Interactions between adsorbed hydrogenated soy phosphatidylcholine (HSPC) vesicles at physiologically high pressures and salt concentrations

Using a surface force balance, we measured normal and shear interactions as a function of surface separation between layers of hydrogenated soy phosphatidylcholine (HSPC) small unilamellar vesicles (SUVs) adsorbed from dispersion at physiologically high salt concentrations (0.15 M NaNO₃). Cryo-scanning electron microscopy shows that each surface is coated by a close-packed HSPC-SUV layer with an overlayer of liposomes on top. A clear attractive interaction between the liposome layers is seen upon approach and separation, followed by a steric repulsion upon further compression. The shear forces reveal low friction coefficients (μ = 0.008–0.0006) up to contact pressures of at least 6 MPa, comparable to those observed in the major joints. The spread in μ-values may be qualitatively accounted for by different local liposome structure at different contact points, suggesting that the intrinsic friction of the HSPC-SUV layers at this salt concentration is closer to the lower limit (μ = ∼0.0006). This low friction is attributed to the hydration lubrication mechanism arising from rubbing of the hydrated phosphocholine-headgroup layers exposed at the outer surface of each liposome, and provides support for the conjecture that phospholipids may play a significant role in biological lubrication.     

Normal and shear forces between surfaces bearing porcine gastric mucin, a high-molecular-weight glycoprotein

A surface force balance was used to measure the normal and shear forces between two mica surfaces each bearing an adsorbed layer of porcine gastric mucin ("Orthana" mucin), genetically similar to human MUC6. This mucin is a highly purified, 546 kDa, weakly negative, polyampholytic molecule with a "dumbbell" structure. Both bare (HP) and hydrophobized (HB) mica substrates were used, and forces were measured under 1 and 30 mg/mL mucin solutions, under pure (no-added-salt) water, and under 0.1 M aqueous Na(+) solution. Normal surface forces were monotonically repulsive in all cases, with onset of repulsion occurring at smaller surface separations, D, in the 0.1 M salt solutions (～ 20 nm, compared with ～40 nm for no added salt). Repulsion on HP mica was greater on surface compression than decompression, an effect, attributed to bridging and slow-relaxing additional adsorption on compression, not seen on HB mica, a difference attributed to the denser coverage of mucin hydrophobic moieties on the HB surface. Friction forces increased with compression in all cases, showing hysteretic behavior on HP but not on HB mica, commensurate with the hysteresis observed in the normal measurements. Low friction coefficients μ (= ?F(s)/?F(n) < 0.05) were seen up to mean pressures

≈ 0.5 to 1.0 MPa, attributed to low interpenetration of the opposed layers together with hydration lubrication effects, with higher μ (up to 0.4) at higher

attributed to interlayer entanglements and to bridging (for the case of HP mica). Shear forces increased only weakly with sliding speed over the range investigated (80-820 nm s(-1)). The lower friction with HB relative to HP mica suggests a selectivity of the HB surface to the hydrophobic moieties of the mucin that in consequence exposes relatively more of the better-lubricating hydrophilic groups. This surface-selectivity effect on lubrication may have a generality extending to other biological macromolecules that contain both hydrophilic and hydrophobic groups.     

Articular cartilage proteoglycans as boundary lubricants: structure and frictional interaction of surface-attached hyaluronan and hyaluronan--aggrecan complexes

Mammalian synovial joints are extremely efficient lubrication systems reaching friction coefficient μ as low as 0.001 at high pressures (up to 100 atm) and shear rates (up to 10(6) to 10(7) Hz); however, despite much previous work, the exact mechanism responsible for this behavior is still unknown. In this work, we study the molecular mechanism of synovial joint lubrication by emulating the articular cartilage superficial zone structure. Macromolecules extracted and purified from bovine hip joints using well-known biochemical techniques and characterized with atomic force microscope (AFM) have been used to reconstruct a hyaluronan (HA)--aggrecan layer on the surface of molecularly smooth mica. Aggrecan forms, with the help of link protein, supramolecular complexes with the surface-attached HA similar to those at the cartilage/synovial fluid interface. Using a surface force balance (SFB), normal and shear interactions between a HA--aggrecan-coated mica surface and bare mica have been examined, focusing, in particular, on the frictional forces. In each stage, control studies have been performed to ensure careful monitoring of the macromolecular surface layers. We found the aggrecan--HA complex to be a much better boundary lubricant than the HA alone, an effect attributed largely to the fluid hydration sheath bound to the highly charged glycosaminoglycan (GAG) segments on the aggrecan core protein. A semiquantitative model of the osmotic pressure is used to describe the normal force profiles between the surfaces and interpret the boundary lubrication mechanism of such layers.     

