Microelectrode technique for in situ measurement of carbon dioxide concentrations in xylem sap of trees

The quandary of sources and sinks of CO2 efflux in tree stems-new insights and future directions

Stem respiration (RS) substantially contributes to the return of photo assimilated carbon to the atmosphere and, thus, to the tree and ecosystem carbon balance. Stem CO2 efflux (ECO2) is often used as a proxy for RS. However, this metric has often been challenged because of the uncertain origin of CO2 emitted from the stem due to post-respiratory processes. In this Insight, we (i) describe processes affecting the quantification of RS, (ii) review common methodological approaches to quantify and model RS and (iii) develop a research agenda to fill the most relevant knowledge gaps that we identified. Dissolution, transport and accumulation of respired CO2 away from its production site, reassimilation of respired CO2 via stem photosynthesis and the enzyme phosphoenolpyruvate carboxylase, axial CO2 diffusion in the gas phase, shifts in the respiratory substrate and non-respiratory oxygen (O2) consumption are the most relevant processes causing divergence between RS and measured stem gas exchange (ECO2 or O2 influx, IO2). Two common methodological approaches to estimate RS, namely the CO2 mass balance approach and the O2 consumption technique, circumvent some of these processes but have yielded inconsistent results regarding the fate of respired CO2. Stem respiration modelling has recently progressed at the organ and tree levels. However, its implementation in large-scale models, commonly operated from a source-driven perspective, is unlikely to reflect adequate mechanisms. Finally, we propose hypotheses and approaches to advance the knowledge of the stem carbon balance, the role of sap pH on RS, the reassimilation of respired CO2, RS upscaling procedures, large-scale RS modelling and shifts in respiratory metabolism during environmental stress.

Partitioning of respired CO2 in newly sprouted Moso bamboo culms

Stem respiration (R _s) plays a vital role in ecosystem carbon cycling. However, the measured efflux on the stem surface (E _s) is not always in situ R _s but only part of it. A previously proposed mass balance framework (MBF) attempted to explore the multiple partitioning pathways of R _s, including sap-flow-transported and internal storage of R _s, in addition to E _s. This study proposed stem photosynthesis as an additional partitioning pathway to the MBF. Correspondingly, a double-chamber apparatus was designed and applied on newly sprouted Moso bamboo (Phyllostachys edulis) in leafless and leaved stages. R _s of newly sprouted bamboo were twice as high in the leafless stage (7.41 ± 2.66 μmol m^-2 s^-1) than in the leaved stage (3.47 ± 2.43 μmol m^-2 s^-1). E _s accounted for ~80% of R _s, while sap flow may take away ~2% of R _s in both leafless and leaved stages. Culm photosynthesis accounted for ~9% and 13% of R _s, respectively. Carbon sequestration from culm photosynthesis accounted for approximately 2% of the aboveground bamboo biomass in the leafless stage. High culm photosynthesis but low sap flow during the leafless stage and vice versa during the leaved stage make bamboo an outstanding choice for exploring the MBF.

Acid waters in tank bromeliads: Causes and potential consequences

PREMISE

The consequences of acidity for plant performance are profound, yet the prevalence and causes of low pH in bromeliad tank water are unknown despite its functional relevance to key members of many neotropical plant communities.

METHODS

We investigated tank water pH for eight bromeliad species in the field and for the widely occurring Guzmania monostachia in varying light. We compared pH changes over time between plant and artificial tanks containing a solution combined from several plants. Aquaporin transcripts were measured for field plants at two levels of pH. We investigated relationships between pH, leaf hydraulic conductance, and CO₂ concentration in greenhouse plants and tested proton pump activity using a stimulator and inhibitor.

RESULTS

Mean tank water pH for the eight species was 4.7 ± 0.06 and was lower for G. monostachia in higher light. The pH of the solution in artificial tanks, unlike in plants, did not decrease over time. Aquaporin transcription was higher for plants with lower pH, but leaf hydraulic conductance did not differ, suggesting that the pH did not influence water uptake. Tank pH and CO₂ concentration were inversely related. Fusicoccin enhanced a decrease in tank pH, whereas orthovanadate did not.

CONCLUSIONS

Guzmania monostachia acidified its tank water via leaf proton pumps, which appeared responsive to light. Low pH increased aquaporin transcripts but did not influence leaf hydraulic conductance, hence may be more relevant to nutrient uptake.

Carbon Dioxide Sensing-Biomedical Applications to Human Subjects

Carbon dioxide (CO2) monitoring in human subjects is of crucial importance in medical practice. Transcutaneous monitors based on the Stow-Severinghaus electrode make a good alternative to the painful and risky arterial "blood gases" sampling. Yet, such monitors are not only expensive, but also bulky and continuously drifting, requiring frequent recalibrations by trained medical staff. Aiming at finding alternatives, the full panel of CO2 measurement techniques is thoroughly reviewed. The physicochemical working principle of each sensing technique is given, as well as some typical merit criteria, advantages, and drawbacks. An overview of the main CO2 monitoring methods and sites routinely used in clinical practice is also provided, revealing their constraints and specificities. The reviewed CO2 sensing techniques are then evaluated in view of the latter clinical constraints and transcutaneous sensing coupled to a dye-based fluorescence CO2 sensing seems to offer the best potential for the development of a future non-invasive clinical CO2 monitor.

Efflux and assimilation of xylem-transported CO2 in stems and leaves of tree species with different wood anatomy

Determining the fate of CO₂ respired in woody tissues is necessary to understand plant respiratory physiology and to evaluate CO₂ recycling mechanisms. An aqueous ¹³ C-enriched CO₂ solution was infused into the stem of 3-4 m tall trees to estimate efflux and assimilation of xylem-transported CO₂ via cavity ring-down laser spectroscopy and isotope ratio mass spectrometry, respectively. Different tree locations (lower stem, upper stem and leafy shoots) and tissues (xylem, bark and leaves) were monitored in species with tracheid, diffuse- and ring-porous wood anatomy (cedar, maple and oak, respectively). Radial xylem CO₂ diffusivity and xylem [CO₂ ] were lower in cedar relative to maple and oak trees, thereby limiting label diffusion. Part of the labeled ¹³ CO₂ was assimilated in cedar (8.7%) and oak (20.6%) trees, mostly in xylem and bark tissues of the stem, while limited solution uptake in maple trees hindered the detection of label assimilation. Little label reached foliar tissues, suggesting substantial label loss along the stem-branch transition following reductions in the radial diffusive pathway. Differences in respiration rates and radial xylem CO₂ diffusivity (lower in conifer relative to angiosperm species) might reconcile discrepancies in efflux and assimilation of xylem-transported CO₂ so far observed between taxonomic clades.

