用热脉冲速度记录仪（HPVR）测定树干液流

树木蒸腾耗水是环境生态平衡(水分)的重要因素。由于树体高大,环境、时间、空间变异因素复杂,测定工作十分困难。在林木生态系统中,水分运动的途径是,树木根部吸收土壤水分,通过树干(木质部上升液流)输送到树冠部,从叶表面蒸腾散失到大气中,即所谓“土壤-植物-大气连续系统”。在此过程中,树干是水流通道的咽喉部位,树干液流量的大小制约着冠部蒸腾量的变化。因此,可以用测定树干部液流的方法确定树冠的蒸腾耗水     

植物水分传输过程中的调控机制研究进展

农田土壤水分的转化利用与调控是以土壤-植物-大气连续体(SPAC)为基础,以植物为核心,其中水分在植物体内的传输与调控研究一直是国际学术研究的前沿性热点课题。本文概述了植物水分传输的驱动力和传输途径,重点从植物的气孔调节、水容调节、渗透调节、水孔蛋白调节、贮水调节、气穴和栓塞调节等方面综述了植物水分传输过程中的调控机制研究进展。通过对植物存在优化调控水分平衡的潜在能力的研究,不仅可充实SPAC系统水分传输理论,而且有助于明确植物对环境的适应机制和高效用水的潜力及其节水调控的效应,对指导干旱半干旱地区农业生产提供理论依据。     

树木抗旱机理研究进展

干旱是树木成活与生长的重要限制因素之一。研究树木的抗旱性,有助于了解树木的干旱适应性机制,探求树木抗旱的适宜调控措施,对充分发挥森林的生态效益具有重要作用。本文从树木的水分关系、净光合速率与蒸腾速率、叶片的δ13C值、木质部导管空穴、光合机制与抗氧化保护机制、渗透调节、脱落酸和C4植物的抗旱机理及树木的抗旱基因与遗传特性(包括水通道蛋白、逆境信号转导、树木抗旱性的遗传基因工程)方面对树木抗旱机理研究进行了综述。     

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。     

植物木质部水力学研究进展

植物为适应陆地环境进化出木质部维管系统,通过水力学机制高效安全的向光合器官长距离运输水分,木质部水分运输对蒸腾、气孔运动、光合碳同化等生理过程有调控和协调作用,被称为植物生理学的支柱。植物水力学作为木质部水分运输的研究内容和手段,已成为整合植物与生态系统功能的中心枢纽。该文首先概述了植物水分运输的水力学机制、运输系统的局限性,以及木质部结构与功能之间的关系;其次,阐述了木质部栓塞的形成机制并详细介绍了栓塞的诱导方法和测试技术,分析了水分运输系统安全与效率之间的权衡关系,总结了植物对环境的响应和干旱致死的预测模型,讨论了测试技术问题及其引发的当前木质部逆压力修复和指数型木质部栓塞脆弱性曲线有效性的争议;最后,总结了目前植物木质部水力学研究的成果,提出了尚待解决的主要问题,探讨了研究机会与方向。     

植物气孔气态失水与SPAC系统液态供水的相互调节作用研究进展

讨论了植物气孔气态失水与SPAC系统液态供水相互作用研究领域的一些重要现象和行为.当植物水力信号和化学信号共同作用促进气孔对叶水势的调节时,植物对叶水势的调节表现为等水行为.气孔对环境湿度变化响应的反馈机制可用来解释土壤干旱条件下气孔和光合的午休现象,以及气孔导度和水流导度之间的相关关系;而气孔对环境湿度变化响应的前馈机制,则可用来解释气孔导度对大气 叶片间水汽饱和差的滞后反应.植物最大限度地利用木质部传输水分的策略,要求气孔快速响应以避免木质部过度气穴化和短时间内将气穴逆转的相应机制.     

应用热技术研究树干液流进展

综述了热技术方法测定树干液流的基本原理和不同适用范围．通过各种热技术方法可以确定树干水分运输格局及其数量;与树木生理指标和环境因素联合测定,可深入分析整树水分导度、气孔导度、边界层导度、水势及树干储水与树木蒸腾之间的关系,探讨树干液流受外界环境因素影响程度及其响应,揭示树木蒸腾内在的调节机制和外在影响因素;热技术可用于长期连续测定地带性森林主要树种蒸腾耗水特征。为正确评价森林的水文效应提供技术支持．     

