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1.
Much of our understanding about how carbon (C) is allocated in plants comes from radiocarbon (14C) pulse‐chase labeling experiments. However, the large amounts of 14C required for decay‐counting mean that these studies have been restricted for the most part to mesocosm or controlled laboratory experiments. Using the enhanced sensitivity for 14C detection available with accelerator mass spectrometry (AMS), we tested the utility of a low‐level 14C pulse‐chase labeling technique for quantifying C allocation patterns and the contributions of different plant components to total ecosystem respiration in a black spruce forest stand in central Manitoba, Canada. All aspects of the field experiment used 14C at levels well below regulated health standards, without significantly altering atmospheric CO2 concentrations. Over 30 days following the label application in late summer (August and September), we monitored the temporal and spatial allocation patterns of labeled photosynthetic products by measuring the amount and 14C content of CO2 respired from different ecosystem components. The mean residence times (MRT) for labeled photosynthetic products to be respired in the understory (feather mosses), canopy (black spruce), and rhizosphere (black spruce roots and associated microbes) were <1, 6, and 15 days, respectively. Respiration from the canopy and understory showed significantly greater influence of labeled photosynthates than excised root and intact rhizosphere respiration. After 30 days,∼65% of the label assimilated had been respired by the canopy,∼20% by the rhizosphere, and∼9% by the understory, with∼6% unaccounted for and perhaps remaining in tissues. Maximum 14C values in root and rhizosphere respiration were reached 4 days after label application. The label was still detectable in root, rhizosphere and canopy respiration after 30 days; these levels of remaining label would not have been detectible had a 13C label been applied. Our results support previous studies indicating that a substantial portion of the C fueling rhizosphere respiration in the growing season may be derived from stored C pools rather than recent photosynthetic products.  相似文献   

2.
The boreal forest is expected to experience the greatest warming of all forest biomes, raising concerns that some of the large quantities of soil carbon in these systems may be added to the atmosphere as CO2. However, nitrogen deposition or fertilization has the potential to increase boreal forest production and retard the decomposition of soil organic matter, hence increasing both tree stand and soil C storage. The major contributors to soil‐surface CO2 effluxes are autotrophic and heterotrophic respiration. To evaluate the effect of nutrient additions on the relative contributions from autotrophic and heterotrophic respiration, a large‐scale girdling experiment was performed in a long‐term nutrient optimization experiment in a 40‐year‐old stand of Norway spruce in northern Sweden. Trees on three nonfertilized plots and three fertilized plots were girdled in early summer 2002, and three nonfertilized and three fertilized plots were used as control plots. Each plot was 0.1 ha and contained around 230 trees. Soil‐surface CO2 fluxes, soil moisture, and soil temperature were monitored in both girdled and nongirdled plots. In late July, the time of the seasonal maximum in soil‐surface CO2 efflux, the total soil‐CO2 efflux in nongirdled plots was 40% lower in the fertilized than in the nonfertilized plots, while the efflux in girdled fertilized and nonfertilized plots was 50% and 60% lower, respectively, than in the corresponding nongirdled controls. We attribute these reductions to losses of the autotrophic component of the total soil‐surface CO2 efflux. The estimates of autotrophic respiration are conservative as root starch reserves were depleted more rapidly in roots of girdled than in nongirdled trees. Thus, heterotrophic activity was overestimated. Calculated on a unit area basis, both the heterotrophic and autotrophic soil respiration was significantly lower in fertilized plots, which is especially noteworthy given that aboveground production was around three times higher in fertilized than in nonfertilized plots.  相似文献   

3.
4.
南方型杨树人工林土壤呼吸及其组分分析   总被引:3,自引:0,他引:3  
唐罗忠  葛晓敏  吴麟  田野  魏勇 《生态学报》2012,32(22):7000-7008
采用开沟隔离法,利用LI-8100型土壤呼吸测定系统,对15年生的南方型杨树(Populus deltoides)人工林土壤呼吸进行了研究,并试图区分根系呼吸和土壤微生物呼吸。结果表明,开沟隔离处理后的10个月内,由于土壤中被截断根系具有自养呼吸和分解作用,土壤呼吸中的根系呼吸与微生物呼吸尚难以区分。尽管如此,研究表明15年生杨树人工林的土壤总呼吸通量为9.74 tC.hm-.2a-1,其中,枯枝落叶等土壤表层凋落物分解所释放的碳通量是2.63 tC.hm-.2a-1,占总量的27.0%;林木根系呼吸与土壤微生物呼吸通量的和为7.11 tC.hm-.2a-1,占总量的73.0%。土壤各组分呼吸速率与10 cm深处的土壤温度之间存在着显著的指数函数关系。不同直径的杨树根系被截断后的活力变化有所不同,根系越粗,存活时间越长。  相似文献   