Adsorption, lubrication, and wear of lubricin on model surfaces: polymer brush-like behavior of a glycoprotein

Using a surface force apparatus, we have measured the normal and friction forces between layers of the human glycoprotein lubricin, the major boundary lubricant in articular joints, adsorbed from buffered saline solution on various hydrophilic and hydrophobic surfaces: i), negatively charged mica, ii), positively charged poly-lysine and aminothiol, and iii), hydrophobic alkanethiol monolayers. On all these surfaces lubricin forms dense adsorbed layers of thickness 60–100 nm. The normal force between two surfaces is always repulsive and resembles the steric entropic force measured between layers of end-grafted polymer brushes. This is the microscopic mechanism behind the antiadhesive properties showed by lubricin in clinical tests. For pressures up to ~6 atm, lubricin lubricates hydrophilic surfaces, in particular negatively charged mica (friction coefficient μ = 0.02–0.04), much better than hydrophobic surfaces (μ > 0.3). At higher pressures, the friction coefficient is higher (μ > 0.2) for all surfaces considered and the lubricin layers rearrange under shear. However, the glycoprotein still protects the underlying substrate from damage up to much higher pressures. These results support recent suggestions that boundary lubrication and wear protection in articular joints are due to the presence of a biological polyelectrolyte on the cartilage surfaces.     

Lubrication and load-bearing properties of human salivary pellicles adsorbed ex vivo on molecularly smooth substrata

In a series of Surface Force Balance experiments, material from human whole saliva was adsorbed to molecularly smooth mica substrata (to form an 'adsorbed salivary film'). Measurements were taken of normal (load bearing, F (n)) and shear (frictional, F (s)*) forces between two interacting surfaces. One investigation involved a salivary film formed by overnight adsorption from undiluted, centrifuged saliva, with the adsorbed film rinsed with pure water before measurement. Measurements were taken under pure water and 70?mM NaNO(3). In a second investigation, a film was formed from and measured under a solution of 7% filtered saliva in 10?mM NaNO(3). F (n) results for both systems showed purely repulsive layers, with an uncompressed thickness of 35-70?nm for the diluted saliva investigation and, prior to the application of shear, 11?nm for the rinsed system. F (s)* was essentially proportional to F (n) for all systems and independent of shear speed (in the range 100-2000?nm s(-1)), with coefficients of friction μ?～?0.24 and μ?～?0.46 for the unrinsed and rinsed systems, respectively. All properties of the rinsed system remained similar when the pure water measurement environment was changed to 70?mM NaNO(3). For all systems studied, shear gave rise to an approximately threefold increase in the range of normal forces, attributed to the ploughing up of adsorbed material during shear to form debris that stood proud of the adsorbed layer. The results provide a microscopic demonstration of the wear process for a salivary film under shear and may be of particular interest for understanding the implications for in vivo oral lubrication under conditions such as rinsing of the mouth cavity. The work is interpreted in light of earlier studies that showed a structural collapse and increase in friction for an adsorbed salivary film in an environment of low ionic strength.     

