Relationships among water transport, mechanical strength and anatomical structure in branch and root xylem of Taxodiaceae species

CHEN Sen; LU Shitong; LI Yan; XIE Jiangbo; YE Linfeng; WANG Zhongyuan

doi:10.11833/j.issn.2095-0756.20210248

Volume 39 Issue 2

Mar. 2022

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CHEN Sen, LU Shitong, LI Yan, XIE Jiangbo, YE Linfeng, WANG Zhongyuan. Relationships among water transport, mechanical strength and anatomical structure in branch and root xylem of Taxodiaceae species[J]. Journal of Zhejiang A&F University, 2022, 39(2): 233-243. doi: 10.11833/j.issn.2095-0756.20210248

Citation:

CHEN Sen, LU Shitong, LI Yan, XIE Jiangbo, YE Linfeng, WANG Zhongyuan. Relationships among water transport, mechanical strength and anatomical structure in branch and root xylem of Taxodiaceae species[J]. Journal of Zhejiang A&F University, 2022, 39(2): 233-243. doi: 10.11833/j.issn.2095-0756.20210248

Relationships among water transport, mechanical strength and anatomical structure in branch and root xylem of Taxodiaceae species

doi: 10.11833/j.issn.2095-0756.20210248

CHEN Sen^{1, 2
,},
LU Shitong^{1, 2},
LI Yan^{1, 2},
XIE Jiangbo^{1, 2, 3},
YE Linfeng^{1, 2},
WANG Zhongyuan^{1, 2
,
,}

1.
State Key Laboratory of Subtropical Silviculture, Zhejiang A&F University, Hangzhou 311300, Zhejiang, China
2.
College of Forestry and Biotechnology, Zhejiang A&F University, Hangzhou 311300, Zhejiang, China
3.
State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, Xinjiang, China

Received Date: 2021-03-28
Rev Recd Date: 2021-09-10

Available Online: 2022-03-25

Publish Date: 2022-03-25

Abstract

Objective This study, with Sequoia sempervirens, Taxodium distichum and T. distichum var. imbricatum in humid regions selected as subjects, is aimed at an analysis of the quantitative relationships among the water transport, anatomical structure, and mechanical strength of the xylem of the branches and roots of the Taxodiaceae species and a discussion of the trade-off between the anatomy and function of the xylem at the organ and cross-species levels. Method Targeting the S. sempervirens, T. distichum and T. distichum var. imbricatum grown in Mount Tianmu, Zhejiang Province, with air injection method employed, efforts were made to measure the specific sapwood conductivity (K_s) and cavitation resistance (P₅₀) before the indexes regarding the water transport such as tracheid wall thickness (T_t), pit membrane area (A_pm), pit aperture area (A_pa), pit membrane diameter (P_d), pit aperture diameter (A_d), hydraulic diameter (D_h)、tracheid density (N_t), aperture opening ratio (R_ap) and pit density (N_pm) were measured and calculated. Then, the wood density (D_w) and thickness-to-span ratio (T_tob) related to mechanical strength were calculated. Finally, an analysis was conducted of the correlation between water transport and anatomical structure. Result The roots demonstrated higher hydraulic conductivity yet weaker cavitation resistance than the branches and K_s in branches of S. sempervirens and T. distichum, and roots of S. sempervirens, T. distichum and T. distichum var. imbricatum was positively correlated with P₅₀, while K_s in branches of T. distichum was slightly negatively correlated with P₅₀ (R²=0.03, P=0.35), showing no efficient-safety trade-off at the organ level. There was a trade-off between K_s and P₅₀ across species, following a negative power function of −2.23 (R²=0.91, P＜0.001). There was significant negative correlation between K_s and D_w (R²=0.37, P＜0.01), K_s and T_tob (R²=0.20, P＜0.01), P₅₀ and T_tob (R²=0.20, P＜0.01) in branches of S.sempervirens. Conclusion The safety and efficiency of the xylem of the branches and roots of the three plants were all low, and given the different structural requirements for high safety and high efficiency of the three plants, there was no efficiency-safety trade-off at the organ level. [Ch, 7 fig. 1 tab. 48 ref.]
- humid region,
- Sequoia sempervirens,
- Taxodium,
- hydraulic efficiency,
- cavitation resistance,
- efficiency-safety trade-off

