δ 13C of CO2 respired in the dark in relation to δ13C of leaf carbohydrates in Phaseolus vulgaris L. under progressive drought

The variations in δ ¹³C in both leaf carbohydrates (starch and sucrose) and CO₂ respired in the dark from the cotyledonary leaves of Phaseolus vulgaris L. were investigated during a progressive drought. As expected, sucrose and starch became heavier (enriched in ¹³C) with decreasing stomatal conductance and decreasing p _i/ p _a during the first half (15 d) of the dehydration cycle. Thereafter, when stomata remained closed and leaf net photosynthesis was near zero, the tendency was reversed: the carbohydrates became lighter (depleted in ¹³C). This may be explained by increased p _i/ p _a but other possible explanations are also discussed. Interestingly, the variations in δ ¹³C of CO₂ respired in the dark were correlated with those of sucrose for both well-watered and dehydrated plants. A linear relationship was obtained between δ ¹³C of CO₂ respired in the dark and sucrose, respired CO₂ always being enriched in ¹³C compared with sucrose by ≈ 6‰. The whole leaf organic matter was depleted in ¹³C compared with leaf carbohydrates by at least 1‰. These results suggest that: (i) a discrimination by ≈ 6‰ occurs during dark respiration processes releasing ¹³C-enriched CO₂; and that (ii) this leads to ¹³C depletion in the remaining leaf material.     

Anaerobic sulfur oxidation in the absence of nitrate dominates microbial chemoautotrophy beneath the pelagic chemocline of the eastern Gotland Basin, Baltic Sea

Oxic–anoxic interfaces harbor significant numbers and activity of chemolithoautotrophic microorganisms, known to oxidize reduced sulfur or nitrogen species. However, measurements of in situ distribution of bulk carbon dioxide (CO₂) assimilation rates and active autotrophic microorganisms have challenged the common concept that aerobic and denitrifying sulfur oxidizers are the predominant autotrophs in pelagic oxic–anoxic interfaces. Here, we provide a comparative investigation of nutrient, sulfur, and manganese chemistry, microbial biomass distribution, as well as CO₂ fixation at the pelagic redoxcline of the eastern Gotland Basin, Baltic Sea. Opposing gradients of oxygen, nitrate, and sulfide approached the detection limits at the chemocline at 204 m water depth. No overlap of oxygen or nitrate with sulfide was observed, whereas particulate manganese was detected down to 220 m. More than 70% of the bulk dark CO₂ assimilation, totaling 9.3 mmol C m⁻² day⁻¹, was found in the absence of oxygen, nitrite, and nitrate and could not be stimulated by their addition. Maximum fixation rates of up to 1.1 μmol C L⁻¹ day⁻¹ were surprisingly susceptible to altered redox potential or sulfide concentration. These results suggest that novel redox-sensitive pathways of microbial sulfide oxidation could account for a significant fraction of chemolithoautotrophic growth beneath pelagic chemoclines. A mechanism of coupled activity of sulfur-oxidizing and sulfur-reducing microorganisms is proposed.     

Links between methanotroph community composition and CH4 oxidation in a pine forest soil

The main gap in our knowledge about what determines the rate of CH₄ oxidation in forest soils is the biology of the microorganisms involved, the identity of which remains unclear. In this study, we used stable-isotope probing (SIP) following ¹³CH₄ incorporation into phospholipid fatty acids (PLFAs) and DNA/RNA, and sequencing of methane mono-oxygenase ( pmoA ) genes, to identify the influence of variation in community composition on CH₄ oxidation rates. The rates of ¹³C incorporation into PLFAs differed between horizons, with low ¹³C incorporation in the organic soil and relatively high ¹³C incorporation into the two mineral horizons. The microbial community composition of the methanotrophs incorporating the ¹³C label also differed between horizons, and statistical analyses suggested that the methanotroph community composition was a major cause of variation in CH₄ oxidation rates. Both PLFA and pmoA -based data indicated that CH₄ oxidizers in this soil belong to the uncultivated 'upland soil cluster α'. CH₄ oxidation potential exhibited the opposite pattern to ¹³C incorporation, suggesting that CH₄ oxidation potential assays may correlate poorly with in situ oxidation rates. The DNA/RNA-SIP assay was not successful, most likely due to insufficient ¹³C-incorporation into DNA/RNA. The limitations of the technique are briefly discussed.     