Xylem transport of root-derived CO2 caused a substantial underestimation of belowground respiration during a growing season

Previous research has indicated that a potentially large portion of root-respired CO₂ can move internally through tree xylem, but these reports are relatively scarce and have generally been limited to short observations. Our main objective was to provide a continuous estimate of the quantity and variability of root-respired CO₂ that moves either internally through the xylem (F_T ) or externally through the soil to the atmosphere (F_S ) over most of a growing season. Nine trees were measured in a Populus deltoides stand for 129 days from early June to mid-October. We calculated F_T as the product of sap flow and dissolved [CO₂ ] in the xylem (i.e., [CO₂ *]) and calculated F_S using the [CO₂ ] gradient method. During the study, stem and soil CO₂ concentrations, temperature, and sap flow were measured continuously. We determined that F_T accounted for 33% of daily total belowground CO₂ flux (i.e., F_S  + F_T ; F_B ) during our observation period that spanned most of a growing season. Cumulative daily F_T was lower than F_S 74% of the time, equivalent to F_S 26% of the time, and never exceeded F_S . One-third of the total CO₂ released by belowground respiration over most of the growing season in this forest stand followed the F_T pathway rather than diffusing into the soil. The magnitude of F_T indicates that measurements of F_S alone substantially underestimate total belowground respiration in some forest ecosystems by systematically underestimating belowground autotrophic respiration. The variability in F_T observed during the growing season demonstrated the importance of making long-term, high-frequency measurements of different flux pathways to better understand physiological and ecological processes and their implications to global change.

Limited vertical CO2 transport in stems of mature boreal Pinus sylvestris trees

Several studies have suggested that CO2 transport in the transpiration stream can considerably bias estimates of root and stem respiration in ring-porous and diffuse-porous tree species. Whether this also happens in species with tracheid xylem anatomy and lower sap flow rates, such as conifers, is currently unclear. We infused 13C-labelled solution into the xylem near the base of two 90-year-old Pinus sylvestris L. trees. A custom-built gas exchange system and an online isotopic analyser were used to sample the CO2 efflux and its isotopic composition continuously from four positions along the bole and one upper canopy shoot in each tree. Phloem and needle tissue 13C enrichment was also evaluated at these positions. Most of the 13C label was lost by diffusion within a few metres of the infusion point indicating rapid CO2 loss during vertical xylem transport. No 13C enrichment was detected in the upper bole needle tissues. Furthermore, mass balance calculations showed that c. 97% of the locally respired CO2 diffused radially to the atmosphere. Our results support the notion that xylem CO2 transport is of limited magnitude in conifers. This implies that the concerns that stem transport of CO2 derived from root respiration biases chamber-based estimates of forest carbon cycling may be unwarranted for mature conifer stands.

TReSpire - a biophysical TRee Stem respiration model

Mechanistic models of plant respiration remain poorly developed, especially in stems and woody tissues where measurements of CO₂ efflux do not necessarily reflect local respiratory activity. We built a process-based model of stem respiration that couples water and carbon fluxes at the organ level (TReSpire). To this end, sap flow, stem diameter variations, xylem and soil water potential, stem temperature, stem CO₂ efflux and nonstructural carbohydrates were measured in a maple tree, while xylem CO₂ concentration and additional stem and xylem diameter variations were monitored in an ancillary tree for model validation. TReSpire realistically described: (1) turgor pressure to differentiate growing from nongrowing metabolism; (2) maintenance expenditures in xylem and outer tissues based on Arrhenius kinetics and nitrogen content; and (3) radial CO₂ diffusivity and CO₂ solubility and transport in the sap solution. Collinearity issues with phloem unloading rates and sugar-starch interconversion rates suggest parallel submodelling to close the stem carbon balance. TReSpire brings a breakthrough in the modelling of stem water and carbon fluxes at a detailed (hourly) temporal resolution. TReSpire is calibrated from a sink-driven perspective, and has potential to advance our understanding on stem growth dynamics, CO₂ fluxes and underlying respiratory physiology across different species and phenological stages.

Elevated CO2 does not affect stem CO2 efflux nor stem respiration in a dry Eucalyptus woodland, but it shifts the vertical gradient in xylem [CO2 ]

To quantify stem respiration (R_S ) under elevated CO₂ (eCO₂ ), stem CO₂ efflux (E_A ) and CO₂ flux through the xylem (F_T ) should be accounted for, because part of respired CO₂ is transported upwards with the sap solution. However, previous studies have used E_A as a proxy of R_S , which could lead to equivocal conclusions. Here, to test the effect of eCO₂ on R_S , both E_A and F_T were measured in a free-air CO₂ enrichment experiment located in a mature Eucalyptus native forest. Drought stress substantially reduced E_A and R_S , which were unaffected by eCO₂ , likely as a consequence of its neutral effect on stem growth in this phosphorus-limited site. However, xylem CO₂ concentration measured near the stem base was higher under eCO₂ , and decreased along the stem resulting in a negative contribution of F_T to R_S , whereas the contribution of F_T to R_S under ambient CO₂ was positive. Negative F_T indicates net efflux of CO₂ respired below the monitored stem segment, likely coming from the roots. Our results highlight the role of nutrient availability on the dependency of R_S on eCO₂ and suggest stimulated root respiration under eCO₂ that may shift vertical gradients in xylem [CO₂ ] confounding the interpretation of E_A measurements.

Isotope ratio laser spectroscopy to disentangle xylem-transported from locally respired CO2 in stem CO2 efflux

Respired CO2 in woody tissues radially diffuses to the atmosphere or it is transported upward with the transpiration stream, making the origin of CO2 in stem CO2 efflux (EA) uncertain, which may confound stem respiration (RS) estimates. An aqueous 13C-enriched solution was infused into stems of Populus tremula L. trees, and real-time measurements of 13C-CO2 and 12C-CO2 in EA were performed via Cavity Ring Down Laser Spectroscopy (CRDS). The contribution of locally respired CO2 (LCO2) and xylem-transported CO2 (TCO2) to EA was estimated from their different isotopic composition. Mean daily values of TCO2/EA ranged from 13% to 38%, evidencing the notable role that xylem CO2 transport plays in the assessment of stem respiration. Mean daily TCO2/EA did not differ between treatments of drought stress and light exclusion of woody tissues, but they showed different TCO2/EA dynamics on a sub-daily time scale. Sub-daily CO2 diffusion patterns were explained by a light-induced axial CO2 gradient ascribed to woody tissue photosynthesis, and the resistance to radial CO2 diffusion determined by bark water content. Here, we demonstrate the outstanding potential of CRDS paired with 13C-CO2 labelling to advance in the understanding of CO2 movement at the plant-atmosphere interface and the respiratory physiology in woody tissues.

