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1.
Karl J. Niklas 《American journal of botany》1999,86(5):688-696
The objective of this study was to determine whether the factor of safety for mechanical stability varied among stems differing in size and age within the superstructure of a large dicot tree. Two factors of safety were selected for study: the quotient of the critical buckling height and the actual length of stems, Hcrit/L, and the quotient of the modulus of rupture (the force per unit area required to break a stem) and the working stress (the force per unit area resulting from the biomass measured distal to a stem), MR/σw. These two dimensionless safety factors were determined for a total of 420 shoot segments comprising much of the aboveground biomass of a Robinia pseudoacacia (Fabaceae) tree measuring 18.7 m in height and 1347 kg in mass, and 0.46 m in diameter (40 yr old) at 1.2 m from the ground. An S-shaped trend was observed when each of the two factors of safety was plotted as a function of stem age. Each factor decreased from a local maximum for the most distal (peripheral) stems in the canopy to a local minimum value for stems ∼10 yr old; each factor increased again to another local maximum for stems 11–18 yr old, and then decreased steadily toward the base of the trunk. This trend was the result of the allometric relationships among stem diameter, length, biomass, and material properties (stiffness and strength) with respect to stem age. Although they were disproportionately more slender than their older counterparts, peripheral stems were sufficiently stiff and strong to sustain the stresses resulting from their weight and that of foliage without deflecting under these loads, yet they were sufficiently flexible to easily bend and thereby presumably provide a mechanism to reduce the drag forces acting on the entire tree. In contrast, the internally imposed mechanical forces acting on progressively older stems increased at a greater rate than the observed rate of increase in stem stiffness, strength, or diameter. The probability of mechanical failure, which must be considered from a demographic perspective (i.e., an age-dependent phenomenon), thus increased from older branches to the base of the trunk. Reports of similar allometric trends based on interspecific comparisons among diverse dicot species comply with the allometry observed for the R. pseudoacacia tree and suggest that the S-shaped trend for the factor of safety holds for stems differing in age drawn from individual trees and for the trunks of conspecifics differing in age drawn from a dense population. 相似文献
2.
SANDRA J. BUCCI FABIAN G. SCHOLZ MARIA LAURA PESCHIUTTA NADIA S. ARIAS FREDERICK C. MEINZER GUILLERMO GOLDSTEIN 《Plant, cell & environment》2013,36(12):2163-2174
Hydraulic architecture was studied in shrub species differing in rooting depth in a cold desert in Southern Argentina. All species exhibited strong hydraulic segmentation between leaves, stems and roots with leaves being the most vulnerable part of the hydraulic pathway. Two types of safety margins describing the degree of conservation of the hydraulic integrity were used: the difference between minimum stem or leaf water potential (Ψ) and the Ψ at which stem or leaf hydraulic function was reduced by 50% (Ψ – Ψ50), and the difference between leaf and stem Ψ50. Leaf Ψ50 – stem Ψ50 increased with decreasing rooting depth. Large diurnal decreases in root‐specific hydraulic conductivity suggested high root vulnerability to embolism across all species. Although stem Ψ50 became more negative with decreasing species‐specific Ψsoil and minimum stem Ψ, leaf Ψ50 was independent of Ψ and minimum leaf Ψ. Species with embolism‐resistant stems also had higher maximum stem hydraulic conductivity. Safety margins for stems were >2.1 MPa, whereas those for leaves were negative or only slightly positive. Leaves acted as safety valves to protect the integrity of the upstream hydraulic pathway, whereas embolism in lateral roots may help to decouple portions of the plant from the impact of drier soil layers. 相似文献
3.