5.
The influence of N availability on C sequestration under prolonged elevated CO2 in terrestrial ecosystems remains unclear. We studied the relationships between C and N dynamics in a pasture seeded to Lolium perenne after 8 years of elevated atmospheric CO2 concentration (FACE) conditions. Fertilizer‐15N was applied at a rate of 140 and 560 kg N ha2?1 y2?1 and depleted 13C‐CO2 was used to increase the CO2 concentration to 60 Pa pCO2. The 13C–15N dual isotopic tracer enabled us to study the dynamics of newly sequestered C and N in the soil by aggregate size and fractions of particulate organic matter (POM), made up by intra‐aggregate POM (iPOM) and free light fraction (LF). Eight years of elevated CO2 did not increase total C content in any of the aggregate classes or POM fractions at both rates of N application. The fraction of new C in the POM fractions also remained largely unaffected by N fertilization. Changes in the fractions of new C and new N (fertilizer‐N) under elevated CO2 were more pronounced between POM classes than between aggregate size classes. Hence, changes in the dynamics of soil C and N cycling are easier to detect in the POM fractions than in the whole aggregates. Within N treatments, fractions of new C and N in POM classes were highly correlated with more new C and N in large POM fractions and less in the smaller POM fractions. Isotopic data show that the microaggregates were derived from the macro‐aggregates and that the C and N associated with the microaggregates turned over slower than the C and N associated with the macroaggregates. There was also isotopic evidence that N immobilized by soil microorganisms was an important source of N in the iPOM fractions. Under low N availability, 3.04 units of new C per unit of fertilizer N were sequestered in the POM fractions. Under high N availability, the ratio of new C sequestered per unit of fertilizer N was reduced to 1.47. Elevated and ambient CO2 concentrations lead to similar 15N enrichments in the iPOM fractions under both low and high N additions, clearly showing that the SOM‐N dynamics were unaffected by prolonged elevated CO2 concentrations.  相似文献   

6.
The rates of the phosphorylation and dephosphorylation of 2-deoxyglucose were measured in rat brain in vivo using tracer kinetic techniques. The rate constant for each reaction was estimated from two separate experiments with different protocols for tracer administration. Tracer amounts of [1-14C]2-deoxyglucose (1 microCi) were injected through the internal carotid artery (intraarterial experiment), or through the atrium (intravenous experiment). Brains were sampled by freeze-blowing at various times after the injection. In the intraarterial experiment, the rate constant for the forward reaction from 2-deoxyglucose to 2-deoxyglucose phosphate was calculated by dividing the initial rate of 2-deoxyglucose phosphate production by the 2-deoxyglucose content in brain. The rate constant for the reverse reaction from 2-deoxyglucose phosphate to 2-deoxyglucose was calculated from the decay constant of 2-deoxyglucose phosphate. The rate constants estimated were 10.1 +/- 1.4%/min (SD) and 3.00 +/- 0.01%/min (SD), respectively, for the forward and reverse reactions. In the intravenous experiment, rate constants for both reactions were estimated by compartmental analysis. By fitting data to program SAAM-27, the rate constants for the forward and reverse reactions were estimated as 11.4 +/- 0.4%/min (SD) and 5.1 +/- 0.4%/min (SD), respectively. The rate constants determined were compared to those for the reactions between glucose and glucose-6-phosphate, estimated previously from labeled glucoses. It is concluded that the rate of glucose utilization measured by the 2-deoxyglucose method reflects the rate of the hexokinase reaction and not the rate of glucose utilization or brain energy utilization.  相似文献   

