Xylem Flow and its Driving Forces in a Tropical Liana: Concomitant Flow-Sensitive NMR Imaging and Pressure Probe Measurements

Abstract: Flow-sensitive NMR imaging and pressure probe techniques were used for measuring xylem water flow and its driving forces (i.e., xylem pressure as well as cell turgor and osmotic pressure gradients) in a tropical liana, Epipremnum aureum. Selection of tall specimens allowed continuous and simultaneous measurements of all parameters at various distances from the root under diurnally changing environmental conditions. Well hydrated plants exhibited exactly linearly correlated dynamic changes in xylem tension and flow velocity. Concomitant multiple-probe insertions along the plant shoot revealed xylem and turgor pressure gradients with changing magnitudes due to environmental changes and plant orientation (upright, apex-down, or horizontal). The data suggest that in upright and - to a lesser extent - in horizontal plants the transpirational water loss by the cells towards the apex during the day is not fully compensated by water uptake through the night. Thus, longitudinal cellular osmotic pressure gradients exist. Due to the tight hydraulic coupling of the xylem and the tissue cells these gradients represent (besides the transpiration-induced tension in the xylem) an additional tension component for anti-gravitational water movement from the roots through the vessels to the apex.     

Ion-mediated flow changes suppressed by minimal calcium presence in xylem sap in Chrysanthemum and Prunus laurocerasus

After the discovery of ion-mediated changes in xylem hydraulic resistance a few years ago, a number of research papers were published that related ion-mediated flow changes in the xylem to various aspects of whole plant functioning and evolutionary diversification of vascular cells. Ion-mediated changes in xylem hydraulic resistance are commonly quantified as the percentile change in hydraulic resistance, relative to the hydraulic resistance measured using a reference fluid, usually (ultra) pure deionized water. In this research the impact was investigated of the complete absence of all ions in deionized water compared with reference fluids containing a minimal amount of free calcium on the quantification of ion-mediated flow changes in stem segments of Chrysanthemum (Dendranthemaxgrandiflorum Tzvelev) and Prunus L. (Prunus laurocerasus L.). The addition of 10 mM KCl to deionized water significantly increased flow rate in Chrysanthemum (17-24%) and Prunus L. (16%). The addition of 1 mM CaCl(2) to the reference fluid reduced this KCl-mediated increase in flow rate to 1-2% in both species. 1 mM Ca(2+) is within the lower range of Ca(2+)-concentrations normally measured in xylem sap of many plant species, and three times lower than the original Ca(2+)-concentration measured in the xylem sap of Chrysanthemum plants used for the present measurements. The present results indicate that the complete removal of cations from the xylem fluid with deionized water causes the major part of the ion-mediated flow change previously reported in the xylem of plants. It is concluded that the use of deionized water as a reference fluid should be avoided. Earlier proposed relationships between ion-mediated changes and water flow in xylem of plants should be re-evaluated if they were based on deionized water as the reference fluid.     

Visualization of lateral water transport pathways in soybean by a time of flight-secondary ion mass spectrometry cryo-system

Water movement between cells in a plant body is the basic phenomenon of plant solute transport; however, it has not been well documented due to limitations in observational techniques. This paper reports a visualization technique to observe water movement among plant cells in different tissues using a time of flight-secondary ion mass spectrometry (Tof-SIMS) cryo-system. The specific purpose of this study is to examine the route of water supply from xylem to stem tissues. The maximum resolution of Tof-SIMS imaging was 1.8 μm (defined as the three pixel step length), which allowed detection of water movement at the cellular level. Deuterium-labelled water was found in xylem vessels in the stem 2.5 min after the uptake of labelled water by soybean plants. The water moved from the xylem to the phloem, cambium, and cortex tissues within 30-60 min after water absorption. Deuterium ion counts in the phloem complex were slightly higher than those in the cortex and cambium tissue seen in enlarged images of stem cell tissue during high transpiration. However, deuterium ion counts in the phloem were lower than those in the cambium at night with no evaporative demand. These results indicate that the stem tissues do not receive water directly from the xylem, but rather from the phloem, during high evaporative demand. In contrast, xylem water would be directly supplied to the growing sink during the night without evaporative demand.     

What are the driving forces for water lifting in the xylem conduit?

