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The 18 O enrichment (Δ 18 O) of cellulose (Δ 18 O Cel ) is recognized as a unique archive of past climate and plant function. However, there is still uncertainty regarding the proportion of oxygen in cellulose (p ex ) that exchanges post-photosynthetically with medium water of cellulose synthesis. Particularly, recent research with C 3 grasses demonstrated that the Δ 18 O of leaf sucrose (Δ 18 O Suc , the parent substrate for cellulose synthesis) can be much higher than predicted from daytime Δ 18 O of leaf water (Δ 18 O LW ), which could alter conclusions on photosynthetic versus post-photosynthetic effects on Δ 18 O Cel via p ex . Here, we assessed p ex in leaves of perennial ryegrass (Lolium perenne) grown at different atmospheric relative humidity (RH) and CO 2 levels, by determinations of Δ 18 O Cel in leaves, Δ 18 O LGDZW (the Δ 18 O of water in the leaf growth-and-differentiation zone) and both Δ 18 O Suc and Δ 18 O LW (adjusted for ε bio , the biosynthetic fractionation between water and carbohydrates) as alternative proxies for the substrate for cellulose synthesis. Δ 18 O LGDZW was always close to irrigation water, and p ex was similar (0.53 ± 0.02 SE) across environments when determinations were based on Δ 18 O Suc . Conversely, p ex was erroneously and variably underestimated (range 0.02-0.44) when based on Δ 18 O LW . The photosynthetic signal fraction in Δ 18 O Cel is much more constant than hitherto assumed, encouraging leaf physiological reconstructions., (© 2024 The Authors. Plant, Cell & Environment published by John Wiley & Sons Ltd.)

Background: The anthropogenic increase of atmospheric CO 2 concentration (c a ) is impacting carbon (C), water, and nitrogen (N) cycles in grassland and other terrestrial biomes. Plant canopy stomatal conductance is a key player in these coupled cycles: it is a physiological control of vegetation water use efficiency (the ratio of C gain by photosynthesis to water loss by transpiration), and it responds to photosynthetic activity, which is influenced by vegetation N status. It is unknown if the c a -increase and climate change over the last century have already affected canopy stomatal conductance and its links with C and N processes in grassland., Results: Here, we assessed two independent proxies of (growing season-integrating canopy-scale) stomatal conductance changes over the last century: trends of δ 18 O in cellulose (δ 18 O cellulose ) in archived herbage from a wide range of grassland communities on the Park Grass Experiment at Rothamsted (U.K.) and changes of the ratio of yields to the CO 2 concentration gradient between the atmosphere and the leaf internal gas space (c a - c i ). The two proxies correlated closely (R 2  = 0.70), in agreement with the hypothesis. In addition, the sensitivity of δ 18 O cellulose changes to estimated stomatal conductance changes agreed broadly with published sensitivities across a range of contemporary field and controlled environment studies, further supporting the utility of δ 18 O cellulose changes for historical reconstruction of stomatal conductance changes at Park Grass. Trends of δ 18 O cellulose differed strongly between plots and indicated much greater reductions of stomatal conductance in grass-rich than dicot-rich communities. Reductions of stomatal conductance were connected with reductions of yield trends, nitrogen acquisition, and nitrogen nutrition index. Although all plots were nitrogen-limited or phosphorus- and nitrogen-co-limited to different degrees, long-term reductions of stomatal conductance were largely independent of fertilizer regimes and soil pH, except for nitrogen fertilizer supply which promoted the abundance of grasses., Conclusions: Our data indicate that some types of temperate grassland may have attained saturation of C sink activity more than one century ago. Increasing N fertilizer supply may not be an effective climate change mitigation strategy in many grasslands, as it promotes the expansion of grasses at the disadvantage of the more CO 2 responsive forbs and N-fixing legumes.