7.
Soil respiration is derived from heterotrophic (decomposition of soil organic matter) and autotrophic (root/rhizosphere respiration) sources, but there is considerable uncertainty about what factors control variations in their relative contributions in space and time. We took advantage of a unique whole‐ecosystem radiocarbon label in a temperate forest to partition soil respiration into three sources: (1) recently photosynthesized carbon (C), which dominates root and rhizosphere respiration; (2) leaf litter decomposition and (3) decomposition of root litter and soil organic matter >1–2 years old. Heterotrophic sources and specifically leaf litter decomposition were large contributors to total soil respiration during the growing season. Relative contributions from leaf litter decomposition ranged from a low of ~1±3% of total soil respiration (6± 3 mg C m?2 h?1) when leaf litter was extremely dry, to a high of 42±16% (96± 38 mg C m?2 h?1). Total soil respiration fluxes varied with the strength of the leaf litter decomposition source, indicating that moisture‐dependent changes in litter decomposition drive variability in total soil respiration fluxes. In the surface mineral soil layer, decomposition of C fixed in the original labeling event (3–5 years earlier) dominated the isotopic signature of heterotrophic respiration. Root/rhizosphere respiration accounted for 16±10% to 64±22% of total soil respiration, with highest relative contributions coinciding with low overall soil respiration fluxes. In contrast to leaf litter decomposition, root respiration fluxes did not exhibit marked temporal variation ranging from 34±14 to 40±16 mg C m?2 h?1 at different times in the growing season with a single exception (88±35 mg C m?2 h?1). Radiocarbon signatures of root respired CO2 changed markedly between early and late spring (March vs. May), suggesting a switch from stored nonstructural carbohydrate sources to more recent photosynthetic products.  相似文献   

8.
Partitioning soil carbon dioxide (CO2) efflux (RS) into autotrophic (RA; including plant roots and closely associated organisms) and heterotrophic (RH) components has received considerable attention, as differential responses of these components to environmental change have profound implications for the soil and ecosystem C balance. The increasing number of partitioning studies allows a more detailed analysis of experimental constraints than was previously possible. We present results of an exhaustive literature search of partitioning studies and analyse global trends in flux partitioning between biomes and ecosystem types by means of a metaanalysis. Across all data, an overall decline in the RH/RS ratio for increasing annual RS fluxes emerged. For forest ecosystems, boreal coniferous sites showed significantly higher (P<0.05) RH/RS ratios than temperate sites, while both temperate or tropical deciduous forests did not differ in ratios from any of the other forest types. While chronosequence studies report consistent declines in the RH/RS ratio with age, no difference could be detected for different age groups in the global data set. Different methodologies showed generally good agreement if the range of RS under which they had been measured was considered, with the exception of studies estimating RH by means of root mass regressions against RS, which resulted in consistently lower RH/RS estimates out of all methods included. Additionally, the time step over which fluxes were partitioned did not affect RH/RS ratios consistently. To put results into context, we review the most common techniques and point out the likely sources of errors associated with them. In order to improve soil CO2 efflux partitioning in future experiments, we include methodological recommendations, and also highlight the potential interactions between soil components that may be overlooked as a consequence of the partitioning process itself.  相似文献   

9.
Partitioning soil respiration (RS) into heterotrophic (RH) and rhizospheric (RR) components is an important step for understanding and modeling carbon cycling in forest ecosystems, but few studies on RR and RH exist in Chinese temperate forests. In this study, we used a trenching plot approach to partition RS in six temperate forests in northeastern China. Our specific objectives were to (1) examine seasonal patterns of soil surface CO2 fluxes from trenched (RT) and untrenched plots (RUT) of these forests; (2) quantify annual fluxes of RS components and their relative contributions in the forest ecosystems; and (3) examine effects of plot trenching on measurements of RS and related environmental factors. The RT maximized in early growing season, but the difference between RUT and RT peaked in later summer. The annual fluxes of RH and RR varied with forest types. The estimated values of RH for the Korean pine (Pinus koraiensis Sieb. et Zucc.), Dahurian larch (Larix gmelinii Rupr.), aspen‐birch (Populous davidiana Dode and Betula platyphylla Suk.), hardwood (Fraxinus mandshurica Rupr., Juglans mandshurica Maxim. and Phellodendron amurense Rupr.), Mongolian oak (Quercus mongolica Fisch.) and mixed deciduous (no dominant tree species) forests averaged 89, 196, 187, 245, 261 and 301 g C m−2 yr−1, respectively; those of RR averaged 424, 209, 628, 538, 524 and 483 g C m−2 yr−1, correspondingly; calculated contribution of RR to RS (RC) varied from 52% in the larch forest to 83% in the pine forest. The annual flux of RR was strongly correlated to biomass of roots <0.5 cm in diameter, while that of RH was weakly correlated to soil organic carbon concentration at A horizon. We concluded that vegetation type and associated carbon metabolisms of temperate forests should be considered in assessing and modeling RS components. The significant impacts of changed soil physical environments and substrate availability by plot trenching should be appropriately tackled in analyzing and interpreting measurements of RS components.  相似文献   

