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.     

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.     

CO2 transfer conductance, leaf structure and carbon isotope composition of Polygonum cuspidatum leaves from low and high altitudes

Anatomy and some physiological characteristics of the leaves in Polygonum cuspidatum Sieb. et Zucc., a dioecious clonal herb, were compared between two populations, one from a lowland in Shizuoka City (10 m above sea level), and another from a highland on Mt. Fuji (2500 m above sea level). Leaf mass per area (LMA) of the highland plants was about twice that of the lowland plants. The greater leaf thickness, thicker mesophyll cell walls and higher mesophyll cell density in the highland leaves contributed to the larger LMA. Although mesophyll area exposed to intercellular airspaces was greater in the highland leaves than in the lowland leaves by 30%, the surface area of chloroplasts facing intercellular airspaces was similar between these leaves. CO₂ transfer conductance inside the leaf (g_i) of the highland leaves (0·75 μmol m^?2 s^?1 Pa^?1) is the lowest recorded for herbaceous plants and was only 40% of that in the lowland leaves. On the other hand, the difference in stomatal conductance was small. δ¹³C values in the leaf dry matter were greater in the highland leaves by 4‰. These data and the estimation of CO₂ partial pressures in the intercellular air spaces and in the chloroplast suggested that the greater dry matter δ¹³C in the highland leaves, indicative of lower long‐term ratio of the chloroplast stroma to the ambient CO₂ partial pressures, would be mainly attributed to their lower g_i.     

植物呼吸释放CO2碳同位素变化研究进展

稳定性碳同位素是研究碳循环的有效手段。植物呼吸释放CO_2的碳同位素(δ~(13)C_R)变化是研究植物或生态系统与大气碳交换的重要方法,并可以揭示植物的生理过程、碳分配方式及其对环境变化的响应。介绍了目前国内外关于植物δ~(13)C_R变化的研究概况,植物不同器官δ~(13)C_R值及其日变化幅度趋势一致:叶片根系树干/茎,不同功能群植物其呼吸释放CO_2碳同位素组成存在差异。但植物δ~(13)C_R值日变化与呼吸底物的相关性在不同的研究中结果并不一致。导致植物呼吸δ~(13)C_R发生变化的主要原因为光合同位素效应、呼吸底物的供给及呼吸代谢中间产物利用、碳代谢相关酶的活性、LEDR(light enhanced dark respiration)、植物的遗传特性及外部环境改变。目前国际上已有较多关于导致植物呼吸δ~(13)C_R发生变化原因的研究,但内在机制的研究尚未完善。该领域研究在国内鲜有报道,因此,亟需加强我国关于植物δ~(13)C_R短期变化及其潜在呼吸代谢机制的研究。     

Photoinhibition of respiratory CO2 release in the green alga Scenedesmus

¹⁴CO₂ evolution of prelabeled Scenedesmus obliquus Kütz, has been followed in the dark and in the light. In the light, no carbon dioxide is evolved. Addition of unlabeled NaHCO, leads to ¹⁴CO₂ release attaining 20 to 30% of the dark rate. Double-reciprocal plots of NaHCO₃ concentrations vs ¹⁴CO₂ release results in a straight line, indicative of competition between exogenously supplied bicarbonate and endogenously evolved carbon dioxide. With this method, it is possible to measure CO₂ evolved by respiration in the light and to show that true photoinhibition of respiration occurs in Scenedesmus . In the light. DCMU substantially increases ¹⁴CO₂ evolution; in the presence of the uncoupler carbonyl cyanide- m -chlorophenylhydrazone. ¹⁴CO₂ evolution is comparable to that in the dark. ¹⁴CO₂ release and oxygen uptake in the dark are only slightly affected by cyanide, indicative of a cyanide-resistant respiration and/or fermentation as the essential CO₂-yielding processes in the presence of cyanide. These results, compared with concurrent ATP levels, lead us to assume that energy charge is not the only factor responsible for photoinhibition of respiration.     