Plant carbon-use-efficiency (CUE), a key parameter in carbon cycle and plant growth models, quantifies the fraction of fixed carbon that is converted into net primary production rather than respired. CUE has not been directly measured, partly because of the difficulty of measuring respiration in light. Here, we explore if CUE is affected by atmospheric CO 2 . Sunflower stands were grown at low (200 μmol mol -1 ) or high CO 2 (1000 μmol mol -1 ) in controlled environment mesocosms. CUE of stands was measured by dynamic stand-scale 13 C labelling and partitioning of photosynthesis and respiration. At the same plant age, growth at high CO 2 (compared with low CO 2 ) led to 91% higher rates of apparent photosynthesis, 97% higher respiration in the dark, yet 143% higher respiration in light. Thus, CUE was significantly lower at high (0.65) than at low CO 2 (0.71). Compartmental analysis of isotopic tracer kinetics demonstrated a greater commitment of carbon reserves in stand-scale respiratory metabolism at high CO 2 . Two main processes contributed to the reduction of CUE at high CO 2 : a reduced inhibition of leaf respiration by light and a diminished leaf mass ratio. This work highlights the relevance of measuring respiration in light and assessment of the CUE response to environment conditions., (© 2016 John Wiley & Sons Ltd.)

Leaks and isotopic disequilibria represent potential errors and artefacts during combined measurements of gas exchange and carbon isotope discrimination (Δ). This paper presents new protocols to quantify, minimize, and correct such phenomena. We performed experiments with gradients of CO2 concentration (up to ±250 μmol mol(-1) ) and δ(13) CCO2 (34‰), between a clamp-on leaf cuvette (LI-6400) and surrounding air, to assess (1) leak coefficients for CO2 , (12) CO2 , and (13) CO2 with the empty cuvette and with intact leaves of Holcus lanatus (C3 ) or Sorghum bicolor (C4 ) in the cuvette; and (2) isotopic disequilibria between net photosynthesis and dark respiration in light. Leak coefficients were virtually identical for (12) CO2 and (13) CO2 , but ∼8 times higher with leaves in the cuvette. Leaks generated errors on Δ up to 6‰ for H. lanatus and 2‰ for S. bicolor in full light; isotopic disequilibria produced similar variation of Δ. Leak errors in Δ in darkness were much larger due to small biological : leak flux ratios. Leak artefacts were fully corrected with leak coefficients determined on the same leaves as Δ measurements. Analysis of isotopic disequilibria enabled partitioning of net photosynthesis and dark respiration, and indicated inhibitions of dark respiration in full light (H. lanatus: 14%, S. bicolor: 58%)., (© 2015 John Wiley & Sons Ltd.)

Leaf-level measurements have shown that mesophyll conductance (g(m)) can vary rapidly in response to CO₂ and other environmental factors, but similar studies at the canopy-scale are missing. Here, we report the effect of short-term variation of CO₂ concentration on canopy-scale g(m) and other CO₂ exchange parameters of sunflower (Helianthus annuus L.) stands in the presence and absence of abscisic acid (ABA) in their nutrient solution. g(m) was estimated from gas exchange and on-line carbon isotope discrimination (Δ(obs)) in a ¹³CO₂/¹²CO₂) gas exchange mesocosm. The isotopic contribution of (photo)respiration to stand-scale Δ(obs) was determined with the experimental approach of Tcherkez et al. Without ABA, short-term exposures to different CO₂ concentrations (C(a) 100 to 900 µmol mol⁻¹) had little effect on canopy-scale g(m) . But, addition of ABA strongly altered the CO₂-response: g(m) was high (approx. 0.5 mol CO₂ m⁻² s⁻¹) at C(a) <200 µmol mol⁻¹ and decreased to <0.1 mol CO₂ m⁻² s⁻¹ at C(a) >400 µmol mol⁻¹. In the absence of ABA, the contribution of (photo)respiration to stand-scale Δ(obs) was high at low C(a) (7.2 ‰) and decreased to <2 ‰ at C(a) > 400 µmol mol⁻¹. Treatment with ABA halved this effect at all C(a) ., (© 2010 Blackwell Publishing Ltd.)