Niklas KJ 《Journal of experimental botany》2000,51(345):797-806
The drag forces, bending moments and stresses acting on stems differing in size and location within the mechanical infrastructure of a large wild cherry (Prunus serotina Ehrh.) tree are estimated and used to calculate the factor of safety against wind-induced mechanical failure based on the mean breaking stress of intact stems and samples of wood drawn from this tree. The drag forces acting on stems are calculated based on stem projected areas and field measurements of wind speed taken within the canopy and along the length of the trunk. The bending moments and stresses resulting from these forces are shown to increase basipetally in a nearly log-log linear fashion toward the base of the tree. The factor of safety, however, varies in a sinusoidal manner such that the most distal stems have the highest factors of safety, whereas stems of intermediate location and portions of the trunk near ground level have equivalent and much lower factors of safety. This pattern of variation is interpreted to indicate that, as a course of normal growth and development, trees similar to the one examined in this study maintain a cadre of stems prone to wind-induced mechanical damage that can reduce the probability of catastrophic tree failure by reducing the drag forces acting on older portions of the tree. Comparisons among real and hypothetical stems with different taper experiencing different vertical wind speed profiles show that geometrically self-similar stems have larger factors of safety than stems tapering according to elastic or stress self-similarity, and that safety factors are less significantly influenced by the 'geometry' of the wind-profile. 相似文献
4.
Seoung‐Bum Son Seul Cham Kim Chan Soon Kang Thomas A. Yersak Yoon‐Chang Kim Chun‐Gyoo Lee Sung‐Hwan Moon Jong Soo Cho Jeong‐Tak Moon Kyu Hwan Oh Se‐Hee Lee 《Liver Transplantation》2012,2(10):1226-1231
This paper reports a Si‐Ti‐Ni ternary alloy developed for commercial application as an anode material for lithium ion batteries. Our alloy exhibits a stable capacity above 900 mAh g?1 after 50 cycles and a high coulombic efficiency of up to 99.7% during cycling. To enable a highly reversible nano‐Si anode, melt spinning is employed to embed nano‐Si particles in a Ti4Ni4Si7 matrix. The Ti4Ni4Si7 matrix fulfills two important purposes. First, it reduces the maximum stress evolved in the nano‐Si particles by applying a compressive stress to mechanically confine Si expansion during lithiation. And second, the Ti4Ni4Si7 matrix is a good mixed conductor that isolates nano‐Si from the liquid electrolyte, thus preventing parasitic reactions responsible for the formation of a solid electrolyte interphase. Given that a coulombic efficiency above 99.5% is rarely reported for Si based anode materials, this alloy's performance suggests a promising new approach to engineering Si anode materials. 相似文献
5.
The electrochemical polarization effect on early adsorption of Flavobacterium breve and Pseudomonas fluorescens P17 to platinum, titanium, stainless steel, copper, aluminum alloy and mild steel was studied. A well‐defined peak characterized the bacterial adsorption dependence on externally applied potential. Maximal adsorption occurred in the potential range of ‐0.5 to 0.5 V (SCE) for all tested metals. A shift of applied potential towards both a positive and a negative direction from the maximal adsorption potential (Emax,ad) was accompanied by a gradual decrease in bacterial adsorption. The extent of bacterial adsorption strongly depended on the nature of the metallic substratum and decreased accordingly as follows: platinum > titanium > stainless steel > aluminum alloy > carbon steel > copper. Adsorption on all tested metals was approximately two orders of magnitude higher with the relatively more hydrophobic F. breve compared to the less hydrophobic P. fluorescens P17. The effect of electrochemical polarization on the initial stages of bacterial adsorption onto metallic substrata is further discussed. 相似文献
6.
Qiang Ma Xian‐Xiang Zeng Junpei Yue Ya‐Xia Yin Tong‐Tong Zuo Jia‐Yan Liang Qi Deng Xiong‐Wei Wu Yu‐Guo Guo 《Liver Transplantation》2019,9(13)
Herein, a composite polymer electrolyte with a viscoelastic and nonflammable interface is designed to handle the contact issue and preclude Li dendrite formation. The composite polymer electrolyte (cellulose acetate/polyethylene glycol/Li1.4Al0.4Ti1.6P3O12) exhibits a wide electrochemical window of 5 V (vs Li+/Li), a high Li+ transference number of 0.61, and an excellent ionic conductivity of above 10?4 S cm?1 at 60 °C. In particular, the intimate contact, low interfacial impedance, and fast ion‐transport process between the electrodes and solid electrolytes can be simultaneously achieved by the viscoelastic and nonflammable layer. Benefiting from this novel design, solid lithium metal batteries with either LiFePO4 or LiCoO2 as cathode exhibit superior cyclability and rate capability, such as a discharge capacity of 157 mA h g?1 after 100 cycles at C/2 and 97 mA h g?1 at 5C for LiFePO4 cathode. Moreover, the smooth and uniform Li surface after long‐term cycling confirms the successful suppression of dendrite formation. The viscoelastic and nonflammable interface modification of solid electrolytes provides a promising and general strategy to handle the interfacial issues and improves the operative safety of solid lithium metal batteries. 相似文献
7.
