Comparison of methods for detecting vulnerability of xylem embolism in Robinia pseudoacacia

在全球变暖的背景下, 植物木质部栓塞脆弱性是林木死亡率升高的重要生理学因素。然而不同方法在长导管树种上建立的栓塞脆弱性曲线存在较大差异。该研究以长导管树种刺槐(Robinia pseudoacacia)为研究对象, 利用自然干燥法、Cochard Cavitron离心机法以及Sperry离心机法建立了栓塞脆弱性曲线, 旨在探讨不同检测方法的合理性。在Sperry离心法中, 使用了两种规格的转子, 从而对“开口导管假象”学说进行了检验。研究结果表明: 自然干燥法建立的栓塞脆弱性曲线为“s”形, 而Cochard Cavitron离心机法和Sperry离心机法建立的栓塞脆弱性曲线为“r”形; 自然干燥法与离心机法建立的曲线存在显著性差异, 且两种离心机法建立的曲线也具有显著性差异。尽管刺槐枝条的导管长度分布表明14.4 cm长的刺槐枝条具有更高比例的开放导管, 但用Sperry离心机法在27.4 cm和14.4 cm长茎段上建立的栓塞脆弱性曲线相似, 表明Sperry离心机法检测刺槐脆弱性曲线时未产生“开口导管假象”, 具有更为可靠的检测结果。     

6个耐旱树种木质部结构与栓塞脆弱性的关系

木质部栓塞脆弱性对干旱响应的研究已成为全球气候变化背景下的热点和重要内容。该文以6个耐旱树种刺槐(Robinia pseudoacacia)、沙棘(Hippophae rhamnoides)、榆树(Ulmus pumila)、元宝枫(Acer truncatum)、旱柳(Salix matsudana)、榛(Corylus heterophylla)为研究对象, 采用Cochard Cavitron离心机技术建立木质部栓塞脆弱曲线, 计算木质部栓塞脆弱性, 利用染色法、硅胶注射法等测定木质部导管直径、导管内径跨度、导管连接度、导管密度、导管长度和木质部密度, 探究木质部结构与栓塞脆弱性的关系, 区分6个耐旱树种木质部结构在抗栓塞性上的差异, 以期建立6个耐旱树种在木质部结构方面的抗栓塞性指标。结果表明: 6个耐旱树种木质部栓塞脆弱性大小为刺槐>榆树>沙棘>旱柳>元宝枫>榛, 其中, 刺槐、沙棘和榆树的栓塞脆弱曲线为“r”形, 而元宝枫、旱柳和榛的栓塞脆弱曲线为“s”形, 脆弱曲线为“r”形的树种与脆弱曲线为“s”形的树种栓塞脆弱性差异极显著(p < 0.01)。线性分析表明: 木质部结构影响各树种的栓塞脆弱性, 其中, 木质部密度影响最大(t = 0.702), 导管直径次之(t = 0.532), 导管长度影响最小(t = 0.010)。     

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。     

7种木本植物根和小枝木质部栓塞的脆弱性

用脆弱曲线表示的植物木质部栓塞脆弱性反映了植物木质部栓塞程度与其水势间的关系。众多学者的研究结果表明,脆弱曲线能够提供有关植物的许多生理生态信息,与植物的木质部结构、部位、分布、抗寒、抗旱性等存在一定关系,但各国学者利用不同材料研究得出的结果各异,为了研究木质部栓塞的这种差异是否由于树木对环境适应性不同引起,选取西北农林科技大学西林校区内自然状况下生长良好的5个耐旱树种:刺槐(RobiniapseudoacaciaL.)、元宝枫(AcertruncatumBge.)(低水势忍耐脱水耐旱树种)、白榆(UlmuspumilaL.)(亚低水势忍耐脱水耐旱树种)、油松(PinustabulaeformisCarr.)、白皮松(PinusbungeanaZucc.ex.Endl.)(高水势延迟脱水耐旱树种),及中生的女贞(LigustrumlucidumAit.)和柳树(SalixmatsudanaKoidz.f.pendulaSchneid.)为研究对象,绘制了它们根和小枝的木质部栓塞脆弱曲线,探讨了中生树种和不同耐旱类型树种根和小枝木质部栓塞脆弱性的差异。结果表明:根和小枝的栓塞脆弱性主要由木质部结构决定,栓塞脆弱性顺序基本一致,小枝容易发生木质部栓塞的,其根也较容易发生栓塞;同一树种根和小枝的木质部栓塞脆弱性与植物的耐旱性有关,与树种的耐旱策略无关;一般是中生树种的栓塞脆弱性:小枝>根;耐旱树种的栓塞脆弱性:根>小枝。     

