Carbon isotopes in terrestrial ecosystem pools and CO2 fluxes

Stable carbon isotopes are used extensively to examine physiological, ecological, and biogeochemical processes related to ecosystem, regional, and global carbon cycles and provide information at a variety of temporal and spatial scales. Much is known about the processes that regulate the carbon isotopic composition (delta(13)C) of leaf, plant, and ecosystem carbon pools and of photosynthetic and respiratory carbon dioxide (CO(2)) fluxes. In this review, systematic patterns and mechanisms underlying variation in delta(13)C of plant and ecosystem carbon pools and fluxes are described. We examine the hypothesis that the delta(13)C of leaf biomass can be used as a reference point for other carbon pools and fluxes, which differ from the leaf in delta(13)C in a systematic fashion. Plant organs are typically enriched in (13)C relative to leaves, and most ecosystem pools and respiratory fluxes are enriched relative to sun leaves of dominant plants, with the notable exception of root respiration. Analysis of the chemical and isotopic composition of leaves and leaf respiration suggests that growth respiration has the potential to contribute substantially to the observed offset between the delta(13)C values of ecosystem respiration and the bulk leaf. We discuss the implications of systematic variations in delta(13)C of ecosystem pools and CO(2) fluxes for studies of carbon cycling within ecosystems, as well as for studies that use the delta(13)C of atmospheric CO(2) to diagnose changes in the terrestrial biosphere over annual to millennial time scales.     

Interspecific variation in bulk tissue, fatty acid and monosaccharide delta(13)C values of leaves from a mesotrophic grassland plant community

The leaves of 37 grass, herb, shrub and tree species were collected from a mesotrophic grassland to assess natural variability in bulk, fatty acid and monosaccharide delta(13)C values of leaves from one plant community. The leaf tissue mean bulk delta(13)C value was -29.3 per thousand. No significant differences between tissue bulk delta(13)C values with life form were determined (P=0.40). On average, C(16:0), C(18:2) and C(18:3) constituted 89% of leaf tissue total fatty acids, whose delta(13)C values were depleted compared to whole leaf tissues. A general interspecific (between different species) trend for fatty acids delta(13)C values was observed, i.e. delta(13)C(16:0)delta(13)C(xylose)>delta(13)C(glucose)>delta(13)C(galactose), was consistently observed. Therefore, we have shown (i) diversity in compound-specific delta(13)C values contributing to leaf bulk delta(13)C values; (ii) interspecific variability between bulk and compound-specific delta(13)C values of leaves of individual grassland species, and (iii) trends between individual fatty acid and monosaccharide delta(13)C values common to leaves of all species within one plant community.     

Variation in the degree of coupling between delta13C of phloem sap and ecosystem respiration in two mature Nothofagus forests

Day-to-day variability in the carbon isotope composition of phloem sap (delta13Chd) and ecosystem respiratory CO2 (delta13CR) were measured to assess the tightness of coupling between canopy photosynthesis (delta13Chd) and ecosystem respiration (delta13CR) in two mature Nothofagus solandri (Hook. f.) forests in New Zealand. Abundant phloem-tapping scale insects allowed repeated, nondestructive access to stem phloem sap 1-2 m above ground. delta13Chd was compared with delta13C predicted by an environmentally driven, process-based canopy photosynthesis model. Keeling plots of within-canopy CO2 were used to estimate delta13CR. By including a lag of 3 d, there was good agreement in the timing and direction of variation in delta13Chd and predictions by the canopy photosynthesis model, suggesting that delta13Chd represents a photosynthesis-weighted, integrative record of canopy photosynthesis and conductance. Significant day-to-day variability in delta13CR was recorded at one of the two forests. At this site, delta13CR reflected variability in delta13Chd only on days with <2 mm rain. We conclude that the degree of coupling between canopy photosynthesis and ecosystem respiration varies between sites, and with environmental conditions at a single site.     