10.
Chronic N additions to forest ecosystems can enhance soil N availability, potentially leading to reduced C allocation to root systems. This in turn could decrease soil CO2 efflux. We measured soil respiration during the first, fifth, sixth and eighth years of simulated atmospheric NO3? deposition (3 g N m?2 yr?1) to four sugar maple‐dominated northern hardwood forests in Michigan to assess these possibilities. During the first year, soil respiration rates were slightly, but not significantly, higher in the NO3?‐amended plots. In all subsequent measurement years, soil respiration rates from NO3?‐amended soils were significantly depressed. Soil temperature and soil matric potential were measured concurrently with soil respiration and used to develop regression relationships for predicting soil respiration rates. Estimates of growing season and annual soil CO2 efflux made using these relationships indicate that these C fluxes were depressed by 15% in the eighth year of chronic NO3? additions. The decrease in soil respiration was not due to reduced C allocation to roots, as root respiration rates, root biomass, and root turnover were not significantly affected by N additions. Aboveground litter also was unchanged by the 8 years of treatment. Of the remaining potential causes for the decline in soil CO2 efflux, reduced microbial respiration appears to be the most likely possibility. Documented reductions in microbial biomass and the activities of extracellular enzymes used for litter degradation on the NO3?‐amended plots are consistent with this explanation.  相似文献   

11.
In this paper we describe measurements and modeling of 18O in CO2 and H2O pools and fluxes at a tallgrass prairie site in Oklahoma. We present measurements of the δ18O value of leaf water, depth‐resolved soil water, atmospheric water vapor, and Keeling plot δ18O intercepts for net soil‐surface CO2 and ecosystem CO2 and H2O fluxes during three periods of the 2000 growing season. Daytime discrimination against C18OO, as calculated from measured above‐canopy CO2 and δ18O gradients, is also presented. To interpret the isotope measurements, we applied an integrated land‐surface and isotope model (ISOLSM) that simulates ecosystem H218O and C18OO stocks and fluxes. ISOLSM accurately predicted the measured isotopic composition of ecosystem water pools and the δ18O value of net ecosystem CO2 and H2O fluxes. Simulations indicate that incomplete equilibration between CO2 and H2O within C4 plant leaves can have a substantial impact on ecosystem discrimination. Diurnal variations in the δ18O value of above‐canopy vapor had a small impact on the predicted δ18O value of ecosystem water pools, although sustained differences had a large impact. Diurnal variations in the δ18O value of above‐canopy CO2 substantially affected the predicted ecosystem discrimination. Leaves dominate the ecosystem 18O‐isoflux in CO2 during the growing season, while the soil contribution is relatively small and less variable. However, interpreting daytime measurements of ecosystem C18OO fluxes requires accurate predictions of both soil and leaf 18O‐isofluxes.  相似文献   

12.
A method based on simultaneous short-term exposure to 14CO2 and 15N2 is described for studying nitrogen fixation and distribution in legumes relative to carbon assimilation and use. Equipment designed to accomodate experiments under natural conditions with very little disturbance of the N2 fixing association is used. It permits continuous measurement and regulation of variables such as air temperature, humidity and CO2 concentration as well as soil aeration. Measurements of distribution and use of assimilates, respiration of nodulated roots, quantitative N2 fixation and the distribution and fate of fixed N as a function of time lead to a precise estimation of C and N budgets for each labelling period. When experiments are done at several phenological stages they give a new insight into the complex C and N interrelations in legume symbiosis.
A series of trials throughout the growth period of Glycine max (L.) Merr. cv. Hodgson demonstrated the sensitivity of the method. The development of the plants from vegetative to reproductive stages was accompanied by a complete change in the distribution patterns of current assimilates and products of nitrogen fixation. Maximum sink strength moved from the leaves to the pods and seeds which ended up receiving 70% of the incoming C and 35% of the fixed N. The fact that up to 85% of fixed N in the plants was in the reproductive organs at maturity can be accounted for by remobilisation from vegetative parts.
The respiration of nodulated roots utilized 33% of carbon translocated to below-ground plant parts before nitrogen fixation started, but as much as 50% during the period of optimal fixation. The advantages and limitations of the isotopic method described are critically discussed as a prelude to future investigations.  相似文献   

