A theoretical model of hydraulic conductivity recovery from embolism with comparison to experimental data on Acer saccharum

A theoretical model of bubble dissolution in xylem conduits of stems was designed using the finite differential method and iterative calculations via computer. The model was based on Fick's, Henry's and Charles' laws and the capillary equation. The model predicted the tempo of recovery from embolism in small diameter branches of woody plants with various xylem structures under different xylem water pressures. The model predicted the time required to recover conductivity in any position in the stem. Repeated iterative solution of the model for different situations yielded an empirical formula to calculate the time for complete recovery of conductivity in stems from a fully embolised initial state. The time, t_p, is given by: where α is a temperature coefficient; D is the coefficient of diffusion of air in wood at 25°C; r_cs is the ratio of the area of total conduit cross section to the stem cross section; Ψ_xp is the stem xylem pressure potential (Pa, where 0 Pa equals atmospheric pressure); τ is solution surface tension (0.072 N m^?1); and D_c and D_s are diameters of the conduits and the stem, respectively (m). The equation is valid only when Ψ_xp > –4τ/D_c. The model predicts no recovery of conductivity when Ψ_xp≤–4τ/D_c. The model agreed with experiments.     

Diurnal variation in xylem hydraulic conductivity in white ash (Fraxinus americana L.), red maple (Acer rubrum L.) and red spruce (Picea rubens Sarg.)

Xylem hydraulic conductivity and percentage loss of conductivity (PLC) were measured on a ring-porous ( Fraxinus americana L., white ash), a diffuse porous ( Acer rubrum L., red maple) and a coniferous ( Picea rubens Sarg., red spruce) tree species in a temperate deciduous forest in central Massachusetts, USA. Measurements were made on current and 1-year-old branch segments in the afternoon and on the following morning. Afternoon PLC was 45 to 70% for the current year's extension growth in both white ash and red maple. Morning PLC was significantly lower (10–40%). Conductivity also varied diurnally suggesting, on average, a 50% recovery from cavitation overnight. Red spruce showed lower PLC and conductivity and a less pronounced night-time recovery. Diurnal variation in hydraulic conductivity and PLC suggests that embolism removal occurred in all three species despite the existence of tension within the xylem. Further evidence for embolism removal was observed with an in situ double-staining experiment in which dyes were fed to a transpiring branch during the late afternoon and the following morning. Examination of stem cross-sections showed that a larger number of vessels were conductive in the morning than on the preceding afternoon. Results of this study suggest that hydraulic capacity is highly dynamic and that conductivity measurements reflect a balance between two processes: cavitation and embolism removal.     

植物木质部栓塞测定技术的研究进展

气候变化引发干旱频度和强度的变化影响植物的生长发育和生态适应。干旱胁迫会引起木质部栓塞并造成水力失效,而如何准确量化木质部抗栓塞的能力对研究植物对干旱的响应过程尤为重要。通常可通过脆弱性曲线量化木质部抗栓塞的能力。目前已经研发出构建木质部栓塞脆弱性曲线的多种方法,但不同方法往往产生不一致的结果。深入理解这些方法的设计原理并在实际应用时比较各方法的异同,对合理解释相关文献数据及准确选择干旱预测模型等尤为重要。本文阐述了自然干燥法、离心法、注气法、声学测定法、同步加速器与X射线显微断层扫描法、光学可视化法及抽气法7种测定木质部栓塞脆弱性的方法,并总结了近年来各测定方法在具体研究中的运用情况及存在的争议。最后,对未来研究测定木质部栓塞脆弱性与实际运用相关方法的选择等提出了展望。     

Stomatal control of xylem embolism

Abstract. The potential role of stomatal closure in the control of xylem embolism is investigated by means of a simple model of hydraulic flow in plants. Maintenance of a maximally efficient conducting system requires the stomata to close in an appropriate fashion as evaporative demand increases in order to prevent shoot water potentials falling below the threshold value at which cavitations occur. The model showed that the optimal stomatal behaviour required depends on soil water availability. Further analysis of the model demonstrated that there could be certain circumstances where loss of a proportion of the conducting tissue by embolisms can, perhaps surprisingly, be beneficial in terms of maximizing stomatal aperture and hence short-term productivity. The results are discussed in relation to the signals controlling stomatal aperture, and it is shown that (1) optimal control cannot be obtained using information on leaf water potential alone, and (2) information relating to soil water potential is a necessary requirement for optimal control.     