Internal transport of CO2 from the root-zone to plant shoot is pH dependent

We investigated the fate of carbon dioxide (CO₂ ) absorbed by roots or internally produced by respiration using gas exchange and stable isotopic labeling. CO₂ efflux from detached leaves supplied with bicarbonate/CO₂ solutions was followed over six cycles. CO₂ effluxes were detected when bicarbonate solution at high pH was used, corresponding to 71-85% of the expected efflux. No CO₂ efflux was detected when CO₂ solutions at low pH were used but CO₂ efflux was subsequently detected as soon as bicarbonate solutions at high pH were supplied. By sealing the leaf and petiole in a plastic bag to reduce diffusion to the atmosphere, a small CO₂ efflux signal (14-30% of the expected efflux) was detected suggesting that CO₂ in the xylem stream can readily escape to the atmosphere before reaching the leaf. When the root-zones of intact plants were exposed to CO₂ solutions, a significant efflux from leaf surface was observed (13% of the expected efflux). However, no signal was detected when roots were exposed to a high pH bicarbonate solution. Isotopic tracer experiments confirmed that CO₂ supplied to the root-zone was transported through the plant and was readily lost to the atmosphere. However, little ¹³ C moved to the shoot when roots were exposed to bicarbonate solutions at pH 8, suggesting that bicarbonate does not pass into the xylem.

Daytime depression in temperature-normalised stem CO2 efflux in young poplar trees is dominated by low turgor pressure rather than by internal transport of respired CO2

Daytime decreases in temperature-normalised stem CO₂ efflux (E_{A_D} ) are commonly ascribed to internal transport of respired CO₂ (F_T ) or to an attenuated respiratory activity due to lowered turgor pressure. The two are difficult to separate as they are simultaneously driven by sap flow dynamics. To achieve combined gradients in turgor pressure and F_T , sap flow rates in poplar trees were manipulated through severe defoliation, severe drought, moderate defoliation and moderate drought. Turgor pressure was mechanistically modelled using measurements of sap flow, stem diameter variation, and soil and stem water potential. A mass balance approach considering internal and external CO₂ fluxes was applied to estimate F_T . Under well-watered control conditions, both turgor pressure and sap flow, as a proxy of F_T , were reliable predictors of E_{A_D} . After tree manipulation, only turgor pressure was a robust predictor of E_{A_D} . Moreover, F_T accounted for < 15% of E_{A_D} . Our results suggest that daytime reductions in turgor pressure and associated constrained growth are the main cause of E_{A_D} in young poplar trees. Turgor pressure is determined by both carbohydrate supply and water availability, and should be considered to improve our widely used but inaccurate temperature-based predictions of woody tissue respiration in global models.

Effects of stem size on stem respiration and its flux components in yellow-poplar (Liriodendron tulipifera L.) trees

Carbon dioxide (CO2) released from respiring cells in the stems of trees (RS) can diffuse radially to the atmosphere (EA) or dissolve in xylem sap and move internally in the tree (FT). Previous studies have observed that EA decreases as stem or branch diameter increases, but the cause of this relationship has not been determined, nor has the relationship been confirmed between stem diameter and RS, which includes both EA and FT. In this study, for the first time the mass balance technique was used to estimate RS of stems of Liriodendron tulipifera L. trees of different diameters, ranging from 16 to 60 cm, growing on the same site. The magnitude of the component fluxes scaled with tree size. Among the five trees, the contribution of EA to RS decreased linearly with increasing stem diameter and sapwood area while the contribution of FT to RS increased linearly with stem diameter and sapwood area. For the smallest tree EA was 86% of RS but it was only 46% of RS in the largest tree. As tree size increased a greater proportion of respired CO2 dissolved in sap and remained within the tree. Due to increase in FT with tree size, we observed that trees of different sizes had the same RS even though they had different EA. This appears to explain why the EA of stems and branches decreases as their size increases.

Inside out: efflux of carbon dioxide from leaves represents more than leaf metabolism

High concentrations of inorganic carbon in the xylem, produced from root, stem, and branch respiration, travel via the transpiration stream and eventually exit the plant through distant tissues as CO2. Unlike previous studies that focused on the efflux of CO2 from roots and woody tissues, we focus on efflux from leaves and the potential effect on leaf respiration measurements. We labeled transported inorganic carbon, spanning reported xylem concentrations, with 13C and then manipulated transpiration rates in the dark in order to vary the rates of inorganic carbon supply to cut leaves from Brassica napus and Populus deltoides. We used tunable diode laser absorbance spectroscopy to directly measure the rate of gross 13CO2 efflux, derived from inorganic carbon supplied from outside of the leaf, relative to gross 12CO2 efflux generated from leaf cells. These experiemnts showed that 13CO2 efflux was dependent upon the rate of inorganic carbon supply to the leaf and the rate of transpiration. Our data show that the gross leaf efflux of xylem-transported CO2 is likely small in the dark when rates of transpiration are low. However, gross leaf efflux of xylem-transported CO2 could approach half the rate of leaf respiration in the light when transpiration rates and branch inorganic carbon concentrations are high, irrespective of the grossly different petiole morphologies in our experiment.

Measuring the ratio of CO2 efflux to O2 influx in tree stem respiration

In recent studies, the ratio of tree stem CO₂ efflux to O₂ influx has been defined as the apparent respiratory quotient (ARQ). The metabolism of carbohydrates, the putative respiratory substrate in trees, is expected to yield an ARQ of 1.0. However, previous studies have reported ARQ values ranging between 0.23 and 0.90. These interesting results may indicate internal transport of respired CO₂ within stems; yet no simple field applicable methods for ARQ measurement have been available. Here, we report on the assembly of a closed circulating system called 'Hampadah', which uses CO₂ and O₂ analyzers to measure air samples from stem chambers. We tested the performance of the Hampadah with samples from 36 trees (Tetragastris panamensis (Engl.) Kuntze). Additionally, we showed the feasibility of measuring ARQ directly from stem chambers, using portable CO₂ and O₂ sensors, in both discrete and continuous modes of operation. The Hampadah measurement proved to be consistent with CO₂ gas standards (R² = 0.999) and with O₂ determined by O₂/Ar measurements with a mass spectrometer (R² = 0.998). The Hampadah gave highly reproducible results for ARQ determination of field samples (±0.01 for duplicates). The portable sensors measurement showed good correlation with the Hampadah in measuring CO₂, O₂ and ARQ (n = 5, R² = 0.97, 0.98 and 0.91, respectively). We have demonstrated here that the Hampadah and the sensors' methods enable accurate ARQ measurements for both laboratory and field research.

The influence of preparation method on measured carbon fractions in tree tissues

Carbon fractions of tree tissues are a key component of forest carbon mass estimation. Several methods have been used to measure carbon fractions, yet no comprehensive comparison between methods has been performed. We found significant differences between carbon fractions derived from four sample preparation methods: oven-drying, vacuum desiccation, freeze-drying, and a new method that consisted of (i) not drying samples, (ii) cutting samples instead of grinding them, (iii) measuring carbon content of samples, (iv) oven-drying remaining sample material and (v) using mass measurements of remaining sample material before and after oven-drying to adjust measured carbon fraction values to an oven-dry basis (minimize the loss of carbon (MLC) method). Oven-drying, freeze-drying and vacuum desiccation resulted in lower average carbon fraction estimates than the MLC method, suggesting that they do not capture as much of the carbon present in tree tissues. Further analysis showed significant, though small, differences in carbon fractions between powdered samples and samples excised from tree core segments with a razor blade. Powdered samples were found to have lower carbon fractions than the excised samples, indicating that some carbon is lost when samples are powdered instead of cut. Utilization of the MLC method captured an average of 1.4% more carbon on an oven-drying basis than freeze-drying, the next best method. Additionally, when applied to different tree tissue types, these methods measured different volatile carbon fractions, indicating that studies attempting to quantify volatile carbon and total carbon fraction in trees should measure all tissue types present.

