δ13C of CO2 respired in the dark in relation to δ13C of leaf metabolites: comparison between Nicotiana sylvestris and Helianthus annuus under drought

The variations of δ¹³C in leaf metabolites (lipids, organic acids, starch and soluble sugars), leaf organic matter and CO₂ respired in the dark from leaves of Nicotiana sylvestris and Helianthus annuus were investigated during a progressive drought. Under well‐watered conditions, CO₂ respired in the dark was ¹³C‐enriched compared to sucrose by about 4‰ in N. sylvestris and by about 3‰ and 6‰ in two different sets of experiments in H. annuus plants. In a previous work on cotyledonary leaves of Phaseolus vulgaris, we observed a constant ¹³C‐enrichment by about 6‰ in respired CO₂ compared to sucrose, suggesting a constant fractionation during dark respiration, whatever the leaf age and relative water content. In contrast, the ¹³C‐enrichment in respired CO₂ increased in dehydrated N. sylvestris and decreased in dehydrated H. annuus in comparison with control plants. We conclude that (i) carbon isotope fractionation during dark respiration is a widespread phenomenon occurring in C₃ plants, but that (ii) this fractionation is not constant and varies among species and (iii) it also varies with environmental conditions (water deficit in the present work) but differently among species. We also conclude that (iv) a discrimination during dark respiration processes occurred, releasing CO₂ enriched in ¹³C compared to several major leaf reserves (carbohydrates, lipids and organic acids) and whole leaf organic matter.     

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.     

The combined effect of constant water deficit and nitrogen supply on WUE, NUE and Δ13C in durum wheat potted plants

Water scarcity and nitrogen shortage are the main constraints on durum wheat productivity. This paper examines the combined effects of a constant water deficit and nitrogen supply (NS) on growth, photosynthesis, stomatal conductance (g_s) and transpiration, instantaneous and time‐integrated water use efficiency (WUE) and nitrogen use efficiency (NUE) and carbon isotope discrimination (Δ¹³C) in durum wheat genotypes grown in pots under greenhouse conditions. Three water levels (40%, 70% and 100% container capacity), two nitrogen doses (high and low N) and four genotypes were assayed in a total of 24 experimental treatments. Water and nitrogen treatments were imposed 2 weeks after plant emergence. The growth, nitrogen content and Δ¹³C of the shoot and the gas exchange in the flag leaf were determined about 2 weeks after anthesis. As expected, both water and NS had a strong positive effect on growth. However, a reduction in water supply had low effect decreasing photosynthesis and transpiration, Δ¹³C and NUE and increasing WUE. On the contrary, increasing the level of nitrogen supplied had a significant negative effect on g_s, which decreased significantly the ratio of intercellular to ambient CO₂ concentrations and Δ¹³C, and increased both instantaneous and time‐integrated WUE. In addition, a higher N level also negatively affected the instantaneous and time‐integrated NUE. The Δ¹³C of shoots correlated significantly and negatively with either instantaneous or time‐integrated measurements of WUE. Moreover, within each NS, Δ¹³C also correlated negatively with the integrated NUE. We concluded that under our experimental conditions, Δ¹³C gives information about the efficiency with which not just water but also nitrogen are used by the plant. In addition, this study illustrates that a steady water limitation may strongly affect biomass without consistent changes in WUE. The lack of effect of the different water regimes on gas exchange, WUE and Δ¹³C illustrate the importance of how stress is imposed during growth.     

Canopy-scale δ13C of photosynthetic and respiratory CO2 fluxes: observations in forest biomes across the United States

The δ¹³C values of atmospheric carbon dioxide (CO₂) can be used to partition global patterns of CO₂ source/sink relationships among terrestrial and oceanic ecosystems using the inversion technique. This approach is very sensitive to estimates of photosynthetic ¹³C discrimination by terrestrial vegetation (Δ_A), and depends on δ¹³C values of respired CO₂ fluxes (δ¹³C_R). Here we show that by combining two independent data streams – the stable isotope ratios of atmospheric CO₂ and eddy‐covariance CO₂ flux measurements – canopy scale estimates of Δ_A can be successfully derived in terrestrial ecosystems. We also present the first weekly dataset of seasonal variations in δ¹³C_R from dominant forest ecosystems in the United States between 2001 and 2003. Our observations indicate considerable summer‐time variation in the weekly value of δ¹³C_R within coniferous forests (4.0‰ and 5.4‰ at Wind River Canopy Crane Research Facility and Howland Forest, respectively, between May and September). The monthly mean values of δ¹³C_R showed a smaller range (2–3‰), which appeared to significantly correlate with soil water availability. Values of δ¹³C_R were less variable during the growing season at the deciduous forest (Harvard Forest). We suggest that the negative correlation between δ¹³C_R and soil moisture content observed in the two coniferous forests should represent a general ecosystem response to the changes in the distribution of water resources because of climate change. Shifts in δ¹³C_R and Δ_A could be of sufficient magnitude globally to impact partitioning calculations of CO₂ sinks between oceanic and terrestrial compartments.     

