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在全球气候变暖背景下,由水分胁迫引起的树木死亡和作物减产等事件频发,水分胁迫带来的损失甚至超过其他环境胁迫所造成的损失总和[1]。研究表明:水力失效是导致植物死亡的主要原因[2]。研究植物的水力功能(水力效率和安全性)是量化植物抗旱能力的关键,而木质部结构是水力功能的基础,因此,结构和功能关系的定量研究是树木干旱胁迫研究的热点之一[3-4]。
导水率(Kh)和比导率(Ks)是衡量植物水力效率的重要参数[5],是植物为适应特定环境条件及物种竞争而形成的不同水力功能特征[6]。Ks越大,说明单位有效面积输送水分的能力越强,输水效率越高。此外,一般用导水率损失50%时的木质部水势(P50)来表征植物栓塞抗性,P50是确定木本植物抗旱极限的一个重要性状[7],不同类群植物的栓塞抗性差异很大[8]。有研究指出:水力效率与栓塞抗性之间可能存在一种权衡关系,即水力效率越大栓塞抗性就越小[9]。虽然水力效率-安全权衡存在与否仍有争议,但现有证据表明:在全球尺度上,效率和安全之间存在微弱的负相关[10-11]。
不同树种的材性、解剖等结构性状会影响树木的水分运输效率和水分利用策略[11-12]。按照次生木质部横截面上的导管管孔特征,双子叶植物可大致分为散孔材和环孔材2种功能类群[13-14]。在面对干旱诱导形成的栓塞产生的环境胁迫时,2种材性树种在水力结构上会进化不同的适应性策略。环孔材树种往往导管直径较大,具有较高的水分运输效率,但在水分胁迫条件下更容易遭受空穴化和栓塞胁迫[12]。而没有明显的早材和晚材区别的散孔材树种,它们较小的导管抵抗水分胁迫产生空穴化和栓塞的能力较强[5, 14]。除导管直径外,导管壁厚度、木材基本密度等对散孔材和环孔材树种水力效率和栓塞抗性都具有一定的影响[15-16]。COCHARD等[17]研究指出:导管壁厚度与栓塞抗性呈正相关,且厚度跨度比与栓塞抗性的相关性显著。此外,木质部在结构上形成了一个相互连通的网络管道结构,导管在木质部网络的空间排列影响了木质部的功能特性[18]。与散孔材树种相比,环孔材树种存在导管密度小、导管连接度低的特性,使其通过相邻导管或纹孔运输水分的概率显著小于散孔材树种,故而环孔材树种导管较散孔材更易发生栓塞[19]。因此,研究木质部结构与水力效率、栓塞抗性的关系,可揭示树木干旱致死机制。
目前,多年干旱和降水降低趋势可能是未来气候的开端[20],在这种气候背景下,物种的抗旱能力是生态系统稳定性与可持续发展的关键。河南省作为气候变化敏感区,自1961年以来,干旱发生频率和发生范围不断扩大[21],这对鸡公山国家级自然保护区内具有生态、经济和药用价值的园林树种的生长和生存产生潜在影响。基于此,本研究选取河南省鸡公山国家级自然保护区内园林树种中的3个散孔材和3个环孔材树种,对2种功能树种的枝条木质部解剖结构和水力功能性状差异进行分析,从植物水力结构的角度探讨植物的水分运输和抗旱性,以期为研究区园林植物的保护和开发提供指导意见。
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研究样地位于河南省信阳市南部的鸡公山国家级自然保护区(31°46′~31°52′N,114°01′~114°06′E),该区地处北亚热带边缘,具有北亚热带向暖温带过渡的季风气候和山地气候特征。四季分明,光、热、水同期,年均气温为15.2 ℃,年均降水量为1 118.7 mm,植被以亚热带常绿阔叶林以及暖温带落叶阔叶林为主。保护区内森林茂密、生物资源丰富,为河南省物种最为丰富的区域之一。
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选取鸡公山国家自然保护区2种材性的6个园林树种作为研究对象(表1),包括3个散孔材树种(枫香Liquidambar formosana、旱柳Salix matsudana、深山含笑Michelia maudiae)和3个环孔材树种(槲树Quercus dentata、楝Melia azedarach、野核桃Juglans cathayensis)。2019年6—9月,随机选取生长状况良好,株高、胸径、冠幅相似的植株,选择向阳生长的当年生枝条,所截枝条基部直径为6~8 mm,长为30~50 cm,每个树种取3~5株树,每株树4~6个重复,每个树种20个重复。
表 1 散孔材和环孔材样树的基本特性
Table 1. Basic characteristics of diffuse- and ring-porous sampled trees
材性 树种 树高/m 胸径/cm 树龄/a 冠幅/m 散孔材 枫香 17.45±0.35 16.26±0.12 15~20 4.10±0.22 旱柳 12.75±0.25 12.18±0.24 10~15 2.14±0.07 深山含笑 12.41±0.43 13.65±0.54 15~20 3.45±0.14 环孔材 槲树 13.32±0.25 13.34±0.27 15~20 5.86±0.08 楝 8.78±0.11 9.15±0.42 10~15 2.54±0.23 野核桃 11.33±0.17 13.42±0.15 15~20 4.02±0.21 说明:数值为平均值±标准误;样本数为20 株·种−1 -