Elena Serena Elisa Figallo Nina Tandon Christopher Cannizzaro Nicola Elvassore Gordana Vunjak-Novakovic 《Experimental cell research》2009,315(20):3611-3619
Exogenous electric fields have been implied in cardiac differentiation of mouse embryonic stem cells and the generation of reactive oxygen species (ROS). In this work, we explored the effects of electrical field stimulation on ROS generation and cardiogenesis in embryoid bodies (EBs) derived from human embryonic stem cells (hESC, line H13), using a custom-built electrical stimulation bioreactor. Electrical properties of the bioreactor system were characterized by electrochemical impedance spectroscopy (EIS) and analysis of electrical currents. The effects of the electrode material (stainless steel, titanium-nitride-coated titanium, titanium), length of stimulus (1 and 90 s) and age of EBs at the onset of electrical stimulation (4 and 8 days) were investigated with respect to ROS generation. The amplitude of the applied electrical field was 1 V/mm. The highest rate of ROS generation was observed for stainless steel electrodes, for signal duration of 90 s and for 4-day-old EBs. Notably, comparable ROS generation was achieved by incubation of EBs with 1 nM H2O2. Cardiac differentiation in these EBs was evidenced by spontaneous contractions, expression of troponin T and its sarcomeric organization. These results imply that electrical stimulation plays a role in cardiac differentiation of hESCs, through mechanisms associated with the intracellular generation of ROS. 相似文献
8.
Comparison of lodging safety factor of untreated and succinic Acid 2,2-dimethylhydrazide-treated shoots of mulberry tree 总被引:1,自引:0,他引:1
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This study examined the lodging resistance of mulberry tree (Morus bombycis Koidz. cv Kenmochi) shoots treated or not treated with succinic acid 2,2-dimethylhydrazide (SADH). The lodging safety factor, an indicator of lodging resistance, was defined as the ratio of critical lodging load to the leaf fresh weight observed, provided that the distribution of the critical lodging load along the stem was similar to that of the leaf fresh weight observed. The critical lodging load was experimentally estimated by loading weights onto the stems. In the untreated trees, the lodging safety factor was maintained at about 3.2. In the SADH-treated trees, the stem elongation was inhibited to about 80% of that in the untreated trees, and the percentage of shoot dry matter partitioned into the leaves was always larger than that of the untreated trees. This dwarfing of the stem caused by SADH increased the critical lodging load supported by the unit stem dry weight, while this large investment of materials in leaves increased the leaf fresh weight supported by the unit stem dry weight. Since the increments canceled each other, the lodging safety factor of the SADH-treated shoots was similar to that of the untreated ones. These results suggest that the shoot formation of the mulberry tree is controlled to maintain the lodging safety factor at a constant level. 相似文献
9.
