore, we critically examine the models and parameterisation and derive recommendations for follow-up studies.

Metabolism is a major driver of hydrogen isotope fractionation recorded in tree-ring glucose of Pinus nigra

Stable isotope abundances convey valuable information about plant physiological processes and underlying environmental controls. Central gaps in our mechanistic understanding of hydrogen isotope abundances impede their widespread application within the plant and biogeosciences. To address these gaps, we analysed intramolecular deuterium abundances in glucose of Pinus nigra extracted from an annually resolved tree-ring series (1961-1995). We found fractionation signals (i.e. temporal variability in deuterium abundance) at glucose H¹ and H² introduced by closely related metabolic processes. Regression analysis indicates that these signals (and thus metabolism) respond to drought and atmospheric CO₂ concentration beyond a response change point. They explain ≈ 60% of the whole-molecule deuterium variability. Altered metabolism is associated with below-average yet not exceptionally low growth. We propose the signals are introduced at the leaf level by changes in sucrose-to-starch carbon partitioning and anaplerotic carbon flux into the Calvin-Benson cycle. In conclusion, metabolism can be the main driver of hydrogen isotope variation in plant glucose.

Anaplerotic flux into the Calvin-Benson cycle: hydrogen isotope evidence for in vivo occurrence in C3 metabolism

As the central carbon uptake pathway in photosynthetic cells, the Calvin-Benson cycle is among the most important biochemical cycles for life on Earth. A carbon flux of anaplerotic origin (i.e. through the chloroplast-localized oxidative branch of the pentose phosphate pathway) into the Calvin-Benson cycle was proposed recently. Here, we measured intramolecular deuterium abundances in leaf starch of Helianthus annuus grown at varying ambient CO₂ concentrations, C_a . Additionally, we modelled deuterium fractionations expected for the anaplerotic pathway and compared modelled with measured fractionations. We report deuterium fractionation signals at H¹ and H² of starch glucose. Below a C_a change point, these signals increase with decreasing C_a consistent with modelled fractionations by anaplerotic flux. Under standard conditions (C_a  = 450 ppm corresponding to intercellular CO₂ concentrations, C_i , of 328 ppm), we estimate negligible anaplerotic flux. At C_a  = 180 ppm (C_i  = 140 ppm), more than 10% of the glucose-6-phosphate entering the starch biosynthesis pathway is diverted into the anaplerotic pathway. In conclusion, we report evidence consistent with anaplerotic carbon flux into the Calvin-Benson cycle in vivo. We propose the flux may help to: maintain high levels of ribulose 1,5-bisphosphate under source-limited growth conditions to facilitate photorespiratory nitrogen assimilation required to build-up source strength; and counteract oxidative stress.

The resilience of perennial grasses under two climate scenarios is correlated with carbohydrate metabolism in meristems

Extreme climatic events (ECEs) such as droughts and heat waves affect ecosystem functioning and species turnover. This study investigated the effect of elevated CO2 on species' resilience to ECEs. Monoliths of intact soil and their plant communities from an upland grassland were exposed to 2050 climate scenarios with or without an ECE under ambient (390 ppm) or elevated (520 ppm) CO2. Ecophysiological traits of two perennial grasses (Dactylis glomerata and Holcus lanatus) were measured before, during, and after ECE. At similar soil water content, leaf elongation was greater under elevated CO2 for both species. The resilience of D. glomerata increased under enhanced CO2 (+60%) whereas H. lanatus mostly died during ECE. D. glomerata accumulated 30% more fructans, which were more highly polymerized, and 4-fold less sucrose than H. lanatus. The fructan concentration in leaf meristems was significantly increased under elevated CO2. Their relative abundance changed during the ECE, resulting in a more polymerized assemblage in H. lanatus and a more depolymerized assemblage in D. glomerata. The ratio of low degree of polymerization fructans to sucrose in leaf meristems was the best predictor of resilience across species. This study underlines the role of carbohydrate metabolism and the species-dependent effect of elevated CO2 on the resilience of grasses to ECE.

