A method for measuring hydraulic conductivity and embolism in xylem

Abstract Hydraulic conductivity of the xylem is computed as the quotient of mass flow rate and pressure gradient. Measurements on excised plant stems can be difficult to interpret because of time-dependent reductions in flow rate, and because of variable degrees of embolism. Using Acer saccharum Marsh. stems, we found that certain perfusing solutions including dilute fixatives (e.g. 0.05% formaldehyde) and acids with pH below 3 (e.g. 10 mol m^?3 oxalic) prevent long-term decline in conductivity. Xylem embolism can be quantified by expressing the initial conductivity as a percentage of the maximum obtained after flow-impeding air emboli have been removed by repeated high-pressure (175 kPa) flushes. Correlation between microbial contamination and declining conductivity suggests that long-term (> 4h) declines are caused by microbial growth within the vessels. Unpredictable trends in short-term (< 4h) measurements may be caused by movements of air emboli in vessels and/or participate matter.     

Natural and experimentally altered hydraulic architecture of branch junctions in Acer saccharum Marsh. and Quercus velutina Lam. trees

 The functional xylem anatomy and the hydraulic conductivity of intact and treated branch junctions of the diffuse-porous sugar maple (Acer saccharum Marsh.) were compared to those of the ring-porous black oak (Quercus velutina Lam.). Maple shoots possessed greater growth intensity than those of oak. The extensive growth of the maple trees resulted in about a two-fold increase in xylem production in the maple branches. Branches were altered by removing a patch of bark from the base of a branch (near a junction) leaving a bridge of bark on the upper or lower side of the branch. The experimentally treated branch junctions revealed that, in oak, most (up to 92%) of the water flows in the lower side of a branch, where most of the large vessels occurred. In maple, most of the conductive tissue was observed to form in the upper side of the branches, which was equally or more conductive than the lower side. A treatment of longitudinal, parallel scratches in the bark-bridge, which reduced earlywood vessel width, substantially decreased conductivity (to only 15%) in oak, but had no effect on conductivity in maple. In maple, such wounding stimulated more wood formation and increased conductivity. In both trees, a narrow bridge at the junction induced more wood formation and higher conductivity in the branch. The mechanisms controlling wood formation and water flow in branch junctions of ring- and diffuse-porous trees are discussed. Received: 14 February 1996 / Accepted: 27 May 1996     

Frequency of partial and missing rings in Acer saccharum in relation to canopy position and growth rate

Locally absent growth rings are known to occur in trees during periods of environmental stress, but little evidence has been available on their frequency in moist temperate forests or on their potential for causing problems in dendroecological studies. In this study, 95 stem disks of Acer saccharum cut from trees representing a wide range of size and age were examined for ring anomalies using three techniques – inspection of disks for partial rings, crossdating of each tree against regional master chronologies, and use of stand history evidence. The number of ring anomalies in Acer saccharum was inversely related to growth rate and vigor. Mean percentage of ring anomalies was 1.3% in dominant canopy trees, 4.6% in trees of the intermediate crown class, and 16.2% in overtopped trees. All of the overtopped trees had partial or missing rings, with a mean of 10 per tree and a range of 2 to 20. Use of crossdating to correct for ring anomalies appeared to be successful in 88% of the canopy trees. However, crossdating results were inconclusive in 32% of the overtopped trees because of low ring width variability, multiple missing rings, and short duration of trouble-free segments. Because overtopped trees are of limited value in reconstructing disturbance history and often cannot be crossdated reliably, we suggest that they normally be excluded from disturbance chronologies.     