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。     

胡杨木质部水分传导对盐胁迫的响应与适应北大核心CSCD

解析植物木质部导水率对逆境的响应和适应对促进植物抗逆性机理研究和受损植被恢复具有重要意义。该文以荒漠河岸林建群种胡杨(Populus euphratica)为研究对象,系统分析了胡杨幼株根、茎、叶水分传输通道对不同浓度盐胁迫的响应和适应。结果表明:(1)胡杨幼株根系对盐胁迫的敏感性高于茎和叶,盐胁迫下根系生长和根尖数显著受到抑制,根木质部易于发生栓塞,导水率明显降低。(2)胡杨幼株茎木质部导水率对盐胁迫的响应依盐浓度而定,轻度(0.05 mol·L–1 Na Cl)和中度(0.15 mol·L–1 Na Cl)盐胁迫下,胡杨可以通过协调导管输水的有效性和安全性来调节木质部的导水率,维持植物正常生长;重度(0.30 mol·L–1 Na Cl)盐胁迫下,胡杨茎木质部导管输水有效性和安全性均明显降低,木质部导水率显著下降,并伴随叶片气孔导度的显著降低,从而严重抑制了胡杨的光合和生长。     

胡杨木质部水分传导对盐胁迫的响应与适应

解析植物木质部导水率对逆境的响应和适应对促进植物抗逆性机理研究和受损植被恢复具有重要意义。该文以荒漠河岸林建群种胡杨(Populus euphratica)为研究对象, 系统分析了胡杨幼株根、茎、叶水分传输通道对不同浓度盐胁迫的响应和适应。结果表明: (1)胡杨幼株根系对盐胁迫的敏感性高于茎和叶, 盐胁迫下根系生长和根尖数显著受到抑制, 根木质部易于发生栓塞, 导水率明显降低。(2)胡杨幼株茎木质部导水率对盐胁迫的响应依盐浓度而定, 轻度(0.05 mol·L^-1 NaCl)和中度(0.15 mol·L^-1 NaCl)盐胁迫下, 胡杨可以通过协调导管输水的有效性和安全性来调节木质部的导水率, 维持植物正常生长; 重度(0.30 mol·L^-1 NaCl)盐胁迫下, 胡杨茎木质部导管输水有效性和安全性均明显降低, 木质部导水率显著下降, 并伴随叶片气孔导度的显著降低, 从而严重抑制了胡杨的光合和生长。     

Interpretation of seasonal changes of xylem embolism and plant hydraulic resistance in Fagus sylvatica

The annual course of xylem embolism in twigs of adult beech trees was monitored, and compared to concurrent changes of tree water status and hydraulic resistances. Xylem embolism was quantified in 1-year-old apical twigs by the hydraulic conductivity as a percentage of the maximum measured after removal of air emboli. Tree and root hydraulic resistances were estimated from water potential differences and sap flux measurements. The considerable degree of twig embolism found in winter (up to 90% loss of hydraulic conductivity) may be attributed to the effect of freeze-thaw cycles in the xylem. A partial recovery from winter embolism occurred in spring, probably because of the production of new functional xylem. Xylem embolism fluctuated around 50% throughout the summer, without significant changes. Almost complete refilling of apical twigs was observed early in autumn. A significant negative correlation was found between xylem embolism and precipitation; thus, an active role of rainfall in embolism reversion is hypothesized. Tree and root hydraulic resistances were found to change throughout the growing period. A marked decrease of hydraulic resistance preceded the refilling of apical twigs in the autumn. Most of the decrease in total tree resistance was estimated to be located in the root compartment.     