Carbon dioxide emission from bamboo culms

Bamboos are one of the fastest growing plants on Earth, and are widely considered to have high ability to capture and sequester atmospheric carbon, and consequently to mitigate climate change. We tested this hypothesis by measuring carbon dioxide (CO2 ) emissions from bamboo culms and comparing them with their biomass sequestration potential. We analysed diurnal effluxes from Bambusa vulgaris culm surface and gas mixtures inside hollow sections of various bamboos using gas chromatography. Corresponding variations in gas pressure inside the bamboo section and culm surface temperature were measured. SEM micrographs of rhizome and bud portions of bamboo culms were also recorded. We found very high CO2 effluxes from culm surface, nodes and buds of bamboos. Positive gas pressure and very high concentrations of CO2 were observed inside hollow sections of bamboos. The CO2 effluxes observed from bamboos were very high compared to their carbon sequestration potential. Our measurements suggest that bamboos are net emitters of CO2 during their lifespan.

Why small fluxes matter: the case and approaches for improving measurements of photosynthesis and (photo)respiration

Since its inception, the Farquhar et al. (1980) model of photosynthesis has been a mainstay for relating biochemistry to environmental conditions from chloroplast to global levels in terrestrial plants. Many variables could be assigned from basic enzyme kinetics, but the model also required measurements of maximum rates of photosynthetic electron transport (J max ), carbon assimilation (Vcmax ), conductance of CO2 into (g s ) and through (g m ) the leaf, and the rate of respiration during the day (R d ). This review focuses on improving the accuracy of these measurements, especially fluxes from photorespiratory CO2, CO2 in the transpiration stream, and through the leaf epidermis and cuticle. These fluxes, though small, affect the accuracy of all methods of estimating mesophyll conductance and several other photosynthetic parameters because they all require knowledge of CO2 concentrations in the intercellular spaces. This review highlights modified methods that may help to reduce some of the uncertainties. The approaches are increasingly important when leaves are stressed or when fluxes are inferred at scales larger than the leaf.

Seasonal and diel variation in xylem CO2 concentration and sap pH in sub-Mediterranean oak stems

Since a substantial portion of respired CO2 remains within the stem, diel and seasonal trends in stem CO2 concentration ([CO2]) are of major interest in plant respiration and carbon budget research. However, continuous long-term stem [CO2] studies are scarce, and generally absent in Mediterranean climates. In this study, stem [CO2] was monitored every 15min together with stem and air temperature, sap flow, and soil water storage during a growing season in 16 stems of Quercus pyrenaica to elucidate the main drivers of stem [CO2] at different temporal scales. Fluctuations in sap pH were also assessed during two growing seasons to evaluate potential errors in estimates of the concentration of CO2 dissolved in xylem sap ([CO2*]) calculated using Henry's law. Stem temperature was the best predictor of stem [CO2] and explained more than 90% and 50% of the variability in stem [CO2] at diel and seasonal scales, respectively. Under dry conditions, soil water storage was the main driver of stem [CO2]. Likewise, the first rains after summer drought caused intense stem [CO2] pulses, suggesting enhanced stem and root respiration and increased resistance to radial CO2 diffusion. Sap flow played a secondary role in controlling stem [CO2] variations. We observed night-time sap pH acidification and progressive seasonal alkalinization. Thus, if the annual mean value of sap pH (measured at midday) was assumed to be constant, night-time sap [CO2*] was substantially overestimated (40%), and spring and autumn sap [CO2*] were misestimated by 25%. This work highlights that diel and seasonal variations in temperature, tree water availability, and sap pH substantially affect xylem [CO2] and sap [CO2*].

Diel growth dynamics in tree stems: linking anatomy and ecophysiology

Impacts of climate on stem growth in trees are studied in anatomical, ecophysiological, and ecological disciplines, but an integrative framework to assess those impacts remains lacking. In this opinion article, we argue that three research efforts are required to provide that integration. First, we need to identify the missing links in diel patterns in stem diameter and stem growth and relate those patterns to the underlying mechanisms that control water and carbon balance. Second, we should focus on the understudied mechanisms responsible for seasonal impacts on such diel patterns. Third, information on stem anatomy and ecophysiology should be integrated in the same experiments and mechanistic plant growth models to capture both diel and seasonal scales.

Stem girdling affects the quantity of CO2 transported in xylem as well as CO2 efflux from soil

There is recent clear evidence that an important fraction of root-respired CO2 is transported upward in the transpiration stream in tree stems rather than fluxing to the soil. In this study, we aimed to quantify the contribution of root-respired CO2 to both soil CO2 efflux and xylem CO2 transport by manipulating the autotrophic component of belowground respiration. We compared soil CO2 efflux and the flux of root-respired CO2 transported in the transpiration stream in girdled and nongirdled 9-yr-old oak trees (Quercus robur) to assess the impact of a change in the autotrophic component of belowground respiration on both CO2 fluxes. Stem girdling decreased xylem CO2 concentration, indicating that belowground respiration contributes to the aboveground transport of internal CO2 . Girdling also decreased soil CO2 efflux. These results confirmed that root respiration contributes to xylem CO2 transport and that failure to account for this flux results in inaccurate estimates of belowground respiration when efflux-based methods are used. This research adds to the growing body of evidence that efflux-based measurements of belowground respiration underestimate autotrophic contributions.

Quantifying the impact of daily and seasonal variation in sap pH on xylem dissolved inorganic carbon estimates in plum trees

In studies on internal CO2 transport, average xylem sap pH (pH(x)) is one of the factors used for calculation of the concentration of dissolved inorganic carbon in the xylem sap ([CO2 *]). Lack of detailed pH(x) measurements at high temporal resolution could be a potential source of error when evaluating [CO2*] dynamics. In this experiment, we performed continuous measurements of CO2 concentration ([CO2]) and stem temperature (T(stem)), complemented with pH(x) measurements at 30-min intervals during the day at various stages of the growing season (Day of the Year (DOY): 86 (late winter), 128 (mid-spring) and 155 (early summer)) on a plum tree (Prunus domestica L. cv. Reine Claude d'Oullins). We used the recorded pH(x) to calculate [CO2*] based on T(stem) and the corresponding measured [CO2]. No statistically significant difference was found between mean [CO2*] calculated with instantaneous pH(x) and daily average pH(x). However, using an average pH(x) value from a different part of the growing season than the measurements of [CO2] and T(stem) to estimate [CO2*] led to a statistically significant error. The error varied between 3.25 ± 0.01% under-estimation and 3.97 ± 0.01% over-estimation, relative to the true [CO2*] data. Measured pH(x) did not show a significant daily variation, unlike [CO2], which increased during the day and declined at night. As the growing season progressed, daily average [CO2] (3.4%, 5.3%, 7.4%) increased and average pH(x) (5.43, 5.29, 5.20) decreased. Increase in [CO2] will increase its solubility in xylem sap according to Henry's law, and the dissociation of [CO2*] will negatively affect pH(x). Our results are the first quantifying the error in [CO2*] due to the interaction between [CO2] and pH(x) on a seasonal time scale. We found significant changes in pH(x) across the growing season, but overall the effect on the calculation of [CO2*] remained within an error range of 4%. However, it is possible that the error could be more substantial for other tree species, particularly if pH(x) is in the more sensitive range (pH(x) > 6.5).