将符合实验要求的枝条样品取下后,立刻浸入水中并用不透光容器保存带回。实验开始之前,将样品置于水中进行60~120 min的暗适应,之后在水下修剪样品,并在样品中间部位剪取一段约16~17 cm的茎段。将茎段基部表皮剥离,随后连接到木质部导水率与栓塞测量系统XYL’EM-Plus(Bronkhorst, Montigny-les-Cormeilles, 法国),用于测量导水率。首先用冲洗液(20 mmol·L−1氯化钾+1 mmol·L−1氯化钙)在低压下测得茎段的原位导水率(K0, kg·m·s−1·MPa−1),用0.15 MPa的压力冲刷枝条,直至枝条管胞内无气泡溢出为止,然后测量最大导水率(Kmax,kg·m·s−1·MPa−1)[22]。最大导水率测量结束后,将茎段放入压力腔中并保持茎段两端露出压力腔外,将压力表连在压力腔上[11]。在一定压力梯度下持续加压一定时间以诱导茎段发生栓塞,这个过程以0.2 或0.3 MPa的增量重复进行(取决于植物),直至导水率损失达90%以上,同时记录相应压力下的导水率(Kh, kg·m·s−1·MPa−1)。导水率损失百分比(PLC)计算如下:PLC=(1−Kh/Kmax)×100%。将所得压力值和PLC值在Origin软件中用sweibull分布函数进行拟合,获得脆弱曲线和导水率损失50%时对应的茎段木质部水势(P50)。比导率(Ks, kg·s−1·m−1·MPa−1)通过Kmax除以无髓、无树皮的基部边材横截面积得到。
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从用于测定脆弱曲线的枝条上截取4段0.5 cm长的茎段。样品经固定、软化后采用番红固绿对染法制作石蜡切片。使用Leica DM 3000光学显微镜在50和400倍镜下摄像(图1)。用Image-J软件分析所摄照片,测量参数包括导管直径(μm)、导管壁厚度(μm)、导管密度(个·mm−2)以及导管壁厚度跨度比。
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根据HACKE等[23]的方法测量样本的木材基本密度。将木材样品树皮用剃刀除去后浸没在蒸馏水中。采用排水法测定木材新鲜样品的体积(V,cm3)。然后将样品放置在75 ℃的烘箱烘48 h,测量干质量(W,g)。木材基本密度(WBD)计算公式为:WBD(g·cm−3)=W/V。
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采用Minitab 18.1软件对数据进行单因素方差分析(one-way ANOVA), 显著性水平为0.05。采用Pearson相关分析检验各功能性状间的相关性。利用Origin软件作图。
Relationship between xylem structure and function of diffuse-porous and ring-porous wood species in Jigongshan Nature Reserve
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摘要:
目的 木质部解剖结构是植物水力功能和水分利用策略的基础。散孔材和环孔材的导管分布和形态存在显著差异,对2种材性树种木质部解剖结构和水力功能的关系进行比较,有助于理解不同材性树种的水分适应机制。 方法 选取河南省信阳市鸡公山自然保护区中的3个散孔材树种(枫香Liquidambar formosana、旱柳Salix matsudana、深山含笑Michelia maudiae)和3个环孔材树种(槲树Quercus dentata、楝Melia azedarach、野核桃Juglans cathayensis),研究2种材性树种的枝水力功能(比导率和栓塞抗性)和解剖结构的关系。 结果 3个环孔材树种比导率更大,3个散孔材树种栓塞抗性更强;在物种水平上,只有散孔材树种深山含笑比导率和栓塞抗性呈显著负相关(P<0.05),存在水力效率-安全权衡。3种散孔材树种相较于3种环孔材树种具有更大的导管密度、导管壁厚度跨度比和更小的导管直径、导管壁厚度。 结论 环孔材树种倾向于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。散孔材树种木质部结构中的导管壁厚度和导管壁厚度跨度比对植物水分传输功能具有重要影响。