13.
Ecosystem respiration (Reco) is one of the largest terrestrial carbon (C) fluxes. The effect of climate change on Reco depends on the responses of its autotrophic and heterotrophic components. How autotrophic and heterotrophic respiration sources respond to climate change is especially important in ecosystems underlain by permafrost. Permafrost ecosystems contain vast stores of soil C (1672 Pg) and are located in northern latitudes where climate change is accelerated. Warming will cause a positive feedback to climate change if heterotrophic respiration increases without corresponding increases in primary production. We quantified the response of autotrophic and heterotrophic respiration to permafrost thaw across the 2008 and 2009 growing seasons. We partitioned Reco using Δ14C and δ13C into four sources–two autotrophic (above – and belowground plant structures) and two heterotrophic (young and old soil). We sampled the Δ14C and δ13C of sources using incubations and the Δ14C and δ13C of Reco using field measurements. We then used a Bayesian mixing model to solve for the most likely contributions of each source to Reco. Autotrophic respiration ranged from 40 to 70% of Reco and was greatest at the height of the growing season. Old soil heterotrophic respiration ranged from 6 to 18% of Reco and was greatest where permafrost thaw was deepest. Overall, growing season fluxes of autotrophic and old soil heterotrophic respiration increased as permafrost thaw deepened. Areas with greater thaw also had the greatest primary production. Warming in permafrost ecosystems therefore leads to increased plant and old soil respiration that is initially compensated by increased net primary productivity. However, barring large shifts in plant community composition, future increases in old soil respiration will likely outpace productivity, resulting in a positive feedback to climate change.  相似文献   

14.
The [14C]deoxyglucose [Sokoloff et al., J. Neurochem. 28, 897-916 (1977)] and [6-14C]glucose [Hawkins et al., Am. J. Physiol. 248, C170-C176 (1985)] quantitative autoradiographic methods were used to measure regional brain glucose utilization in awake rats. The spatial resolution and qualitative appearance of the autoradiograms were similar. In resting animals, there was no significant difference between the two methods among 18 gray and three white matter structures over a fourfold range in glucose utilization rates (coefficient of correlation = 0.97). In rats given increasing frequencies of photoflash visual stimulation, the two methods gave different results for glucose utilization within visual pathways. The linearity of the metabolic response was studied in the superior colliculus using an on-off checkerboard stimulus between 0 and 33 Hz. The greatest increment in activity occurred between 0 and 4 Hz stimulation with both methods, probably representing recruitment of neuronal elements into activity. Above 4 Hz, there was a progressive increase in labeling with [14C]deoxyglucose up to 1.7 times control at 33 Hz. With [6-14C]-glucose, there was no further increment in change above a 30% increase seen at 4 Hz. Measurement of tissue glucose revealed no drop in the visually stimulated structures compared to control. We interpret these results to indicate that, with increasing rates of physiological activity, the products of deoxyglucose metabolism accumulate progressively, but the products of glucose metabolism are removed from brain in 10 min.  相似文献   

15.
Soil carbon is returned to the atmosphere through the process of soil respiration, which represents one of the largest fluxes in the terrestrial C cycle. The effects of climate change on the components of soil respiration can affect the sink or source capacity of ecosystems for atmospheric carbon, but no current techniques can unambiguously separate soil respiration into its components. Long‐term free air CO2 enrichment (FACE) experiments provide a unique opportunity to study soil C dynamics because the CO2 used for fumigation has a distinct isotopic signature and serves as a continuous label at the ecosystem level. We used the 13C tracer at the Duke Forest FACE site to follow the disappearance of C fixed before fumigation began in 1996 (pretreatment C) from soil CO2 and soil‐respired CO2, as an index of belowground C dynamics during the first 8 years of the experiment. The decay of pretreatment C as detected in the isotopic composition of soil‐respired CO2 and soil CO2 at 15, 30, 70, and 200 cm soil depth was best described by a model having one to three exponential pools within the soil system. The majority of soil‐respired CO2 (71%) originated in soil C pools with a turnover time of about 35 days. About 55%, 50%, and 68% of soil CO2 at 15, 30, and 70 cm, respectively, originated in soil pools with turnover times of less than 1 year. The rest of soil CO2 and soil‐respired CO2 originated in soil pools that turn over at decadal time scales. Our results suggest that a large fraction of the C returned to the atmosphere through soil respiration results from dynamic soil C pools that cannot be easily detected in traditionally defined soil organic matter standing stocks. Fast oxidation of labile C substrates may prevent increases in soil C accumulation in forests exposed to elevated [CO2] and may consequently result in shorter ecosystem C residence times.  相似文献   

