Zhang Yong-Jiang, Rockwell Fulton E, Wheeler James K, Holbrook N Michele
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138.
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138
Plant Physiol. 2014 Aug;165(4):1557-1565. doi: 10.1104/pp.114.243105. Epub 2014 Jun 19.
Declines in leaf hydraulic conductance (K) with increasing water stress have been attributed to cavitation of the leaf xylem. However, in the leaves of conifers, the reversible collapse of transfusion tracheids may provide an alternative explanation. Using Taxus baccata, a conifer species without resin, we developed a modified rehydration technique that allows the separation of declines in K into two components: one reversible and one irreversible upon relaxation of water potential to -1 MPa. We surveyed leaves at a range of water potentials for evidence of cavitation using cryo-scanning electron microscopy and quantified dehydration-induced structural changes in transfusion tracheids by cryo-fluorescence microscopy. Irreversible declines in K did not occur until leaf water potentials were more negative than -3 MPa. Declines in K between -2 and -3 MPa were reversible and accompanied by the collapse of transfusion tracheids, as evidenced by cryo-fluorescence microscopy. Based on cryo-scanning electron microscopy, cavitation of either transfusion or xylem tracheids did not contribute to declines in K in the reversible range. Moreover, the deformation of transfusion tracheids was quickly reversible, thus acting as a circuit breaker regulating the flux of water through the leaf vasculature. As transfusion tissue is present in all gymnosperms, the reversible collapse of transfusion tracheids may be a general mechanism in this group for the protection of leaf xylem from excessive loads generated in the living leaf tissue.
随着水分胁迫加剧，叶片水力导度（K）下降，这归因于叶片木质部的空化现象。然而，在针叶树的叶片中，输导管胞的可逆塌陷可能提供了另一种解释。我们以无树脂的针叶树种欧洲红豆杉为材料，开发了一种改良的复水技术，该技术可将K的下降分为两个部分：一个是可逆的，另一个在水势恢复到-1 MPa时是不可逆的。我们在一系列水势下对叶片进行了调查，使用低温扫描电子显微镜寻找空化的证据，并通过低温荧光显微镜对输导管胞中脱水诱导的结构变化进行了量化。直到叶片水势比-3 MPa更负时，K才出现不可逆下降。在-2至-3 MPa之间K的下降是可逆的，并伴随着输导管胞的塌陷，低温荧光显微镜证明了这一点。基于低温扫描电子显微镜，在可逆范围内，输导或木质部管胞的空化对K的下降没有影响。此外，输导管胞的变形可迅速逆转，因此起到了断路器的作用，调节通过叶片脉管系统的水流。由于所有裸子植物中都存在输导组织，输导管胞的可逆塌陷可能是该类群中保护叶片木质部免受活叶组织中产生的过大负荷影响的一种普遍机制。