Assimilation of xylem-transported CO2 is dependent on transpiration rate but is small relative to atmospheric fixation

The effect of transpiration rate on internal assimilation of CO2 released from respiring cells has not previously been quantified. In this study, detached branches of Populus deltoides were allowed to take up (13)CO2-labelled solution at either high (high label, HL) or low (low label, LL) (13)CO2 concentrations. The uptake of the (13)CO2 label served as a proxy for the internal transport of respired CO2, whilst the transpiration rate was manipulated at the leaf level by altering the vapour pressure deficit (VPD) of the air. Simultaneously, leaf gas exchange was measured, allowing comparison of internal CO2 assimilation with that assimilated from the atmosphere. Subsequent (13)C analysis of branch and leaf tissues revealed that woody tissues assimilated more label under high VPD, corresponding to higher transpiration, than under low VPD. More (13)C was assimilated in leaf tissue than in woody tissue under the HL treatment, whereas more (13)C was assimilated in woody tissue than in leaf tissue under the LL treatment. The ratio of (13)CO2 assimilated from the internal source to CO2 assimilated from the atmosphere was highest for the branches under the HL and high VPD treatment, but was relatively small regardless of VPD×label treatment combination (up to 1.9%). These results showed that assimilation of internal CO2 is highly dependent on the rate of transpiration and xylem sap [CO2]. Therefore, it can be expected that the relative contribution of internal CO2 recycling to tree carbon gain is strongly dependent on factors controlling transpiration, respiration, and photosynthesis.

Long-term stem CO2 concentration measurements in Norway spruce in relation to biotic and abiotic factors

Stem CO(2) concentrations (stem [CO(2)]) undergo large temporal variations that need to be understood to better link tree physiological processes to biosphere-atmosphere CO(2) exchange. During 19 months, stem [CO(2)] was continuously measured in mature subalpine Norway spruce trees (Picea abies) and jointly analysed with stem, soil and air temperatures, sap flow rates, stem radius changes and CO(2) efflux rates from stem and soil on different time scales. Stem [CO(2)] exhibited a strong seasonality, of which over 80% could be explained with stem and soil temperatures. Both physical equilibrium processes of CO(2) between water and air according to Henry's law as well as physiological effects, including sap flow and local respiration, concurrently contributed to these temporal variations. Moreover, the explanatory power of potential biological drivers (stem radius changes, sap flow and soil respiration) varied strongly with season and temporal resolution. We conclude that seasonal and daily courses of stem [CO(2)] in spruce trees are a combined effect of physical equilibrium and tree physiological processes. Furthermore, we emphasize the relevance of axial diffusion of CO(2) along air-filled spaces in the wood, and potential wound response processes owing to sensor installation.

Transport of root-respired CO₂ via the transpiration stream affects aboveground carbon assimilation and CO₂ efflux in trees

Upward transport of CO₂ via the transpiration stream from belowground to aboveground tissues occurs in tree stems. Despite potentially important implications for our understanding of plant physiology, the fate of internally transported CO₂ derived from autotrophic respiratory processes remains unclear. We infused a ¹³CO₂-labeled aqueous solution into the base of 7-yr-old field-grown eastern cottonwood (Populus deltoides) trees to investigate the effect of xylem-transported CO₂ derived from the root system on aboveground carbon assimilation and CO₂ efflux. The ¹³C label was transported internally and detected throughout the tree. Up to 17% of the infused label was assimilated, while the remainder diffused to the atmosphere via stem and branch efflux. The largest amount of assimilated ¹³C was found in branch woody tissues, while only a small quantity was assimilated in the foliage. Petioles were more highly enriched in ¹³C than other leaf tissues. Our results confirm a recycling pathway for respired CO₂ and indicate that internal transport of CO₂ from the root system may confound the interpretation of efflux-based estimates of woody tissue respiration and patterns of carbohydrate allocation.

Pulse labeling of dissolved (13) C-carbonate into tree xylem: developing a new method to determine the fate of recently fixed photosynthate

Stable carbon isotopes are often employed as tracers in plant and soil systems to study the fate and transformations of carbon as is it assimilated by the forest canopies and then translocated into the soil matrix and soil microorganisms. This experiment tested a new method of (13) C-labeling. We dissolved (13) C-carbonate into 12 mL of water and injected it into the xylem of a 6-cm diameter tree. The isotopic composition of foliage, stem CO(2) , and phloem contents were measured before the experiment and up to two weeks after the pulse label. Isotopic enrichments of 6.1‰ and 7.7‰ were observed in stem CO(2) and phloem contents, respectively. No enrichment in bulk foliage was observed. The pulse came through the phloem five days after the label was injected, consistent with expectations based on transport rates through the tree. The application of this xylem pulse-labeling method may provide new insights into labile carbon sequestration in trees, perhaps even in much larger trees. Furthermore, the method could be applied under experimental treatments that would elucidate the mechanisms controlling the fate and transformation of recently fixed photosynthate in forests.

Storage and transpiration have negligible effects on delta13C of stem CO2 efflux in large conifer trees

Stem respiration rates are often quantified by measuring the CO(2) efflux from stems into chambers. It has been suggested that these measurements underestimate respiration because some of the respired CO(2) can be either retained or transported upwards in the transpiration stream. If the stem CO(2) efflux does not represent all respired CO(2), then the interpretation of its isotopic signal may be compromised as well. The C-isotope composition of the respired CO(2) and the measured efflux could differ due to (i) the release of CO(2) produced elsewhere into the stem and transported upwards in xylem water (soil CO(2) or root respired CO(2)); (ii) the retention or release of CO(2) storage pools within the tree stem and (iii) the removal of CO(2) by the transpiration stream. We investigated the effects of these processes in large conifer trees using two manipulative experiments: a labelling experiment and a crown removal experiment. The labelling experiment used an extreme enrichment of dissolved CO(2) in soil water to assess the C uptake by the roots. In this experiment, we found no contamination of the stem CO(2) pool despite clear evidence that the water itself had been taken up. The crown removal experiment tested for vertical CO(2) flux in xylem water by eliminating transpiration. Here, we found no change in the delta(13)C of stem CO(2) efflux (delta(EA); P > 0.05). We concluded that for these large conifers, sap-flow influenced neither delta(13)C of stem efflux nor that of the stem CO(2) pool. By parameterizing Henry's Law for conditions inside the stem, we estimated the transport flux to represent 1-3% of the stem CO(2) efflux to the atmosphere. Finally, assuming a 2 per thousand difference between delta(13)C of root and stem respiration, we estimated that potential contamination of delta(EA) by root respired CO(2) would be < 0.1 per thousand. Thus, neither the release of soil or root CO(2), nor storage in the stem, nor vertical transport of CO(2) in the xylem sap had any detectable influence on delta(13)C of the CO(2) measured in stem efflux.

