树干光合维持整树水力功能的补偿作用

Differing from leaf photosynthesis that depends on stomatal uptake of CO2, stem green tissue mainly uses internal CO2 for re-assimulation, which could make up for deficiency of the photosynthate delivered from leaf and balance carbon loss in stem under stress condition. The few carbohydrates produced by stem photosynthesis are not mainly for construction purpose, but for maintaining and repairing xylem hydraulic system with the advantage of proximity. The stem photosynthesis in this way could compensate for hydraulic function and to a certain extent relieve draught and high temperature stresses. It is one of draught-resistant mechanisms that has received few attention. In connection with the current problems concerning with the relation between leaf and stem photosynthesis, we hypothesize that the functional coordination between leaf photosynthesis and hydraulic transfer would rely on available carbon sources in the stem, and the change of stem photosynthesis and the subsequent alternation of hydraulic conductance would trigger adaptive adjustment of leaf photosynthesis. We will design shading experiment to establish a stem photosynthesis gradient, together with the measurement of sap flow, leaf photosynthesis rate, stem sap flow and leaf/soil water potential - based whole tree hydraulic conductance, fluorescence parameters as well as soluble sugars in stem tissue, to investigate maintenance and regulation of whole tree hydraulic function by stem photosynthesis. The results will help to understand non-foliage photosynthesis response to environment and its contribution to plant carbon balance, which is of special academic importance.

与依靠气孔吸收CO2进行光合反应的叶片不同，树干绿色组织主要利用内部的CO2进行再同化，弥补逆境下由叶片输入光合产物的不足，平衡茎部碳损失。但是，树干光合生产的碳水化合物用于结构建造有限，更多是借助邻近优势为维护和修复木质部水力系统提供部分碳源和能量，补偿水力功能，一定程度上缓解干旱和高温胁迫，是至今未被足够重视的重要抗旱机制之一。针对目前有关叶片与树干光合相互关系研究的存在问题，我们提出“叶片光合与水力传导的功能协同有赖于树干可利用的碳源，树干光合的变化和随之出现的水力导度变动将引发叶片光合的适应性调整”的理论假说。通过设置遮光实验形成不同大小的树干光合作用，整合树干液流、叶片光合速率、基于液流和叶片/土壤水势梯度的整树水力导度、树干组织荧光参数和可溶性糖含量的观测研究，探讨树干光合对整树水力功能的维持和调节机理，研究结果对认识非叶光合对环境的响应和对植物碳平衡的贡献，有特殊的学术价值。

本项目通过设计树干遮光处理，产生不同的光照强度以改变尾叶桉树干光合能力，继而解除遮光处理，开展野外树干液流的长期观测、水势和非结构碳水化合物（NSC）的非定期测定，结合温室土壤水分控制实验，研究尾叶桉树干光合作用维持和修复木质部水力功能的补偿作用，从水分动态和碳代谢的角度，探究了树木对土壤水分亏缺甚至干旱胁迫的响应机制。项目执行过程中，发表SCI论文7篇，培养博士和硕士研究生各2名。主要研究结果及结论如下：.（1）遮光处理改变了尾叶桉树干合作用的强度，导致茎组织NSC浓度、整树水力导度（Kplant）和液流速率（Js）的下降，木质部导水损失率（PLC）升高；不同土壤水分条件下，茎组织NSC浓度和整株导水率之间均存在显著正相关关系；表明树干光合通过影响木质组织NSC的有效性，对于维持水分传输完整性发挥水力补偿效用。解除遮光后，Kplant和Js的显著恢复地证实了这一结论。.（2）重度干旱胁迫下，遮光处理引起尾叶桉幼苗叶片气孔导度（gs）和最大净光合效率（Amax）的显著下降，gs和Amax均与Kplant存在一定程度上的正相关关系。表明树干遮光处理对叶片气体交换的影响程度与土壤水分条件有关，反映出叶片光合与水力传导的功能协同有赖于树干可利用的碳源供应，树干光合的变化和伴随的Kplant的变动，引发了叶片光合的适应性调整。.（3）在干旱胁迫下，树干光合对修复和维持木质部水力传输效率及整树水分利用的补偿作用更加明显，尾叶桉木质部的栓塞传感能力可能取决于木质部负压水平。结合第（2）点的结论，突显了碳水协同关系对理解树干光合作用增强植物抗旱性的重要性。.（4）干旱胁迫处理导致尾叶桉幼苗木质部PLC达84.4%，干季较低的土壤水分含量同样使尾叶桉树木Kplant和Js下降，显示干旱胁迫对整树水分利用的明显影响。另外，干旱胁迫没有引起叶片和茎组织NSC浓度的显著下降，却促进了淀粉向可溶性糖的转化。结合第（3）点的结论，我们认为，这是树干光合为水力修复提供物质和能量、部分弥补栓塞脆弱性的缺陷的间接证明。

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