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。     

Climate-related trends in sapwood biophysical properties in two conifers: avoidance of hydraulic dysfunction through coordinated adjustments in xylem efficiency, safety and capacitance

In the Pacific north‐west, the Cascade Mountain Range blocks much of the precipitation and maritime influence of the Pacific Ocean, resulting in distinct climates east and west of the mountains. The current study aimed to investigate relationships between water storage and transport properties in populations of Douglas‐fir (Pseudotsuga menziesii) and ponderosa pine (Pinus ponderosa) adapted to both climates. Sapwood thickness, capacitance, vulnerability to embolism, and axial and radial conductivity were measured on samples collected from trunks of mature trees. The sapwood of ponderosa pine was three to four times thicker than Douglas‐fir. Radial conductivity was higher in west‐side populations of both species, but axial conductivity was higher in the east‐side populations and in Douglas‐fir. Eastern populations of both species had sapwood that was more vulnerable to embolism than west‐side populations. Sapwood capacitance was similar between species, but was about twice as great in east‐side populations (580 kg m^?3 MPa^?1) as in west‐side populations (274 kg m^?3 MPa^?1). Capacitance was positively correlated with both mean embolism pressure and axial conductivity across species and populations, suggesting that coordinated adjustments in xylem efficiency, safety and water storage capacity may serve to avoid embolism along a gradient of increasing aridity.     

Drought resistance of Quercus pubescens as a function of root hydraulic conductance, xylem embolism and hydraulic architecture

Water relations, xylem embolism, root and shoot hydraulic conductance of both young plants in the field and potted seedlings of Quercus pubescens have been studied with the aim of investigating whether these variables may account for the well known adaptation of this oak species to arid habitats. Our data revealed that Q. pubescens is able to maintain high leaf relative water contents under water stress conditions. In fact, relative water contents measured in summer (July) did not differ from those recorded in April. This was apparently achieved by compensating water loss by an equal amount of water uptake. Such a drought avoidance strategy was made possible by the recorded high hydraulic efficiency of stems and roots under water stress. In fact, root hydraulic conductance of field-grown plants was maintained high in summer when the percentage loss of hydraulic conductance of stems was lowest. The hydraulic architecture of young plants of Q. pubescens measured in terms of partitioning of hydraulic resistances along the water pathway revealed that the highest hydraulic resistance was located in stems of the current year's growth. This hydraulic architecture is interpreted as consistent with the adaptation of Q. pubescens to arid habitats as a consequence of the recorded seasonal changes in water relation parameters as well as in root and stem hydraulics.     

Diurnal and seasonal variation in root xylem embolism in neotropical savanna woody species: impact on stomatal control of plant water status

Vulnerability to water-stress-induced embolism and variation in the degree of native embolism were measured in lateral roots of four co-occurring neotropical savanna tree species. Root embolism varied diurnally and seasonally. Late in the dry season, loss of root xylem conductivity reached 80% in the afternoon when root water potential (psi root) was about -2.6 MPa, and recovered to 25-40% loss of conductivity in the morning when psi root was about -1.0 MPa. Daily variation in psi root decreased, and root xylem vulnerability and capacitance increased with rooting depth. However, all species experienced seasonal minimum psi root close to complete hydraulic failure independent of their rooting depth or resistance to embolism. Predawn psi root was lower than psi soil when psi soil was relatively high (> -0.7 MPa) but became less negative than psi soil, later in the dry season, consistent with a transition from a disequilibrium between plant and soil psi induced by nocturnal transpiration to one induced by hydraulic redistribution of water from deeper soil layers. Shallow longitudinal root incisions external to the xylem prevented reversal of embolism overnight, suggesting that root mechanical integrity was necessary for recovery, consistent with the hypothesis that if embolism is a function of tension, refilling may be a function of internal pressure imbalances. All species shared a common relationship in which maximum daily stomatal conductance declined linearly with increasing afternoon loss of root conductivity over the course of the dry season. Daily embolism and refilling in roots is a common occurrence and thus may be an inherent component of a hydraulic signaling mechanism enabling stomata to maintain the integrity of the hydraulic pipeline in long-lived structures such as stems.     