CO2浓度倍增和土壤干旱对两种幼龄沙生灌木碳分配的影响

利用大型环境生长箱研究了两种幼龄沙地优势灌木柠条 (Caraganaintermedia) 和羊柴 (Hedysarummon golicum) 对CO2 浓度倍增和土壤干旱交互作用的响应。CO2 浓度倍增并没有改善两种沙生灌木叶片的水分状况, 而土壤干旱使叶片的相对含水量 (RWC) 显著降低。在土壤水分充足条件下, CO2 浓度倍增促进两种沙生灌木植株生长, 在干旱条件下则主要促进根的生长, 提高根冠比。土壤干旱显著减少了植株生物量, 但相对促进了根的生长, 特别是显著提高了羊柴的根冠比。CO2 倍增使稳定性碳同位素组分 (δ13 C) 降低, 但土壤干旱使之增加。两种沙生灌木叶片与根部的δ13 C值呈极显著线性关系, 羊柴的斜率大于柠条的, 表明前者叶片与根部在光合产物分配上具有较高的生态可塑性, 这和干旱条件下羊柴的根冠比增加相关联。羊柴的“源库”调节特性反映了对土壤水分胁迫具有较高的耐性。     

Effects of intraleaf variations in carbonic anhydrase activity and gas exchange on leaf C18OO isoflux in Zea mays

Variation in the C18OO content of atmospheric CO2 (delta18Oa) can be used to distinguish photosynthesis from soil respiration, which is based on carbonic anhydrase (CA)-catalyzed 18O exchange between CO2 and 18O-enriched leaf water (delta18Ow). Here we tested the hypothesis that mean leaf delta18Ow and assimilation rates can be used to estimate whole-leaf C18OO flux (isoflux), ignoring intraleaf variations in CA activity and gas exchange parameters. We observed variations in CA activity along the leaf (> 30% decline from the leaf center toward the leaf ends), which were only partially correlated to those in delta18Ow (7 to 21 per thousand), delta18O and delta13C of leaf organic matter (25 to 30 per thousand and -12.8 to -13.2 per thousand, respectively), and substomatal CO2 concentrations (intercellular CO2 concentrations, c(i), at the leaf center were approximately 40% of those at the leaf tip). The combined effect of these variations produced a leaf-integrated isoflux that was different from that predicted based on bulk leaf values. However, because of canceling effects among the influencing parameters, isoflux overestimations were only approximately 10%. Conversely, use of measured parameters from a leaf segment could produce large errors in predicting leaf-integrated C18OO fluxes.     

C-isotope composition of CO2 respired by shoots and roots: fractionation during dark respiration?

The CO₂ respired by leaves is ¹³C-enriched relative to leaf biomass and putative respiratory substrates (Ghashghaie et al., Phytochemistry Reviews 2, 145–161, 2003), but how this relates to the ¹³C content of root, or whole plant respiratory CO₂ is unknown. The C isotope composition of respiratory CO₂ (δ_R) from shoots and roots of sunflower (Helianthus annuus L.), alfalfa (Medicago sativa L.), and perennial ryegrass (Lolium perenne L.) growing in a range of conditions was analysed. In all instances plants were grown in controlled environments with CO₂ of constant concentration and δ¹³C. Respiration of roots and shoots of individual plants was measured with an open CO₂ exchange system interfaced with a mass spectrometer. Respiratory CO₂ from shoots was always ¹³C-enriched relative to that of roots. Conversely, shoot biomass was always ¹³C-depleted relative to root biomass. The δ-difference between shoot and root respiratory CO₂ was variable, and negatively correlated with the δ-difference between shoot and root biomass (r² = 0.52, P = 0.023), suggesting isotope effects during biosynthesis. ¹³C discrimination in respiration (R) of shoots, roots and whole plants (e_Shoot, e_Root, e_Plant) was assessed as e = (δ_Substrate − δ_R)/(1 + δ_R/1000), where root and shoot substrate is defined as imported C, and plant substrate is total photosynthate. Estimates were obtained from C isotope balances of shoots, roots and whole plants of sunflower and alfalfa using growth and respiration data collected at intervals of 1 to 2 weeks. e_plant and e_Shoot differed significantly from zero. e_plant ranged between −0.4 and −0.9‰, whereas e_Shoot was much greater (−0.6 to −1.9‰). e_Root was not significantly different from zero. The present results help to resolve the apparent conflict between leaf- and ecosystem-level ¹³C discrimination in respiration.     