Drought-Induced Accumulation of Root Exudates Supports Post-drought Recovery of Microbes in Mountain Grassland

Droughts strongly affect carbon and nitrogen cycling in grasslands, with consequences for ecosystem productivity. Therefore, we investigated how experimental grassland communities interact with groups of soil microorganisms. In particular, we explored the mechanisms of the drought-induced decoupling of plant photosynthesis and microbial carbon cycling and its recovery after rewetting. Our aim was to better understand how root exudation during drought is linked to pulses of soil microbial activity and changes in plant nitrogen uptake after rewetting. We set up a mesocosm experiment on a meadow site and used shelters to simulate drought. We performed two ¹³C-CO₂ pulse labelings, the first at peak drought and the second in the recovery phase, and traced the flow of assimilates into the carbohydrates of plants and the water extractable organic carbon and microorganisms from the soil. Total microbial tracer uptake in the main metabolism was estimated by chloroform fumigation extraction, whereas the lipid biomarkers were used to assess differences between the microbial groups. Drought led to a reduction of aboveground versus belowground plant growth and to an increase of ¹³C tracer contents in the carbohydrates, particularly in the roots. Newly assimilated ¹³C tracer unexpectedly accumulated in the water-extractable soil organic carbon, indicating that root exudation continued during the drought. In contrast, drought strongly reduced the amount of ¹³C tracer assimilated into the soil microorganisms. This reduction was more severe in the growth-related lipid biomarkers than in the metabolic compounds, suggesting a slowdown of microbial processes at peak drought. Shortly after rewetting, the tracer accumulation in the belowground plant carbohydrates and in the water-extractable soil organic carbon disappeared. Interestingly, this disappearance was paralleled by a quick recovery of the carbon uptake into metabolic and growth-related compounds from the rhizospheric microorganisms, which was probably related to the higher nitrogen supply to the plant shoots. We conclude that the decoupling of plant photosynthesis and soil microbial carbon cycling during drought is due to reduced carbon uptake and metabolic turnover of rhizospheric soil microorganisms. Moreover, our study suggests that the maintenance of root exudation during drought is connected to a fast reinitiation of soil microbial activity after rewetting, supporting plant recovery through increased nitrogen availability.

Land use in mountain grasslands alters drought response and recovery of carbon allocation and plant-microbial interactions

Mountain grasslands have recently been exposed to substantial changes in land use and climate and in the near future will likely face an increased frequency of extreme droughts. To date, how the drought responses of carbon (C) allocation, a key process in the C cycle, are affected by land-use changes in mountain grassland is not known.We performed an experimental summer drought on an abandoned grassland and a traditionally managed hay meadow and traced the fate of recent assimilates through the plant-soil continuum. We applied two ¹³ CO ₂ pulses, at peak drought and in the recovery phase shortly after rewetting.Drought decreased total C uptake in both grassland types and led to a loss of above-ground carbohydrate storage pools. The below-ground C allocation to root sucrose was enhanced by drought, especially in the meadow, which also held larger root carbohydrate storage pools.The microbial community of the abandoned grassland comprised more saprotrophic fungal and Gram(+) bacterial markers compared to the meadow. Drought increased the newly introduced AM and saprotrophic (A+S) fungi:bacteria ratio in both grassland types. At peak drought, the ¹³C transfer into AM and saprotrophic fungi, and Gram(-) bacteria was more strongly reduced in the meadow than in the abandoned grassland, which contrasted the patterns of the root carbohydrate pools.In both grassland types, the C allocation largely recovered after rewetting. Slowest recovery was found for AM fungi and their ¹³C uptake. In contrast, all bacterial markers quickly recovered C uptake. In the meadow, where plant nitrate uptake was enhanced after drought, C uptake was even higher than in control plots. Synthesis. Our results suggest that resistance and resilience (i.e. recovery) of plant C dynamics and plant-microbial interactions are negatively related, that is, high resistance is followed by slow recovery and vice versa. The abandoned grassland was more resistant to drought than the meadow and possibly had a stronger link to AM fungi that could have provided better access to water through the hyphal network. In contrast, meadow communities strongly reduced C allocation to storage and C transfer to the microbial community in the drought phase, but in the recovery phase invested C resources in the bacterial communities to gain more nutrients for regrowth. We conclude that the management of mountain grasslands increases their resilience to drought.