• The mechanism controlling the use of stored carbon in respiration is poorly understood. Here, we explore if the reliance on stores as respiratory substrate depends on day length. • Lolium perenne (perennial ryegrass) was grown in continuous light (275 μmol photons m(-2) s(-1) ) or in a 16 : 8 h day : night regime (425 μmol m(-2) s(-1) during the photoperiod), with the same daily photosynthetic photon flux density (PPFD). Plants in stands were labelled with (13)CO(2) : (12)CO(2) for various time intervals. The rates and isotopic signatures of shoot- and root-respired CO(2) were measured after labelling, and water-soluble carbohydrates were determined in biomass. The tracer kinetics in respired CO(2) was analysed with compartmental models to infer the sizes, half-lives and contributions of respiratory substrate pools. • Stores were the main source for respiration in both treatments (c. 60% of all respired carbon). But, continuous light slowed the turnover (+270%) and increased the size (+160%) of the store relative to the 16 : 8 h day : night regime. This effect corresponded with a greatly elevated fructan content. Yet, day length had no effect on sizes and half-lives of other pools serving respiration. • We suggest that the residence time of respiratory carbon was strongly influenced by partitioning of carbon to fructan stores., (© The Authors (2010). Journal compilation © New Phytologist Trust (2010).)

* The kinetic characteristics of the main sources of ecosystem respiration are quite unknown, partly because of methodological constraints. Here, we present a new open-top chamber (OTC) apparatus for continuous 13C/12C labelling and measurement of ecosystem CO2 fluxes, and report the tracer kinetics of nighttime respiration of a temperate grassland. * The apparatus includes four dynamic flow-through OTCs, a unit mixing CO2-free air with 13C-depleted CO2, and a CO2 analyser and an online isotope ratio mass spectrometer. * The concentration (367 +/- 6.5 micromol mol(-1)) and carbon isotopic composition, delta13C, (-46.9 +/- 0.4 per thousand) of CO2 in the OTCs were stable during photosynthesis as a result of high air through flux and minimal incursion through the buffered vent. Soil CO2 efflux was not affected by pressure effects during respiration measurements. The labelling kinetics of respiratory CO2 measured in the field agreed with that of excised soil + vegetation blocks measured in a laboratory-based system. The kinetics fitted a two-source system (r(2) = 0.97), with a rapidly labelled source (half-life 2.6 d) supplying 48% of respiration, and the other source (52%) releasing no tracer during 14 d of labelling. * Of the two sources supplying ecosystem respiration, one was closely connected to current photosynthesis (approximately autotrophic respiration) and the other was provided by decomposition of structural plant biomass (approximately heterotrophic respiration).

Effects of the arbuscular mycorrhizal fungus (AMF) Glomus hoi on the carbon economy of perennial ryegrass (Lolium perenne) were investigated by comparing nonmycorrhizal and mycorrhizal plants of the same size, morphology and phosphorus status. Plants were grown in the presence of CO2 sources with different C isotope composition (delta13C -1 or -44). Relative respiration and gross photosynthesis rates, and belowground allocation of C assimilated during one light period ('new C'), as well as its contribution to respiration, were quantified by the concerted use of 13CO2/12CO2 steady-state labelling and 13CO2/12CO2 gas-exchange techniques. AMF (G. hoi) enhanced the relative respiration rate of the root + soil system by 16%, inducing an extra C flow amounting to 3% of daily gross photosynthesis. Total C flow into AMF growth and respiration was estimated at < 8% of daily gross photosynthesis. This was associated with a greater amount of new C allocated belowground and respired in mycorrhizal plants. AMF colonization affected the sources supplying belowground respiration, indicating a greater importance of plant C stores in supplying respiration and/or the participation of storage pools within fungal tissues. When ontogenetic and nutritional effects were accounted for, AMF increased belowground C costs, which were not compensated by increased photosynthesis rates. Therefore the instantaneous relative growth rate was lower in mycorrhizal plants.