Interpretation of stem CO2 efflux measurements

It is known that stem CO2 efflux differs somewhat both temporally and spatially from actual stem respiration, but relations between these two are not fully understood. A physical model of CO2 diffusion and advection by xylem sap flow is developed to interpret the CO2 flux signal from the stem. Model predictions are compared against measured CO2 efflux data from a field-grown 16-m Pinus sylvestris L. tree. The ratio of CO2 efflux to CO2 production is predicted to be much larger in the upper part of the tree than in the lower part as the xylem sap carries the respired CO2 upwards. The model also predicts the temperature dependency of real respiration to be higher than that of the CO2 efflux due to the slowness of diffusion. The relation between stem respiration and CO2 efflux depends strongly on the sap flow rate, radial diffusion resistance and stem geometry and size. The model may be used to scale individual CO2 efflux measurements to evaluate the respiration rate of whole trees and forests.

Root-derived CO(2) efflux via xylem stream rivals soil CO(2) efflux

Respiration consumes a large portion of annual gross primary productivity in forest ecosystems and is dominated by belowground metabolism. Here, we present evidence of a previously unaccounted for internal CO(2) flux of large magnitude from tree roots through stems. If this pattern is shown to persist over time and in other forests, it suggests that belowground respiration has been grossly underestimated. Using an experimental Populus deltoides plantation as a model system, we tested the hypothesis that a substantial portion of the CO(2) released from belowground autotrophic respiration remains within tree root systems and is transported aboveground through the xylem stream rather than diffusing into the soil atmosphere. On a daily basis, the amount of CO(2) that moved upward from the root system into the stem via the xylem stream (0.26 mol CO(2) m(-2) d(-1)) rivalled that which diffused from the soil surface to the atmosphere (0.27 mol CO(2) m(-2) d(-1)). We estimated that twice the amount of CO(2) derived from belowground autotrophic respiration entered the xylem stream as diffused into the soil environment. Our observations indicate that belowground autotrophic respiration consumes substantially more carbohydrates than previously recognized and challenge the paradigm that all root-respired CO(2) diffuses into the soil atmosphere.

Assimilation of xylem-transported 13C-labelled CO2 in leaves and branches of sycamore (Platanus occidentalis L.)

Previous reports have shown that CO(2) dissolved in xylem sap in tree stems can move upward in the transpiration stream. To determine the fate of this dissolved CO(2), the internal transport of respired CO(2) at high concentration from the bole of the tree was simulated by allowing detached young branches of sycamore (Platanus occidentalis L.) to transpire water enriched with a known quantity of (13)CO(2) in sunlight. Simultaneously, leaf net photosynthesis and CO(2) efflux from woody tissue were measured. Branch and leaf tissues were subsequently analysed for (13)C content to determine the quantity of transported (13)CO(2) label that was fixed. Treatment branches assimilated an average of 35% (SE=2.4) of the (13)CO(2) label taken up in the treatment water. The majority was fixed in the woody tissue of the branches, with smaller amounts fixed in the leaves and petioles. Overall, the fixation of internally transported (13)CO(2) label by woody tissues averaged 6% of the assimilation of CO(2) from the atmosphere by the leaves. Woody tissue assimilation rates calculated from measurements of (13)C differed from rates calculated from measurements of CO(2) efflux in the lower branch but not in the upper branch. The results of this study showed unequivocally that CO(2) transported in xylem sap can be fixed in photosynthetic cells in the leaves and branches of sycamore trees and provided evidence that recycling of xylem-transported CO(2) may be an important means by which trees reduce the carbon cost of respiration.

General trait relationships in stems: a study on the performance and interrelationships of several functional and structural parameters involved in corticular photosynthesis

We addressed corticular photosynthesis, focusing on parameters of underlying dark and light reactions as well as structural differentiation. To unveil general stem traits and underlying principles that may be valid across several tree species, CO(2) exchange rates and chlorophyll-fluorescence parameters were measured in current-year to 3-year-old stems of five deciduous tree species (including climax and pioneer species). Across species, dark CO(2) efflux rates (R(d)) of stems exhibited a common regression relationship with photosynthetic rates (A) and light-adapted quantum efficiency of photosystem II (PSII) (Delta F/Fm'), a pattern analogous to leaf trait correlations. Furthermore, A and Delta F/Fm' were closely interrelated to each other. Consistent correlations of stem structure and function were also assessed among species. Changes in tissue structure during ageing significantly affected several stem functional parameters. Stem CO(2) efflux during the dark and corticular photosynthetic rates declined with increasing stem age as well as light-adapted quantum efficiency of PSII. Furthermore, a strong relationship between stem R(d) and peridermal PFD-transmittance (T) as well as between R(d) and total bark chlorophyll was evident. Consistent results were found for the relationships between corticular photosynthesis (or primary photosynthetic reactions like Delta F/Fm') and selected structural traits. The found correlation patterns among functional and/or structural traits of stems and their concordance with leaf trait relationships may aid in identifying underlying mechanisms and scaling relationships that link traits to plant and ecosystem function.

CO2 efflux, CO2 concentration and photosynthetic refixation in stems of Eucalyptus globulus (Labill.)

In spite of the importance of respiration in forest carbon budgets, the mechanisms by which physiological factors control stem respiration are unclear. An experiment was set up in a Eucalyptus globulus plantation in central Portugal with monoculture stands of 5-year-old and 10-year-old trees. CO(2) efflux from stems under shaded and unshaded conditions, as well as the concentration of CO(2) dissolved in sap [CO(2)(*)], stem temperature, and sap flow were measured with the objective of improving our understanding of the factors controlling CO(2) release from stems of E. globulus. CO(2) efflux was consistently higher in 5-year-old, compared with 10-year-old, stems, averaging 3.4 versus 1.3 mumol m(-2) s(-1), respectively. Temperature and [CO(2)(*)] both had important, and similar, influences on the rate of CO(2) efflux from the stems, but neither explained the difference in the magnitude of CO(2) efflux between trees of different age and size. No relationship was found between efflux and sap flow, and efflux was independent of tree volume, suggesting the presence of substantial barriers to the diffusion of CO(2) from the xylem to the atmosphere in this species. The rate of corticular photosynthesis was the same in trees of both ages and only reduced CO(2) efflux by 7%, probably due to the low irradiance at the stem surface below the canopy. The younger trees were growing at a much faster rate than the older trees. The difference between CO(2) efflux from the younger and older stems appears to have resulted from a difference in growth respiration rather than a difference in the rate of diffusion of xylem-transported CO(2).