A review of new research progress on the vulnerability of xylem embolism of woody plants

Xylem cavitation/embolism is the blockage of xylem conduits when woody plants suffer from water stress under drought and other environmental conditions, the study of embolism has become a hot and key topic under global climate change. Recent researches on the relationship between the vulnerability of xylem embolism and hydraulic architecture/drought tolerance have made some progress, however, scholars reached different conclusions based on results from different regions or different materials. This paper reviews the current achievements and controversial viewpoints, which includes indicator of xylem embolism vulnerability (P₅₀), method of vulnerability curve establishment, the relationship between embolism vulnerability and hydraulic architecture (vessel diameter, vessel length, pit area, wood density, fiber and fiber tracheid) and the relationship between embolism vulnerability and drought tolerance of woody plants. Future studies should use Cochard Cavitron centrifuge and Sperry centrifuge coupled with traditional methods to establish vulnerability curves, calculate P₅₀, analyze the difference among different organisms (root, stem, leaf), and measure physiological and ecological indexes. Future studies should be aimed to explore the relationship between the vulnerability of xylem embolism and hydraulic architecture/drought tolerance and to assess drought tolerance ability of different species under future climate change.     

A technique for measuring xylem hydraulic conductance under high negative pressures

A technique for measuring hydraulic conductances of excised xylem segments exposed to high negative pressures is described. A centrifugal force is used to generate negative pressures (P) in the sample and to create a positive hydrostatic pressure difference (ΔP) between its two ends. ΔP forces water through the sample at a flow rate (F) determined optically during centrifugation. The sample hydraulic conductance k is derived from F and ΔP. The sample vulnerability curve is given by the dependence of k on P. Results for Cedrus atlantica Manetti and Laurus nobilis L. shoots are given. The technique is appropriate for the analysis of xylem refilling under negative pressure.     

木本植物木质部栓塞脆弱性研究新进展

木质部空穴化和栓塞是木本植物在干旱等条件下遭受水分胁迫时产生的木质部输水功能障碍, 在全球气候变化的大背景下, 栓塞脆弱性对干旱响应的研究已成为热点和重要内容。近年来有关木质部栓塞脆弱性与植物输水结构和耐旱性的关系已有大量研究并取得一定成果, 但是, 不同学者在不同地区对不同材料的研究结果存在很大不同。该文就近年来这一研究领域取得的成果及争议问题进行了概括和总结, 主要涉及木质部栓塞脆弱性(P₅₀)及脆弱曲线的建立方法、木质部栓塞脆弱性与木质部结构(导管直径、导管长度、纹孔膜、木质部密度、纤维及纤维管胞)间的关系和木质部栓塞脆弱性与耐旱性的关系, 并对未来工作进行展望, 提出在未来的工作中应对同一树种使用Cochard Cavitron离心机法、Sperry离心机技术与传统方法建立的脆弱曲线进行比较验证、计算P₅₀值、分析植物个体器官水平差异(根、茎、叶)、测定树种生理生态指标, 探索植物栓塞脆弱性与输水结构和耐旱性的关系, 从而评估不同类型植物在未来气候变化下的耐旱能力。     

Ion-mediated increase in the hydraulic conductivity of Laurel stems: role of pits and consequences for the impact of cavitation on water transport

Changes in hydraulic conductivity (K(h)) were measured in stems of Laurus nobilis L. during perfusion with KCl, NaCl or sucrose solutions. Ionic solutes induced marked increase of K(h) with respect to deionized water but sucrose had no effect. The kinetics of KCl-induced K(h) increase was measured together with changes in [K(+)] of the perfused solution. K(h) increases were paralleled by increases in the [K(+)](out)/[K(+)](in) ratio. Samples of different lengths or with increasing percentage loss of conductivity (PLC) due to xylem cavitation were tested, with the aim of increasing radial flow through intervessel pits. KCl solutions enhanced the K(h) of 12-cm-long samples with a concentration-dependent effect up to 100 mm KCl. DeltaK(h) increased from 3 to 30% in 1.5- and 12-cm-long samples, respectively and remained constant for longer samples. Increasing PLC induced an exponential increase in DeltaK(h). PLC measured with KCl solutions was significantly less than that measured with deionized water, suggesting that measurements of PLC can be affected by the composition of the perfused solution. Experiments support the hypothesis that the 'ionic effect' is mediated by physico-chemical changes of pectins of the pit membranes and raise the possibility that plants might alter the ionic composition of the xylem sap to alleviate the hydraulic impact of cavitation.     