木本植物水力系统对干旱胁迫的响应机制

干旱导致树木死亡对生态系统功能和碳平衡有重大影响。植物水分运输系统失调是引发树木死亡的主要机制。然而, 树木对干旱胁迫响应的多维性和复杂性, 使人们对植物水分运输系统在极端干旱条件下的响应以及植物死亡机理的认识还不清楚。该文首先评述衡量植物抗旱性的指标, 着重介绍可以综合评价植物干旱抗性特征的新参数——气孔安全阈值(SSM)。SSM越高, 表明气孔和水力性状之间的协调性越强, 木质部栓塞的可能性越低, 水力策略越保守。然后, 阐述木本植物应对干旱胁迫的一般响应过程。之后, 分别综述植物不同器官(叶、茎和根)对干旱胁迫的响应机制。植物达到死亡临界阈值的概率和时间, 取决于相关生理和形态学特征的相互作用。最后, 介绍木本植物水力恢复机制, 并提出3个亟待开展的研究问题: (1)改进叶片水分运输(木质部和木质部外水力导度)的测量方法, 量化4种不同途径的叶肉水分运输的相对贡献; (2)量化叶片表皮通透性变化, 以便更好地理解植物水分利用策略; (3)深入研究树木水碳耦合机制, 将个体结构和生理特征与群落/景观格局和过程相关联, 以便更好地评估和监测干旱诱导树木死亡的风险。     

树木储存水对水力限制的补偿研究进展

水力限制假说认为水分传输阻力与水势梯度协同调节气孔气体交换、影响CO₂的吸收进而限制树木的高生长,比较合理地解释了不同生境树木极限高度和高生长的差异.但该理论并未考虑到水力结构和其他生物学特征在树木向上生长的同时会进行适应性调整,以减弱逐渐增加的水分传输阻力.树木储存水对缓解木质部的水力限制、控制叶片水势的波动具有重要的生物学意义,也可能是气孔调节和水分状况变化格型的重要决定因素,其对水力限制可起到部分补偿作用.本文对储存水影响树木水分利用过程中的水力限制进行了综述,探讨了储存水补偿水力限制的可能性机理以及相对应的研究方法,并对未来的研究方向进行了展望.     

木本植物木质部栓塞修复机制:研究进展与问题

维持木本植物体内长距离的水分运输对于植物生存、生长和发育非常重要,但因水分在木质部张力状态下处于亚稳定状态而易发生空穴化和栓塞,导致水力导度降低、生产力下降、甚至植物死亡。面对水分胁迫诱导的空穴化,植物可通过形成抵抗空穴化的解剖结构降低栓塞发生频率,或(和)通过活跃的代谢修复栓塞,其中对木质部栓塞及其修复的发生频率、条件、机制等的认识仍有很大分歧。为此,该文首先综述了木质部栓塞修复过程及时间动态、木质部栓塞形成及修复的发生频率。然后,总结了木质部导管"新的再充水"栓塞修复过程中的4种主要假说:(1)渗透调节假说;(2)反渗透调节假说;(3)韧皮部驱动再充水假说;(4)韧皮部卸载假说。在此基础上,比较了针叶树种和木本被子植物木质部栓塞形成与修复的差异,并分析了木质部栓塞阻力与修复能力之间的权衡关系。最后,提出了木本植物木质部栓塞与修复研究的4个优先研究问题:(1)改进木质部栓塞测定技术;(2)验证"新的再充水"栓塞修复机制假说及引发木质部再充水的信号;(3)阐明木质部栓塞与修复特性的树种间差异及其可能的权衡关系;(4)加强碳代谢和水通道蛋白表达与木质部栓塞及其修复关系的生理生化研究。     

Water stress induced cavitation and embolism in some woody plants

A comparison was made of the relative vulnerability of xylem conduits to cavitation and embolism in three species [ Thuja occidentalis L., Tsuga canadensis (L.) Carr. and Acer saccharum Marsh.]. Waterlogged samples of wood were air dehydrated while measuring relative water loss, loss of hydraulic conductance, cumulative acoustic emissions (= cavitations) and xylem water potential. Most cavitation events and loss of hydraulic conductance occurred while water potential declined from – 1 to –6 MPa. There were differences in vulnerability between species. Other people have hypothesized that large xylem conduits (e.g. vessels) should be more vulnerable to cavitations than small conduits (e.g. tracheids). Our findings are contrary to this hypothesis. Under water stress, the vessel bearing wood retained water better than tracheid bearing wood. However, within a species large conduits were more prone to cavitation than small conduits.     

Capacitive effect of cavitation in xylem conduits: results from a dynamic model

Embolisms decrease plant hydraulic conductance and therefore reduce the ability of the xylem to transport water to leaves provided that embolized conduits are not refilled. However, as a xylem conduit is filled with gas during cavitation, water is freed to the transpiration stream and this transiently increases xylem water potential. This capacitive effect of embolism formation on plant function has not been explicitly quantified in the past. A dynamic model is presented that models xylem water potential, xylem sap flow and cavitation, taking into account both the decreasing hydraulic conductance and the water release effect of xylem embolism. The significance of the capacitive effect increases in relation to the decreasing hydraulic conductance effect when transpiration rate is low in relation to the total amount of water in xylem conduits. This ratio is typically large in large trees and during drought.     

