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1.
Plant and mycorrhizal regulation of rhizodeposition   总被引:31,自引:7,他引:24  
2.
Despite the fundamental importance of rhizosphere C-flow in managed and natural systems, reliable measurement/resolution of C-flow and assessment of its consequences have largely remained elusive to soil biologists. Techniques involving both radioactive (14C) and stable (13C) isotopes of carbon have made some progress in terms of studying rhizosphere C-flow. Pulse-chase techniques have been used effectively to study dynamics of C-transfer to the rhizosphere and rhizosphere microbial biomass. The information obtained through pulse-chase is strongly dependent on the chase period following the labelling event. Continuous labelling is primarily used to determine plant inputs to soil over an extended time period and includes all kinds of C input – from root turnover, root respiration, root exudation, production of mucilage, etc. One of the main constraints to both approaches is that distinguishing root from microbial respiration is difficult, if not impossible. 13C techniques have gone some way towards resolving this difficulty, although 13C signatures in the plant–soil system are not easy to interpret and detailed resolution of carbon flow through different components of the rhizosphere biomass is unlikely to be achieved in such an inherently `noisy' system. Recent developments in molecular biology now provide a new opportunity to resolve rhizosphere C-flow and its implications. Reporter gene systems where, for example, rhizobacteria are marked with lux and unstable gfp reporters, overcome the difficulty of distinguishing root and microbial C fluxes and complement the isotopic and more traditional approaches. Reporter systems have now begun to resolve the competitive C sink strengths of different components of the rhizosphere microbial community and assess how a rhizobacterial inoculum may change C-flow in applications such as disease control and rhizoremediation of contaminated land. Fusion of reporter genes to nutrient (N and P) starvation genes in rhizobacteria has also enabled in situ characterisation of nutrient depletion around the root and assessment of the impact of changes in C-flow (such as those induced by climate change) on nutrient depletion dynamics. The availability of an integrated approach involving isotopic, molecular biological and other techniques now offers an exciting new era where reliable measurement and resolution of rhizosphere C-flow (and its consequences) can contribute to our understanding of ecosystem function and to management of crop-microbe interactions.  相似文献
3.
CO2浓度升高对植物-土壤系统地下部分碳流通的影响   总被引:12,自引:1,他引:11  
目前 ,由于化石燃料的燃烧和土地利用的改变 ,每年释放到大气中的碳大约有 7Gt[2 4 ] ,其中 ,有 3Gt留在大气中 ,2Gt被固定在深海中 ,另 2Gt被植物固定在生态系统中[19,4 8] ,事实上 ,陆地生态系统中的碳大部分都贮存在土壤中[4 4 ] ,所以植物与土壤之间的碳流通对全球碳循环极为重要。大气CO2 浓度升高有可能通过生态系统中的各种生理过程来改变植物 -土壤系统中碳通量的变化 ,使输入土壤的碳量增加 ,另一方面 ,地下部分碳通量的增加使土体成为一个潜在的碳汇 ,有可能缓解大气中CO2 浓度的升高。但有关高CO2 对地下部分植物…  相似文献
4.
Root border cells take up and release glucose-C   总被引:6,自引:0,他引:6  
BACKGROUND AND AIMS: Border cells are released from the root tips of many plant species, and can remain viable in the rhizosphere for 1 week. Whether border cells are capable of controlled glucose exchange with their environment was investigated. METHODS: Border cells were removed from Zea mays L. root tips, and immersed in (14)C-labelled D-glucose. In one experiment, the hexose transport inhibitor, phlorizin, was used to investigate active glucose uptake from a range of glucose concentrations. In another experiment, glucose efflux from border cells was monitored over time. KEY RESULTS: Glucose uptake by the border cells increased with increasing glucose concentration from 0.2 to 20 mm. At 0.2 mm glucose, uptake was mainly active, as evidenced by the approx. 60 % inhibition with phlorizin. At 2 and 20 mm glucose, however, uptake was mainly via diffusion, as phlorizin inhibition was negligible. Glucose efflux increased with time for live border cells in both 2 and 20 mm glucose. There was no clear efflux/time pattern for heat-killed border cells. CONCLUSIONS: Border cells actively take up glucose, and also release it. Under our experimental conditions, glucose uptake and efflux were of similar order of magnitude. In the rhizosphere net glucose exchange will almost certainly depend on local soil conditions.  相似文献
5.