A new measurement technique reveals rapid post-illumination changes in the carbon isotope composition of leaf-respired CO2

We describe an open leaf gas exchange system coupled to a tunable diode laser (TDL) spectroscopy system enabling measurement of the leaf respiratory CO(2) flux and its associated carbon isotope composition (delta(13)C(Rl)) every 3 min. The precision of delta(13)C(Rl) measurement is comparable to that of traditional mass spectrometry techniques. delta(13)C(Rl) from castor bean (Ricinus communis L.) leaves tended to be positively related to the ratio of CO(2) produced to O(2) consumed [respiratory quotient (RQ)] after 24-48 h of prolonged darkness, in support of existing models. Further, the apparent fractionation between respiratory substrates and respired CO(2) within 1-8 h after the start of the dark period was similar to previous observations. In subsequent experiments, R. communis plants were grown under variable water availability to provide a range in delta(13)C of recently fixed carbohydrate. In leaves exposed to high light levels prior to the start of the dark period, CO(2) respired by leaves was up to 11 per thousand more enriched than phloem sap sugars within the first 10-15 min after plants had been moved from the light into the dark. The (13)C enrichment in respired CO(2) then decreased rapidly to within 3-7 per thousand of phloem sap after 30-60 min in the dark. This strong enrichment was not observed if light levels were low prior to the start of the dark period. Measurements of RQ confirmed that carbohydrates were the likely respiratory substrate for plants (RQ > 0.8) within the first 60 min after illumination. The strong (13)C enrichment that followed a high light-to-dark transition coincided with high respiration rates, suggesting that so-called light-enhanced dark respiration (LEDR) is fed by (13)C-enriched metabolites.     

Short-term dynamics of the carbon isotope composition of CO2 emitted from a wheat agroecosystem--physiological and environmental controls

Understanding environmental and physiological controls of the variations in δ(13) C of CO(2) respired (δ(13) C(R)) from different compartments of an ecosystem is important for separation of CO(2) fluxes and to assess coupling between assimilation and respiration. In a wheat field, over 3 days we characterised the temporal dynamics of δ(13) C(R) from shoots and roots, from the soil and from the whole agroecosystem. To evaluate the basis of potential variations in δ(13) C(R), we also measured δ(13) C in different organic matter pools, as well as meteorological and gas exchange parameters. We observed strong diel variations up to ca. 6% in shoot, root and soil δ(13) C(R), but not in δ(13) C of the putative organic substrates for respiration, which varied by not more than ca. 1% within 24 h. Whole ecosystem-respired CO(2) was least depleted in (13) C in the afternoon and most negative in the early morning. We assume that temporally variable respiratory carbon isotope fractionation and changes in fluxes through metabolic pathways, rather than photosynthetic carbon isotope fractionation, governs the δ(13) C of respired CO(2) at the diel scale, and thus provides insights into the metabolic processes related to respiration under field conditions.     

Metabolic origin of carbon isotope composition of leaf dark-respired CO2 in French bean

The carbon isotope composition (delta(13)C) of CO(2) produced in darkness by intact French bean (Phaseolus vulgaris) leaves was investigated for different leaf temperatures and during dark periods of increasing length. The delta(13)C of CO(2) linearly decreased when temperature increased, from -19 per thousand at 10 degrees C to -24 per thousand at 35 degrees C. It also progressively decreased from -21 per thousand to -30 per thousand when leaves were maintained in continuous darkness for several days. Under normal conditions (temperature not exceeding 30 degrees C and normal dark period), the evolved CO(2) was enriched in (13)C compared with carbohydrates, the most (13)C-enriched metabolites. However, at the end of a long dark period (carbohydrate starvation), CO(2) was depleted in (13)C even when compared with the composition of total organic matter. In the two types of experiment, the variations of delta(13)C were linearly related to those of the respiratory quotient. This strongly suggests that the variation of delta(13)C is the direct consequence of a substrate switch that may occur to feed respiration; carbohydrate oxidation producing (13)C-enriched CO(2) and beta-oxidation of fatty acids producing (13)C-depleted CO(2) when compared with total organic matter (-27.5 per thousand). These results are consistent with the assumption that the delta(13)C of dark respired CO(2) is determined by the relative contributions of the two major decarboxylation processes that occur in darkness: pyruvate dehydrogenase activity and the Krebs cycle.     