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气候变暖加剧了全球范围内干旱程度^[1]。CHOAT等^[2]基于全球尺度对植物水力性状进行的Meta分析表明：所有的森林群落都易受到水力破坏的影响。然而，以往的研究主要集中于干旱区和半干旱区，对湿润地区植物的研究较少。不同环境中植物木质部性状的变异和性状权衡与组合的不同，反映植物个体及其群落获取环境资源方式的不同，因此植物采取的生活策略也存在差异^[3]。植物生理生态功能，如光合速率、木质部输水效率和栓塞抗性，与木质部结构密切相关^[4-9]。对植物生理生态变化与木质部解剖结构在不同环境的变异规律进行研究，有助于理解植物响应气候变化的机制和物种分布的限制。植物的生长受木质部水力效率与栓塞抗性权衡的制约^[10]。对于裸子植物来说，木质部管胞很大程度上决定了水力效率^[11]，但是单位面积木质部管胞直径越大，数量越少，就更易发生水力障碍。此外，管胞壁上的纹孔膜面积也会随着管胞直径的增加而增加，进一步降低植物的栓塞抗性^[12]。因此“安全”的水力系统通常以降低水力效率为代价^[13]。然而，关于植物对水力效率和栓塞抗性适应机制的研究仍存在争议。早期的植物干旱敏感性研究发现：木质部性状反映了植物水分利用上的效率-安全权衡^[14]。木本植物不存在效率-安全权衡或存在较弱的权衡^[15]，大多数物种在不同生境和生理状态下表现出低效率和低安全性，这与木质部性状间的权衡相矛盾^[15-17]。木质部效率-安全权衡可能与一系列甚至单个解剖性状的权衡有关^[18]。然而，木质部在不考虑效率-安全权衡情况下的结构-功能关系尚未得到研究。权衡可能存在与木质部安全、水力效率和机械强度之间^[19-20]。LENS等^[18]研究表明：许多物种的平均栓塞抗性与木质部密度(D_w)和管道厚度跨度比(T_tob)密切相关^[21-22]。这3种特性之间似乎存在复杂的权衡^[23]，而且没有一个有说服力的结果来解释相当数量的低效率和低安全性物种，是理解植物木质部进化时的挑战^[15]。此外，在干旱胁迫下，植物的枝和根都会采取各自的适应策略。有研究发现：植物的叶比枝条更容易发生栓塞现象^[24-25]。然而，一些对潮湿生境中生长的木本植物的研究表明：叶和枝的栓塞抗性几乎没有差别^[26]，甚至叶片栓塞抗性比枝更强^[27-28]。然而，WU等^[29]研究发现：欧亚槭Acer pseudoplatanus、栓皮槭A. campestre、欧榛Corylus avellana根系的栓塞抗性大于枝条。枝的栓塞抗性低于根可能是植物对干旱胁迫的适应策略，通过保留根部的完整性来优化复水后的栓塞修复能力。相较于生长在干旱环境中的植物，北美红杉Sequoia sempervirens和落羽杉属Taxodium植物的木质部栓塞抗性可能较弱，因此在全球变暖导致干旱强度和频度增加的背景下，湿润地区植物可能会面临更大的威胁。本研究对杉科Taxodiaceae北美红杉、落羽杉Taxodium distichum、池杉T. distichum var. imbricatum的木质部水力功能、解剖结构性状和机械强度进行了测定，比较了3种植物枝/根木质部水力功能和解剖结构的差异，探讨了木质部水分运输、解剖结构和机械强度的定量关系，以期在器官水平和种内水平上，为评估木质部解剖学基础与木质部功能的权衡是否存在因果关系提供依据。

3. 讨论

3.1. 枝/根水力功能性状差异

本研究表明：3种植物根的导水率均高于枝条。根系吸收水分是土壤-植物-大气连续体的关键环节^[38-39]。被子植物和裸子植物的根比枝更容易受到干旱的影响^{[13, 40]}，这与本研究结果一致。有研究表明：在干旱胁迫发生时，植物更倾向于增加地下生物量分配，发展根系，提高根冠比以增加抗旱性^[41-42]。植物在严重干旱条件下保存根系的活力，提高了干旱胁迫过后的恢复能力^[43]。生长在湿润地区的树木木质部栓塞抗性(尤其是根栓塞抗性)比干旱地区的树木弱，面对愈发严重、频繁的干旱，湿润地区的树木面临水力失效的风险更高。