Wood day capacitance is related to water content,wood density,and anatomy across 30 temperate tree species

Water released from wood during transpiration (capacitance) can meaningfully affect daily water use and drought response. To provide context for better understanding of capacitance mechanisms, we investigated links between capacitance and wood anatomy. On twigs of 30 temperate angiosperm tree species, we measured day capacitance (between predawn and midday), water content, wood density, and anatomical traits, that is, vessel dimensions, tissue fractions, and vessel–tissue contact fractions (fraction of vessel circumference in contact with other tissues). Across all species, wood density (WD) and predawn lumen volumetric water content (VWC_L-pd) together were the strongest predictors of day capacitance (r²_adj = .44). Vessel–tissue contact fractions explained an additional ~10% of the variation in day capacitance. Regression models were not improved by including tissue lumen fractions. Among diffuse-porous species, VWC_L-pd and vessel–ray contact fraction together were the best predictors of day capacitance, whereas among semi/ring-porous species, VWC_L-pd, WD and vessel–fibre contact fraction were the best predictors. At predawn, wood was less than fully saturated for all species (lumen relative water content = 0.52 ± 0.17). Our findings imply that day capacitance depends on the amount of stored water, tissue connectivity and the bulk wood properties arising from WD (e.g., elasticity), rather than the fraction of any particular tissue.     

不同水分条件下两个树种木质部栓塞对P素添加的响应

 在两种水分供给(干旱胁迫和适宜水分,土壤含水量分别为田间持水量的30%～40%和70%～80%)下,研究了耐旱树种元宝枫(Acer truncatum)和 中生树种女贞(Ligustrum lucidum )木质部栓塞(以导水率(Percentage loss of hydraulic conductivity, PLC)损失程度衡量)对P素添加的 响应。结果发现,两个树种PLC的日变化均呈现出先上升后降低的规律,表明木质部栓塞的形成与恢复是植物体的一种平常事件;除适宜水分条 件的女贞外,P素可以显著提高元宝枫和遭受干旱胁迫时女贞的PLC;两种水分条件下,干旱胁迫时元宝枫木质部栓塞明显高于适宜水分供给时 。女贞的PLC在两种水分状况下无显著差异;树种间,干旱胁迫促进了元宝枫木质部的栓塞形成,明显高于同等水分条件下的女贞。该研究结果 证实了“木质部限流耐旱假设”。     

The relevance of xylem network structure for plant hydraulic efficiency and safety

The xylem is one of the two long distance transport tissues in plants, providing a low resistance pathway for water movement from roots to leaves. Its properties determine how much water can be transported and transpired and, at the same time, the plant's vulnerability to transport dysfunctions (the formation and propagation of emboli) associated to important stress factors, such as droughts and frost. Both maximum transport efficiency and safety against embolism have classically been attributed to the properties of individual conduits or of the pit membrane connecting them. But this approach overlooks the fact that the conduits of the xylem constitute a network. The topology of this network is likely to affect its overall transport properties, as well as the propagation of embolism through the xylem, since, according to the air-seeding hypothesis, drought-induced embolism propagates as a contact process (i.e., between neighbouring conduits). Here we present a model of the xylem that takes into account its system-level properties, including the connectivity of the xylem network. With the tools of graph theory and assuming steady state and Darcy's flow we calculated the hydraulic conductivity of idealized wood segments at different water potentials. A Monte Carlo approach was adopted, varying the anatomical and topological properties of the segments within biologically reasonable ranges, based on data available from the literature. Our results showed that maximum hydraulic conductivity and vulnerability to embolism increase with the connectivity of the xylem network. This can be explained by the fact that connectivity determines the fraction of all the potential paths or conduits actually available for water transport and spread of embolism. It is concluded that the xylem can no longer be interpreted as the mere sum of its conduits, because the spatial arrangement of those conduits in the xylem network influences the main functional properties of this tissue. This brings new arguments into the long-standing discussion on the efficiency vs. safety trade-off in the plants' xylem.     

Wood allocation trade-offs between fiber wall,fiber lumen,and axial parenchyma drive drought resistance in neotropical trees

Functional relationships between wood density and measures of xylem hydraulic safety and efficiency are ambiguous, especially in wet tropical forests. In this meta-analysis, we move beyond wood density per se and identify relationships between xylem allocated to fibers, parenchyma, and vessels and measures of hydraulic safety and efficiency. We analyzed published data of xylem traits, hydraulic properties and measures of drought resistance from neotropical tree species retrieved from 346 sources. We found that xylem volume allocation to fiber walls increases embolism resistance, but at the expense of specific conductivity and sapwood capacitance. Xylem volume investment in fiber lumen increases capacitance, while investment in axial parenchyma is associated with higher specific conductivity. Dominant tree taxa from wet forests prioritize xylem allocation to axial parenchyma at the expense of fiber walls, resulting in a low embolism resistance for a given wood density and a high vulnerability to drought-induced mortality. We conclude that strong trade-offs between xylem allocation to fiber walls, fiber lumen, and axial parenchyma drive drought resistance in neotropical trees. Moreover, the benefits of xylem allocation to axial parenchyma in wet tropical trees might not outweigh the consequential low embolism resistance under more frequent and severe droughts in a changing climate.     