Hironobu Koseki Akihiko Yonekura Takayuki Shida Itaru Yoda Hidehiko Horiuchi Yoshitomo Morinaga Katsunori Yanagihara Hideyuki Sakoda Makoto Osaki Masato Tomita 《PloS one》2014,9(10)
Biofilms forming on the surface of biomaterials can cause intractable implant-related infections. Bacterial adherence and early biofilm formation are influenced by the type of biomaterial used and the physical characteristics of implant surface. In this in vitro research, we evaluated the ability of Staphylococcus epidermidis, the main pathogen in implant-related infections, to form biofilms on the surface of the solid orthopaedic biomaterials, oxidized zirconium-niobium alloy, cobalt-chromium-molybdenum alloy (Co-Cr-Mo), titanium alloy (Ti-6Al-4V), commercially pure titanium (cp-Ti) and stainless steel. A bacterial suspension of Staphylococcus epidermidis strain RP62A (ATCC35984) was added to the surface of specimens and incubated. The stained biofilms were imaged with a digital optical microscope and the biofilm coverage rate (BCR) was calculated. The total amount of biofilm was determined with the crystal violet assay and the number of viable cells in the biofilm was counted using the plate count method. The BCR of all the biomaterials rose in proportion to culture duration. After culturing for 2–4 hours, the BCR was similar for all materials. However, after culturing for 6 hours, the BCR for Co-Cr-Mo alloy was significantly lower than for Ti-6Al-4V, cp-Ti and stainless steel (P<0.05). The absorbance value determined in the crystal violet assay and the number of viable cells on Co-Cr-Mo were not significantly lower than for the other materials (P>0.05). These results suggest that surface properties, such as hydrophobicity or the low surface free energy of Co-Cr-Mo, may have some influence in inhibiting or delaying the two-dimensional expansion of biofilm on surfaces with a similar degree of smoothness. 相似文献
10.
Inhibiting sulfate-reducing bacteria in biofilms on steel with antimicrobial peptides generated in situ 总被引:7,自引:0,他引:7
A. Jayaraman P. J. Hallock R. M. Carson C.-C. Lee F. B. Mansfeld T. K. Wood 《Applied microbiology and biotechnology》1999,52(2):267-275
In batch and continuous fermentations, the reduction in corrosion of SAE 1018 mild steel and 304 stainless steel caused by
inhibition of the reference sulfate-reducing bacterium (SRB) Desulfovibrio vulgaris by a protective, antimicrobial-producing Bacillus brevis biofilm was investigated. The presence of D. vulgaris produced a thick black precipitate on mild steel and a higher corrosion rate in batch cultures than that seen in a mono-culture
of non-antimicrobial-producing Pseudomonas fragi K upon the addition of SRB to the aerobic P. fragi K biofilm. In continuous reactors, the polarization resistance R
p decreased for stainless steel and increased for mild steel upon the addition of SRB to a P. fragi K biofilm. Addition of either 200 μg/ml ampicillin, chloramphenicol, or ammonium molybdate to batch and continuous reactors
after SRB had colonized the metal was ineffective in killing SRB, as inferred from the lack of change in both R
p and the impedance spectra. However, when ampicillin was added prior to SRB colonization, the growth of SRB was completely
inhibited on stainless steel in continuous reactors. Prior addition of ampicillin was only able to delay the growth of SRB
on mild steel in continuous reactors. External addition of the purified peptide antimicrobial agent gramicidin S prior to
the addition of SRB also inhibited the growth of SRB on stainless steel in continuous reactors, and the SRB were also inhibited
on stainless steel in both batch and continuous reactors by producing gramicidin S in situ in a protective biofilm when the
gramicidin-S-overproducing strain Bacillus brevis 18 was used.
Received: 29 October 1998 / Received revision: 18 February 1999 / Accepted: 26 February 1999 相似文献
11.
A A Arrage N Vasishtha D Sundberg G Bausch H L Vincent D C White 《Journal of industrial microbiology & biotechnology》1995,15(4):277-282
A laminar flow biofilm-monitoring system was used to determine the efficacies of three antifouling (AF) coatings and five fouling-release (FR) coatings againstVibrio harveyi attachment. On-line measurements of tryptophan fluorescence and bioluminescence from each coating, normalized to an upstream stainless steel coupon, were used to determine the effects of AF and FR surfaces on biofilm formation. The AF coatings consisted of 5, 10, and 35 wt% Sea Nine 211 (C9211) incorporated into a vinyl copolymer. Both the 10 and 35 wt% coatings significantly inhibited biofilm biomass development measured by tryptophan fluorescence compared to the stainless steel control.V. harveyi bioluminescence was significantly greater than tryptophan fluorescence in cells attached to these coatings, suggesting that bioluminescence expression may be a marker for cellular stress or toxicity in biofilms. Five different polydimethylsiloxane (PDMS) FR coatings did not inhibit biofilm formation under low flow conditions. However, four PDMS coatings demonstrated decreased biomass levels compared to stainless steel after exposure to a shear stress of 330 dynes cm–2. There was no toxic additive in these coatings; bioluminescence and tryptophan fluorescence were proportional. 相似文献
12.