Responses of wild C4 and C3 grass (Poaceae) species to elevated atmospheric CO2 concentration: a meta-analytic test of current theories and perceptions

C4 plants contribute ≈ 20% of global gross primary productivity, and uncertainties regarding their responses to rising atmospheric CO₂ concentrations may limit predictions of future global change impacts on C4-dominated ecosystems. These uncertainties have not yet been considered rigorously due to expectations of C4 low responsiveness based on photosynthetic theory and early experiments. We carried out a literature review (1980–97) and meta-analysis in order to identify emerging patterns of C4 grass responses to elevated CO₂, as compared with those of C3 grasses. The focus was on nondomesticated Poaceae alone, to the exclusion of C4 dicotyledonous and C4 crop species. This provides a clear test, controlled for genotypic variability at family level, of differences between the CO₂-responsiveness of these functional types. Eleven responses were considered, ranging from physiological behaviour at the leaf level to carbon allocation patterns at the whole plant level. Results were also assessed in the context of environmental stress conditions (light, temperature, water and nutrient stress), and experimental growing conditions (pot size, experimental duration and fumigation method). Both C4 and C3 species increased total biomass significantly in elevated CO₂, by 33% and 44%, respectively. Differing tendencies between types in shoot structural response were revealed: C3 species showed a greater increase in tillering, whereas C4 species showed a greater increase in leaf area in elevated CO₂. At the leaf level, significant stomatal closure and increased leaf water use efficiency were confirmed in both types, and higher carbon assimilation rates were found in both C3 and C4 species (33% and 25%, respectively). Environmental stress did not alter the C4 CO₂-response, except for the loss of a significant positive CO₂-response for above-ground biomass and leaf area under water stress. In C3 species, stimulation of carbon assimilation rate was reduced by stress (overall), and nutrient stress tended to reduce the mean biomass response to elevated CO₂. Leaf carbohydrate status increased and leaf nitrogen concentration decreased significantly in elevated CO₂ only in C3 species. We conclude that the relative responses of the C4 and C3 photosynthetic types to elevated CO₂ concur only to some extent with expectations based on photosynthetic theory. The significant positive responses of C4 grass species at both the leaf and the whole plant level demand a re-evaluation of the assumption of low responsiveness in C4 plants at both levels, and not only with regard to water relations. The combined shoot structural and water use efficiency responses of these functional types will have consequential implications for the water balance of important catchments and range-lands throughout the world, especially in semiarid subtropical and temperate regions. It may be premature to predict that C4 grass species will lose their competitive advantage over C3 grass species in elevated CO₂.     

Carbon Isotopic Fractionation Does Not Occur during Dark Respiration in C3 and C4 Plants

The magnitude of possible carbon isotopic fractionation during dark respiration was investigated with isolated mesophyll cells from mature leaves of common bean (Phaseolus vulgaris L.), a C3 plant, and corn (Zea mays L.), a C4 plant. Mesophyll protoplasts were extracted from greenhouse-grown leaves and incubated in culture solutions containing different carbohydrate substrates (fructose, glucose, and sucrose) with known [delta]13C values. The CO2 produced by protoplasts after incubation in the dark was collected, purified, and analyzed for its carbon isotope ratio. From observations of the isotope ratios of the substrate and respired CO2, we calculated the carbon isotope discrimination associated with metabolism of each of these substrates. In eight of the 10 treatment combinations, the carbon isotope ratio discrimination was not significantly different from 0. In the remaining two treatment combinations, the carbon isotope ratio discrimination was 1[per mille (thousand) sign]. From these results, we conclude that there is no significant carbon isotopic discrimination during mitochondrial dark respiration when fructase, glucose, or sucrose are used as respiratory substrates.     