Late summer photosynthesis and storage carbohydrates in walnut (Juglans regia L.): Feed-back and feed-forward effects

The effect of late summer - autumn limitation of phloem export on growth, photosynthesis and storage carbohydrate accumulation, was evaluated in walnut (Juglans regia L.). This was done by girdling current years shoots, with either all or with only a third of the leaves left in place. Nineteen days after girdling, photosynthesis was greatly reduced and after 46 days, it was about 70% lower in both girdling treatments compared to the control (ungirdled shoots). This reduction is consistent with a feed-back effect of an increased carbohydrate content of the leaves. At the end of the experiment (46 days after girdling), the radial growth of girdled shoots was increased at their base but not at their apical part compared to the control. Girdling increased the accumulation of sucrose in the bark at the base of the shoot and of starch in the bark and in the wood of the shoot apical part. The activity of ADP-glucose pyrophosphorylase in wood increased in the apical part of girdled shoots. The results suggest that a high availability of carbohydrates elicits a feed-forward action on the shoot sink size and activity (radial growth and storage carbohydrate accumulation). Further, for the first time in tree wood we found an increased total activity of AGP induced by an increased assimilate availability. Moreover, the results indicated that, in late summer - autumn, CO₂ uptake by leaves of the deciduous tree walnut is strongly dependent on export of photosynthates from the crown. Therefore, carbon uptake in this period depends largely on the availability of effective storage sinks where newly produced assimilates can be accumulated.

Elevated CO2 maintains grassland net carbon uptake under a future heat and drought extreme

Extreme climatic events (ECEs) such as droughts and heat waves are predicted to increase in intensity and frequency and impact the terrestrial carbon balance. However, we lack direct experimental evidence of how the net carbon uptake of ecosystems is affected by ECEs under future elevated atmospheric CO2 concentrations (eCO2). Taking advantage of an advanced controlled environment facility for ecosystem research (Ecotron), we simulated eCO2 and extreme cooccurring heat and drought events as projected for the 2050s and analyzed their effects on the ecosystem-level carbon and water fluxes in a C3 grassland. Our results indicate that eCO2 not only slows down the decline of ecosystem carbon uptake during the ECE but also enhances its recovery after the ECE, as mediated by increases of root growth and plant nitrogen uptake induced by the ECE. These findings indicate that, in the predicted near future climate, eCO2 could mitigate the effects of extreme droughts and heat waves on ecosystem net carbon uptake.

Quantification of Deuterium Isotopomers of Tree-Ring Cellulose Using Nuclear Magnetic Resonance

Stable isotopes in tree rings are important tools for reconstruction of past climate. Deuterium (D) is of particular interest since it may contain climate signals and report on tree physiology. Measurements of the D/H ratio of tree-ring cellulose have proven difficult to interpret, presumably because the D/H ratio of the whole molecule blends the abundances of the seven D isotopomers of cellulose. Here we present a method to measure the abundance of the D isotopomers of tree-ring cellulose by nuclear magnetic resonance spectroscopy (NMR). The method transforms tree-ring cellulose into a glucose derivative that gives highly resolved, quantifiable deuterium NMR spectra. General guidelines for measurement of D isotopomers by NMR are described. The transformation was optimized for yield and did not alter the original D isotopomer abundances, thus, conserving the original signals recorded in wood cellulose. In the tree-ring samples tested, the abundances of D isotopomers varied by approximately +/-10% (2% standard error). This large variability can only be caused by biochemistry processes and shows that more information is present in D isotopomer abundances, compared to the D/H ratio. Therefore, measurements of the D isotopomer distribution of tree rings may be used to obtain information on long-term adaptations to environmental changes and past climate change.

