排序方式: 共有96条查询结果,搜索用时 46 毫秒
81.
Origin, fate and significance of CO2 in tree stems 总被引:1,自引:1,他引:0
Although some CO2 released by respiring cells in tree stems diffuses directly to the atmosphere, on a daily basis 15–55% can remain within the tree. High concentrations of CO2 build up in stems because of barriers to diffusion in the inner bark and xylem. In contrast with atmospheric [CO2 ] of c. 0.04%, the [CO2 ] in tree stems is often between 3 and 10%, and sometimes exceeds 20%. The [CO2 ] in stems varies diurnally and seasonally. Some respired CO2 remaining in the stem dissolves in xylem sap and is transported toward the leaves. A portion can be fixed by photosynthetic cells in woody tissues, and a portion diffuses out of the stem into the atmosphere remote from the site of origin. It is now evident that measurements of CO2 efflux to the atmosphere, which have been commonly used to estimate the rate of woody tissue respiration, do not adequately account for the internal fluxes of CO2 . New approaches to quantify both internal and external fluxes of CO2 have been developed to estimate the rate of woody tissue respiration. A more complete assessment of internal fluxes of CO2 in stems will improve our understanding of the carbon balance of trees. 相似文献
82.
Blaise?TF?Alako Antoine?Veldhoven Sjozef?van Baal Rob?Jelier Stefan?Verhoeven Ton?Rullmann Jan?Polman Guido?JensterEmail author 《BMC bioinformatics》2005,6(1):51
Background
High throughput microarray analyses result in many differentially expressed genes that are potentially responsible for the biological process of interest. In order to identify biological similarities between genes, publications from MEDLINE were identified in which pairs of gene names and combinations of gene name with specific keywords were co-mentioned. 相似文献83.
Scaling up: the next challenge in environmental microbiology 总被引:1,自引:0,他引:1
Great strides have recently been made in identifying and characterizing the staggering diversity of microorganisms conducting primary and secondary production, nutrient transformation and mineralization processes that underlie ecosystem and regional biogeochemical, trophodynamic and ecological change. We are now faced with the challenge of assigning and coupling function to structure in highly complex and interactive microbial communities mediating such change. Previous and ongoing ecophysiological work has shown that microbial processes controlled by environmental variables and limiting resources are highly specific in terms of what, when, where and why they are active, not to mention how they impact ecosystem dynamics. As such, it is imperative that we assess the activities and roles of key microbial 'players' along the appropriate environmental scales and gradients catalysing ecological change. Here, we discuss conceptual and technical challenges for some key microbially mediated environmental processes, problems and extremes that require synthesizing our growing knowledge of microbial community structure with emerging knowledge of function in aquatic ecosystems. We emphasize the importance of assessing ecological change over a range of relevant time scales that vary from minutes to millennia and spatial scales that range from microscale aggregates to ocean basins. 相似文献
84.
85.
Use of X-ray computed microtomography for non-invasive determination of wood anatomical characteristics 总被引:7,自引:0,他引:7
Steppe K Cnudde V Girard C Lemeur R Cnudde JP Jacobs P 《Journal of structural biology》2004,148(1):11-21
Quantitative analysis of wood anatomical characteristics is usually performed using classical microtomy yielding optical micrographs of stained thin sections. It is time-consuming to obtain high quality cross-sections from microtomy, and sections can be damaged. This approach, therefore, is often impractical for those who need quick acquisition of quantitative data on vessel characteristics in wood. This paper reports results of a novel approach using X-ray computed microtomography (microCT) for non-invasive determination of wood anatomy. As a case study, stem wood samples of a 2-year-old beech (Fagus sylvatica L.) and a 3-year-old oak (Quercus robur L.) tree were investigated with this technique, beech being a diffuse-porous and oak a ring-porous tree species. MicroCT allowed non-invasive mapping of 2-D transverse cross-sections of both wood samples with micrometer resolution. Self-developed software 'microCTanalysis' was used for image processing of the 2-D cross-sections in order to automatically determine the inner vessel diameters, the transverse cross-sectional surface area of the vessels, the vessel density and the porosity with computer assistance. Performance of this new software was compared with manual analysis of the same micrographs. The automatically obtained results showed no significant statistical differences compared to the manual measurements. Visual inspection of the microCT slices revealed very good correspondence with the optical micrographs. Statistical analysis confirmed this observation in a more quantitative way, and it was, therefore, argued that anatomical analysis of optical micrographs can be readily substituted by automated use of microCT, and this without loss of accuracy. Furthermore, as an additional application of microCT, the 3-D renderings of the internal microstructure of the xylem vessels for both the beech and the oak sample could be reconstructed, clearly showing the complex nature of vessel networks. It can be concluded that the use of microCT in wood science offers an interesting potential for all those who need quantitative data of wood anatomical characteristics in either the 2-D or the 3-D space. 相似文献
86.