After Renner had shown convincingly in 1925 that the transpirational water loss generates tensions larger than 0.1 MPa (i.e. negative pressures) in the xylem of cut leafy twigs the Cohesion Theory proposed by Böhm, Askenasy, Dixon and Joly at the end of the 19th century was immediately accepted by plant physiologists. Introduction of the pressure chamber technique by Scholander et al. in 1965 enforced the general belief that tension is the only driving force for water lifting although substantial criticism regarding the technique and/or the Cohesion Theory was published by several authors. As typical for scientific disciplines, the advent of minimal‐ and non‐invasive techniques in the last decade as well as the development of a new, reliable method for xylem sap sampling have challenged this view. Today, xylem pressure gradients, potentials, ion concentrations and volume flows as well as cell turgor pressure gradients can be monitored online in intact transpiring higher plants, and within a given physiological context by using the pressure probe technique and high‐resolution NMR imaging techniques, respectively. Application of the pressure probe technique to transpiring plants has shown that negative absolute pressures (down to ? 0.6 MPa) and pressure gradients can exist temporarily in the xylem conduit, but that the magnitude and (occasionally) direction of gradients contrasts frequently the belief that tension is the only driving force. This seems to be particularly the case for plants faced with problems of height, drought, freezing and salinity as well as with cavitation of the tensile water. Reviewing the current data base shows that other forces come into operation when exclusively tension fails to lift water against gravity due to environmental conditions. Possible candidates are longitudinal cellular and xylem osmotic pressure gradients, axial potential gradients in the vessels as well as gel‐ and gas bubble‐supported interfacial gradients. The multiforce theory overcomes the problem of the Cohesion Theory that life on earth depends on water being in a highly metastable state.     

An Impasse in Plant Water Relations?

Balling and Zimmermann [Planta 182 (1990), 325–338] used a pressure probe to measure directly negative pressures in the xylem of transpiring plants. They obtained data that challenge the standard framework that plant physiologists use when thinking about plant water relations, and, most notably, found a substantial discrepancy between their measurements of xylem pressure and of leaf water potential measured with a Scholander pressure bomb. Their data are critically examined and it is shown that most of them can be accommodated within the established principles of plant water relations. In particular, there are several reasons, consistent with the established principles, why leaf water potential and xylem pressure may differ.     

Effects of Osmotic Potential in Nutrient Solution on Diurnal Growth of Tomato Fruit

Tomato fruit on plants grown in circulating nutrient solutionexhibited a diurnal cycle in growth rate, measured as a changein diameter, with a maximum during thc day. The diurnal growthcycle was less evident in those fruit grown at high electricalconductivity (17 mS), or on days of reduced irradiance. Girdledfruit of low conductivity plants grew at a much reduced ratewith a diurnal cycle in reverse to that of ungirdled fruit,while girdled fruit of high conductivity plants showed no diurnalgrowth. The evidence suggests that phloem and xylem water transportinto fruit operate on opposite diurnal cycles. Partitioning of available xylem water in detached fruit betweenthe calyx and berry, as well as within the berry, was determinedby berry size and relative humidity in the air. Although berrytranspiration rate was unaffected by conductivity treatmentduring plant growth, water uptake capacity was greatly reducedin the berry from high conductivity plants, suggesting an increasedresistance in the xylem transport system within the fruit. Key words: Salinity, electrical conductivity, tomato fruit, xylem transport, transpiration     

Heavy browsing affects the hydraulic capacity of Ceanothus rigidus (Rhamnaceae)

Defoliation by herbivores can reduce carbon assimilation, change plant water relations, and even shift the biotic structure of plant communities. In this study, we took advantage of a long-term deer exclosure experiment to examine the consequences of persistent deer herbivory on plant water relations and the xylem structure–function relationships in Ceanothus rigidus, a maritime chaparral shrub in coastal California. Browsed plants had thicker stems with many intertwined short distal twigs, and significantly higher sapwood-to-leaf area ratios than their non-browsed counterparts. Leaf area-specific hydraulic conductivity was similar in both browsed and non-browsed plants, but xylem area-specific conductivity was significantly lower in the browsed plants. Vessel diameters were equivalent in both plant groups, but the number of vessels on a transverse area basis was nearly 40 % lower in the browsed plants, accounting for their lower transport efficiency. Mid-day in situ water potentials and losses of hydraulic conductivity due to embolism were similar in both groups of plants but stomatal conductance was higher in the browsed shrubs in the early part of the growing season. We discuss our findings in the context of whole-plant ecophysiology, and explore the consequences of herbivory on hormonal signals, wood anatomy, and xylem function.     