The interaction of rising CO2 and temperatures with water use efficiency

Abstract. Recent data concerning the impact of elevated atmospheric CO₂ upon water use efficiency (WUE) and the related measure, instantaneous transpiration efficiency (ITE), are reviewed. It is concluded from both short and long-term studies that, at the scale of the individual leaf or plant, an increase in WUE or ITE is generally observed in response to increased atmospheric CO₂ levels. However, the magnitude of this increase may decline with time. The opinion that elevated CO₂ may substantially decrease transpiration at the regional scale is discussed. The mechanisms by which elevated CO₂ may cause a change in these measures are discussed in terms of stomatal conductance, assimilation and respiration responses to elevated CO₂. Finally, recent experimental data and model outputs concerning the impact of the interaction of increased temperature with elevated CO₂ on WUE, ITE and yield are reviewed. It is concluded that substantially more data is required before reliable predictions about the regional scale response of WUE and catchment hydrology can be made.     

Photosynthesis and the influence of CO2-enrichment on δ13C values in a C3 halophyte

Abstract Shifts in ?¹³C of the graminaceous C₃ halophyte Puccinellia nuttalliana (Schultes) Hitch. can be induced by salinization. To investigate this phenomenon, three approaches were taken: assay of carboxylases, CO₂-enrichment studies, and gas exchange analysis. Although ribulose-1,5-bisphosphate carboxylase activity decreased with salinity, phosphoenolpyruvate carboxylase activity did not increase and its levels were not atypical of C₃ plants. When plants were grown at four NaCl concentrations under atmospheres of 310 and 1300 cm³ m^?3 CO₂, the CO₂-enrichment enhanced the effects of salinity on ?¹³C. This is consistent with a biophysical explanation for salt-induced shifts in ?¹³C, whereby there is a steepening of the CO₂ diffusion gradient into the leaf. Gas exchange analysis indicated that intercellular CO₂ concentrations were depressed in the leaves of salt-affected plants. This resulted from a greatly decreased stomatal conductance coupled with only small effects on intrinsic photosynthetic capacity. Water-use efficiency was enhanced.     

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.     

Role of land cover changes for atmospheric CO2 increase and climate change during the last 150 years

We assess the role of changing natural (volcanic, aerosol, insolation) and anthropogenic (CO₂ emissions, land cover) forcings on the global climate system over the last 150 years using an earth system model of intermediate complexity, CLIMBER‐2. We apply several datasets of historical land‐use reconstructions: the cropland dataset by Ramankutty & Foley (1999) (R&F), the HYDE land cover dataset of Klein Goldewijk (2001) , and the land‐use emissions data from Houghton & Hackler (2002) . Comparison between the simulated and observed temporal evolution of atmospheric CO₂ and δ¹³CO₂ are used to evaluate these datasets. To check model uncertainty, CLIMBER‐2 was coupled to the more complex Lund–Potsdam–Jena (LPJ) dynamic global vegetation model. In simulation with R&F dataset, biogeophysical mechanisms due to land cover changes tend to decrease global air temperature by 0.26°C, while biogeochemical mechanisms act to warm the climate by 0.18°C. The net effect on climate is negligible on a global scale, but pronounced over the land in the temperate and high northern latitudes where a cooling due to an increase in land surface albedo offsets the warming due to land‐use CO₂ emissions. Land cover changes led to estimated increases in atmospheric CO₂ of between 22 and 43 ppmv. Over the entire period 1800–2000, simulated δ¹³CO₂ with HYDE compares most favourably with ice core during 1850–1950 and Cape Grim data, indicating preference of earlier land clearance in HYDE over R&F. In relative terms, land cover forcing corresponds to 25–49% of the observed growth in atmospheric CO₂. This contribution declined from 36–60% during 1850–1960 to 4–35% during 1960–2000. CLIMBER‐2‐LPJ simulates the land cover contribution to atmospheric CO₂ growth to decrease from 68% during 1900–1960 to 12% in the 1980s. Overall, our simulations show a decline in the relative role of land cover changes for atmospheric CO₂ increase during the last 150 years.     