Can isotopic fractionation during respiration explain the 13C-enriched sporocarps of ectomycorrhizal and saprotrophic fungi?

The mechanism behind the (13)C enrichment of fungi relative to plant materials is unclear and constrains the use of stable isotopes in studies of the carbon cycle in soils. Here, we examined whether isotopic fractionation during respiration contributes to this pattern by comparing delta(13)C signatures of respired CO(2), sporocarps and their associated plant materials, from 16 species of ectomycorrhizal or saprotrophic fungi collected in a Norway spruce forest. The isotopic composition of respired CO(2) and sporocarps was positively correlated. The differences in delta(13)C between CO(2) and sporocarps were generally small, < +/-1 per thousand in nine out of 16 species, and the average shift for all investigated species was 0.04 per thousand. However, when fungal groups were analysed separately, three out of six species of ectomycorrhizal basidiomycetes respired (13)C-enriched CO(2) (up to 1.6 per thousand), whereas three out of five species of polypores respired (13)C-depleted CO(2) (up to 1.7 per thousand; P < 0.05). The CO(2) and sporocarps were always (13)C-enriched compared with wood, litter or roots. Loss of (13)C-depleted CO(2) may have enriched some species in (13)C. However, that the CO(2) was consistently (13)C-enriched compared with plant materials implies that other processes must be found to explain the consistent (13)C-enrichment of fungal biomass compared with plant materials.     

Apparent respiratory discrimination is correlated with growth rate in the shoot apex of sunflower (Helianthus annuus)

The literature offers no consensus as to whether the delta(13)C of respired CO(2) is identical to that of the respiratory substrate, perhaps because of differences in measurement technique and growth conditions. To address this issue, the delta(13)C of respired CO(2) from growing sunflower shoot apices was measured and compared with that of soluble carbohydrates extracted from the respiring tissues. Shoot apices were studied because any influence of growth and biosynthesis was expected to be maximally expressed in these rapidly growing tissues. The two most probable substrates, starch and soluble sugars, were similar in delta(13)C (P=0.46). The delta(13)C of respired CO(2) was enriched in (13)C compared with these putative substrates (P<0.0001). This apparent enrichment ranged from 2.2 per thousand-5.7 per thousand, and decreased with relative growth rate (P<0.0001). The respiratory enrichment was counterbalanced by a depletion in the tissue constructed from the residual carbohydrates. The depletion varied from 2.2 per thousand to 3.0 per thousand relative to soluble carbohydrates (P<0.05), as predicted from mass-balance arguments. These results support the idea that respired CO(2) is enriched relative to its substrates. Variation in growth rates may help to explain the variable amounts of respiratory discrimination described in the literature.     

Elevated CO2, rhizosphere processes,and soil organic matter decomposition

The rhizosphere is one of the key fine-scale components of C cycles. This study was undertaken to improve understanding of the potential effects of atmospheric CO2 increase on rhizosphere processes. Using C isotope techniques, we found that elevated atmospheric CO2 significantly increased wheat plant growth, dry mass accumulation, rhizosphere respiration, and soluble C concentrations in the rhizosphere. When plants were grown under elevated CO2 concentration, soluble C concentration in the rhizosphere increased by approximately 60%. The degree of elevated CO2 enhancement on rhizosphere respiration was much higher than on root biomass. Averaged between the two nitrogen treatments and compared with the ambient CO2 treatment, wheat rhizosphere respiration rate increased 60% and root biomass only increased 26% under the elevated CO2 treatment. These results indicated that elevated atmospheric CO2 in a wheat-soil system significantly increased substrate input to the rhizosphere due to both increased root growth and increased root activities per unit of roots. Nitrogen treatments changed the effect of elevated CO2 on soil organic matter decomposition. Elevated CO2 increased soil organic matter decomposition (22%) in the nitrogen-added treatment but decreased soil organic matter decomposition (18%) without nitrogen addition. Soil nitrogen status was therefore found to be important in determining the directions of the effect of elevated CO2 on soil organic matter decomposition.     