树木树液上升机理研究进展

水分在植物体内的运输一直是很多植物生理生态学家所关注的一个重要问题。介绍了内聚力学说的基本假设和其存在争议,总结了近年来这一研究领域的几个热点问题,主要包括：（1）木质部栓塞及其恢复机理;（2）木质部压力探针和压力室法测定的木质部张力值不一致的现象及其可能原因;（3）补偿压学说;（4）不同界面层张力以及输水管道的毛细作用力、薄壁细胞膨压和木质部渗透压、逆向蒸腾等在树木汁液上升中的贡献;（5）最近发现的存在于木质部导管伴胞和韧皮部薄壁细胞等质膜中的水孔蛋白在植物水分运输中的调控作用等。这些方面在解释树木的树液上升中都起着重要的作用。     

Migration and the genetic covariance between habitat preference and performance

Studies of the genetic covariance between habitat preference and performance have reported conflicting outcomes ranging from no covariance to strong covariance. The causes of this variability remain unclear. Here we show that variation in the magnitude of genetic covariance can result from variability in migration regimes. Using data from walking stick insects and a mathematical model, we find that genetic covariance within populations between host plant preference and a trait affecting performance on different hosts (cryptic color pattern) varies in magnitude predictably among populations according to migration regimes. Specifically, genetic covariance within populations is high in heterogeneous habitats where migration between populations locally adapted to different host plants generates nonrandom associations (i.e., linkage disequilibrium) between alleles at color pattern and host preference loci. Conversely, genetic covariance is low in homogeneous habitats where a single host exists and migration between hosts does not occur. Our results show that habitat structure and patterns of migration can strongly affect the evolution and variability of genetic covariance within populations.     

A carbon cost-gain model explains the observed patterns of xylem safety and efficiency

Efficient water transport from the soil to the leaves is essential for plant function, while building and maintaining the water transport structure in the xylem require a major proportion of the assimilated carbon of the tree. Xylem transport also faces additional challenges as water in the xylem is under tension and therefore cavitation cannot be completely avoided. We constructed a model that calculates the xylem structure that maximizes carbon-use efficiency while simultaneously taking into account pit structure in increasing the resistance to water transport and constricting the spreading of embolisms. The optimal xylem structure predicted by the model was found to correspond well to the generally observed trends: xylem conduits grew in size from the apex towards the base while simultaneously decreasing in number, and vulnerability to cavitation increased with conduit size. These trends were caused primarily by the axial water potential gradient in the xylem. The pits have to be less porous near the apex where water potential is lower to restrict the spreading of embolisms, while whole-plant carbon-use efficiency demands that conduit size decreases and conduit number increases simultaneously. The model predictions remained qualitatively the same regardless of the exact optimality criterion used for defining carbon-use efficiency.     

Structure and function of bordered pits: new discoveries and impacts on whole-plant hydraulic function

Bordered pits are cavities in the lignified cell walls of xylem conduits (vessels and tracheids) that are essential components in the water-transport system of higher plants. The pit membrane, which lies in the center of each pit, allows water to pass between xylem conduits but limits the spread of embolism and vascular pathogens in the xylem. Averaged across a wide range of species, pits account for > 50% of total xylem hydraulic resistance, indicating that they are an important factor in the overall hydraulic efficiency of plants. The structure of pits varies dramatically across species, with large differences evident in the porosity and thickness of pit membranes. Because greater porosity reduces hydraulic resistance but increases vulnerability to embolism, differences in pit structure are expected to correlate with trade-offs between efficiency and safety of water transport. However, trade-offs in hydraulic function are influenced both by pit-level differences in structure (e.g. average porosity of pit membranes) and by tissue-level changes in conduit allometry (average length, diameter) and the total surface area of pit membranes that connects vessels. In this review we address the impact of variation in pit structure on water transport in plants from the level of individual pits to the whole plant.     

Phytoextraction: simulating uptake and translocation of arsenic in a soil-plant system

The uptake, transport, and accumulation of metals by plants are functions central to successful phytoextraction. This study investigates the uptake and translocation of arsenic from a contaminated sandy soil by a mature Chinese brake fern (Pteris vittata L.). An existing mathematical model for the coupled transport of water, heat, and solutes in the soil-plant-atmosphere continuum (CTSPAC) was modified to examine the flow of water as well as the uptake and translocation of total arsenic in the xylem of the fern. This model was calibrated using greenhouse measurements before its application. Simulation results showed that about 20% of the soil arsenic was removed by the fern in 10 d, of which about 90% of the arsenic was stored in the fronds and 10% in the roots. Although arsenic mass in the plant tissues increased consecutively with time, arsenic concentration in the xylem sap of the root tips has a typical diurnal distribution pattern: increasing during the day and decreasing at night, resulting from daily variations of frond surface water transpiration. The largest difference in simulated arsenic concentration in the root tips between the day and night was about 5%. This study also suggests that the use of transpiration stream concentration factor (TSCF), which is defined as the ratio of chemical concentration in the xylem sap to that in the external solution, to evaluate the translocation efficiency of arsenic for the hyperaccumulator Chinese brake fern (Pteris vittata L.) could be limited.     