Bacterially mediated precipitation in marine stromatolites 总被引:4,自引:0,他引:4
Stromatolites are laminated, lithified (CaCO3 ) sedimentary deposits formed by precipitation and/or sediment accretion by cyanobacterial–bacterial mat communities. Stromatolites have been associated with these communities as far back as the Precambrian era some 2+ billion years ago. The means by which microbial communities mediate the precipitation processes have remained unclear, and are the subject of considerable debate and speculation. Two alternative explanations for microbially mediated precipitation include: (i) cyanobacterial photosynthesis increases pH in a system supersaturated in respect of CaCO3 , resulting in CaCO3 precipitation and then laminated lithification, and (ii) decomposition of cyanobacterial extracellular organic matter (e.g. sheaths, mucilage and organic acids) by microheterotrophs leads to release of organic-bound Ca2+ ions and CaCO3 precipitation. We evaluated these explanations by examining metabolically active, lithifying stromatolitic mat communities from Highborne Cay, Bahamas, using microautoradiography. Microautoradiographic detection of 14 CO2 fixation and 3 H organic matter ( d -glucose and an amino acid mixture) utilization by photosynthetically active cyanobacteria and microheterotrophs, combined with community-level uptake experiments, indicate that bacteria, rather than cyanobacteria are the dominant sites of CaCO3 deposition. In the oligotrophic waters in which stromatolites exist, microheterotrophs are reliant on the photosynthetic community as a main source of organic matter. Therefore, autotrophic production indirectly controls microbially mediated precipitation and stromatolite formation in these shallow marine environments. 相似文献
87.
Skin-friction results are presented for fouling-release (FR) hull coatings in the unexposed, clean condition and after dynamic exposure to diatomaceous biofilms for 3 and 6 months. The experiments were conducted in a fully developed turbulent channel flow facility spanning a wide Reynolds number range. The results show that the clean FR coatings tested were hydraulically smooth over much of the Reynolds number range. Biofilms, however, resulted in an increase in skin-friction of up to 70%. The roughness functions for the biofilm-covered surfaces did not display universal behavior, but instead varied with the percentage coverage by the biofilm. The effect of the biofilm was observed to scale with its mean thickness and the square root of the percentage coverage. A new effective roughness length scale (keff) for biofilms based on these parameters is proposed. Boundary layer similarity-law scaling is used to predict the impact of these biofilms on the required shaft power for a mid-sized naval surface combatant at cruising speed. The increase in power is estimated to be between 1.5% and 10.1% depending on the biofilm thickness and percentage coverage. 相似文献
88.
In studies on internal CO2 transport, average xylem sap pH (pHx) is one of the factors used for calculation of the concentration of dissolved inorganic carbon in the xylem sap ([]). Lack of detailed pHx measurements at high temporal resolution could be a potential source of error when evaluating [] dynamics. In this experiment, we performed continuous measurements of CO2 concentration ([CO2]) and stem temperature (Tstem), complemented with pHx measurements at 30‐min intervals during the day at various stages of the growing season (Day of the Year (DOY): 86 (late winter), 128 (mid‐spring) and 155 (early summer)) on a plum tree (Prunus domestica L. cv. Reine Claude d'Oullins). We used the recorded pHx to calculate [] based on Tstem and the corresponding measured [CO2]. No statistically significant difference was found between mean [] calculated with instantaneous pHx and daily average pHx. However, using an average pHx value from a different part of the growing season than the measurements of [CO2] and Tstem to estimate [] led to a statistically significant error. The error varied between 3.25 ± 0.01% under‐estimation and 3.97 ± 0.01% over‐estimation, relative to the true [] data. Measured pHx did not show a significant daily variation, unlike [CO2], which increased during the day and declined at night. As the growing season progressed, daily average [CO2] (3.4%, 5.3%, 7.4%) increased and average pHx (5.43, 5.29, 5.20) decreased. Increase in [CO2] will increase its solubility in xylem sap according to Henry's law, and the dissociation of [] will negatively affect pHx. Our results are the first quantifying the error in [] due to the interaction between [CO2] and pHx on a seasonal time scale. We found significant changes in pHx across the growing season, but overall the effect on the calculation of [] remained within an error range of 4%. However, it is possible that the error could be more substantial for other tree species, particularly if pHx is in the more sensitive range (pHx > 6.5). 相似文献
89.
Changes in stem water content influence sap flux density measurements with thermal dissipation probes 总被引:1,自引:0,他引:1
Lidewei L. Vergeynst Maurits W. Vandegehuchte Mary Anne McGuire Robert O. Teskey Kathy Steppe 《Trees - Structure and Function》2014,28(3):949-955
Key message
Stem WC may decline during the day. Zero-flow dT m increases when WC decreases. Use of nighttime dT m in the calculation of sap flux density during the day might introduce errors.Abstract
There is increasing evidence of diel variation in water content of stems of living trees as a result of changes in internal water reserves. The interplay between dynamic water storage and sap flow is of current interest, but the accuracy of measurement of both variables has come into question. Fluctuations in stem water content may induce inaccuracy in thermal-based measurements of sap flux density because wood thermal properties are dependent on water content. The most widely used thermal method for measuring sap flux density is the thermal dissipation probe (TDP) with continuous heating, which measures the influence of moving sap on the temperature difference between a heated needle and a reference needle vertically separated in the flow stream. The objective of our study was to investigate how diel fluctuations in water content could influence TDP measurements of sap flux density. We analysed the influence of water content on the zero-flow maximum temperature difference, dT m, which is used as the reference for calculating sap flux density, and present results of a dehydration experiment on cut branch segments of American sycamore (Platanus occidentalis L.). We demonstrate both theoretically and experimentally that dT m increases when stem water content declines. Because dT m is measured at night when water content is high, this phenomenon could result in underestimations of sap flux density during the day when water content is lower. We conclude that diel dynamics in water content should be considered when TDP is used to measure sap flow. 相似文献90.