荒漠河岸林植物木质部导水与栓塞特征及其对干旱胁迫的响应

 以同处于干旱区的塔里木河下游(铁干里克)和黑河下游(乌兰图格)断面为研究区, 比较了荒漠河岸林主要建群种胡杨(Populus euphratica)、柽柳(Tamarix spp.)、疏叶骆驼刺(Alhagi sparsifolia)和花花柴(Karelinia caspia)在长期遭受不同干旱胁迫下的根、枝条木质部导水力和栓塞化程度的变化特征, 并分析了木质部导水对干旱胁迫的响应及适应策略。结果表明: 1) 黑河下游荒漠河岸林植物的导水能力显著高于塔里木河下游, 其中柽柳、胡杨、疏叶骆驼刺和花花柴根木质部的初始比导率(K_s0)分别高11.97、6.74、7.10和3.73倍, 枝条的K_s0分别高9.48、3.65、2.07和1.88倍, 地下水埋深导致的干旱胁迫程度不同是诱发荒漠植物导水能力差异的根本原因; 2)柽柳耐干旱能力最强, 适应范围较宽, 而花花柴、疏叶骆驼刺的耐旱性相对较弱, 适生范围较窄, 这可能与植物的根系分布有关; 3)干旱胁迫较轻时, 枝条木质部是荒漠河岸林植物水分传输的主要阻力部位, 干旱胁迫严重时, 根木质部是限制植株水流的最大阻碍部位; 4)荒漠河岸林植物主要通过调节枝条木质部的水流阻力来适应干旱胁迫, 且其适应策略与干旱胁迫程度有关, 干旱胁迫轻时, 植物通过限制枝条木质部水流来协调整株植物的均匀生长; 干旱胁迫严重时, 植物通过牺牲劣势枝条、增强优势枝条水流来提高植株整体生存的机会。     

Cutting xylem under tension or supersaturated with gas can generate PLC and the appearance of rapid recovery from embolism

We investigated the common assumption that severing stems and petioles under water preserves the hydraulic continuity in the xylem conduits opened by the cut when the xylem is under tension. In red maple and white ash, higher percent loss of conductivity (PLC) in the afternoon occurred when the measurement segment was excised under water at native xylem tensions, but not when xylem tensions were relaxed prior to sample excision. Bench drying vulnerability curves in which measurement samples were excised at native versus relaxed tensions showed a dramatic effect of cutting under tension in red maple, a moderate effect in sugar maple, and no effect in paper birch. We also found that air injection of cut branches (red and sugar maple) at pressures of 0.1 and 1.0 MPa resulted in PLC greater than predicted from vulnerability curves for samples cut 2 min after depressurization, with PLC returning to expected levels for samples cut after 75 min. These results suggest that sampling methods can generate PLC patterns indicative of repair under tension by inducing a degree of embolism that is itself a function of xylem tensions or supersaturation of dissolved gases (air injection) at the moment of sample excision. Implications for assessing vulnerability to cavitation and levels of embolism under field conditions are discussed.     

The effects of open-air fumigation with sulphur dioxide on the decomposition of sycamore (Acer pseudoplatanus L.) leaf litters from polluted and unpolluted woodlands

Sycamore (Acer pseudoplatanus L.) leaf litters from 15 woodlands exposed to a broad range of ambient sulphur dioxide (SO₂) concentrations were fumigated with environmentally realistic concentrations (ll-20nmol mol^?1) of SO₂, for 166 d in an open-air fumigation experiment. Fumigation of the sycamore litters significantly increased sulphate-S and proton leaching, and decreased calcium, magnesium and potassium concentrations in leachates and leaf tissues. Leaf litters from relatively unpolluted woodlands showed a tendency to lose higher amounts of sulphate-S, calcium, magnesium and nitrate-N in leachates than litters from polluted sites when exposed to elevated levels of SO₂ in treatment plots. Fumigation inhibited the decomposition rates (CO₂ evolution) of the leaf litters. Marked changes in the composition of the saprotrophic fungal communities in SO₂-fumigated leaf litters were also recorded, but fungal communities and responses to SO₂, were similar between woodlands. There was no evidence from our data to suggest that resistance to SO₂, was developed in decomposer mycofloras in woodlands more frequently polluted by the gas.     

A comparative study of a theoretical neural net model with MEG data from epileptic patients and normal individuals

Objective  

The aim of this study was to compare a theoretical neural net model with MEG data from epileptic patients and normal individuals.     