Identification and characterization of Arabidopsis thaliana genes involved in xylem secondary cell walls

The xylem of higher plants offers support to aerial portions of the plant body and serves as conduit for the translocation of water and nutrients. Terminal differentiation of xylem cells typically involves deposition of thick secondary cell walls. This is a dynamic cellular process accompanied by enhanced rates of cellulose deposition and the induction of synthesis of specific secondary-wall matrix polysaccharides and lignin. The secondary cell wall is essential for the function of conductive and supportive xylem tissues. Recently, significant progress has been made in identifying the genes responsible for xylem secondary cell wall formation. However, our present knowledge is still insufficient to account for the molecular processes by which this complex system operates. To acquire further information about xylem secondary cell walls, we initially focused our research effort on a set of genes specifically implicated in secondary cell wall formation, as well as on loss-of-function mutants. Results from two microarray screens identified several key candidate genes responsible for secondary cell wall formation. Reverse genetic analyses led to the identification of a glycine-rich protein involved in maintaining the stable structure of protoxylem, which is essential for the transport of water and nutrients. A combination of expression analyses and reverse genetics allows us to systematically identify new genes required for the development of physical properties of the xylem secondary wall.     

Accumulation of weathered pp'-DDE in xylem sap of grafted watermelon

Movement of weathered p,p'-dichlorodiphenyldichloroethane (p,p'-DDE) from contaminated soil to the rhizosphere pore water to the xylem sap of grafted watermelon was studied under green house conditions. p,p'-DDE concentrations in pore water and xylem sap was compared in intact plants, homografted, and compatible heterografts of Cucurbita pepo spp. pepo and Citrullus lanatus plants. An average p,p'-DDE concentrations in pore water of contaminated soil ranged from 0.36 microg/L to 0.55 microg/L and there were no statistically significant among the cultivars. Conversely, the xylem sap p,p'-DDE concentration of heterografted watermelon having a zucchini rootstock and watermelon scion was 71 microg/L and it was greater than intact watermelon plants (0.49 microg/L) but less than that of intact plants of zucchini (141 microg/L). Homografting showed no effect on xylem sap p,p'-DDE concentrations of the identical cultivars. The bio-concentration factors (BCFs) which is an average p,p'-DDE concentration in xylem sap over average p,p'-DDE in pore water were 344, 325, 197, 1.28, and 0.89 for intact plant of zucchini, homografted zucchini, heterografted watermelon, homografted watermelon, and intact plant of watermelon, respectively. Xylem sap p,p'-DDE concentrations of the heterografted watermelon plants were clearly influenced by plant phylogeny and enhanced by the zucchini rootstock compared to intact watermelon plants.     

MRI of long-distance water transport: a comparison of the phloem and xylem flow characteristics and dynamics in poplar, castor bean, tomato and tobacco

We used dedicated magnetic resonance imaging (MRI) equipment and methods to study phloem and xylem transport in large potted plants. Quantitative flow profiles were obtained on a per-pixel basis, giving parameter maps of velocity, flow-conducting area and volume flow (flux). The diurnal xylem and phloem flow dynamics in poplar, castor bean, tomato and tobacco were compared. In poplar, clear diurnal differences in phloem flow profile were found, but phloem flux remained constant. In tomato, only small diurnal differences in flow profile were observed. In castor bean and tobacco, phloem flow remained unchanged. In all plants, xylem flow profiles showed large diurnal variation. Decreases in xylem flux were accompanied by a decrease in velocity and flow-conducting area. The diurnal changes in flow-conducting area of phloem and xylem could not be explained by pressure-dependent elastic changes in conduit diameter. The phloem to xylem flux ratio reflects what fraction of xylem water is used for phloem transport (Münch's counterflow). This ratio was large at night for poplar (0.19), castor bean (0.37) and tobacco (0.55), but low in tomato (0.04). The differences in phloem flow velocity between the four species, as well as within a diurnal cycle, were remarkably small (0.25-0.40 mm s(-1)). We hypothesize that upper and lower bounds for phloem flow velocity may exist: when phloem flow velocity is too high, parietal organelles may be stripped away from sieve tube walls; when sap flow is too slow or is highly variable, phloem-borne signalling could become unpredictable.     