Hydrogen exchange during cellulose synthesis distinguishes climatic and biochemical isotope fractionations in tree rings

The abundance of the hydrogen isotope deuterium (D) in tree rings is an attractive record of climate; however, use of this record has proved difficult so far, presumably because climatic and physiological influences on D abundance are difficult to distinguish. Using D labelling, we created a D gradient in trees. Leaf soluble sugars of relatively low D abundance entered cellulose synthesis in stems containing strongly D-labelled water. We used nuclear magnetic resonance (NMR) spectroscopy to quantify D in the C-H groups of leaf glucose and of tree-ring cellulose. Ratios of D abundances of individual C-H groups of leaf glucose depended only weakly on leaf D labelling, indicating that the D abundance pattern was determined by physiological influences. The D abundance pattern of tree-ring cellulose revealed C-H groups that exchanged strongly (C(2)-H) or weakly (C(6)-H2) with water during cellulose synthesis. We propose that strongly exchanging C-H groups of tree-ring cellulose adopt a climate signal stemming from the D abundance of source water. C-H groups that exchange weakly retain their D abundance established in leaf glucose, which reflects physiological influences. Combining both types of groups may allow simultaneous reconstruction of climate and physiology from tree rings.

[Frequency of hypothyroidism after De Quervain thyroiditis and contribution of ultrasonographic thyroid volume measurement]

Subacute thyroiditis conventionally recovers without after-effect. Nevertheless some data relate a 5 to 9% occurrence of final hypothyroidism 6 months after the acute stage. We herein studied end-stage hypothyroidism occurrence and effect of thyroid volume alterations on hormonal course during thyroiditis. Twenty-nine cases of subacute thyroiditis were studied. Final thyroid function remained normal in 15 patients (51%) and undetermined in 5 patients (17%). Final hypothyroidism (TSH: 4.5-14.5 microU/mL) occurred in 9 patients (31%). Mean thyroid volume was increased in acute stage in patients with final normal thyroid function (16.6+/-5.7cm(3)) and decreased by 63% during follow-up; final mean thyroid volume was 6.1+/-1.3cm(3). Mean thyroid volume was normal in acute stage in final hypothyroid patients (10.7+/-3.0cm(3)) and decreased by 72% during follow-up; final mean thyroid volume was 3.3+/-1.1cm(3). End-stage mean thyroid volume was significantly lower in final hypothyroid patients (p<0.05) compared to patients with final normal thyroid function. We conclude that the occurrence of final hypothyroidism is underestimated after subacute thyroiditis. Ultrasonographic follow-up might be helpful in the detection of final hypothyroid-risk patients: thyroid volume not increased in acute stage and lower than 5cm(3) during follow-up is one of the ultrasonographic features of these patients.

Photosystem II photochemical efficiency, zeaxanthin and antioxidant contents in the poikilohydric Ramonda serbica during dehydration and rehydration

Changes in photochemical efficiency, non-radiative energy dissipation (NRD), de-epoxidation state of xanthophyll cycle components (DPS) and contents of the antioxidants ascorbic acid and glutathione were studied in leaves of the poikilohydric Ramonda serbica Panc. (Gesneriaceae) during cycles of dehydration and subsequent rehydration. In drying leaves, the intrinsic efficiency of PS II photochemistry and the photon yield of PS II electron transport showed strong progressive decreases. Simultaneously, the fraction of excitation energy dissipated as heat in the PS II antenna increased markedly. The energy-dependent component of non-photochemical quenching (NPQ) showed an increase in dehydrating leaves down to relative water contents (RWC) values near 30%. Further decreases in RWC below these values caused a decrease in NPQ. Accordingly, DPS showed a similar behaviour, with a sharp increase and a subsequent decrease at very low RWC, although the maximum DPS was reached at slightly lower RWC than that for the maximum NPQ. The pools of reduced ascorbate and glutathione increased strongly when the RWC values fell below 40% and remained high in fully dehydrated leaves. When plants were re-watered photosynthetic efficiency, NRD, DPS and antioxidant contents recovered their initial control values. However, during rehydration, the zeaxanthin content showed a transient increase, as did NPQ, indicating an increasing demand for non-radiative dissipation. On the other hand, the contents of reduced ascorbate and reduced glutathione decreased but were still relatively high in the initial phase of rehydration, when the rate of photosynthetic electron transport, proton pumping and NRD were still relatively low. These results indicate that several photoprotective mechanisms are operating in R. serbica. Protection from photo-oxidation and photoinhibition appears to be achieved by co-ordinated contributions by ascorbate, glutathione and zeaxanthin-mediated NPQ. This variety of photoprotective mechanisms may be essential for conferring desiccation-tolerance.