Here, we analysed the transition from heterotrophic to autotrophic growth of the epigeal species sunflower (Helianthus annuus), and how transition is affected by CO(2). Growth analysis and steady-state (13)CO(2)/(12)CO(2) and (15)NO(3) (-)/(14)NO(3) (-) labelling were used to quantify reserve- and current assimilation-derived carbon (C) and nitrogen (N) allocation to shoots and roots in the presence of 200 and 1,000 micromol CO(2) mol(-1) air. Growth was not influenced by CO(2) until cotyledons unfolded. Then, C accumulation at elevated CO(2) increased to a rate 2-2.5 times higher than in sub-ambient CO(2) due to increased unit leaf rate (+120%) and leaf expansion (+60%). CO(2) had no effect on mobilization and allocation of reserve-derived C and N, even during the transition period. Export of autotrophic C from cotyledons began immediately following the onset of photosynthetic activity, serving roots and shoots near-simultaneously. Allocation of autotrophic C to shoots was increased at sub-ambient CO(2). The synchrony in transition from heterotrophic to autotrophic supply for different sinks in sunflower contrasts with the sequential transition reported for species with hypogeal germination.

A continuous-flow isotope-ratio mass spectrometer (CF-IRMS, custom-made GasBenchII and Delta(plus)Advantage, ThermoFinnigan) was installed on a grassland site and interfaced with a closed-path infrared gas analyser (IRGA). The CF-IRMS and IRGA were housed in an air-conditioned travel van. Air was sampled at 1.5 m above the 0.07-m tall grassland canopy, drawn through a 17-m long PTFE tube at a rate of 0.25 L s(-1), and fed to the IRGA and CF-IRMS in series. The IRMS was interfaced with the IRGA via a stainless steel capillary inserted 0.5 m into the sample air outlet tube of the IRGA (forming an open split), a gas-tight pump, and a sample loop attached to the eight-port Valco valve of the continuous-flow interface. Air was pumped through the 0.25-mL sample loop at 10 mL s(-1) (a flushing frequency of 40 Hz). Air samples were analysed at intervals of approx. 2.8 min. Whole system precision was tested in the field using air mixed from pure CO2 and CO2-free air by means of mass flow controllers. The standard deviation of repeated single measurements was 0.21-0.07 per thousand for delta13C and 0.34-0.14 per thousand for delta18O of CO2 in air with mixing ratios ranging between 200-800 micromol mol(-1). The CO2 peak area measured by the IRMS was proportional to the CO2 mixing ratio (r2 = 1.00), allowing estimation of sample air CO2 mixing ratio from IRMS data. A 1-day long measurement cycle of CO2, delta13C and delta18O of air sampled above the grassland canopy was used to test the system for Keeling plot applications. Delta18O exhibited a clear diurnal cycle (4 per thousand range), but short-term (1-h interval) variability was small (average SD 0.38 per thousand). Yet, the correlation between delta18O and CO2 mixing ratio was relatively weak, and this was true for both the whole data set and 1-h subsets. Conversely, the delta13C of all 541 samples measured during the 25.2-h interval fitted well the Keeling regression (r2 = 0.99), yielding an intercept of -27.40 per thousand (+/-0.07 per thousand SE). Useful Keeling regressions (r2 > 0.9, average r2 = 0.96) also resulted from data collected over 1-h intervals of the 12-h long twilight and dark period. These indicated that 13C content of ecosystem respiration was approx. constant near -27.6 per thousand. The precision of the present system is similar to that of current techniques used in ecosystem studies which employ flask sampling and a laboratory-based CF-IRMS. Sampling (and measurement) frequency is greatly increased relative to systems based on flask sampling, and sampling time (0.025 s per sample) is decreased. These features increase the probability for sampling the entire CO2 range which occurs in a given time window. The system obviates sample storage problems, greatly minimises handling needs, and allows extended campaigns of high frequency sampling and analysis with minimal attendance., (Copyright 2004 John Wiley & Sons, Ltd.)