Gas exchange and carbon isotope composition of Ananas comosus in response to elevated CO2 and temperature

Ananas comosus L. (Merr.) (pineapple) was grown at three day/night temperatures and 350 (ambient) and 700 (elevated) μ mol mol^–1 CO₂ to examine the interactive effects of these factors on leaf gas exchange and stable carbon isotope discrimination ( Δ ,‰). All data were collected on the youngest mature leaf for 24 h every 6 weeks. CO₂ uptake (mmol m^–2 d^–1) at ambient and elevated CO₂, respectively, were 306 and 352 at 30/20 °C, 175 and 346 at 30/25 °C and 187 and 343 at 35/25 °C. CO₂ enrichment enhanced CO₂ uptake substantially in the day in all environments. Uptake at night at elevated CO₂, relative to that at ambient CO₂, was unchanged at 30/20 °C, but was 80% higher at 30/25 °C and 44% higher at 35/25 °C suggesting that phosphoenolpyruvate carboxylase was not CO₂-saturated at ambient CO₂ levels and a 25 °C night temperature. Photosynthetic water use efficiency (WUE) was higher at elevated than at ambient CO₂. Leaf Δ -values were higher at elevated than at ambient CO₂ due to relatively higher assimilation in the light. Leaf Δ was significantly and linearly related to the fraction of total CO₂ assimilated at night. The data suggest that a simultaneous increase in CO₂ level and temperature associated with global warming would enhance carbon assimilation, increase WUE, and reduce the temperature dependence of CO₂ uptake by A. comosus .     

The respiratory source of CO2

Abstract Approximately half of the carbon plants fix in photosynthesis is lost in dark respiration. The major pathways for dark respiration and their control are briefly discussed in the context of a growing plant. It is suggested that whole-plant respiration may be largely ADP-limited and that fine control of the respiratory network serves to select the respiratory substrate and to partition carbon between the numerous possible fates within the network. The striking stoichiometry between whole-plant growth and respiration is reviewed, and the relationships between substrate-limited growth and ADP-limited respiration are discussed.     

Terrestrial higher-plant response to increasing atmospheric [CO2] in relation to the global carbon cycle

Terrestrial higher plants exchange large amounts of CO₂ with the atmosphere each year; c. 15% of the atmospheric pool of C is assimilated in terrestrial-plant photosynthesis each year, with an about equal amount returned to the atmosphere as CO₂ in plant respiration and the decomposition of soil organic matter and plant litter. Any global change in plant C metabolism can potentially affect atmospheric CO₂ content during the course of years to decades. In particular, plant responses to the presently increasing atmospheric CO₂ concentration might influence the rate of atmospheric CO₂ increase through various biotic feedbacks. Climatic changes caused by increasing atmospheric CO₂ concentration may modulate plant and ecosystem responses to CO₂ concentration. Climatic changes and increases in pollution associated with increasing atmospheric CO₂ concentration may be as significant to plant and ecosystem C balance as CO₂ concentration itself. Moreover, human activities such as deforestation and livestock grazing can have impacts on the C balance and structure of individual terrestrial ecosystems that far outweigh effects of increasing CO₂ concentration and climatic change. In short-term experiments, which in this case means on the order of 10 years or less, elevated atmospheric CO₂ concentration affects terrestrial higher plants in several ways. Elevated CO₂ can stimulate photosynthesis, but plants may acclimate and (or) adapt to a change in atmospheric CO₂ concentration. Acclimation and adaptation of photosynthesis to increasing CO₂ concentration is unlikely to be complete, however. Plant water use efficiency is positively related to CO₂ concentration, implying the potential for more plant growth per unit of precipitation or soil moisture with increasing atmospheric CO₂ concentration. Plant respiration may be inhibited by elevated CO₂ concentration, and although a naive C balance perspective would count this as a benefit to a plant, because respiration is essential for plant growth and health, an inhibition of respiration can be detrimental. The net effect on terrestrial plants of elevated atmospheric CO₂ concentration is generally an increase in growth and C accumulation in phytomass. Published estimations, and speculations about, the magnitude of global terrestrial-plant growth responses to increasing atmospheric CO₂ concentration range from negligible to fantastic. Well-reasoned analyses point to moderate global plant responses to CO₂ concentration. Transfer of C from plants to soils is likely to increase with elevated CO₂ concentrations because of greater plant growth, but quantitative effects of those increased inputs to soils on soil C pool sizes are unknown. Whether increases in leaf-level photosynthesis and short-term plant growth stimulations caused by elevated atmospheric CO₂ concentration will have, by themselves, significant long-term (tens to hundreds of years) effects on ecosystem C storage and atmospheric CO₂ concentration is a matter for speculation, not firm conclusion. Long-term field studies of plant responses to elevated atmospheric CO₂ are needed. These will be expensive, difficult, and by definition, results will not be forthcoming for at least decades. Analyses of plants and ecosystems surrounding natural geological CO₂ degassing vents may provide the best surrogates for long-term controlled experiments, and therefore the most relevant information pertaining to long-term terrestrial-plant responses to elevated CO₂ concentration, but pollutants associated with the vents are a concern in some cases, and quantitative knowledge of the history of atmospheric CO₂ concentrations near vents is limited. On the whole, terrestrial higher-plant responses to increasing atmospheric CO₂ concentration probably act as negative feedbacks on atmospheric CO₂ concentration increases, but they cannot by themselves stop the fossil-fuel-oxidation-driven increase in atmospheric CO₂ concentration. And, in the very long-term, atmospheric CO₂ concentration is controlled by atmosphere-ocean C equilibrium rather than by terrestrial plant and ecosystem responses to atmospheric CO₂ concentration.     