Relationships between embolism,stem water tension,and diameter changes

A model for embolism in the sapflow process was developed, in which embolism is described as a physical process linked to real physical properties of the conduits and the thermodynamic state of water. Different mechanisms leading to embolism and their effect on water relations and especially diurnal diameter changes in a tree were examined. The mechanisms of heterogeneous nucleation, air-seeding, and bubble growth have been considered. The significance of embolism has been revealed here by examining diameter changes, which is an easily measurable quantity under field conditions. The most fundamental effects of embolism on sapflow are decrease in permeability and release of water from embolizing conduits to the transpiration stream. These can be indirectly detected by observing diameter changes. If possible changes in elasticity are not accounted for, embolism generally tends to enhance the amplitude of the diurnal diameter changes due to reduced permeability and increased tensions. In the case of reduced elasticity, embolism gives rise to smaller amplitudes of diameter changes.     

A plumber's-eye view of xylem water transport in woody plants

We present a practical for university-level students aimed at measuring and comparing xylem hydraulic properties of co-existing plant species. After sampling branches of several woody species in the field, their main hydraulic properties were measured using a simple set-up. Hydraulic conductivity (K_h ) was calculated as the ratio between water flow through a plant segment and the pressure gradient driving the flow. The percent reduction in conductivity due to xylem embolism (i.e. air-filled conduits) was estimated by comparing K_h before and after flushing the measure segments to remove all native embolism. Raw hydraulic conductivity was standardised by cross-sectional wood area or supported leaf area to obtain more meaningful measures of conducting capacity. The results showed differences among study species, particularly between conifers and angiosperms. These differences are briefly discussed in terms of wood anatomy and the general biology of the species. Overall the practical provides a good opportunity for students to appreciate the main aspects of xylem water transport and the constraints it imposes on plant water relations.     

Wood anatomy of some trees with diffuse- and ring-porous wood: Some functional and ecological interpretations

Abstract

The xylem conduit dimensions (i.e. their width and length) have been measured in 1-year-old internodes, nodes and node-to-petiole (N-P) junctions of three species with diffuse-porous wood, namely Ceratonia siliqua L., Laurus nobilis L. and Olea europaea L. as well as of three species with ring-porous wood, namely Quercus ilex L., Q. suber L. and Q. pubescens Willd‥ The xylem conduit diameter and length distributions have been related to the drought resistance strategies adopted by the six species. C. siliqua and Q. ilex (drought avoiding water spenders) showed the widest xylem conduits (each species within its characteristic pattern of wood anatomy). This is consistent with their high demand of efficient water transport to leaves. L. nobilis (drought avoiding water saver) showed relatively narrow xylem conduits, efficient enough, however, to assure water supply to leaves at the reduced transpiration rate exhibited by the species. O. europaea, Q. suber and Q. pubescens (drought tolerants) showed the narrowest xylem conduits but also the longest ones. The xylem system of C. siliqua and Q. ilex represented a good compromise between efficiency and safety of the water transport, the former as due to wide xylem conduits, the latter to the reduced xylem conduit length as well as to the strong «hydraulic constrictions» at their nodes and N-P junctions. The ecological interpretation of such hydraulic architecture is discussed.     

A Microfluidic Pump/Valve Inspired by Xylem Embolism and Transpiration in Plants

In plants, transpiration draws the water upward from the roots to the leaves. However, this flow can be blocked by air bubbles in the xylem conduits, which is called xylem embolism. In this research, we present the design of a biomimetic microfluidic pump/valve based on water transpiration and xylem embolism. This micropump/valve is mainly composed of three parts: the first is a silicon sheet with an array of slit-like micropores to mimic the stomata in a plant leaf; the second is a piece of agarose gel to mimic the mesophyll cells in the sub-cavities of a stoma; the third is a micro-heater which is used to mimic the xylem embolism and its self-repairing. The solution in the microchannels of a microfluidic chip can be driven by the biomimetic “leaf” composed of the silicon sheet and the agarose gel. The halting and flowing of the solution is controlled by the micro-heater. Results have shown that a steady flow rate of 1.12 µl/min can be obtained by using this micropump/valve. The time interval between the turning on/off of the micro-heater and the halt (or flow) of the fluid is only 2∼3 s. This micropump/valve can be used as a “plug and play” fluid-driven unit. It has the potential to be used in many application fields.