δ13C of wood in growth-rings indicates cambial activity of drought-stressed trees of Eucalyptus globulus

1. Growth, density and δ¹³C of wood and leaf area were measured in two adjacent stands of 6 year-old Eucalyptus globulus growing in the 600–700 mm year^–1 rainfall region of south-western Australia. Study sites were identical except for differences in the availability of water owing to physical properties of soil profiles and location of sites within the landscape.
2. Abundance of ¹³C (expressed as δ¹³C) in wood of trees growing on the drought-prone site (– 24·8‰±1·4) was greater than in other trees (– 25·8‰±1·2, P <0·001) throughout the 6 years and, with further development, the δ¹³C signatures of wood may become useful indices of drought-susceptibility in plantations within a few years of establishment. The seasonal pattern of δ¹³C of wood appeared to reflect seasonal variation in water availability and duration of cambial activity.
3. Basic density of wood of trees growing on the more drought-prone site (496±14·0 kg m^–3) was reduced compared to other trees (554±5·3 kg m^–3, P <0·001). δ¹³C of wood across boundaries of growth-rings suggested that drought stopped cambial activity resulting in less production of late wood and less dense wood.
4. The stand growing on the drought-prone site had reduced growth, wood yield and leaf area but identical specific leaf area. Annual growth was correlated with the previous season's rainfall. Together, these results suggested that within the same evaporative climate, drought reduces growth primarily by reducing leaf area and that there is a lag between onset of drought and reduced productivity.     

Stomatal acclimation to increased CO2 concentration in a Florida scrub oak species Quercus myrtifolia Willd

Native scrub‐oak communities in Florida were exposed for three seasons in open top chambers to present atmospheric [CO₂] (approx. 350 μmol mol^?1) and to high [CO₂] (increased by 350 μmol mol^?1). Stomatal and photosynthetic acclimation to high [CO₂] of the dominant species Quercus myrtifolia was examined by leaf gas exchange of excised shoots. Stomatal conductance (g_s) was approximately 40% lower in the high‐ compared to low‐[CO₂]‐grown plants when measured at their respective growth concentrations. Reciprocal measurements of g_s in both high‐ and low‐[CO₂]‐grown plants showed that there was negative acclimation in the high‐[CO₂]‐grown plants (9–16% reduction in g_s when measured at 700 μmol mol^?1), but these were small compared to those for net CO₂ assimilation rate (A, 21–36%). Stomatal acclimation was more clearly evident in the curve of stomatal response to intercellular [CO₂] (c_i) which showed a reduction in stomatal sensitivity at low c_i in the high‐[CO₂]‐grown plants. Stomatal density showed no change in response to growth in high growth [CO₂]. Long‐term stomatal and photosynthetic acclimation to growth in high [CO₂] did not markedly change the 2·5‐ to 3‐fold increase in gas‐exchange‐derived water use efficiency caused by high [CO₂].     

Diurnal variation of Δ13CO2, ΔC18O16O and evaporative site enrichment of δH218O in Piper aduncum under field conditions in Trinidad

Concurrent measurements of gas exchange, instantaneous isotope discrimination (Δ) against ¹³CO₂ and C¹⁸O¹⁶O, and extent of ¹⁸O enrichment in H₂O at the evaporative sites, were followed in a tropical forest pioneer, Piper aduncum, on two different days in Trinidad during February 1995. Δ¹³CO₂ differed from that predicted from measurements of internal:external CO₂ concentration (C_i/C_a) and showed a wide range of values which decreased throughout the course of the day. Derivation of C_c (the CO₂ concentration at the carboxylation site) was not possible using carbon isotope discrimination under field conditions in situ and was derived assuming a constant value of internal transfer conductance (g_w). Under low rates of assimilation the derived C_c/C_a, like C_i/C_a, remained relatively stable over the course of both days and ΔC¹⁸O¹⁶O followed evaporative demand. Lower values of ΔC¹⁸O¹⁶O on day 2 occurred in response to the indirect effect of increased leaf-to-air vapour pressure deficits (VPD) and reduced stomatal conductance. For the first time, direct determination of the δH₂¹⁸O of transpired water vapour (δ_t) allowed derivation of evaporative site enrichment without the prerequisite of isotopic steady state (ISS) defined in the Craig and Gordon model. Generally, δ_t was less enriched than the source water (δ_s) in the morning and more enriched in the afternoon, which would be predicted from an increase and decrease in ambient VPD, respectively. On both days, leaves of P. aduncum approached ISS (indicated where δ_t≈δ_s) between 1300 and 1500 h. Evaporative site enrichment was maintained into the late afternoon, despite a decrease in ambient VPD. The data presented provide a greater insight into the natural variation in isotopic discrimination under field conditions, which may help to refine models of terrestrial biome discrimination.     

Sensitivity of stomata and water use efficiency to high CO2

Abstract The observed responses of stomata to carbon dioxide are reviewed, and the interaction of other known factors on the sensitivity to CO₂ are summarized. The role of stomatal response to CO₂ is discussed, and it is argued that while the effect of the CO₂ response in normal daily stomatal behaviour is presently poorly understood the stomatal response to CO₂ will have major impact in improving water use efficiency in future CO₂ atmospheres. However, the attenuation of this increase is emphasized so that increases at the crop level will probably be much smaller than those observed at the single leaf assimilation level.     