Origin, fate and significance of CO2 in tree stems

Although some CO(2) released by respiring cells in tree stems diffuses directly to the atmosphere, on a daily basis 15-55% can remain within the tree. High concentrations of CO(2) build up in stems because of barriers to diffusion in the inner bark and xylem. In contrast with atmospheric [CO(2)] of c. 0.04%, the [CO(2)] in tree stems is often between 3 and 10%, and sometimes exceeds 20%. The [CO(2)] in stems varies diurnally and seasonally. Some respired CO(2) remaining in the stem dissolves in xylem sap and is transported toward the leaves. A portion can be fixed by photosynthetic cells in woody tissues, and a portion diffuses out of the stem into the atmosphere remote from the site of origin. It is now evident that measurements of CO(2) efflux to the atmosphere, which have been commonly used to estimate the rate of woody tissue respiration, do not adequately account for the internal fluxes of CO(2). New approaches to quantify both internal and external fluxes of CO(2) have been developed to estimate the rate of woody tissue respiration. A more complete assessment of internal fluxes of CO(2) in stems will improve our understanding of the carbon balance of trees.

Stem respiration and carbon dioxide efflux of young Populus deltoides trees in relation to temperature and xylem carbon dioxide concentration

Oxidative respiration is strongly temperature driven. However, in woody stems, efflux of CO(2) to the atmosphere (E (A)), commonly used to estimate the rate of respiration (R (S)), and stem temperature (T (st)) have often been poorly correlated, which we hypothesized was due to transport of respired CO(2) in xylem sap, especially under high rates of sap flow (f (s)). To test this, we measured E (A), T (st), f (s) and xylem sap CO(2) concentrations ([CO(2)*]) in 3-year-old Populus deltoides trees under different weather conditions (sunny and rainy days) in autumn. We also calculated R (S) by mass balance as the sum of both outward and internal CO(2) fluxes and hypothesized that R (S) would correlate better with T (st) than E (A). We found that E (A) sometimes correlated well with T (st), but not on sunny mornings and afternoons or on rainy days. When the temperature effect on E (A) was accounted for, a clear positive relationship between E (A) and xylem [CO(2)*] was found. [CO(2)*] varied diurnally and increased substantially at night and during periods of rain. Changes in [CO(2)*] were related to changes in f (s) but not T (st). We conclude that changes in both respiration and internal CO(2) transport altered E (A). The dominant component flux of R (S) was E (A). However, on a 24-h basis, the internal transport flux represented 9-18% and 3-7% of R (S) on sunny and rainy days, respectively, indicating that the contribution of stem respiration to forest C balance may be larger than previously estimated based on E (A) measurements. Unexpectedly, the relationship between R (S) and T (st) was sometimes weak in two of the three trees. We conclude that in addition to temperature, other factors such as water deficits or substrate availability exert control on the rate of stem respiration so that simple temperature functions are not sufficient to predict stem respiration.

Resistance to radial CO2 diffusion contributes to between-tree variation in CO2 efflux of Populus deltoides stems

Rates of CO₂ efflux of stems and branches are highly variable among and within trees and across stands. Scaling factors have only partially succeeded in accounting for the observed variations. In this study, the resistance to radial CO₂ diffusion was quantified for tree stems of an eastern cottonwood (Populus deltoides Bartr. ex Marsh.) clone by direct manipulation of the CO₂ concentration ([CO₂]) of xylem sap under controlled conditions. Tree-specific linear relationships between rates of stem CO₂ efflux (J_O) and xylem [CO₂] were found. The resistance to radial CO₂ diffusion differed 6-fold among the trees and influenced the balance between the amount of CO₂ retained in the xylem v. that which diffused to the atmosphere. Therefore, we hypothesised that variability in the resistance to radial CO₂ diffusion might be an overlooked cause for the inconsistencies and large variations in woody tissue CO₂ efflux. It was found that transition from light to dark conditions caused a rapid increase in J_O and xylem [CO₂], both in manipulated trees and in an intact tree with no sap manipulation. This resulted in an increased resistance to radial CO₂ diffusion during the dark, at least for trees with smaller daytime resistances. Stem diameter changes measured in the intact tree supported the idea that higher actual respiration rates occurred at night owing to higher metabolism in relation to an improved water status and higher turgor pressure.

Measurement of stem respiration of sycamore (Platanus occidentalis L.) trees involves internal and external fluxes of CO2 and possible transport of CO2 from roots

CO(2) released by respiring cells in tree stems can either diffuse to the atmosphere or dissolve in xylem sap. In this study, the internal and external fluxes of CO(2) released from respiring stems of five sycamore (Platanus occidentalis L.) trees were calculated. Mean rates of stem respiration were highest in mid-afternoon and lowest at night, and were positively correlated with air temperature. Over a 24 h period, on average 34% of the CO(2) released by respiring cells in the measured stem segment remained within the tree. CO(2) efflux to the atmosphere consisted of similar proportions of CO(2) derived from local respiring cells (55%) and CO(2) that had been transported in the xylem (45%), indicating that CO(2) efflux does not accurately estimate respiration. A portion of the efflux of transported CO(2) appeared to have originated in the root system. A modification of the method for calculating stem respiration based on internal and external fluxes of CO(2) was developed to separate efflux due to local respiration from efflux of transported CO(2).

Temperature dependency of bark photosynthesis in beech (Fagus sylvatica L.) and birch (Betula pendula Roth.) trees

Temperature dependencies of stem dark respiration (R(d)) and light-driven bark photosynthesis (A(max)) of two temperate tree species (Fagus sylvatica and Betula pendula) were investigated to estimate their probable influence on stem carbon balance. Stem R(d) was found to increase exponentially with increasing temperatures, whereas A(max) levelled off or decreased at the highest temperatures chosen (35-40 degrees C). Accordingly, a linear relationship between respiratory and assimilatory metabolism was only found at moderate temperatures (10-30 degrees C) and the relationship between stem R(d) and A(max) clearly departed from linearity at chilling (5 degrees C) and at high temperatures (35-40 degrees C). As a result, the proportional internal C-refixation rate also decreased non-linearly with increasing temperature. Temperature response of photosystem II (PSII) photochemistry was also assessed. Bark photochemical yield (Delta F/F(m)') followed the same temperature pattern as bark CO(2) assimilation. Maximum quantum yield of PSII (F(v)/F(m)) decreased drastically at freezing temperatures (-5 degrees C), while from 30 to 40 degrees C only a marginal decrease in F(v)/F(m) was found. In in situ measurements during winter months, bark photosynthesis was found to be strongly reduced. Low temperature stress induced an active down-regulation of PSII efficiency as well as damage to PSII due to photoinhibition. All in all, the benefit of bark photosynthesis was negatively affected by low (<5 degrees C) as well as high temperatures (>30 degrees C). As the carbon balance of tree stems is defined by the difference between photosynthetic carbon gain and respiratory carbon loss, this might have important implications for accurate modelling of stem carbon balance.