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。     

木本植物水力学结构之导管长度研究进展

导管作为多数被子植物木质部水分运输的主要通道, 了解其结构及功能对研究被子植物水力学特性及植物对环境的适应性有着重要的作用。导管长度作为导管解剖特征之一, 对水分运输的安全性及有效性有着重要的影响。该文概述了导管长度测量及计算的方法, 导管长度在种内及种间的分布, 导管长度与导管直径的关系, 导管长度与导水率的关系及导管长度对建立栓塞脆弱曲线的影响, 并对未来导管长度的研究工作重点提出了建议: 1)改进灌注物质, 使灌注更加充分且更利于观察、提高计算精度、发展活体动态测量技术; 2)建立导管在植物不同器官及整体的分布网络以及不同生活型、不同地区的导管长度数据库; 3)对导管直径在导管方向的变化, 导管长度与其他导管特性之间的关系进行研究; 4)光学测量建立栓塞脆弱曲线技术的兴起, 可为解决离心机法建立栓塞脆弱曲线的真实与准确与否的争议提供新的方向。更深入地了解导管长度在植物水力功能中担负的角色, 可以为耐旱、抗旱品种选育提供理论基础。     

Vessel length as a key hydraulic structure in woody plants: A review

导管作为多数被子植物木质部水分运输的主要通道, 了解其结构及功能对研究被子植物水力学特性及植物对环境的适应性有着重要的作用。导管长度作为导管解剖特征之一, 对水分运输的安全性及有效性有着重要的影响。该文概述了导管长度测量及计算的方法, 导管长度在种内及种间的分布, 导管长度与导管直径的关系, 导管长度与导水率的关系及导管长度对建立栓塞脆弱曲线的影响, 并对未来导管长度的研究工作重点提出了建议: 1)改进灌注物质, 使灌注更加充分且更利于观察、提高计算精度、发展活体动态测量技术; 2)建立导管在植物不同器官及整体的分布网络以及不同生活型、不同地区的导管长度数据库; 3)对导管直径在导管方向的变化, 导管长度与其他导管特性之间的关系进行研究; 4)光学测量建立栓塞脆弱曲线技术的兴起, 可为解决离心机法建立栓塞脆弱曲线的真实与准确与否的争议提供新的方向。更深入地了解导管长度在植物水力功能中担负的角色, 可以为耐旱、抗旱品种选育提供理论基础。     

简易注气法测定刺槐木质部导管组织长度

导管组织在木本植物水力学特性及水分关系中的作用逐渐受到关注,但其长度的传统测试装置和方法相对繁琐。本研究在注气法原理基础上进行改进,改进后装置简易、操作便捷。结果表明,本方法可以成功测定刺槐(Robinia pseudoacacia L.)导管的长度及相关参数:气体传导耗散系数为0.111±0.012;最大导管长度为56.9±1.5 cm;平均导管长度为19.6±2.5 cm;最大分布导管长度为9.8±1.3 cm。枝条注气方向对测试结果没有明显影响。测试过程中注气端压强变化不明显,对导管长度及相关参数的测定和计算影响不显著。本方法与传统注气法相比具有更高的精确度。     

中国大型真菌红色名录评估方法和程序

在中国大型真菌红色名录评估过程中, 根据大型真菌与动植物在生物学特性上的差异, 对IUCN物种红色名录标准做了适当调整, 形成了“中国大型真菌红色名录评估等级与标准”, 即: (1)依据可见的分布地点和子实体数量来估计、推测或判断种群的波动以及种群成熟个体数量的变化; (2)以一定的时间段代替世代时长来计算种群的变化情况; (3)将“疑似灭绝”作为一个独立的评估等级。评估基本数据来自于中国菌物名录数据库、真菌分类文献资料和专家咨询, 评估过程包括任务分工、数据收集、评估物种确定、建立评估表、初评、专家评审和编制红色名录等步骤, 最终完成《中国生物多样性红色名录——大型真菌卷》的编制。     