A kinetic model for lipoxygenases based on experimental data with the lipoxygenase of reticulocytes

A comprehensive kinetic model for lipoxygenase catalysis is proposed which includes the simultaneous occurrence of dioxygenase and hydroperoxidase activities and is based on the assumption of a single binding site for substrate fatty acid and product. The aerobic reaction of purified lipoxygenase from rabbit reticulocytes with 9,12(Z,Z)-octadecadienoic acid (linoleic acid) as substrate was studied. The rate constants and the dissociation constants of this enzyme were calculated for the model from progress curves; the model describes correctly the experimental data. The following kinetic features of the reticulocyte enzyme are assumed to apply generally to lipoxygenases. (a) The enzyme shows autoactivation by its product. (b) The rate-limiting step is the hydrogen abstraction. (c) Both substrate fatty acid and its product are competitive inhibitors of the lipoxygenase. (d) Lowering the oxygen concentration enhances the degree of substrate inhibition, whereas product inhibition is not influenced. (e) If substrate is in excess the oxygen concentration determines the share of dioxygenase and hydroperoxidase activities of the enzyme. As predicted from the model it was found that at low concentrations of oxygen the regio- and stereo-specificities of the dioxygenation are diminished. During the autoactivation phase the steady-state approximation does not hold.     

A new method to model change in cutaneous blood flow due to mechanical skin irritation part I: comparison between experimental and numerical data

Mechanical skin irritation creates vasodilation. Vasodilation of vascular networks induces increase in blood volume and blood velocity. Both can be measured by Laser Doppler Velocimetry. We propose in this article a method permitting comparison between experimental and numerical results. Experimental data was obtained by Laser Doppler Velocimetry. Numerical results were obtained by a continuous model of the vascular network. The model consists of three layers. First and last layer are described by anisotropic and heterogeneous porous media. They represent the irrigation and the drainage of the vascular system. The intermediate layer is described by a lumped parameter model that does not permit horizontal fluxes. All vessels are compliant. The permeabilities depend on the volumes of the specific layer. Skin irritation is modeled by a change in compliance of small arterial blood vessels. The comparison between experimental and numerical data is based on the model proposed by Bonner and Nossal [1981. Model for laser Doppler measurements of blood flow. Appl. Opt. 20, 2097-2107]. The model describes the Doppler frequency spectrum S(omega) as a function of the optical phenomena creating the frequency shift. The comparison is based on the model of the first moment M1 approximately integralomegaS(omega)domega. The variables of the first moment can be determined by results of the numerical model. We have shown, that it exists a linear relation between the change in compliance and the following increase in first moment. Using this linear relation experimental and numerical data can be compared.     

A discrepancy between the experimental and theoretical data on energy migration from B800 to B850 in LH-2 antennary complexes in purple bacteria

A discrepancy between the times of excitation transfer from B800 to B850 bacteriochlorophyll fractions in LH-2 complexes of purple bacteria was revealed. The experimental value (0.7-0.8 ps from literature sources) are at least four times lower than that (> 3.2 ps) calculated theoretically on the basis of recently obtained atomic structure of LH2. Possible reasons for this discrepancy are discussed.     

Decoupling the influence of leaf and root hydraulic conductances on stomatal conductance and its sensitivity to vapour pressure deficit as soil dries in a drained loblolly pine plantation

The study examined the relationships between whole tree hydraulic conductance ( K _tree) and the conductance in roots ( K _root) and leaves ( K _leaf) in loblolly pine trees. In addition, the role of seasonal variations in K _root and K _leaf in mediating stomatal control of transpiration and its response to vapour pressure deficit ( D ) as soil-dried was studied. Compared to trunk and branches, roots and leaves had the highest loss of conductivity and contributed to more than 75% of the total tree hydraulic resistance. Drought altered the partitioning of the resistance between roots and leaves. As soil moisture dropped below 50%, relative extractable water (REW), K _root declined faster than K _leaf. Although K _tree depended on soil moisture, its dynamics was tempered by the elongation of current-year needles that significantly increased K _leaf when REW was below 50%. After accounting for the effect of D on g _s, the seasonal decline in K _tree caused a 35% decrease in g _s and in its sensitivity to D , responses that were mainly driven by K _leaf under high REW and by K _root under low REW. We conclude that not only water stress but also leaf phenology affects the coordination between K _tree and g _s and the acclimation of trees to changing environmental conditions.     