Modelling environmental controls on ecosystem photosynthesis and the carbon isotope composition of ecosystem-respired CO2 in a coastal Douglas-fir forest

We developed and applied an ecosystem-scale model that calculated leaf CO₂ assimilation, stomatal conductance, chloroplast CO₂ concentration and the carbon isotope composition of carbohydrate formed during photosynthesis separately for sunlit and shaded leaves within multiple canopy layers. The ecosystem photosynthesis model was validated by comparison to leaf-level gas exchange measurements and estimates of ecosystem-scale photosynthesis from eddy covariance measurements made in a coastal Douglas-fir forest on Vancouver Island. A good agreement was also observed between modelled and measured δ ¹³C values of ecosystem-respired CO₂ ( δ _R). The modelled δ _R values showed strong responses to variation in photosynthetic photon flux density (PPFD), air temperature, vapour pressure deficit (VPD) and available soil moisture in a manner consistent with leaf-level studies of photosynthetic ¹³C discrimination. Sensitivity tests were conducted to evaluate the effect of (1) changes in the lag between the time of CO₂ fixation and the conversion of organic matter back to CO₂; (2) shifts in the proportion of autotrophic and heterotrophic respiration; (3) isotope fractionation during respiration; and (4) environmentally induced changes in mesophyll conductance, on modelled δ _R values. Our results indicated that δ _R is a good proxy for canopy-level C _c/ C _a and ¹³C discrimination during photosynthetic gas exchange, and therefore has several applications in ecosystem physiology.     

The Nostoc-Gunnera symbiosis: carbon fixation and translocation

The in vitro specific activity of ribulose-1,5-bisphosphate carboxylase (Rubisco; EC 4. 1. 1. 39) and the dark and light in vivo CO₂ fixation activities were determined in the cyanobiont of Gunnera . Compared to the free-living isolate Nostoc PCC 9231, the in vitro Rubisco activity was high, while the in vivo CO₂ fixation was very low. Light did not significantly influence CO₂ fixation if the cyanobiont was left in the sliced Gunnera tissues, while a small light stimulation was found for CO₂ fixation of the freshly-isolated cyanobiont. The adjacent non-infected Gunnera tissue showed a very low CO₂ fixation. A rapid translocation of fixed ¹⁴CO₂ from leaves towards apical parts of the plant was apparent, in particular to the symbiotic tissue. The ¹⁴C label appeared mainly in soluble form in this tissue and was rapidly catabolised as shown by ¹⁴C chase experiments. Also, short-term experiments revealed that maximum ¹⁴C accumulation occurred in the symbiotic tissue showing the highest rates of nitrogen fixation (Söderbäck et al. 1990), about 10–15 mm from the plant apex. The data were taken to indicate that there is a modification in the photosynthetic light reaction of the cyanobiont and that the cyanobiont lives heterotrophically in the dark on photo-synthate rapidly delivered from nearby leaves of the host plant.     

Diurnal and seasonal variation in the carbon isotope composition of leaf dark-respired CO2 in velvet mesquite (Prosopis velutina)

We evaluated diurnal and seasonal patterns of carbon isotope composition of leaf dark-respired CO₂ ( δ ¹³C_l) in the C₃ perennial shrub velvet mesquite ( Prosopis velutina ) across flood plain and upland savanna ecosystems in the south-western USA. δ ¹³C_l of darkened leaves increased to maximum values late during daytime periods and declined gradually over night-time periods to minimum values at pre-dawn. The magnitude of the diurnal shift in δ ¹³C_l was strongly influenced by seasonal and habitat-related differences in soil water availability and leaf surface vapour pressure deficit. δ ¹³C_l and the cumulative flux-weighted δ ¹³C value of photosynthates were positively correlated, suggesting that progressive ¹³C enrichment of the CO₂ evolved by darkened leaves during the daytime mainly resulted from short-term changes in photosynthetic ¹³C discrimination and associated shifts in the δ ¹³C signature of primary respiratory substrates. The ¹³C enrichment of dark-respired CO₂ relative to photosynthates across habitats and seasons was 4 to 6‰ at the end of the daytime period (1800 h), but progressively declined to 0‰ by pre-dawn (0300 h). The origin of night-time and daytime variations in δ ¹³C_l is discussed in terms of the carbon source(s) feeding respiration and the drought-induced changes in carbon metabolism.     