Carbon flow in the littoral food web of an oligotrophic lake   总被引:6,自引:3,他引:3  
Benthic food web dynamics and carbon flow were examined in the littoral zone of Lake Coleridge, a large deep oligotrophic lake, using radioactive and stable isotope techniques in conjunction with analyses of stomach contents of the fauna. We specifically address two hypotheses: (1) that macrophytes only contribute to the carbon flow to higher trophic levels when they have decayed; and (2) that epiphytic algae is the major source of carbon for macroinvertebrates, and thus fish, with only minor contributions from phytoplankton or terrestrial sources. Epiphytic diatoms were a major component of the stomach contents of the gastropod snail Potamopyrgus antipodarum, and of chironomids. Animal remains were also common in the diet of some chironomids, while amorphous organic matter predominated in the stomachs of oligochaetes. A variety of epiphytic algal taxa was found in trichopteran larvae. Feeding rate of P. antipodarum measured with radioactive tracers increased by 10× on decayed macrophytes (Elodea) compared with live material, while feeding rates on characean algae increased by a factor of 3 when decayed material was presented. However, assimilation rates were less than 20% on decayed material compared with 48–52% on live material. Potential carbon sources were easily distinguished based on their 13C values, although isotopic ratios showed significant variation among sites. Epiphytic algae showed less variation among sites than macrophytes and were depleted by 4–5 compared with macrophytes. Detrital material, organic matter in the sediments and plankton were significantly depleted in 13C relative to macrophytes and slightly depleted relative to epiphytic algae. Most macroinvertebrate taxa showed a similar pattern among sites to macrophytes and epiphytic algae. P. antipodarum and chironomids were slightly enriched compared with epiphytic algae. Ratios for the common bully (Gobiomorphus cotidianus) were generally consistent with a diet dominated by chironomids, while there was some evidence for terrestrial inputs for koaro (Galaxias brevipinnis) and juvenile brown trout. Epiphytic algae appear to underpin much of the production in the littoral zone of this oligotrophic lake, with trichopteran and chironomid larvae mediating carbon flows from algae to fish. Macrophytes do not make a major contribution directly to carbon flow to higher trophic levels even when decayed. The lack of a direct link between macrophytes and higher trophic levels is due to the faunal composition, including a lack of large herbivores.  相似文献
6.
The rhizosphere is a major sink for photo-assimilated carbon and quantifying inputs into this sink is one of the main goals of rhizosphere biology as organic carbon lost from plant roots supports a higher microbial population in the rhizosphere compared to bulk soil. Two fundamentally different14CO2 labelling strategies have been developed to estimate carbon fluxes through the rhizosphere — continuous feeding of shoots with labelled carbon dioxide and pulse-chase experiments. The biological interpretation that can be placed on the results of labelling experiments is greatly biased by the technique used. It is the purpose of this paper to assess the advantages, disadvantages and the biological interpretation of both continuous and pulse labelling and to consider how to partition carbon fluxes within the rhizosphere.  相似文献
7.
Challenges for mycorrhizal research into the new millennium   总被引:4,自引:0,他引:4  
Some future directions for mycorrhizal research are discussed. Data from The IPCC Third Assessment Reports 2001 on elevated carbon dioxide the concentration and consequences regarding global climate change, form the basis for arguments that a better understanding of the mycorrhizal activities with regard to controlling carbon flow and plant nutrient flow is essential. Mycorrhiza can be regarded as energy and nutrient switches in the plant roots and it is suggested that more studies on mycorrhizal carbon relations are necessary. The impact of the genomic research era on mycorrhizal research is suggested to be of major importance during the years to come, and it is claimed that stronger interactions between different disciplines will be essential.  相似文献
8.
Measurements of photosynthesis and community respiration in Lake Apopka, Florida, U.S.A. indicate that this lake may be heterotrophic, and that the source of extra organic carbon is internal rather than external to the lake. This large and shallow lake (area 124 km2, mean depth 1.7 m) was dominated by macrophytes until hurricane-associated winds disrupted the plants in 1947, and the lake switched to a turbid, algal state. A layer of flocculent, organic sediments covers the lakebed to an average depth of 45 cm and winds regularly resuspend the upper portion into the water column. We used the diel oxygen curve method to estimate production and respiration and also reanalyzed the results of five past studies of production in the lake. The production measurements did not support the hypothesis that the flocculent layer represented excess algal production since 1947. Community respiration exceeded gross production on 60 out of 76 days sampled with statistically significant negative net production found in two of the three studies using the light and dark bottle oxygen method. External supplies of organic carbon are relatively small and are balanced by losses through the outlet. If the lake is heterotrophic, the excess respiration is most likely supported by the remains of macrophytes deposited in the sediments prior to the switch to an algal state. Similar sediment oxidation probably occurs in other shallow lakes that switch from the macrophyte to the algal state.  相似文献
9.
Abstract Threonine and lysine are two of the economically most important essential amino acids. They are produced industrially by species of the genera Corynebacterium and Brevibacterium . The branched biosynthetic pathway of these amino acids in corynebacteria is unusual in gene organization and in the control of key enzymatic steps with respect to other microorganisms. This article reviews the molecular control mechanisms of the biosynthetic pathways leading to threonine and lysine in corynebacteria, and their implications in the production of these amino acids. Carbon flux can be redirected at branch points by gene disruption of the competing pathways for lysine or threonine. Removal of bottlenecks has been achieved by amplification of genes which encode feedback resistant aspartokinase and homoserine dehydrogenase (obtained by in vitro directed mutagenesis).  相似文献
10.
Microbial food web in a large shallow lake (Lake Balaton, Hungary)   总被引:2,自引:2,他引:0  
Seasonal variations of phyto-, bacterio- and colourless flagellate plankton were followed across a year in the large shallow Lake Balaton (Hungary). Yearly average chlorophyll-a concentration was 11 µg 1–1, while the corresponding values of bacterioplankton and heterotrophic nanoflagellate (HNF) plankton biomass (fresh weight) were 0.24 mg 1–1 and 0.35 mg 1–1, respectively. About half of planktonic primary production was channelled through bacterioplankton on the yearly basis. However, there was no significant correlation between phytoplankton biomass and bacterial abundance. Bacterial specific growth rates were in the range of 0.009 and 0.09 h–1, and ended to follow the seasonal changes in water temperature. In some periods of the year, predator-prey relationships between the HNF and bacterial abundance were obvious. The estimated HNF grazing on bacteria varied between 3% and 227% of the daily bacterial production. On an annual basis, 87% of bacterial cell production was grazed by HNF plankton.  相似文献
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