16.
In this study, we investigated the impact of elevated atmospheric CO2 (ambient + 350 μmol mol–1) on fine root production and respiration in Scots pine (Pinus sylvestris L.) seedlings. After six months exposure to elevated CO2, root production measured by root in-growth bags, showed significant increases in mean total root length and biomass, which were more than 100% greater compared to the ambient treatment. This increased root length may have lead to a more intensive soil exploration. Chemical analysis of the roots showed that the roots in the elevated treatment accumulated more starch and had a lower C/N-ratio. Specific root respiration rates were significantly higher in the elevated treatment and this was probably attributed to increased nitrogen concentrations in the roots. Rhizospheric respiration and soil CO2 efflux were also enhanced in the elevated treatment. These results clearly indicate that under elevated atmospheric CO2 root production and development in Scots pine seedlings is altered and respiratory carbon losses through the root system are increased.  相似文献   

17.
The effects of drought stress and season on both allocation of photosynthates to stems and leaves and potential for stem rubber synthesis were studied in guayule ( Parthenium argentatum Gray USDA line 11604). Two-year-old plants grown under field conditions in the Negev desert of Israel were subjected to different irrigation regimes, and water status was assessed by measuring the relative water content (RWC). Undetached plant tips were exposed to a 1 h pulse of 14CO2, followed by a 24 h chase. 14C fixed and translocated to different plants parts and notably 14C incorporation into rubber and resin fractions was determined. The potential of detached branch slices to incorporate [14C]-acetate into rubber was also studied. A higher percentage of fixed 14C was translocated from shoot tips in winter (28–30%) than in summer (15–18%). The percentage of [14C]-acctate incorporated into the rubber fraction by stem slices was maximal in winter (20%) and minimal in summer (3–5%) in both cases in the absence of drought stress. In summer the translocation of photosynthates into stems was inversely related to plant RWC, dropping from 18% three days after irrigation to 3% 14 days later, and the potential of stems to synthesise rubber was high under drought conditions and low in well irrigated plants.  相似文献   

18.
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 pi/pa of a plant to changing relative humidity.  相似文献   

19.
Abstract: A continuous dual 13CO2 and 15NH415NO3 labelling experiment was undertaken to determine the effects of ambient (350μmol mol-1) or elevated (700μmol mol-1) atmospheric CO2 concentrations on C and N uptake and allocation within 3-year-old beech ( Fagus sylvatica L.) during leafing. After six weeks of growth, total carbon uptake was increased by 63 % (calculated on total C content) under elevated CO2 but the carbon partitioning was not altered. 56 % of the new carbon was found in the leaves. On a dry weight basis was the content of structural biomass in leaves 10 % lower and the lignin content remained unaffected under elevated as compared to ambient [CO2]. Under ambient [CO2] 37 %, and under elevated [CO2] 51 %, of the lignin C of the leaves derived from new assimilates. For both treatments, internal N pools provided more than 90 % of the nitrogen used for leaf-growth and the partitioning of nitrogen was not altered under elevated [CO2]. The C/N ratio was unaffected by elevated [CO2] at the whole plant level, but the C/N ratio of the new C and N uptake was increased by 32 % under elevated [CO2].  相似文献   

20.
Whole bean plants, ev. Cockfield, grown in pots crowded or well-spaced (50 or 10 plants m2, respectively) were treated with 14CO2 at the pod-fill stage (25 modes) and the radioactivity in each leaf was determined after 30 min. With spaced plants the uptake was greatest in the mid-stem leaves and was proportional to leaf area. In contrast, 70% of the total assimilation took place in the upper six leaves of crowded plants and there was a steady decrease down the stem.
When 14CO2 was fed to single leaves of similar crowded plants the resultant distribution of labelled assimilates varied with the position of the treated leaf. After 6 h, 67% of the 14C fixed by a mid-stem leaf (node 13) was recovered from the beans, whereas 76% of that from an upper leaf (node 23) had accumulated along the stem. Due to the shading of mid-stem leaves at the higher planting densities, seed yield becomes increasingly dependent upon re-distribution of assimilates from stem to beans.  相似文献   

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