Carbon dioxide fixation in orchid aerial roots

Acidity fluctuation, CO₂ gas exchange, δ¹³C value, PEP carboxylase and RuBP carboxylase activities in aerial roots of selected thick-leaved orchid hybrids ( Arachnis and Aranthera ) were studied. Both aerial roots and leaves showed acidity fluctuation over a 24 h period. Dark acidification in aerial roots was enhanced at low temperature (15°C). Aerial roots had δ¹³C values close to those of leaves which have been previously demonstrated to possess crassulacean acid metabolism. Variation in δ¹³C values along the length of the roots was observed; the root tip having a less negative δ¹³C value (—13.34%‰) than the older portions of the roots (—14.55%‰). There was no net CO₂ fixation by aerial root, although ¹⁴³²CO₂ fixation was observed in light and in darkness. The pattern of fluctuation in activities of PEP carboxylase and RuBP carboxylase in aerial roots was similar to that obtained for the leaves. In both aerial roots and leaves, PEP carboxylase activity was several times higher than that of RuBP carboxylase.     

Effects of mycorrhizal colonization on biomass production and nitrogen fixation of black locust (Robinia pseudoacacia) seedlings grown under elevated atmospheric carbon dioxide

Interactive effects of elevated atmospheric CO₂ and arbuscular mycorrhizal (AM) fungi on biomass production and N₂ fixation were investigated using black locust ( Robinia pseudoacacia ). Seedlings were grown in growth chambers maintained at either 350 μmol mol⁻¹ or 710 μmol mol⁻¹ CO₂. Seedlings were inoculated with Rhizobium spp. and were grown with or without AM fungi. The ¹⁵N isotope dilution method was used to determine N source partitioning between N₂ fixation and inorganic fertilizer uptake. Elevated atmospheric CO₂ significantly increased the percentage of fine roots that were colonized by AM fungi. Mycorrhizal seedlings grown under elevated CO₂ had the greatest overall plant biomass production, nodulation, N and P content, and root N absorption. Additionally, elevated CO₂ levels enhanced nodule and root mass production, as well as N₂ fixation rates, of non- mycorrhizal seedlings. However, the relative response of biomass production to CO₂ enrichment was greater in non-mycorrhizal seedlings than in mycorrhizal seedlings. This study provides strong evidence that arbuscular mycorrhizal fungi play an important role in the extent to which plant nutrition of symbiotic N₂-fixing tree species is affected by enriched atmospheric CO₂.     

Different carbon support for respiration and secondary production in unproductive lakes

This study investigates the allocation of allochthonous organic carbon (AlloOC) to pelagic respiration and biomass production in unproductive lakes. Metabolic process rates and stable isotopic composition (δ¹³C) of crustacean zooplankton and respired CO₂ were measured in the epilimnion of 13 forest lakes in northern Sweden. The δ¹³C of zooplankton was low (−31.2 to −38.0‰) compared to that of respired CO₂ (−28.4 to −30.6‰), implying that the relative importance of AlloOC was lower for zooplankton (ca 40%) than for respiration (ca 80%). Combining δ¹³C and carbon flux data revealed that a large amount of metabolized AlloOC was lost in respiration, compared to the amount transferred to zooplankton (<3%). Thus, despite large respiratory losses, AlloOC was still important for zooplankton growth, implying a high supply of AlloOC in comparison to phytoplankton generated organic carbon in the lakes.     

PYRUVATE METABOLISM IN THE LOBSTER NERVE AS AFFECTED BY THE PARTIAL PRESSURE OF CARBON DIOXIDE: OBSERVATIONS ON THE SYNTHESIS OF ACETYLCHOLINE AND ON METABOLIC COMPARTMENTATION

Abstract— In the lobster nerve the fixation of CO, at various levels of _pCO2 was studied by the incorporation of [l-¹⁴C]pyruvate. Incorporation of ¹⁴C was solely dependent on CO₂ fixation since the C-1 was decarboxylated in the formation of acetyl-CoA. Paired-nerve studies with [2-¹⁴C]pyruvate afforded a study of pyruvate metabolism in the lobster nerve. [I¹⁴C]Pyruvate was incorporated to nearly the same extent at all levels of pCO₂ including zero pCO₂, a finding that suggested metabolic recycling of CO₂. The magnitude of the metabolic recycling of C-1 of pyruvate or pyruvate dismutation was estimated to be nearly 20 per cent of total CO₂ fixation. Re-evaluation of the relative contributions of the CO₂ fixation. and acetyl-CoA pathways on the basis of more extensive data gave a ratio of 2:3.
The pCO₂ affected synthesis of ACh and the level of citrate. With increasing pCO₂, the specific radioactivity of ACh decreased much more than the content of ACh. The decrease in the specific radioactivity of ACh but not that of citrate further suggested metabolic compartmentation. The implication of these findings is discussed.
Alanine functioned as a metabolic sink for the incorporated pyruvate. Pyruvate levels were estimated to be approximately 0.1 nmol/mg of protein.     