Tropic leaf movements, photosynthetic gas exchange, leaf δ13C and chlorophyll a fluorescence of three soybean species in response to water availability

Midday leaf angle, photosynthetic gas exchange, stable carbon isotope ratio ( δ ¹³C), and chlorophyll a fluorescence among three wild soybeans, Glycine soja, G . tomentella and G . tabacina, from habitats with different water availability were examined. Plants grown under low water availability had reduced leaf area, photosynthetic and electron transport rates, more positive δ ¹³C values, and more vertical midday leaf angles. The three species differed in midday leaflet orientation, leaf size, photosynthesis and fluorescence responses to water availability. The species differences were consistent with the water availability of their habitat. G . soja , which grows in the wettest habitats, was shown to be the most photosynthetically susceptible to low water treatment and tended to have the most vertical midday leaflet angles. In contrast, G . tabacina , distributed in the driest habitats, had the least vertical midday leaflet angles and the lowest photosynthetic sensitivity to low water availability. G . tomentella , inhabiting an intermediate habitat, had intermediate midday leaf angles and photosynthetic responses. Our results support the hypothesis that paraheliotropic leaf movements respond in concert with photosynthetic characteristics in soybean leaves such that water use efficiency is enhanced and the risk of photoinhibition under water stress conditions is reduced.     

Correlating δ13C and δ18O in cellulose of trees

We measured the carbon and oxygen isotopic composition of stem cellulose of Pinus sylvestris, Picea abies, Fagus sylvatica and Fraxinus excelsior. Several sites along a transect of a small valley in Switzerland were selected which differ in soil moisture conditions. At every site, six trees per species were sampled, and a sample representing a mean value for the period from 1940 to 1990 was analysed. For all species, the mean site δ¹³C and δ¹⁸O of stem cellulose are related to the soil moisture availability, whereby higher isotope ratios are found at drier sites. This result is consistent with isotope fractionation models when assuming enhanced stomatal resistance (thus higher δ¹³C of incorporated carbon) and increased oxygen isotope enrichment in the leaf water (thus higher δ¹⁸O) at the dry sites. δ¹⁸ O-δ¹³C plots reveal a linear relationship between the carbon and oxygen isotopes in cellulose. To interpret this relationship we developed an equation which combines the above-mentioned fractionation models. An important new parameter is the degree to which the leaf water enrichment is reflected in the stem cellulose. In the combined model the slope of the δ¹⁸O-δ¹³C plot is related to the sensitivity of the p_i/p_a of a plant to changing relative humidity.     

Acquisition and within-plant allocation of 13C and 15N in CO2-enriched Quercus robur plants

We assessed the effects of doubling atmospheric CO₂ concentration, [CO₂], on C and N allocation within pedunculate oak plants (Quercus robur L.) grown in containers under optimal water supply. A short-term dual ¹³CO₂ and ¹⁵NO₃^? labelling experiment was carried out when the plants had formed their third growing flush. The 22-week exposure to 700 μl l^?1 [CO₂] stimulated plant growth and biomass accumulation (+53% as compared with the 350 μl l^?1 [CO₂] treatment) but decreased the root/shoot biomass ratio (-23%) and specific leaf area (-18%). Moreover, there was an increase in net CO₂ assimilation rate (+37% on a leaf dry weight basis; +71% on a leaf area basis), and a decrease in both above- and below-ground CO₂ respiration rates (-32 and -26%, respectively, on a dry mass basis) under elevated [CO₂]. ¹³C acquisition, expressed on a plant mass basis or on a plant leaf area basis, was also markedly stimulated under elevated [CO₂] both after the 12-h ¹³CO₂ pulse phase and after the 60-h chase phase. Plant N content was increased under elevated CO₂ (+36%), but not enough to compensate for the increase in plant C content (+53%). Thus, the plant C/N ratio was increased (+13%) and plant N concentration was decreased (-11%). There was no effect of elevated [CO₂] on fine root-specific ¹⁵N uptake (amount of recently assimilated ¹⁵N per unit fine root dry mass), suggesting that modifications of plant N pools were merely linked to root size and not to root function. N concentration was decreased in the leaves of the first and second growing flushes and in the coarse roots, whereas it was unaffected by [CO₂] in the stem and in the actively growing organs (fine roots and leaves of the third growth flush). Furthermore, leaf N content per unit area was unaffected by [CO₂]. These results are consistent with the short-term optimization of N distribution within the plants with respect to growth and photosynthesis. Such an optimization might be achieved at the expense of the N pools in storage compartments (coarse roots, leaves of the first and second growth flushes). After the 60-h ¹³C chase phase, leaves of the first and second growth flushes were almost completely depleted in recent ¹³C under ambient [CO₂], whereas these leaves retained important amounts of recently assimilated ¹³C (carbohydrate reserves?) under elevated [CO₂].