Many microorganisms are reported to influence the corrosive behaviour of mild steel and stainless steel in different habitats. In this study, 40 bacterial strains were isolated from corroded mild steel and stainless steel coupons in the nitrate supplemented environments. The corrosion abilities of the isolates against the mild steel and stainless steel coupons were tested with or without additional nitrate sources. The presence of bacterial isolates alone stimulated the corrosion of mild steel coupons. Most of the bio-corrosion processes of mild steel coupons were mitigated by adding nitrate supplement with bacterial isolates. The effects of bacterial isolates and additional nitrogen sources on corrosion of stainless steels were varied. Not all bacterial isolates stimulated the corrosion on stainless steel during the study period. Unlike the effects on mild steel coupons, additional NaNO3 might stimulate, retard the corrosion rate by the bacterial isolates or have limited effects. Similar results were obtained when NH4NO3 was used. Phylogenetic analysis demonstrated that all isolates were closely related. The majority of the bacterial isolates from corroded metal coupons were identified as Bacillus species. Others were identified as Pseudomonas sp., Marinobacter sp., and Halomonas species. The results prove that the isolated aerobic microorganisms do play a role in the corrosion process of stainless and mild steel. Adding additional nitrate sources might be a tool to mitigate corrosion of mild steel which was stimulated by the presence of bacteria. However, to prevent the corrosion of stainless steels, it might need a trial and errors approach in each case. 相似文献
13.
Grapevine petioles are more sensitive to drought induced embolism than stems: evidence from in vivo MRI and microcomputed tomography observations of hydraulic vulnerability segmentation 总被引:1,自引:0,他引:1
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Uri Hochberg Caetano Albuquerque Shimon Rachmilevitch Herve Cochard Rakefet David‐Schwartz Craig R. Brodersen Andrew McElrone Carel W. Windt 《Plant, cell & environment》2016,39(9):1886-1894
The ‘hydraulic vulnerability segmentation’ hypothesis predicts that expendable distal organs are more susceptible to water stress‐induced embolism than the main stem of the plant. In the current work, we present the first in vivo visualization of this phenomenon. In two separate experiments, using magnetic resonance imaging or synchrotron‐based microcomputed tomography, grapevines (Vitis vinifera) were dehydrated while simultaneously scanning the main stems and petioles for the occurrence of emboli at different xylem pressures (Ψx). Magnetic resonance imaging revealed that 50% of the conductive xylem area of the petioles was embolized at a Ψx of ?1.54 MPa, whereas the stems did not reach similar losses until ?1.9 MPa. Microcomputed tomography confirmed these findings, showing that approximately half the vessels in the petioles were embolized at a Ψx of ?1.6 MPa, whereas only few were embolized in the stems. Petioles were shown to be more resistant to water stress‐induced embolism than previously measured with invasive hydraulic methods. The results provide the first direct evidence for the hydraulic vulnerability segmentation hypothesis and highlight its importance in grapevine responses to severe water stress. Additionally, these data suggest that air entry through the petiole into the stem is unlikely in grapevines during drought. 相似文献
14.
E. T. NILSEN 《Plant, cell & environment》1992,15(4):455-461
Stem and leaf photosynthetic responses to environmental parameters were studied in Spartium junceum L., a legume with chlorophyllous stems. Stem net photosynthesis (Pn) was consistently lower than leaf Pn. The low stem Pn was due to lower quantum yield, lower mesophyll conductance and lower CO2-saturated Pn than that of leaf Pn. Stomatal limitations to leaf and stem Pn were similar (25%). Water stress caused a greater reduction in leaf Pn than that of stems. Leaf Pn was also reduced in water-stressed plants following rehydration. The reduced leaf Pn was associated with a reduced photon saturated Pn rate and a reduced CO2 saturated Pn rate. Apparent quantum yield, mesophyll conductance and stomatal limitation of leaves were unaffected by water-stress. Stem Pn following rehydration was not influenced by the water-stress treatment. In general, leaf Pn was more responsive to environmental parameters and more sensitive to water stress than stem Pn. These data support the hypothesis that stem Pn has greater tolerance of water stress, but is limited to low Pn by biochemical means compared to leaves. 相似文献
15.