Friction coefficients for mechanically damaged bovine articular cartilage

We used a pin-on-disc tribometer to measure the friction coefficient of both pristine and mechanically damaged cartilage samples in the presence of different lubricant solutions. The experimental set up maximizes the lubrication mechanism due to interstitial fluid pressurization. In phosphate buffer solution (PBS), the measured friction coefficient increases with the level of damage. The main result is that when poly(ethylene oxide) (PEO) or hyaluronic acid (HA) are dissolved in PBS, or when synovial fluid (SF) is used as lubricant, the friction coefficients measured for damaged cartilage samples are only slightly larger than those obtained for pristine cartilage samples, indicating that the surface damage is in part alleviated by the presence of the various lubricants. Among the lubricants considered, 100 mg/mL of 100,000 Da MW PEO in PBS appears to be as effective as SF. We attempted to discriminate the lubrication mechanism enhanced by the various compounds. The lubricants viscosity was measured at shear rates comparable to those employed in the friction experiments, and a quartz crystal microbalance with dissipation monitoring was used to study the adsorption of PEO, HA, and SF components on collagen type II adlayers pre-formed on hydroxyapatite. Under the shear rates considered the viscosity of SF is slightly larger than that of PBS, but lower than that of lubricant formulations containing HA or PEO. Neither PEO nor HA showed strong adsorption on collagen adlayers, while evidence of adsorption was found for SF. Combined, these results suggest that synovial fluid is likely to enhance boundary lubrication. It is possible that all three formulations enhance lubrication via the interstitial fluid pressurization mechanism, maximized by the experimental set up adopted in our friction tests.     

Lubrication and load-bearing properties of human salivary pellicles adsorbed ex vivo on molecularly smooth substrata

In a series of Surface Force Balance experiments, material from human whole saliva was adsorbed to molecularly smooth mica substrata (to form an ‘adsorbed salivary film’). Measurements were taken of normal (load bearing, F _n) and shear (frictional, F _s*) forces between two interacting surfaces. One investigation involved a salivary film formed by overnight adsorption from undiluted, centrifuged saliva, with the adsorbed film rinsed with pure water before measurement. Measurements were taken under pure water and 70 mM NaNO₃. In a second investigation, a film was formed from and measured under a solution of 7% filtered saliva in 10 mM NaNO₃. F _n results for both systems showed purely repulsive layers, with an uncompressed thickness of 35–70 nm for the diluted saliva investigation and, prior to the application of shear, 11 nm for the rinsed system. F _s* was essentially proportional to F _n for all systems and independent of shear speed (in the range 100–2000 nm s^?1), with coefficients of friction  μ ～ 0.24 and μ ～ 0.46 for the unrinsed and rinsed systems, respectively. All properties of the rinsed system remained similar when the pure water measurement environment was changed to 70 mM NaNO₃. For all systems studied, shear gave rise to an approximately threefold increase in the range of normal forces, attributed to the ploughing up of adsorbed material during shear to form debris that stood proud of the adsorbed layer. The results provide a microscopic demonstration of the wear process for a salivary film under shear and may be of particular interest for understanding the implications for in vivo oral lubrication under conditions such as rinsing of the mouth cavity. The work is interpreted in light of earlier studies that showed a structural collapse and increase in friction for an adsorbed salivary film in an environment of low ionic strength.     

Dependence of nanoscale friction and adhesion properties of articular cartilage on contact load

Boundary lubrication of articular cartilage by conformal, molecularly thin films reduces friction and adhesion between asperities at the cartilage-cartilage contact interface when the contact conditions are not conducive to fluid film lubrication. In this study, the nanoscale friction and adhesion properties of articular cartilage from typical load-bearing and non-load-bearing joint regions were studied in the boundary lubrication regime under a range of physiological contact pressures using an atomic force microscope (AFM). Adhesion of load-bearing cartilage was found to be much lower than that of non-load-bearing cartilage. In addition, load-bearing cartilage demonstrated steady and low friction coefficient through the entire load range examined, whereas non-load-bearing cartilage showed higher friction coefficient that decreased nonlinearly with increasing normal load. AFM imaging and roughness calculations indicated that the above trends in the nanotribological properties of cartilage are not due to topographical (roughness) differences. However, immunohistochemistry revealed consistently higher surface concentration of boundary lubricant at load-bearing joint regions. The results of this study suggest that under contact conditions leading to joint starvation from fluid lubrication, the higher content of boundary lubricant at load-bearing cartilage sites preserves synovial joint function by minimizing adhesion and wear at asperity microcontacts, which are precursors for tissue degeneration.     