图5表1参32 Abstract:Objective The anatomical structure of xylem is the basis of plant hydraulic function and water use strategy. There are significant differences in vessel distribution and morphology between diffuse-porous and ring-porous wood species. The purpose of this study is to compare anatomical structure and hydraulic function of xylem of the two wood species, as well as the quantitative relationship between structure and function, so as to understand the water adaptation mechanism of different wood species. Method Three diffuse-porous species (Liquidambar formosana, Salix matsudana, Michelia maudiae) and three ring-porous species (Quercus dentata, Melia azedarach, Juglans cathayensis) in Jigongshan Nature Reserve, Henan Province, were selected to study the branch hydraulic function (specific conductivity, Ks and embolism resistance, P50) and anatomical structure, and the relationship between hydraulic function and anatomical structure was quantitatively analyzed. Result Ks of the three ring-porous species was higher and P50 of the three diffuse-porous species was stronger. At the species level, Ks of M. maudiae was negatively correlated with P50 (P<0.05), and there was a tradeoff between hydraulic efficiency and safety. Compared with the three ring-porous wood species, the three diffuse-porous species had higher vessel density, vessel thickness span ratio, smaller vessel diameter and vessel wall thickness. Conclusion The ring-porous species tend to significantly improve their water delivery efficiency by increasing vessel diameter to avoid water potential decline and reduce the risk of embolism. The ratio of vessel wall thickness and vessel thickness span in the xylem structure of the diffuse-porous species has important effects on plant water transfer function. [Ch, 5 fig. 1 tab. 32 ref.] -
表 1 散孔材和环孔材样树的基本特性
Table 1. Basic characteristics of diffuse- and ring-porous sampled trees
材性 树种 树高/m 胸径/cm 树龄/a 冠幅/m 散孔材 枫香 17.45±0.35 16.26±0.12 15~20 4.10±0.22 旱柳 12.75±0.25 12.18±0.24 10~15 2.14±0.07 深山含笑 12.41±0.43 13.65±0.54 15~20 3.45±0.14 环孔材 槲树 13.32±0.25 13.34±0.27 15~20 5.86±0.08 楝 8.78±0.11 9.15±0.42 10~15 2.54±0.23 野核桃 11.33±0.17 13.42±0.15 15~20 4.02±0.21 说明:数值为平均值±标准误;样本数为20 株·种−1 -
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https://zlxb.zafu.edu.cn/article/doi/10.11833/j.issn.2095-0756.20210628