3.2. 效率-安全权衡

本研究表明：3种植物枝/根均不存在效率-安全权衡，但是在跨物种间发现了权衡。GLEASON等^[15]的研究发现了许多既低效又不安全的植物。与松科Pinaceae植物的K_s和P₅₀相比^[44]，3种植物都属于低效率、低安全的物种。有研究表明：在管胞系统中构建冗余^[45]或者新的木质部^{[15, 21]}，可以在栓塞积累的情况下维持叶片的水分供应。叶经济学光谱表明：一些物种具备“缓慢投资-收益”能力，即这些物种获取和利用资源的速度很慢，因此只需要低效率和低安全性^{[23-24, 46]}。此外，大量研究表明：北美红杉和落羽杉属植物除了具有效率-安全权衡以外还有其他重要性状(如木质部的机械支持)的解剖学基础^{[15, 23]}，本研究结果也证实了这一观点。北美红杉枝的K_s与D_w呈显著负相关，根的K_s与厚度跨度比的相关系数为−0.55；落羽杉根的P₅₀与厚度跨度比的相关系数为−0.46；池杉根的P₅₀与木质部密度相关系数为0.38。这些结果表明：3种植物水力效率与机械强度之间存在着平衡，这与PRATT等^[23]的研究结果一致。这是湿润区树种对水分充足生境适应的结果，因为水分充足，植物木质部输送水分安全性方面的威胁弱；反而高大的裸子植物其机械强度的需求在增加。此外，在跨物种水平上的幂函数曲线有助于解释本研究种内和其他研究中不存在效率-安全权衡的原因：幂函数的左端斜率为0，右端斜率为无穷大，因此，当所测量的数据处于幂函数两端时，效率-安全之间的权衡会因测量误差的微小变化而产生剧烈波动；只有当研究物种的K_s和P₅₀处于幂函数曲线的中心部分，或所研究物种的K_s和P₅₀变化范围很大时，这种权衡才会变得显著^[47]。

一系列甚至单个解剖性状的权衡都有可能对效率-安全权衡进行解释。LENS等^[18]研究了7种槭属Acer植物的解剖性状以及水力性状和解剖性状的相关关系网络发现：几乎所有测量到的解剖特征都是效率-安全权衡的因果基础。在本研究中，3种植物的枝和根都没有发现权衡关系。北美红杉枝的K_s和P₅₀与D_h、T_t、A_d呈正相关，根的K_s和P₅₀也随着D_h、N_t的增加呈升高的趋势。落羽杉根的K_s和P₅₀与D_h、P_d有较强的正相关关系。池杉枝的K_s和P₅₀也与D_h、T_t存在正相关关系。

由于导水率和栓塞抗性的结构需求不同，无法实现效率-安全权衡。这种对结构需求的独立性仅在本研究的3种植物中发现，然而，这种独立的物种特异性增加了对水力系统进化的理解^[48]。

4. 结论

本研究表明：3种杉科植物不存在效率-安全权衡。效率-安全权衡不存在的原因是植物对高安全性和高效率的结构性需求不同，表明效率-安全权衡的结构基础并不存在。跨物种间输水效率与栓塞抗性存在的负幂函数关系或许存在于更多物种。效率、安全性和机械强度之间的权衡关系表明了木质部水力功能是由多种木质部共同决定的。为全面了解木质部导水系统，需要在整株水平(包括根、茎、枝、叶柄和叶)上进行更多的系统研究。

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物种	海拔/m	坡度/(°)	树龄/a	树高/m	胸径/cm	冠幅/m
北美红杉	40.43±0.14	0.62±0.07	20.79±0.64	17.21±0.46	18.14±0.31	2.70±0.09
落羽杉	40.42±0.12	0.48±0.03	16.89±0.31	23.26±0.48	16.73±0.47	2.45±0.09
池杉	42.33±0.51	1.83±0.06	17.28±0.54	23.78±0.34	17.44±0.46	2.61±0.09
说明：数值为平均值±标准误