Diurnal variation of the delta 13C of pine needle respired CO2 evolved in darkness

The delta 13C of pine needle CO2 evolved in darkness (delta 13Cr) for slash pine trees (Pinus elliottii) was determined by placing recently collected pine needles in darkness and collecting respired CO2 over a short time period (<15 min). Delta 13Cr measurements were made over several 24 h periods to test the hypothesis that significant variation in delta 13Cr would be observed during a diurnal cycle. The delta 13Cr measurements from the 24 h time series trials showed a consistent midday 13C-enrichment (5-10 per thousand) relative to bulk biomass. The delta 13Cr values became more 13C-depleted at night and following shading, and approached bulk-biomass delta 13C values by dawn. The effect of night-time respired 13C-enriched CO2 on the delta 13C value of the remaining assimilate is shown to be minimal (13C depleted by 0.22 per thousand) under field conditions for P. elliottii needles.     

Biomass allocation in old-field annual species grown in elevated CO2 environments: no evidence for optimal partitioning

Increased atmospheric carbon dioxide supply is predicted to alter plant growth and biomass allocation patterns. It is not clear whether changes in biomass allocation reflect optimal partitioning or whether they are a direct effect of increased growth rates. Plasticity in growth and biomass allocation patterns was investigated at two concentrations of CO₂ ([CO₂]) and at limiting and nonlimiting nutrient levels for four fast‐ growing old‐field annual species. Abutilon theophrasti, Amaranthus retroflexus, Chenopodium album, and Polygonum pensylvanicum were grown from seed in controlled growth chamber conditions at current (350 μmol mol^?1, ambient) and future‐ predicted (700 μmol mol^?1, elevated) CO₂ levels. Frequent harvests were used to determine growth and biomass allocation responses of these plants throughout vegetative development. Under nonlimiting nutrient conditions, whole plant growth was increased greatly under elevated [CO₂] for three C3 species and moderately increased for a C4 species (Amaranthus). No significant increases in whole plant growth were observed under limiting nutrient conditions. Plants grown in elevated [CO₂] had lower or unchanged root:shoot ratios, contrary to what would be expected by optimal partitioning theory. These differences disappeared when allometric plots of the same data were analysed, indicating that CO₂‐induced differences in root:shoot allocation were a consequence of accelerated growth and development rates. Allocation to leaf area was unaffected by atmospheric [CO₂] for these species. The general lack of biomass allocation responses to [CO₂] availability is in stark contrast with known responses of these species to light and nutrient gradients. We conclude that biomass allocation responses to elevated atmospheric [CO₂] are not consistent with optimal partitioning predictions.     

植物生理生态指标对大气CO2浓度倍增响应的整合分析

对 8 4篇文献有关植物对大气CO2 浓度倍增响应进行整合分析(一种对同一主题下多个独立实验进行综合的统计学方法),发现环境因素(土壤水分亏缺、土壤低氮、高温和高浓度O3 )显著地影响植物对高CO2 浓度的响应。无任何环境胁迫时,高CO2 浓度对C3 植物的 12个植物生理生态指标产生负效应,对另 12个则表现正效应,负响应最强的前 5个指标为:气孔导度(gs) >暗呼吸速率(Rd) >单位叶重中的氮含量(Nm) >单位叶重中蛋白质含量(Prm) >单位叶结构重量中氮含量(Ns);正响应最强烈的前 5个指标为:根生物量(Br) >地上部生物量(Bs) >单位叶重中淀粉含量(St) >光饱和时的光合速率(A) >总生物量(Bt)。可见植物的气体交换和生物量受高CO2 浓度影响较大,叶化学成分的变化则以淀粉、单位叶重含氮量和单位叶重蛋白质含量较为明显。无任何胁迫时,C3 植物的总生物量和光饱和时的光合速率分别提高 30.0 1%和 40.36 %;气孔导度下降 30.39%。     