Experiments on Ultrasonic Lubrication Using a Piezoelectrically-assisted Tribometer and Optical Profilometer

Friction and wear are detrimental to engineered systems. Ultrasonic lubrication is achieved when the interface between two sliding surfaces is vibrated at a frequency above the acoustic range (20 kHz). As a solid-state technology, ultrasonic lubrication can be used where conventional lubricants are unfeasible or undesirable. Further, ultrasonic lubrication allows for electrical modulation of the effective friction coefficient between two sliding surfaces. This property enables adaptive systems that modify their frictional state and associated dynamic response as the operating conditions change. Surface wear can also be reduced through ultrasonic lubrication. We developed a protocol to investigate the dependence of friction force reduction and wear reduction on the linear sliding velocity between ultrasonically lubricated surfaces. A pin-on-disc tribometer was built which differs from commercial units in that a piezoelectric stack is used to vibrate the pin at 22 kHz normal to the rotating disc surface. Friction and wear metrics including effective friction force, volume loss, and surface roughness are measured without and with ultrasonic vibrations at a constant pressure of 1 to 4 MPa and three different sliding velocities: 20.3, 40.6, and 87 mm/sec. An optical profilometer is utilized to characterize the wear surfaces. The effective friction force is reduced by 62% at 20.3 mm/sec. Consistently with existing theories for ultrasonic lubrication, the percent reduction in friction force diminishes with increasing speed, down to 29% friction force reduction at 87 mm/sec. Wear reduction remains essentially constant (49%) at the three speeds considered.     

Lubrication regimes in mesothelial sliding

To function normally, the lungs, heart, and other organs must undergo changes in shape and size, sliding against surrounding body walls. It is not known whether the delicate mesothelial surfaces covering these organs and body wall are in contact during sliding, or if hydrodynamic pressure in the lubricating liquid increases separation between their surfaces. To address this question, we measured the coefficient of friction (mu) of the mesothelial surface of nine rat-abdominal walls sliding in saline on a smooth glass surface. Sliding at physiological velocities of 0.0123-6.14 cm/s with normal stresses of 50-200 Pa, mu varied with velocity (P<0.001). On average, mu was relatively high at low speeds (0.078 at 0.041 cm/s), decreased to a minimum at intermediate speeds (0.034 at 1.23 cm/s), and increased slightly again at higher speeds (0.045 at 6.14 cm/s), consistent with a mixed lubrication regime in which there is at least partial hydrodynamic separation of surfaces. We conclude that mesothelial surfaces, sliding under physiological conditions, are protected from excessive shear by hydrodynamic pressures that increase separation of surfaces.     

Normal and frictional interactions of purified human statherin adsorbed on molecularly-smooth solid substrata

Human salivary statherin was purified from parotid saliva and adsorbed to bare hydrophilic (HP) mica and STAI-coated hydrophobic (HB) mica in a series of Surface Force Balance experiments that measured the normal (F(n)) and friction forces (F(s)*) between statherin-coated mica substrata. Readings were taken both in the presence of statherin solution (HP and HB mica) and after rinsing (HP mica). F(n) measurements showed, for both substrata, monotonic steric repulsion that set on at a surface separation D ~20 nm, indicating an adsorbed layer whose unperturbed thickness was ca 10 nm. An additional longer-ranged repulsion, probably of electrostatic double-layer origin, was observed for rinsed surfaces under pure water. Under applied pressures of ~1 MPa, each surface layer was compressed to a thickness of ca 2 nm on both types of substratum, comparable with earlier estimates of the size of the statherin molecule. Friction measurements, in contrast with F(n) observations, were markedly different on the two different substrata: friction coefficients, μ ≡ ?F(s)*/?F(n), on the HB substratum (μ ≈ 0.88) were almost an order of magnitude higher than on the HP substratum (μ ≈ 0.09 and 0.12 for unrinsed and rinsed, respectively), and on the HB mica there was a lower dependence of friction on sliding speed than on the HP mica. The observations were attributed to statherin adsorbing to the mica in multimer aggregates, with internal re-arrangement of the protein molecules within the aggregate dependent on the substratum to which the aggregate adsorbed. This internal re-arrangement permitted aggregates to be of similar size on HP and HB mica but to have different internal molecular orientations, thus exposing different moieties to the solution in each case and accounting for the very different friction behaviour.     