Re-evaluation of proposed C4 photosynthetic characteristics in the genus Larix

It has been suggested previously that Japanese larch ( Larix kaempferi ) exhibits characteristics of C₄ photosynthesis. To further evaluate this suggestion, stable carbon isotope ratios were determined for leaf and bark tissue of Larix gmelini, L. kaempferi, L. laricina, L. Iyallii, L. occidentalis , and L. sibirica. All δ¹³C values were more negative than –22‰. Short-term labeling with ¹⁴CO₂ showed that phosphoglyceric acid and other phosphorylated compounds were the first products of photosynthesis in L. sibirica. Both of these results strongly suggest that the initial fixation of atmospheric CO₂ in these six Larix species is accomplished solely via the C₃ photosynthetic pathway.     

Enrichment and characterization of a sulfate-reducing toluene-degrading microbial consortium by combining in situ microcosms and stable isotope probing techniques

A toluene-degrading microbial consortium was enriched directly in a BTEX-contaminated aquifer under sulfate-reducing conditions using in situ microcosms consisting of toluene-loaded activated carbon pellets. Degradation of toluene and concomitant sulfide production by the consortium was subsequently demonstrated in laboratory microcosms. The consortium was physiologically and phylogenetically characterized by isotope tracer experiments using nonlabeled toluene, [¹³C]-α-toluene or [¹³C₇]-toluene as growth substrates. Cells incubated with [¹³C]-α-toluene or [¹³C₇]-toluene incorporated 8–15 at.%¹³C and 51–57 at.%¹³C into total lipid fatty acids, respectively, indicating a lower specific incorporation of ¹³C from [¹³C₇]-toluene. In order to identify the toluene-assimilating bacteria, the incorporation of carbon from both [¹³C]-α-toluene and [¹³C₇]-toluene into rRNA was analyzed by stable isotope probing. Time and buoyant density-resolved 16S rRNA gene-based terminal restriction fragment length polymorphism profiles, combined with cloning and sequencing, revealed that an uncultured bacterium (99% sequence similarity) related to the genus Desulfocapsa was the main toluene-degrading organism in the consortium. The ratio of the respective terminal restriction fragments changed over time, indicating trophic interactions within this consortium.     

Air pollution in the past recorded in width and stable carbon isotope composition of annual growth rings of Douglas-fir

Abstract. Tree-ring indices (TRIs) of annual growth rings in stems of Douglas-fir ( Pseudotsuga menziesii ) growing near a copper smelter showed reduced growth during two multi-year time periods in the past. These periods coincided with World Wars I and II, which are known to represent periods of particularly high SO₂ emissions from the smelter. Reduced growth was correlated with less negative stable carbon isotope composition (δ¹³C) in cellulose purified from wood formed in such years. Based on current models for ¹³C/¹²C in plants, these results indicate that exposure to air pollution resulted in reduced concentration of CO₂ in the intercellular air spaces of the needles. This is consistent with the hypothesis that stomatal closure resulted in impaired photosynthesis and reduced growth during past episodes of high air pollution. The pollution-related change in δ¹³C was superimposed on a change with time in δ¹³C, independent of growth, by - 1.4 per mil from 1902 to 1984.     

Using stable isotopes to determine soil carbon input differences under ambient and elevated atmospheric CO2 conditions

Quantitative estimates of soil C input under ambient (35 Pa) and elevated (60 Pa) CO₂-partial pressure (pCO₂) were determined in a Free-Air Carbon dioxide Enrichment (FACE) experiment. To facilitate ¹³C-tracing, Trifolium repens L. was grown in a soil with an initial δ¹³C distinct by at least 5‰ from the δ¹³C of T. repens grown under ambient or elevated pCO₂. A shift in δ¹³C of the soil organic C was detected after one growing season. Calculated new soil C inputs in soil under ambient and elevated pCO₂ were 2 and 3 t ha^–1, respectively. Our findings suggest that under elevated CO₂ conditions, soil C sequestration may be altered by changes in plant biomass production and quality.     