弄岗北热带喀斯特季节性雨林15 ha监测样地的树种组成与空间分布

北热带喀斯特季节性雨林(northern tropical karst seasonal rain forest)是在我国热带北缘喀斯特地区分布的典型森林植被类型之一。由于富钙偏碱的地球化学背景及多样性的生境类型, 同时受季风气候影响, 该森林呈现群落结构多样、树种组成丰富、特有成分突出等特点。基于大型固定监测样地对该森林树种组成与空间分布进行的研究, 是探明该区域生物多样性形成与维持机制的基础。我们于2011年底建立了广西弄岗北热带喀斯特季节性雨林15 ha监测样地, 依照CTFS (Center for Tropical Forest Science)全球森林生物多样性监测规范, 定位并调查了样地内每一棵胸径≥1 cm的木本植物。结果表明: (1)样地内有监测树种223种, 隶属于56科157属; 独立个体总数为68,010株(含分枝为95,471株), 平均胸径为4.84 cm; (2)树种科、属的区系均以热带成分为主, 大戟科、马鞭草科、梧桐科等为优势科; (3)个体数最多的11个树种的个体数之和占到总个体数的51.64%, 前58个树种的占90.19%; 稀有种有75种, 占总树种数的33.63%; (4)群落结构稳定且更新良好, 主要优势种的径级结构均呈倒“J”形, 无明显断层; (5)树种分布在空间上表现出明显差异, 黄梨木(Boniodendron minus)等强耐旱型树种分布于山顶周围; 蚬木(Excentrodendron tonkinense)等树种分布于山坡中部; 对叶榕(Ficus hispida)等喜湿耐荫型树种分布于山谷周围; (6)胸径>20 cm的个体较多分布在山坡中下部, 极少分布在山顶周围; 胸径10–20 cm的个体较均匀分布于整个样地; 分枝和萌枝较多分布在山顶周围; (7)种–面积散点图在2–7 ha的取样面积下分化形成两条曲线, 表明了树种数量组成在空间上具有强烈异质性。研究初步认为: 强烈生境异质性及独特地质背景可能是影响该喀斯特森林物种组成及空间分布的重要因素。     

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.     

Xylem vulnerability to cavitation can be accurately characterised in species with long vessels using a centrifuge method

Vulnerability to cavitation curves describe the decrease in xylem hydraulic conductivity as xylem pressure declines. Several techniques for constructing vulnerability curves use centrifugal force to induce negative xylem pressure in stem or root segments. Centrifuge vulnerability curves constructed for long‐vesselled species have been hypothesised to overestimate xylem vulnerability to cavitation due to increased vulnerability of vessels cut open at stem ends that extend to the middle or entirely through segments. We tested two key predictions of this hypothesis: (i) centrifugation induces greater embolism than dehydration in long‐vesselled species, and (ii) the proportion of open vessels changes centrifuge vulnerability curves. Centrifuge and dehydration vulnerability curves were compared for a long‐ and short‐vesselled species. The effect of open vessels was tested in four species by comparing centrifuge vulnerability curves for stems of two lengths. Centrifuge and dehydration vulnerability curves agreed well for the long‐ and short‐vesselled species. Centrifuge vulnerability curves constructed using two stem lengths were similar. Also, the distribution of embolism along the length of centrifuged stems matched the theoretical pressure profile induced by centrifugation. We conclude that vulnerability to cavitation can be accurately characterised with vulnerability curves constructed using a centrifuge technique, even in long‐vesselled species.     

Water relations of Robinia pseudoacacia L.: do vessels cavitate and refill diurnally or are R‐shaped curves invalid in Robinia?

Since 2005, an unresolved debate has questioned whether R‐shaped vulnerability curves (VCs) might be an artefact of the centrifuge method of measuring VCs. VCs with R‐shape show loss of stem conductivity from approximately zero tension, and if true, this suggests that some plants either refill embolized vessels every night or function well with a high percentage of vessels permanently embolized. The R‐shaped curves occur more in species with vessels greater than half the length of the segments spun in a centrifuge. Many have hypothesized that the embolism is seeded by agents (bubbles or particles) entering the stem end and travelling towards the axis of rotation in long vessels, causing premature cavitation. VCs were measured on Robinia pseudoacacia L. by three different techniques to yield three different VCs; R‐shaped: Cavitron P₅₀ = 0.30 MPa and S‐shaped: air injection P₅₀ = 1.48 MPa and bench top dehydration P₅₀ = 3.57 MPa. Stem conductivity measured in the Cavitron was unstable and is a function of vessel length when measured repeatedly with constant tension, and this observation is discussed in terms of stability of air bubbles drawn into cut‐open vessels during repeated Cavitron measurement of conductivity; hence, R‐shaped curves measured in a Cavitron are probably invalid.     

Vulnerability curves by centrifugation: is there an open vessel artefact,and are ‘r’ shaped curves necessarily invalid?