Meta-analysis*of*the Response*of*Plant Ecophysiological Variables to Doubled Atmospheric CO2 Concentrations

Eighty-four published papers were synthesized using meta-analysis (a statistical method to summarize the different individual studies under the same subject), with results of the environmental factors (soil water deficit, low soil nitrogen, high temperature and high concentration O3) affecting significantly the response of plant to elevated atmospheric CO2 concentrations. Under unstressed condition, the overall effect sizes of twelve ecophysiological variables of C3 plants were negative, other twelve positive. For negative effect group, the first five variables ranked as: stomatal conductance ( g s)> leaf dark respiration rate （Rd）>leaf nitrogen content on a mass basis ( N m)>leaf protein content on a mass basis ( Prm)>leaf nitrogen content on a structural mass basis ( Ns); for positive one, root biomass ( Bt)>shoot biomass ( Bs)>leaf starch content on a mass basis ( St )>light saturated net photosynthetic rate ( A )>total biomass ( Bt). The Bt and A of C3 plants increased 30.01% and 40.36% respectively under unstressed condition, while g s decreased 30.39%.     

Large daily variation in 13C-enrichment of leaf-respired CO2 in two Quercus forest canopies

The use of the 13C : 12C isotopic ratio (delta13C) of leaf-respired CO2 to trace carbon fluxes in plants and ecosystems is limited by little information on temporal variations in delta13C of leaf dark-respired CO2 (delta13Cr) under field conditions. Here, we explored variability in delta13Cr and its relationship to key respiratory substrates from collections of leaf dark-respired CO2, carbohydrate extractions and gas exchange measurements over 24-h periods in two Quercus canopies. Throughout both canopies, delta13Cr became progressively 13C-enriched during the photoperiod, by up to 7%, then 13C-depleted at night relative to the photoperiod. This cycle could not be reconciled with delta13C of soluble sugars (delta13Css), starch (delta13Cst), lipids (delta13Cl), cellulose (delta13Cc) or with calculated photosynthetic discrimination (Delta). However, photoperiod progressive enrichment in delta13Cr was correlated with cumulative carbon assimilation (r2 = 0.91). We concluded that there is considerable short-term variation in delta13Cr in forest canopies, that it is consistent with current hypotheses for 13C fractionation during leaf respiration, that leaf carbohydrates cannot be used as surrogates for delta13Cr, and that diel changes in leaf carbohydrate status could be used to predict changes in delta13Cr empirically.     

Seasonal patterns of 13C partitioning between shoots and nodulated roots of N2- or nitrate-fed Pisum sativum L

The effect of nitrogen source (N(2) or nitrate) on carbon assimilation by photosynthesis and on carbon partitioning between shoots and roots was investigated in pea (Pisum sativum L. 'Baccara') plants at different growth stages using (13)C labelling. Plants were grown in the greenhouse on different occasions in 1999 and 2000. Atmospheric [CO(2)] and growth conditions were varied to alter the rate of photosynthesis. Carbon allocation to nodulated roots was unaffected by N source. At the beginning of the vegetative period, nodulated roots had priority for assimilates over shoots; this priority decreased during later stages and became identical to that of the shoot during seed filling. Carbon allocation to nodulated roots was always limited by competition with shoots, and could be predicted for each phenological stage: during vegetative and flowering stages a single, negative exponential relationship was established between sink intensity (percentage of C allocated to the nodulated root per unit biomass) and net photosynthesis. At seed filling, the amount of carbon allocated to the nodulated root was directly related to net photosynthesis. Respiration of nodulated roots accounted for more than 60 % of carbon allocated to them during growth. Only at flowering was respiration affected by N supply: it was significantly higher for strictly N(2)-fixing plants (83 %) than for plants fed with nitrate (71 %). At the vegetative stage, the increase in carbon in nodulated root biomass was probably limited by respiration losses.