Photosynthesis and carbon metabolism in photoautotrophic cell suspensions of Chenopodium rubrum from different phases of batch growth

Rapidly dividing photoautotrophic cell suspensions from Chenopodium rubrum L. assimilated about 85 μmol CO₂ (mg chlorophyll)⁻¹ h⁻¹. During the late stationary phase of culture growth, CO₂ fixation rate was reduced to about 60 μmol CO₂ (mg chlorophyll)⁻¹ h⁻¹. Actively dividing cells characteristically incorporated a smaller proportion of ¹⁴C into starch than cells from non-dividing stationary phases. In rapidly dividing cells, [¹⁴C]-turnover from free sugars, sugar-phosphates, organic and amino acids was substantially higher compared to non-dividing cells from stationary growth phase. Higher proportions of photosynthetically fixed carbon were channelled into proteins, lipids and structural components in actively dividing cells than in non-dividing cells. In the latter. ¹⁴C was preferentially channeled into starch, and a striking increase in starch accumulation was observed. The transfer of non-dividing, stationary growth-phase cells into fresh culture medium resulted in an increase in the maximum extractable activities of some enzymes involved in the glycolytic and dark respiratory pathways and in the citric acid cycle. In contrast, the maximum extractable activities of the chloroplastic enzymes, ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.38) and NADP⁺-glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13) were highest after the cells had reached the stationary growth phase.     

Biosynthesis of aromatic amino acids by highly purified spinach chloroplasts – Compartmentation and regulation of the reactions

The ability of chloroplasts to synthesize aromatic amino acids from CO₂ was investigated using highly purified, intact spinach ( Spinacia oleracea L. cv. Viking II) chloroplasts and ¹⁴CO₂. Incorporation of ¹⁴C into aromatic amino acids was very low, however, and this was assumed to be due to lack of phosphoenolpyruvate (PEP), one of the substrates for the shikimate/arogenate pathway leading to aromatic amino acids in chloroplasts. Therefore, the glycolytic enzymes phosphoglycerate mutase (EC 2.7.5.3) and enolase (EC 4.2.1.11) were added to the ¹⁴CO₂ fixation medium in order to convert labelled 3-phosphoglycerate exported from the intact chloroplasts to 2-phosphoglycerate and PEP. In this way a part of the glycolytic pathway was reconstituted outside the chloroplasts to substitute for the cytoplasm lost on isolation. The presence of both enzymes in the medium increased incorporation of ¹⁴C into Tyr and Phe more than ten-fold and incorporation into Trp about two-fold, while total ¹³CO₂ fixation rates were not affected. Our results suggest that chloroplasts do not contain phosphoglycerate mutase or enolase, and that, in vivo, PEP is synthesized in the cytoplasm and imported to the chloroplast stroma for the biosynthesis of aromatic amino acids. The biosynthesis of all three aromatic amino acids was under feedback control. Using expected physiological concentrations (below 100 μ M ), each of the aromatic amino acids exerted a strict feedback inhibition of its own biosynthesis only.     

A new method to measure carbon isotope composition of CO2 respired by trees: stem CO2 equilibration

1.  Applying Keeling plot techniques to derive δ¹³C of respiratory input in a closed non-equilibrated chamber can lead to large errors because steady-state diffusion rules are violated in a non-steady-state environment. To avoid these errors, respiratory δ¹³C can be derived using equilibrated closed chambers.
2.  We introduce a new method to obtain stem respired CO₂δ¹³C (δ_{st - r}) with closed equilibrated stem chambers (E-SC). We present a theoretical model describing the equilibration process, test the model against field data and find excellent agreement. The method is further tested by comparing it with closed non-equilibrated stem chambers (NE-SC); we found no difference between these methods.
3.  Our theoretical model to describe CO₂ diffusion from the respiratory pool into the chamber and the equation to derive the δ¹³C of the efflux are general. They could be applied to other ecosystem components (e.g. soils).
4.  Our method is easy to implement, cost effective, minimizes sources of error and allows for rigorous leak detection. One major limitation is its inability to detect rapid change; the equilibration process requires 15 ± 2 h. A second limitation is that it cannot be used for species that produce abundant pitch at sites of stem wounding (e.g. Pseudotsuga menziesii ).
5.  Investigating δ¹³C of CO₂ respired by different ecosystem components is necessary to interpret δ¹³C of ecosystem respiration. This parameter has major implications with respect to global carbon cycle science.     