Vulnerability curves using the 'Cavitron' centrifuge rotor yield anomalous results when vessels extend from the end of the stem segment to the centre ('open-to-centre' vessels). Curves showing a decline in conductivity at modest xylem pressures ('r' shaped) have been attributed to this artefact. We determined whether the original centrifugal method with its different rotor is influenced by open-to-centre vessels. Increasing the proportion of open-to-centre vessels by shortening stems had no substantial effect in four species. Nor was there more embolism at the segment end versus centre as seen in the Cavitron. The dehydration method yielded an 'r' shaped curve in Quercus gambelii that was similar to centrifuged stems with 86% open-to-centre vessels. Both 'r' and 's' (sigmoidal) curves from Cercocarpus intricatus were consistent with each other, differing only in whether native embolism had been removed. An 'r' shaped centrifuge curve in Olea europaea was indistinguishable from the loss of conductivity caused by forcing air directly across vessel end-walls. We conclude that centrifuge curves on long-vesselled material are not always prone to the open vessel artefact when the original rotor design is used, and 'r' shaped curves are not necessarily artefacts. Nevertheless, confirming curves with native embolism and dehydration data is recommended.     

Does sample length influence the shape of xylem embolism vulnerability curves? A test with the Cavitron spinning technique

The Cavitron spinning technique is used to construct xylem embolism vulnerability curves (VCs), but its reliability has been questioned for species with long vessels. This technique generates two types of VC: sigmoid ‘s’‐shaped and exponential, levelling‐off ‘r’‐shaped curves. We tested the hypothesis that ‘r’‐shaped VCs were anomalous and caused by the presence of vessels cut open during sample preparation. A Cavitron apparatus was used to construct VCs from samples of different lengths in species with contrasting vessel lengths. The results were compared with VCs obtained using other independent techniques. When vessel length exceeded sample length, VCs were ‘r’‐shaped and anomalous. Filling vessels cut open at both ends with air before measurement produced more typical ‘s’‐shaped VCs. We also found that exposing segments of 11 woody species in a Cavitron at the pressure measured in planta before sampling considerably increased the degree of embolism above the native state level for species with long vessels. We concluded that open vessels were abnormally more vulnerable to cavitation than intact vessels. We recommend restricting this technique to species with short conduits. The relevance of our conclusions for other spinning techniques is discussed.     

Vulnerability to cavitation in Olea europaea current‐year shoots: further evidence of an open‐vessel artifact associated with centrifuge and air‐injection techniques

Different methods have been devised to analyze vulnerability to cavitation of plants. Although a good agreement between them is usually found, some discrepancies have been reported when measuring samples from long‐vesseled species. The aim of this study was to evaluate possible artifacts derived from different methods and sample sizes. Current‐year shoot segments of mature olive trees (Olea europaea), a long‐vesseled species, were used to generate vulnerability curves (VCs) by bench dehydration, pressure collar and both static‐ and flow‐centrifuge methods. For the latter, two different rotors were used to test possible effects of the rotor design on the curves. Indeed, high‐resolution computed tomography (HRCT) images were used to evaluate the functional status of xylem at different water potentials. Measurements of native embolism were used to validate the methods used. The pressure collar and the two centrifugal methods showed greater vulnerability to cavitation than the dehydration method. The shift in vulnerability thresholds in centrifuge methods was more pronounced in shorter samples, supporting the open‐vessel artifact hypothesis as a higher proportion of vessels were open in short samples. The two different rotor designs used for the flow‐centrifuge method revealed similar vulnerability to cavitation. Only the bench dehydration or HRCT methods produced VCs that agreed with native levels of embolism and water potential values measured in the field.     

No evidence for an open vessel effect in centrifuge-based vulnerability curves of a long-vesselled liana (Vitis vinifera)

Vulnerability to cavitation curves are used to estimate xylem cavitation resistance and can be constructed using multiple techniques. It was recently suggested that a technique that relies on centrifugal force to generate negative xylem pressures may be susceptible to an open vessel artifact in long-vesselled species. Here, we used custom centrifuge rotors to measure different sample lengths of 1-yr-old stems of grapevine to examine the influence of open vessels on vulnerability curves, thus testing the hypothesized open vessel artifact. These curves were compared with a dehydration-based vulnerability curve. Although samples differed significantly in the number of open vessels, there was no difference in the vulnerability to cavitation measured on 0.14- and 0.271-m-long samples of Vitis vinifera. Dehydration and centrifuge-based curves showed a similar pattern of declining xylem-specific hydraulic conductivity (K(s)) with declining water potential. The percentage loss in hydraulic conductivity (PLC) differed between dehydration and centrifuge curves and it was determined that grapevine is susceptible to errors in estimating maximum K(s) during dehydration because of the development of vessel blockages. Our results from a long-vesselled liana do not support the open vessel artifact hypothesis.