The water relations of the root–soil interface

The difference in hydrostatic pressure between the xylem of the leaf and the soil depends, for a given transpiration rate, on the series of hydraulic resistances encountered along this pathway. Many studies have shown that the sum of the resistances in the plant and the soil is too small to account for the fall in water pressure between the leaf xylem and the soil, especially when plants are growing in sandy soils, which are prone to dry rapidly. A resistance at the root–soil interface, caused possibly by poor contact between the roots and the soil, has been proposed to account for the discrepancy. We explored the resistance in the pathway from soil to leaf using a technique that allows precise and continuous non-destructive measurement of the hydrostatic pressure in the leaf xylem. When the soil was leached with water, the fall in leaf water status as the soil dried was reasonably well described by a simple physical model without the need to invoke an interfacial resistance. However, when the soil was flushed with a nutrient solution with an osmotic pressure of 70kPa, the hydrostatic pressure in the leaf xylem fell several times faster than that in the soil. We suggest that solutes accumulated either in the root or just outside it, creating large osmotic pressures, which gave the appearance of an interfacial resistance.     

Intact plant MRI for the study of cell water relations, membrane permeability, cell-to-cell and long distance water transport

Water content and hydraulic conductivity, including transport within cells, over membranes, cell-to-cell, and long-distance xylem and phloem transport, are strongly affected by plant water stress. By being able to measure these transport processes non-invasely in the intact plant situation in relation to the plant (cell) water balance, it will be possible explicitly or implicitly to examine many aspects of plant function, plant performance, and stress responses. Nuclear magnetic resonance imaging (MRI) techniques are now available that allow studying plant hydraulics on different length scales within intact plants. The information within MRI images can be manipulated in such a way that cell compartment size, water membrane permeability, water cell-to-cell transport, and xylem and phloem flow hydraulics are obtained in addition to anatomical information. These techniques are non-destructive and non-invasive and can be used to study the dynamics of plant water relations and water transport, for example, as a function of environmental (stress) conditions. An overview of NMR and MRI methods to measure such information is presented and hardware solutions for minimal invasive intact plant MRI are discussed.     

自然和人工生境被子植物枝木质部结构与功能差异

水是植物生存与生长的基础条件, 水分有效性影响植物木质部解剖结构、水力功能, 使之形成特定的适应特征。因此, 对比自然与人工生境中同一植物的水力功能与解剖结构差异, 有助于理解植物对水分环境的适应机理。该研究以湿润区三角槭(Acer buergerianum)、青冈(Cyclobalanopsis glauca)和女贞(Ligustrum lucidum)为研究材料, 对比分析了自然和人工生境中各物种的栓塞抗性(导水率损失50%时的水势(P₅₀))、输水效率(比导率(K_s))和解剖结构(导管直径(D)、导管壁厚(T)、导管密度(N)、木质部密度(WD)、厚度跨度比(t/b)²)特征, 探究了同生境种内与跨生境、跨物种水平的效率-安全权衡关系, 量化分析了水力功能与解剖结构的关系。结果发现: 1) 3种被子植物在自然生境中K_s更大, P₅₀更小, 与其更大的D、更小的(t/b)²有关。2)同生境种内K_s与P₅₀不存在权衡。3)功能性状和解剖结构相关分析表明: 同生境种内D与P₅₀不存在显著的相关关系; 除自然生境女贞外, T、(t/b)²均与P₅₀正相关。相对于人工生境, 在水分有效性低或无额外浇灌的自然生境中, 植物通过增大导管直径显著提高其输水效率, 从而避免水势下降、降低潜在栓塞风险。     

An analysis of long-distance water transport in the soybean stem using H215O

The lateral water movement in the intact stem of a transpiring soybean plant was analyzed quantitatively by a real-time measurement system utilizing labeled water, H(2)(15)O and gamma ray detectors. A large volume of water escaping from xylem vessels during its transport was detected. The escape of water was not influenced by evaporation from the stem surface or mass flow in the sieve tubes. It was assumed that the total amount of water transported through xylem vessels was kept almost completely constant along the internode. As a result, most of the escaped water was found to re-enter the xylem vessels, i.e. water exchange occurred. The analysis of radiographs of tritiated water suggested that the self-diffusion effect of water was strong for lateral water movement, although another driving force besides thermal motion was included in the process, and that the process was also affected by the water permeability of the plasma membrane. An analysis based on a mathematical model showed that the net volume of water which escaped from xylem vessels was not dependent on the transpiration rate of the plant.