Combined Effect of Textured Patterns and Graphene Flake Additives on Tribological Behavior under Boundary Lubrication

A ball-on-plate wear test was employed to investigate the effectiveness of graphene (GP) nanoparticles dispersed in a synthetic-oil-based lubricant in reducing wear. The effect by area ratio of elliptically shaped dimple textures and elevated temperatures were also explored. Pure PAO4 based oil and a mixture of this oil with 0.01 wt% GP were compared as lubricants. At pit area ratio of 5%, GP-base oil effectively reduced friction and wear, especially at 60 and 100°C. Under pure PAO4 oil lubrication, the untextured surfaces gained low friction coefficients (COFs) and wear rates under 60 and 100°C. With increasing laser—texture area ratio, the COF and wear rate decreased at 25 and 150°C but increased at 60 and 100°C. Under the GP-based oil lubrication, the textured surface with 5% area ratio achieved the lowest COF among those of the area ratios tested at all test temperatures. Meanwhile, the textured surface with 20% area ratio obtained the highest COF among those of the area ratios. With the joint action of GP and texture, the textured surface with 10% area ratio exhibited the best anti-wear performance among all of the textured surfaces at all test temperatures.     

High pressure response of fruit jams contaminated with Listeria monocytogenes

High pressure is an alternative to thermal processing and is used to preserve food. Listeria monocytogenes is a bacterium which grows at low temperature, is able to multiply under vacuum, and is responsible for food poisoning. Pressures of 100, 200, 300 and 400 MPa were used for 5, 10 and 15 min at 20 degrees C on pure culture, and on apple and plum jam baby food artificially contaminated with Listeria. Pure culture was also to test pressures of 200, 300, 350 and 400 MPa at 5 degrees C for 30 min. The results were analysed statistically and showed that there were no significant differences between pressures of 100 and 200 MPa at 5, 10 and 15 min. However, at 300 MPa, there were significant differences at 15 min. When the pressure treatment was 400 MPa, significant differences were observed at pressure times of 5, 10 and 15 min. The results were fitted to a linear curve. In pure culture, no viable cells were detected after high pressure treatment of 350 MPa for 30 min at 5 degrees C. The use of low temperature helps to maintain the sensory properties of the product.     

Influence of the stiffness of bone defect implants on the mechanical conditions at the interface--a finite element analysis with contact

The study focused on the influence of the implant material stiffness on stress distribution and micromotion at the interface of bone defect implants. We hypothesized that a low-stiffness implant with a modulus closer to that of the surrounding trabecular bone would yield a more homogeneous stress distribution and less micromotion at the interface with the bony bed. To prove this hypothesis we generated a three-dimensional, non-linear, anisotropic finite element (FE) model. The FE model corresponded to a previously developed animal model in sheep. A prismatic implant filled a standardized defect in the load-bearing area of the trabecular bone beneath the tibial plateau. The interface was described by face-to-face contact elements, which allow press fits, friction, sliding, and gapping. We assumed a physiological load condition and calculated contact pressures, shear stresses, and shear movements at the interface for two implants of different stiffness (titanium: E=110GPa; composite: E=2.2GPa). The FE model showed that the stress distribution was more homogeneous for the low-stiffness implant. The maximum pressure for the composite implant (2.1 MPa) was lower than for the titanium implant (5.6 MPa). Contrary to our hypothesis, we found more micromotion for the composite (up to 6 microm) than for the titanium implant (up to 4.5 microm). However, for both implants peak stresses and micromotion were in a range that predicts adequate conditions for the osseointegration. This was confirmed by the histological results from the animal studies.     