F. Plhák 《Biologia Plantarum》1992,34(1-2):109-113
Nitrate content in lucerne stems and leaf blades immersed by cut ends in distilled water or in KNO3 solution increased with the increase in KNO3 concentration and with the duration of exposure under irradiance of 100 or 230 W m?2 PAR. The nitrate content increased from basal stem parts to apical stem parts and leaves. Nitrate was transported mainly with transpiration stream. Some flow variations occurred in stems causing time changes in nitrate content in different parts of stems. 相似文献
16.
In this study, the applicability of plasma nitriding treatment in the production of non-magnetic and corrosion resistant layer on 316L stainless steel implant material was investigated. 316L stainless steel substrates were plasma nitrided at temperatures of 350 ℃, 375 ℃, 400 ℃, 425 ℃ and 450 ℃ for 2 h in a gas mixture of 50% N2-50% H2, respectively. It was determined that the treatment temperature is the most important factor on the properties of the corrosion resistant layer of 316L stainless steel. The results show that s-phase formed at the temperatures under 400 ℃, and at the temperatures above 400 ℃, instead of s-phase, CrN and y'-Fe4N phases were observed in the modified layer. The electrical resistivity and surface roughness of the modified layer increase with treatment temperature. Under 400 ℃ the corrosion resistance increased with the temperature, above 400 ℃ it decreased with the increase in treatment temperature. It was analyzed that the electrical resistivity and the soft (ideal) ferro- magnetic properties of 316L stainless steel increased with treatment temperature during nitriding treatment. Also, plasma ni- triding at low temperatures provided magnetic behavior close to the ideal untreated 316L stainless steel. 相似文献
17.
Mechanical Differences Between Free-standing and Supported Wheat Plants, Triticum aestivum L. 总被引:2,自引:0,他引:2
The effect of wind sway on the mechanical characteristics ofthe anchorage roots and the stem was investigated in maturewinter wheat (Triticum aestivumL., cv. Hereward). Wheat plantswere field-grown, either supported by a frame, which preventedwind sway, or unsupported (free-standing) and the morphologyand mechanical properties of the stems and the anchorage, coronal, roots were measured. Wind sway had little influence on either the stem height orear weight of the plants but did affect the mechanical propertiesof the stem. Stems of supported plants were weaker and moreflexible than the stems of free-standing plants. There werealso differences in the anchorage systems between the treatments:supported plants had just under half as many coronal anchorage roots as the free-standing plants. This reducedthe anchorage strength of supported plants by a third. These differences in mechanical structure meant that the free-standingplants were more resistant to stem buckling and more resistantto anchorage failure. However, considering the difference inthe need for mechanical strength in plants from the two regimes,these differences were small. This suggests that wheat has inherentmechanical integrity and, as a monocotyledon with no secondarythickening, it differs little structurally between environments. Triticum aestivumL.; thigmomorphogenesis; anchorage; safety factor; mechanical stimulation 相似文献
18.
Field measurements and a computer model were used to determine how stem shape and arrangement of stems in space affect interception of photosynthetically active radiation (PAR) and CO2 uptake under otherwise optimal conditions for four species of columnar cacti (Carnegiea gigantea, Lophocereus schottii, Pachycereus pringlei, and Stenocereus thurberi). In simulations where the number of widely spaced stems was increased from 1 to 19 but plant volume remained constant, surface area and PAR interception increased, leading to 3-fold increases in whole-plant CO2 uptake. Increasing the distance between stems from 0 cm to infinity decreased self-shading and increased predicted CO2 uptake 4-fold. Stem length, diam, ribbing characteristics, and spine coverage also influenced PAR interception. The model indicated that the observed higher frequency of branches on the south side of the trunk of C. gigantea had only a slight, though positive, effect on CO2 uptake for single-branched plants. Because of its greater surface area (A), a five-stemmed plant of C. gigantea typical for a field site near Tucson, Arizona was predicted to have 52% more CO2 uptake than a single-stemmed plant of the same volume (V). Although large A/V decreases water storage per unit transpiring area, whole-plant CO2 uptake can be increased when A/V is increased by branching for these constant-volume plants. However, the stems must be arranged to avoid excessive self-shading and thus keep the area below PAR compensation small. 相似文献
19.