A general method of deriving the best binding site model consistent with experimental binding data

An analysis of binding data is presented which yields the best binding site model consistent with the experimental data. The analysis is applicable to homotropic binding and yields the number of independent sites, number of interacting sites (dimers and tetramers of sites), intrinsic association constants, and degree of interaction. The information is derived from the roots of a binding polynomial constructed by the fitted Adair constants.     

A comparison of marine planktonic and sediment core diatoms in Hong Kong with emphasis on Pseudo-nitzschia

Potentially toxic diatoms belonging to the genus Pseudo-nitzschia were observed for the first time in plankton samples from Hong Kong collected in 1996. To determine whether potentially toxic diatoms had become more common during the last six decades, three gravity cores were taken from the anaerobic sediments of Kowloon Bay in Victoria Harbour. Anaerobic sediments are thought to be ideal for palaeoecological reconstructions because their vertical stratigraphy is undisturbed by bioturbidation. Analysis of the Kowloon Bay sediment cores indicated that very few individual diatoms belonged to the genus Pseudo-nitzschia, even though Pseudo-nitzschia was found in abundance in many of the plankton samples taken from a nearby site. The relative absence of Pseudo-nitzschia frustules was interpreted as indicating that these thin walled, poorly silicified, planktonic diatoms failed to preserve in the saline (32–34‰), slightly alkaline (pH 7.6–7.8), anaerobic sediments of Kowloon Bay. Dissolution of thinly silicified diatoms rather than predation was believed to be the reason for their virtual absence in the core. The anaerobic conditions near the bottom of Kowloon Bay and the shallowness of the Bay, 12 m, makes predation an unlikely explanation. Diatom abundance declined in the sediment cores below a depth of 15 cm (ca 1955). This was attributed to the decrease in nutrient loading to Victoria Harbour prior to 1955 rather than enhance diatom dissolution in the deeper sediments. Benthic diatoms became proportionately more abundant below the15 cm core depth. This revised version was published online in August 2006 with corrections to the Cover Date.     

A model to estimate risk of infection with human herpesvirus 8 associated with transfusion from cross-sectional data

In cross-sectional studies of infectious diseases, the data typically consist of: age at the time of study, status (presence or absence) of infection, and a chronology of events possibly associated with the disease. Motivated by a study of how human herpesvirus 8 (HHV-8) is transmitted among children with sickle cell anemia in Uganda, we have developed a flexible parametric approach for combining current-status data with a history of blood transfusions. We model heterogeneity in transfusion-associated risk by a child-specific random effect. We present an extension of the model to accommodate the fact that there is no gold standard for HHV-8 infection and infection status was assessed by a serological assay. The parameters are estimated via maximum likelihood. Finally, we present results from applying various parameterizations of the model to the Ugandan study.     

Tryptophanyl fluorescence lifetime distribution of hyperthermophilic beta-glycosidase from molecular dynamics simulation: a comparison with the experimental data

A molecular dynamics simulation approach has been utilized to understand the unusual fluorescence emission decay observed for beta-glycosidase from the hyperthermophilic bacterium Solfolobus sulfotaricus (Sbeta gly), a tetrameric enzyme containing 17 tryptophanyl residues for each subunit. The tryptophanyl emission decay of Sbeta gly results from a bimodal distribution of fluorescence lifetimes with a short-lived component centered at 2.5 ns and a long-lived one at 7.4 ns (Bismuto E, Nucci R, Rossi M, Irace G, 1999, Proteins 27:71-79). From the examination of the trajectories of the side chains capable of causing intramolecular quenching for each tryptophan microenvironment and using a modified Stern-Volmer model for the emission quenching processes, we calculated the fluorescence lifetime for each tryptophanyl residue of Sbeta gly at two different temperatures, i.e., 300 and 365 K. The highest temperature was chosen because in this condition Sbeta gly evidences a maximum in its catalytic activity and is stable for a very long time. The calculated lifetime distributions overlap those experimentally determined. Moreover, the majority of trytptophanyl residues having longer lifetimes correspond to those originally identified by inspection of the crystallographic structure. The tryptophanyl lifetimes appear to be a complex function of several variables, such as microenvironment viscosity, solvent accessibility, the chemical structure of quencher side chains, and side-chain dynamics. The lifetime calculation by MD simulation can be used to validate a predicted structure by comparing the theoretical data with the experimental fluorescence decay results.