Artificial composite bone as a model of human trabecular bone: the implant-bone interface

The use of artificial bones in implant testing has become popular due to their low variability and ready availability. However, friction coefficients, which are critical to load transfer in uncemented implants, have rarely been compared between human and artificial bone, particularly for wet and dry conditions. In this study, the static and dynamic friction coefficients for four commercially used titanium surfaces (polished, Al(2)O(3) blasted, plasma sprayed, beaded) acting on the trabecular component of artificial bones (Sawbones) were compared to those for human trabecular bone. Artificial bones were tested in dry and wet conditions and normal interface stress was varied (0.25, 0.5, 1.0MPa). Friction coefficients were mostly lower for artificial bones than real bone. In particular, static friction coefficients for the dry polished surface were 20% of those for real bone and 42-61% for the dry beaded surface, with statistical significance (alpha<0.05). Less marked differences were observed for dynamic friction coefficients. Significant but non-systematic effects of normal stress or wet/dry condition on friction coefficients were observed within each surface type. These results indicate that the use of artificial bone models for pre-clinical implant testing that rely on interface load transfer with trabecular bone for mechanical integrity can be particularly sensitive to surface finish and lubrication conditions.     

Synthesis and Lubrication of Phosphorylcholine Derivative

##正## The monomer of phosphorylcholine derivative, O-(5-(2-methacryloxy)-3, 3-dimethyl-3-azapentyl)-O'-(ω-hydroxy-octyl)-phosphatequaternary ammonium salt, was designed and synthesized successfully. It was characterized by the spectra of ~1HNMR and Mass spectra (ESI+), and every signal was assigned. Then the lubricating characteristics of the phosphorylcholinederivative were investigated on the tribological setup of ball-oh-flat. The Ultra-High Molecular Weight Polyethylene(UHMWPE) flat was rotated against a stainless steel ball with 6 mm diameter. The load was 2.3 N, which corresponded to amaximal Hertz contact pressure of 29 MPa. Water, phosphorylcholine derivative, and Acrylic Acid (AA) solution were used aslubricants, respectively. Compared with AA, the phosphorylcholine derivative shows significant lubrication. It can be stronglyhydrated under water due to the charged segment in chemical structure. The thick water layers within the chains serves asboundary lubricants, and this is thought to be the molecular origins of lubricating behavior.     

Permeability of Iris germanica's multiseriate exodermis to water, NaCl, and ethanol

The exodermis of Iris germanica roots is multiseriate. Its outermost layer matures first with typical Casparian bands and suberin lamellae. But as subsequent layers mature, the Casparian band extends into the tangential and anticlinal walls of their cells. Compared with roots in which the endodermis represents the major transport barrier, the multiseriate exodermis (MEX) was expected to reduce markedly radial water and solute transport. To test this idea, precocious maturation of the exodermis was induced with a humid air gap inside a hydroponic chamber. Hydraulic conductivity (Lp(pc)) was measured on completely submerged roots (with an immature exodermis) and on air-gap-exposed root regions (with two mature exodermal layers) using a pressure chamber. Compared with regions of roots with no mature exodermal layers, the mature MEX reduced Lp(pc) from 8.5×10(-8) to 3.9×10(-8) m s(-1) MPa(-1). Puncturing the MEX increased Lp(pc) to 19×10(-8) m s(-1) MPa(-1), indicating that this layer constituted a substantial hydraulic resistance within the root (75% of the total). Alternatively, a root pressure probe was used to produce pressure transients from which hydraulic conductivity was determined, but this device measured mainly flow through the endodermis in these wide-diameter roots. The permeability of roots to NaCl and ethanol was also reduced in the presence of two mature MEX layers. The data are discussed in terms of the validity of current root models and in terms of a potential role for I. germanica MEX during conditions of drought and salt stress.     