Partitioning of CO2 Production Between Glucose and Lactate in Excised Sympathetic Ganglia, with Implications for Brain

Abstract: Chains of lumbar sympathetic ganglia from 15-day-old chicken embryos were incubated for 4 h at 36°C in a bicarbonate-buffered salt solution equilibrated with 5% CO₂-95% O₂. Glucose (1–10 m M ), lactate (1–10 m M ), [U-¹⁴C]glucose, [1-¹⁴C]glucose, [6-¹⁴C]glucose, and [U-¹⁴C]lactate were added as needed. ¹⁴CO₂ output was measured continuously by counting the radioactivity in gas that had passed through the incubation chamber. Lactate reduced the output of CO₂ from [U-¹⁴C]glucose, and glucose reduced that from [U-¹⁴C]lactate. When using uniformly labeled substrates in the presence of 5.5 m M glucose, the output of CO₂ from lactate exceeded that from glucose when the lactate concentration was >2 m M . The combined outputs at each concentration tested were greater than those from either substrate alone. The ¹⁴CO₂ output from [1-¹⁴C]glucose always exceeded that from [6-¹⁴C]glucose, indicating activity of the hexose monophosphate shunt. Lactate reduced both of these outputs, with the maximum difference between them during incubation remaining constant as the lactate concentration was increased, suggesting that lactate may not affect the shunt. Modeling revealed many details of lactate metabolism as a function of its concentration. Addition of a blood-brain barrier to the model suggested that lactate can be a significant metabolite for brain during hyperlactemia, especially at the high levels reached physiologically during exercise.     

The influence of leaf thickness on the CO2 transfer conductance and leaf stable carbon isotope ratio for some evergreen tree species in Japanese warm-temperate forests

1. The influence of leaf thickness on internal conductance for CO₂ transfer from substomatal cavity to chloroplast stroma ( g _i) and carbon isotope ratio (δ¹³C) of leaf dry matter was investigated for some evergreen tree species from Japanese temperate forests. g _i was estimated based on the combined measurements of gas exchange and concurrent carbon isotope discrimination.
2. Leaves with thicker mesophyll tended to have larger leaf dry mass per area (LMA), larger surface area of mesophyll cells exposed to intercellular air spaces per unit leaf area ( S _mes) and smaller volume ratio of intercellular spaces to the whole mesophyll (mesophyll porosity).
3. g _i of these leaves was correlated positively to S _mes but negatively to mesophyll porosity. The variation in g _i among these species would be therefore primarily determined by variation of the conductance in liquid phase rather than that in gas phase.
4. δ¹³C was positively correlated to mesophyll thickness and leaf nitrogen content on an area basis. However, g _i values did not correlate to δ¹³C. These results suggest that difference in δ¹³C among the species was not caused by the variation in g _i, but mainly by the difference in long-term photosynthetic capacity.
5. Comparison of our results with those of previous studies showed that the correlation between leaf thickness and g _i differed depending on leaf functional types (evergreen, deciduous or annual). Differences in leaf properties among these functional types were discussed.     

Physiological and isotopic (δ13C and δ18O) responses of three tropical tree species to water and nutrient availability

Water-use efficiency and stable isotope composition were studied in three tropical tree species. Seedlings of Tectona grandis , Swietenia macrophylla and Platymiscium pinnatum were grown at either high or low water supply, and with or without added fertilizer. These three species previously exhibited low, intermediate and high whole-plant water-use efficiency ( TE ) when grown at high water supply in unfertilized soil. Responses of TE to water and nutrient availability varied among species. The TE was calculated as experiment-long dry matter production divided by cumulative water use. Species-specific offsets were observed in relationships between TE and whole-plant ¹³C discrimination (Δ¹³C_p). These offsets could be attributed to a breakdown in the relationship between Δ¹³C_p and the ratio of intercellular to ambient CO₂ partial pressures ( c _i/ c _a) in P. pinnatum , and to variation among species in the leaf-to-air vapour pressure difference ( v ). Thus, a plot of v · TE against c _i/ c _a showed a general relationship among species. Relationships between δ ¹⁸O of stem dry matter and stomatal conductance ranged from strongly negative for S. macrophylla to no relationship for T. grandis . Results suggest inter-specific variation among tropical tree species in relationships between stable isotope ratios ( δ ¹³C and δ ¹⁸O) and the gas exchange processes thought to affect them.