Changes in axial hydraulic conductivity along elongating leaf blades in relation to xylem maturation in tall fescue

Xylem maturation in elongating leaf blades of tall fescue ( Festuca arundinacea ) was studied using staining and microcasting. Three distinctive regions were identified in the blade: (1) a basal region, in which elongation was occurring and protoxylem (PX) vessels were functioning throughout; (2) a maturation region, in which elongation had stopped and narrow (NMX) and large (LMX) metaxylem vessels were beginning to function; (3) a distal, mature region in which most of the longitudinal water movements occurred in the LMX. The axial hydraulic conductivity ( K _h) was measured in leaf sections from all these regions and compared with the theoretical axial hydraulic conductivity ( K _t) computed from the diameter of individual inner vessels. K _t was proportional to K _h throughout the leaf, but K _t was about three times K _h. The changes in K _h and K _t along the leaf reflected the different stages of xylem maturation. In the basal 60 mm region, K _h was about 0.30±0.07 mmol s⁻¹ mm MPa⁻¹. Beyond that region, K _h rapidly increased with metaxylem element maturation to a maximum value of 5.0±0.3 mmol s⁻¹ mm MPa⁻¹, 105 mm from the leaf base. It then decreased to 3.5±0.2 mmol s⁻¹ mm MPa⁻¹ near the leaf tip. The basal expanding region was observed to restrict longitudinal water movement. There was a close relationship between the water deposition rate in the elongation zone and the sum of the perimeters of PX vessels. The implications of this longitudinal vasculature on the partitioning of water between growth and transpiration is discussed.     

Relationships among cytokinins, ethylene and poly amines during the stratification-germination process in seeds of Acer saccharum

Relatively high levels of dihydrozeatin and trans-zeatin were detected in unstratified seeds of Acer saccharum Marsh. Both cytokinins increased substantially over the first 20 days of stratification at 5°C and then fell rapidly to values well below original levels by first germination on day 55. In seeds held at 20°C, a non-afterripening temperature, cytokinin levels remained constant for the first 10 days and then declined to their lowest levels by day 20. Levels of putrescine, spermidine and spermine in the radicles and cotyledons did not change during the full course of the afterripening process, but large increments were noted during radicle emergence. A large increase in ethylene production at germination suggests that competition for S-adenosyl-methionine by the ethylene and polyamine biosynthetic pathways did not inhibit synthesis of ethylene or polyamines during seedling emergence and establishment. In seeds stratified at 20°C, ethylene showed an exceptionally large peak early in the stratification period, but polyamine contents remained low throughout the test. The present results are consistent with the hypothesis that cytokinins play a significant role in overcoming the metabolic block present in dormant seeds. This conclusion is supported by data showing that high levels of cytokinins develop concurrently with the start of tissue differentiation and at the time when abscisic acid and phenolic inhibitors decline in stratifying seeds. Changes in ethylene and polyamine contents did not correlate with any events in the afterripening process; however, large increases in levels of these substances were closely associated with the germinative process and, in the case of polyamines, specifically with the start of cell division.     

A new method for vulnerability analysis of small xylem areas reveals that compression wood of Norway spruce has lower hydraulic safety than opposite wood

Compression wood is formed at the underside of conifer twigs to keep branches at their equilibrium position. It differs from opposite wood anatomically and subsequently in its mechanical and hydraulic properties. The specific hydraulic conductivity (k_s) and vulnerability to drought‐induced embolism (loss of conductivity versus water potential ψ) in twigs of Norway spruce [Picea abies (L.) Karst.] were studied via cryo‐scanning electron microscope observations, dye experiments and a newly developed ‘Micro‐Sperry’ apparatus. This new technique enabled conductivity measurements in small xylem areas by insertion of syringe cannulas into twig samples. The hydraulic properties were related to anatomical parameters (tracheid diameter, wall thickness). Compression wood exhibited 79% lower k_s than opposite wood corresponding to smaller tracheid diameters. Vulnerability was higher in compression wood despite its narrower tracheids and thicker cell walls. The P₅₀ (ψ at 50% loss of conductivity) was ?3.6 MPa in opposite but only ?3.2 MPa in compression wood. Low hydraulic efficiency and low hydraulic safety indicate that compression wood has primarily a mechanical function.