滨海盐碱地刺槐(Robinia pseudoacacia)混交林土壤水盐动态

以固氮树种刺槐与绒毛白蜡、榆树、臭椿三树种的混交林及其纯林为研究对象,研究了刺槐与不同树种混交对土壤水分、盐分年动态变化的影响。研究结果表明：（1）刺槐与3个树种混交,刺槐臭椿混交林生长最好,均高于各自纯林。（2）混交林一定程度改善了土壤含水量及层次分布,土壤含水量整体趋势均表现出0～60 cm表层土中含水量高于各自纯林,而深层土壤含水量低的特点,只有8月份纯林和混交林的土壤含水量没有显著差别。不同树种在具体层次上略有差异;（3）混交林降低了土壤含盐量,改变了土壤盐分层次分布和年变化规律。不同月份间土壤含盐量随土壤深度、混交树种的变化而变化。深层土壤含盐量高,表层土壤含盐量低,混交林含盐量低于纯林且存在树种差异。混交林与纯林含盐量均雨季低,旱季高;但在某一具体月份各层次含盐量差别不大;不同土层含水量、含盐量与天然降水之间有明显的相关性,天然降水是混交林及纯林土壤水分的主要来源。     

放牧对黄河低阶地盐化草场土壤水盐空间异质性的影响

选定黄河低阶地地形大致相近的放牧草场与封育草场,按实地情况分别设置长770m（放牧）、370m（放牧）和240m（封育）3条样线,采用校正后的便携式WET Sensor以10m为间隔进行土壤水盐的分层（0～10cm,10～20cm）测定,并对其进行地统计学分析.结果表明：黄河低阶地盐化草场表层土壤水盐的变异系数和相关性高于下层,土壤水分的变异系数低于土壤盐分且层间的相关性较高;放牧使土壤水盐之间的相关程度增大,变异降低,且使土壤水分存在一定的冗余.所研究草场的土壤水盐表现为中等或强烈空间自相关,封育盐化草场水盐空间相关尺度的差异较小,而放牧草场则相反.持续放牧已经成为一种稳定影响盐化草场水盐空间分布的结构性因素,使盐化草场的水盐空间异质性减弱.盐生植物在草场内随机分布且不受放牧干扰,是封育盐化草场随机作用较强的最可能原因;而放牧草场内牲畜的啃食和践踏对盐生植物和土壤结构改变所形成的反馈作用大大降低了这些植物对水分特别是盐分的再分配,进而导致放牧草场水盐的含量趋于增大且分布格局趋于简单.     

Na~ and Water Uptake in Relation to the Radial Reflection Coefficient of Root in Arrowleaf Saltbush Under Salt Stress

The response of halophyte arrowleaf saltbush(Atriplex triangularis Willd)plants to a gradient of salt stress were investigatedwith hydroponically cultured seedlings.Under salt stress,both the Na~ uptake into root xylem and negative pressures inxylem vessels increased with the elevation of salinity(up to 500 mol/m~3)in the root environment.However,the increment innegative pressures in root xylem far from matches the decrease in the osmotic potential of the root bathing solutions,evenwhen the osmotic potential of xylem sap is taken into consideration.The total water potential of xylem sap in arrowleafsaltbush roots was close to the osmotic potential of root bathing solutions when the salt stress was low,but a progressivelyincreased gap between the water potential of xylem sap and the osmotic potential of root bathing solutions was observedwhen the salinity in the root environment was enhanced.The maximum gap was 1.4 MPa at a salinity level of 500 mol/m~3without apparent dehydration of the tested plants.This discrepancy could not be explained with the current theories inplant physiology.The radial reflection coefficient of root in arrowleaf saltbush decreased with the enhanced salt stress wasand accompanied by an increase in the Na~ uptake into xylem sap.However,the relative Na~ in xylem exudates based onthe corresponding NaCl concentration in the root bathing solutions showed a tendency of decrease.The results showedthat the reduction in the radial reflection coefficient of roots in the arrowleaf saltbush did not lead to a mass influx of NaClinto xylem when the radial reflection coefficient of the root was considerably small;and that arrowleaf saltbush could usesmall xylem pressures to counterbalance the salt stresses,either with the uptake of large amounts of salt,or with thedevelopment of xylem pressures dangerously negative.This strategy could be one of the mechanisms behind the highresistance of arrowleaf saltbush plants to salt stress.