Zhang Yong-Jiang, Hochberg Uri, Rockwell Fulton E, Ponomarenko Alexandre, Chen Ya-Jun, Manandhar Anju, Graham Adam C, Holbrook N Michele
School of Biology and Ecology, University of Maine, Orono, ME, 04469, USA.
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, 02138, USA.
New Phytol. 2023 Feb;237(4):1242-1255. doi: 10.1111/nph.18584. Epub 2023 Jan 3.
The hydraulic system of vascular plants and its integrity is essential for plant survival. To transport water under tension, the walls of xylem conduits must approximate rigid pipes. Against this expectation, conduit deformation has been reported in the leaves of a few species and hypothesized to function as a 'circuit breaker' against embolism. Experimental evidence is lacking, and its generality is unknown. We demonstrated the role of conduit deformation in protecting the upstream xylem from embolism through experiments on three species and surveyed a diverse selection of vascular plants for conduit deformation in leaves. Conduit deformation in minor veins occurred before embolism during slow dehydration. When leaves were exposed to transient increases in transpiration, conduit deformation was accompanied by large water potential differences from leaf to stem and minimal embolism in the upstream xylem. In the three species tested, collapsible vein endings provided clear protection of upstream xylem from embolism during transient increases in transpiration. We found conduit deformation in diverse vascular plants, including 11 eudicots, ginkgo, a cycad, a fern, a bamboo, and a grass species, but not in two bamboo and a palm species, demonstrating that the potential for 'circuit breaker' functionality may be widespread across vascular plants.
维管植物的液压系统及其完整性对植物生存至关重要。为了在张力下运输水分，木质部导管壁必须近似于刚性管道。与这一预期相反，已有报道称少数物种的叶片中存在导管变形现象，并推测其起到防止栓塞的“断路器”作用。但缺乏实验证据，其普遍性也未知。我们通过对三个物种进行实验，证明了导管变形在保护上游木质部免受栓塞方面的作用，并对多种维管植物叶片中的导管变形情况进行了调查。在缓慢脱水过程中，小叶脉中的导管变形在栓塞之前就已发生。当叶片蒸腾作用瞬间增强时，导管变形伴随着叶片与茎之间较大的水势差，且上游木质部的栓塞极少。在所测试的三个物种中，可塌陷的叶脉末梢在蒸腾作用瞬间增强时，为上游木质部提供了明显的防栓塞保护。我们在多种维管植物中发现了导管变形现象，包括11种双子叶植物、银杏、一种苏铁、一种蕨类植物、一种竹子和一种禾本科植物，但在两种竹子和一种棕榈科植物中未发现，这表明“断路器”功能的潜力可能在维管植物中广泛存在。