Carbon isotope discrimination in C3-C4 intermediates

Carbon isotope discrimination in C₃–C₄ intermediates is determined by fractionations during diffusion and the biochemical fractionations occurring during CO₂ fixation. These biochemical fractionations in turn depend on the fractionation by Rubisco in the mesophyll, the amount of CO₂ fixation. These biochemical fractionations in turn depend on the fractionation by Rubisco in the mesophyll, the amount of CO₂ fixation occurring in the bundle sheath, the extent of bundle-sheath leakiness and the contribution which C₄-cycle activity makes to the CO₂ pool there. In most instances, carbon isotope discrimination in C₃–C₄ intermediates is C₃-like because only a small fraction of the total carbon fixed is fixed in the bundle sheath. In particular, this must be the case for Flaveria intermediates which initially fix substantial amounts of CO₂ into C₄-acids. In C₃–C₄ intermediates that refix photorespiratory CO₂ alone, it is possible for carbon isotope discrimination to be greater than in C₃-species, particularly at low CO₂ pressures or at high leaf temperatures. Short-term measurements of carbon isotope discrimination and gas exchange of leaves can be used to study the photosynthetic pathways of C₃-C₄ intermediates and their hybrids as has recently been done for C₃ and C₄ species.     

The input and fate of new C in two forest soils under elevated CO2

The aim of this study was to estimate (i) the influence of different soil types on the net input of new C into soils under CO₂ enrichment and (ii) the stability and fate of these new C inputs in soils. We exposed young beech–spruce model ecosystems on an acidic loam and calcareous sand for 4 years to elevated CO₂. The added CO₂ was depleted in ¹³C, allowing to trace new C inputs in the plant–soil system. We measured CO₂‐derived new C in soil C pools fractionated into particle sizes and monitored respiration as well as leaching of this new C during incubation for 1 year. Soil type played a crucial role in the partitioning of C. The net input of new C into soils under elevated CO₂ was about 75% greater in the acidic loam than in the calcareous sand, despite a 100% and a 45% greater above‐ and below‐ground biomass on the calcareous sand. This was most likely caused by a higher turnover of C in the calcareous sand as indicated by 30% higher losses of new C from the calcareous sand than from the acidic loam during incubation. Therefore, soil properties determining stabilization of soil C were apparently more important for the accumulation of C in soils than tree productivity. Soil fractionation revealed that about 60% of the CO₂‐derived new soil C was incorporated into sand fractions. Low natural ¹³C abundance and wide C/N ratios show that sand fractions comprise little decomposed organic matter. Consistently, incubation indicated that new soil C was preferentially respired as CO₂. During the first month, evolved CO₂ consisted to 40–55% of new C, whereas the fraction of new C in bulk soil C was 15–23% only. Leaching of DOC accounted for 8–23% of the total losses of new soil C. The overall effects of CO₂ enrichment on soil C were small in both soils, although tree growth increased significantly on the calcareous sand. Our results suggest that the potential of soils for C sequestration is limited, because only a small fraction of new C inputs into soils will become long‐term soil C.     