植物枝干光合(Pg)固定其自身呼吸所释放的CO2,有效减少植物向大气的CO2排放量。以古尔班通古特沙漠优势木本植物白梭梭(Haloxylon persicum)为研究对象,利用LI-COR 6400便携式光合仪与特制光合叶室(P-Chamber)相结合,观测白梭梭叶片、不同径级枝干的光响应及光合日变化特征;同时监测环境因子(大气温湿度、光合有效辐射、土壤温度及含水量等)与叶片/枝干性状指标(叶绿素含量、含水量、干物质含量、碳/氮含量等),揭示叶片/枝干光合的主要影响因子;采用破坏性取样,量化个体水平上叶片与枝干的总表面积,阐明枝干光合对植株个体碳平衡的贡献。研究结果显示:(1)白梭梭叶片叶绿素含量是枝干叶绿素含量的12-16倍,各径级枝干叶绿素含量差异不显著;(2)枝干光饱和点低于叶片,枝干不同径级(由粗至细),暗呼吸速率和枝干光合逐渐减小;(3)光合有效辐射、土壤含水量和空气温湿度是影响叶片光合的主要因子,对枝干光合无显著影响;(4)枝干光合可以固定其自身呼吸产生CO2的73%,最高可达90%,枝干光合固定CO2约占个体水平固碳量的15.4%。研究结果表明,忽视枝干光合的贡献来预测未来气候变化背景下荒漠生态系统碳过程,可能存在根本性缺陷,并且在估算枝干呼吸时,需要考虑枝干是否存在光合作用,以提高枝干呼吸的准确性。 相似文献
20.
Spark plasma sintering synthesis of porous nanocrystalline titanium alloys for biomedical applications 总被引:1,自引:0,他引:1
The reason for the extended use of titanium and its alloys as implant biomaterials stems from their lower elastic modulus, their superior biocompatibility and improved corrosion resistance compared to the more conventional stainless steel and cobalt-based alloys [Niinomi, M., Hattori, T., Niwa, S., 2004. Material characteristics and biocompatibility of low rigidity titanium alloys for biomedical applications. In: Jaszemski, M.J., Trantolo, D.J., Lewandrowski, K.U., Hasirci, V., Altobelli, D.E., Wise, D.L. (Eds.), Biomaterials in Orthopedics. Marcel Dekker Inc., New York, pp. 41-62]. Nanostructured titanium-based biomaterials with tailored porosity are important for cell-adhesion, viability, differentiation and growth. Newer technologies like foaming or low-density core processing were recently used for the surface modification of titanium alloy implant bodies to stimulate bone in-growth and improve osseointegration and cell-adhesion, which in turn play a key role in the acceptance of the implants. We here report preliminary results concerning the synthesis of mesoporous titanium alloy bodies by spark plasma sintering. Nanocrystalline cp Ti, Ti-6Al-4V, Ti-Al-V-Cr and Ti-Mn-V-Cr-Al alloy powders were prepared by high-energy wet-milling and sintered to either full-density (cp Ti, Ti-Al-V) or uniform porous (Ti-Al-V-Cr, Ti-Mn-V-Cr-Al) bulk specimens by field-assisted spark plasma sintering (FAST/SPS). Cellular interactions with the porous titanium alloy surfaces were tested with osteoblast-like human MG-63 cells. Cell morphology was investigated by scanning electron microscopy (SEM). The SEM analysis results were correlated with the alloy chemistry and the topographic features of the surface, namely porosity and roughness. 相似文献