CO2 fluxes and respiration of branch segments of sycamore (Platanus occidentalis L.) examined at different sap velocities, branch diameters, and temperatures

Respiration of stems and branches of trees (R(S)) has typically been estimated by measuring radial CO(2) efflux from woody tissue (E(A)) and rates of efflux are often scaled temporally using a temperature relationship (Q(10)). High concentrations of CO(2) in xylem sap ([CO(2)*]) have been shown to affect E(A), and the transport of CO(2) in the xylem stream has been suggested as a mechanism to explain field observations of temperature-independent fluctuations in E(A). Sap velocity and temperature were manipulated in detached branch segments of sycamore (Platanus occidentalis L.) under controlled conditions to quantify these effects. Within individual branches of similar size, E(A) and [CO(2)*] were greater at low sap velocity, while the amount of respired CO(2) transported in sap (transport flux, F(T)) was greater at high sap velocity. E(A) was linearly correlated with [CO(2)*]. In branches of three diameter classes (1, 2, and 3 cm), volume-based E(A), F(T), and R(S) did not differ, but surface-area based CO(2) fluxes increased with diameter class. Regardless of diameter, E(A) accounted for only 30% of respired CO(2) at high sap velocity, while at low sap velocity, E(A) accounted for 71% of respired CO(2). E(A), F(T), and R(S) measured at 5, 20, and 35 degrees C at the same sap velocity showed a typical exponential response to temperature. However, at the lowest temperature, E(A) accounted for only 18% of the CO(2) released from respiring cells compared with 44% at the highest temperature, perhaps due to the effect of temperature on the solubility of CO(2) in water. These results directly demonstrate the transport of respired CO(2) in the xylem stream and may help to explain inconsistencies in stem and branch respiration measurements made in situ.

Relationship between stem CO2 efflux, stem sap velocity and xylem CO2 concentration in young loblolly pine trees

We measured diel patterns of stem surface CO2 efflux (Es, micromol m(-2) s(-1)), sap velocity (vs, mm s(-1)) and xylem CO2 concentration ([CO2]) (Xs, %) in 8-year-old loblolly pine trees during the spring to determine how vs and Xs influence Es. All trees showed a strong diel hysteresis between Es and stem temperature, where at a given temperature, Es was lower during the day than at night. Diel variations in temperature-independent Es were correlated with vs (R2= 0.54), such that at maximum vs, Es was reduced between 18 and 40%. However, this correlation may not represent a cause-and-effect relationship. In a subset of trees, vs was artificially reduced by progressively removing the tree canopy. Reducing vs to near zero had no effect on Es and did not change the diel hysteretic response to temperature. Diel Xs tended to decrease with vs and increase with Es, however, in defoliated trees, large increases in Xs, when vs approximately 0, had no effect on Es. We conclude that at this time of the year, Es is driven primarily by respiration of cambium and phloem tissues and that sap flow and xylem transport of CO2 had no direct influence on Es.

Conditions leading to high CO2 (>5 kPa) in waterlogged-flooded soils and possible effects on root growth and metabolism

AIMS

Soil waterlogging impedes gas exchange with the atmosphere, resulting in low P(O2) and often high P(CO2). Conditions conducive to development of high P(CO2) (5-70 kPa) during soil waterlogging and flooding are discussed. The scant information on responses of roots to high P(CO2) in terms of growth and metabolism is reviewed.

SCOPE

P(CO2) at 15-70 kPa has been reported for flooded paddy-field soils; however, even 15 kPa P(CO2) may not always be reached, e.g. when soil pH is above 7. Increases of P(CO2) in soils following waterlogging will develop much more slowly than decreases in P(O2); in soil from rice paddies in pots without plants, maxima in P(CO2) were reached after 2-3 weeks. There are no reliable data on P(CO2) in roots when in waterlogged or flooded soils. In rhizomes and internodes, P(CO2) sometimes reached 10 kPa, inferring even higher partial pressures in the roots, as a CO2 diffusion gradient will exist from the roots to the rhizomes and shoots. Preliminary modelling predicts that when P(CO2) is higher in a soil than in roots, P(CO2) in the roots would remain well below the P(CO2) in the soil, particularly when there is ventilation via a well-developed gas-space continuum from the roots to the atmosphere. The few available results on the effects of P(CO2) at > 5 kPa on growth have nearly all involved sudden increases to 10-100 kPa P(CO2); consequently, the results cannot be extrapolated with certainty to the much more gradual increases of P(CO2) in waterlogged soils. Nevertheless, rice in an anaerobic nutrient solution was tolerant to 50 kPa CO2 being suddenly imposed. By contrast, P(CO2) at 25 kPa retarded germination of some maize genotypes by 50%. With regard to metabolism, assuming that the usual pH of the cytoplasm of 7.5 was maintained, every increase of 10 kPa CO2 would result in an increase of 75-90 mM HCO3(-) in the cytoplasm. pH maintenance would depend on the biochemical and biophysical pH stats (i.e. regulatory systems). Furthermore, there are indications that metabolism is adversely affected when HCO3(-) in the cytoplasm rises above 50 mM, or even lower; succinic dehydrogenase and cytochrome oxidase are inhibited by HCO3(-) as low as 10 mM. Such effects could be mitigated by a decrease in the set point for the pH of the cytoplasm, thus lowering levels of HCO3(-) at the prevailing P(CO2) in the roots.

CONCLUSIONS

Measurements are needed on P(CO2) in a range of soil types and in roots of diverse species, during waterlogging and flooding. Species well adapted to high P(CO2) in the root zone, such as rice and other wetland plants, thrive even when P(CO2) is well over 10 kPa; mechanisms of adaptation, or acclimatization, by these species need exploration.

Carbon dioxide concentrations are very high in developing oilseeds

A new method has been developed to rapidly determine the total inorganic carbon concentration (gaseous [CO2] + aqueous [CO(2)] + [HCO3-] + [CO3(2)-]) in developing seeds. Seeds are rapidly dissected and homogenized in 1 N HCl in gas-tight vials. The headspace gas is then analyzed by infrared gas analysis. Developing rapeseed (Brassica napus L.) and soybean [Glycine max (L.) Merr.] seeds were analyzed and found to have up to 40 and 12 mM total inorganic carbon, respectively. These concentrations are ca. 600-2000-fold higher than in ambient air or values reported for leaves. Carbon dioxide concentrations in rapeseed peaked during the stage of maximum oil synthesis and declined as seeds matured. The consequences for seed metabolism, physiology and carbon economy are discussed.