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.     

Effect of abscisic acid on CO2 exchange in Lemna gibba

The effects of abscisic acid (ABA) on photosynthesis, dark respiration, and photorespiration were studied in Lemna gibba L. plants. The initial concentration of ABA in the nutrient solution was 10⁻⁷M and in a few experiments, 10⁻⁶M. The cultures were grown in the same solution for time periods ranging from one hour to 12 days. Net photosynthesis, measured as CO₂ uptake by infrared gas analyser technique, was inhibited after four hours of ABA treatment and reached a minimum after four to seven days depending on the time of the year. After 12 days a substantial recovery of photosynthesis was observed. Dark respiration was significantly stimulated after two to seven days of ABA treatment but then returned to the control level. The transient effects of ABA on photosynthesis and dark respiration corresponded to the previously measured time course of [¹⁴C]-ABA uptake by Lemna . Photorespiration measured as oxygen inhibition of photosynthesis was not affected by ABA.     

Experimental evidence for diel variations of the carbon isotope composition in leaf, stem and phloem sap organic matter in Ricinus communis

Carbon isotope fractionation in metabolic processes following carboxylation of ribulose-1,5-bisphosphate (RuBP) is not as well described as the discrimination during photosynthetic CO(2) fixation. However, post-carboxylation fractionation can influence the diel variation of delta(13)C of leaf-exported organic matter and can cause inter-organ differences in delta(13)C. To obtain a more mechanistic understanding of post-carboxylation modification of the isotopic signal as governed by physiological and environmental controls, we combined the modelling approach of Tcherkez et al., which describes the isotopic fractionation in primary metabolism with the experimental determination of delta(13)C in leaf and phloem sap and root carbon pools during a full diel course. There was a strong diel variation of leaf water-soluble organic matter and phloem sap sugars with relatively (13)C depleted carbon produced and exported during the day and enriched carbon during the night. The isotopic modelling approach reproduces the experimentally determined day-night differences in delta(13)C of leaf-exported carbon in Ricinus communis. These findings support the idea that patterns of transitory starch accumulation and remobilization govern the diel rhythm of delta(13)C in organic matter exported by leaves. Integrated over the whole 24 h day, leaf-exported carbon was enriched in (13)C as compared with the primary assimilates. This may contribute to the well-known--yet poorly explained--relative (13)C depletion of autotrophic organs compared with other plant parts. We thus emphasize the need to consider post-carboxylation fractionations for studies that use delta(13)C for assessing environmental effects like water availability on ratio of mole fractions of CO(2) inside and outside the leaf (e.g. tree ring studies), or for partitioning of CO(2) fluxes at the ecosystem level.     

The effect of elevated CO2 on the chemical composition and construction costs of leaves of 27 C3 species

We determined the proximate chemical composition as well as the construction costs of leaves of 27 species, grown at ambient and at a twice-ambient partial pressure of atmospheric CO₂. These species comprised wild and agricultural herbaceous plants as well as tree seedlings. Both average responses across species and the range in response were considered. Expressed on a total dry weight basis, the main change in chemical composition due to CO₂ was the accumulation of total non-structural carbohydrates (TNC). To a lesser extent, decreases were found for organic N compounds and minerals. Hardly any change was observed for total structural carbohydrates (cellulose plus hemicellulose), lignin and lipids. When expressed on a TNC-free basis, decreases in organic N compounds and minerals were still present. On this basis, there was also an increase in the concentration of soluble phenolics. In terms of glucose required for biosynthesis, the increase in costs for one chemical compound – TNC – was balanced by a decrease in the costs for organic N compounds. Therefore, the construction costs, the total amount of glucose required to produce 1 g of leaf, were rather similar for the two CO₂ treatments; on average a small decrease of 3% was found. This decrease was attributable to a decrease of up to 30% in the growth respiration coefficient, the total CO₂ respired [mainly for N AD(P)H and ATP] in the process of constructing 1 g of biomass. The main reasons for this reduction were the decrease in organic N compounds and the increase in TNC.