Surfactants may shed novel light on water transport in plants under negative pressure

当考虑到流经植物的水对我们生物圈的运作以及人类人口具有极其重要的意义时，大多数人会非常惊讶地得知，植物到底是如何完成长途水分运输的仍然是未知的。虽然有大量证据表明植物木质部在负压下进行水分运输，但为什么这个系统不会因为吸入气泡而不断失灵，这一直是一个长期存在的问题。本项目旨在研究表面活性剂在植物水力运输系统中的功能意义，特别是它们在相邻管道之间的边界坑处形成干旱诱导的血栓形成中的作用。特别是，将检验以下三个假设：(1)表面活性剂存在于种子植物的木质部汁液中，很可能是不溶性脂类(包括磷脂、半乳糖脂和蛋白质)；(2)由于其纤维状介质和表面活性剂的存在，管内坑膜可用作纳米气泡生成器，预计它们具有动态(而不是静态)的表面张力；(3)木质部汁液表面活性剂可能通过保持气泡小而稳定，促进负压下的水的传输。这些假说将通过对三种被子植物和两种裸子植物木质部汁液的化学分析以及光学和透射电子显微镜的染色方法来验证。测量将集中在木质部汁液表面活性剂的动态表面张力、导管间坑膜的孔隙率、液压通道的润湿性以及木质部汁液中表面活性物质包裹的纳米气泡的出现。实验工作将与模拟方法相结合，以预测凹坑膜多孔纤维素网中空气-水弯月面的行为，以及通过气泡断裂事件形成纳米气泡。我们还将对木质部汁液进行更多的实验，以检验不溶性表面活性剂保持气泡小而稳定的想法。这个创新的项目将有助于我们理解木质部汁液中的表面活性剂，这一点多年来一直被忽视。通过更好地了解植物如何在负压下运输水分，这一提议将对仿生应用产生影响，并将增加我们对植物水分利用和抗旱性的理解，鉴于当前对气候变化的担忧，这一点尤其相关。

When considering that the flow of water through plants has utmost importance for the functioning of our biosphere as well as the human population, most people would be extremely surprised to learn that it is still unknown how exactly plants accomplish long-distance water transport. While there is massive evidence for water transport in plant xylem under negative pressure, it has been a long-standing question why this system is not constantly failing by pulling in air-bubbles. This project aims to investigate the functional implications of surfactants in the hydraulic transport system of plants, and especially their role in drought-induced embolism formation at the bordered pit level between neighbouring conduits. In particular, the following three hypotheses will be tested: (1) Surfactants occur in xylem sap of seed plants and are most likely insoluble lipids (including phospholipids, galactolipids, and proteins); (2) Interconduit pit membranes may function as nanobubble generators due to their fibrous medium and the presence of surfactants, which are expected to have a dynamic (instead of static) surface tension, and (3) xylem sap surfactants may contribute to water transport under negative pressure by keeping air-bubbles small and stable. These hypotheses will be tested by applying chemical analyses of xylem sap and staining methods for light and transmission electron microscopy on three angiosperm and two gymnosperm species. Measurements will focus on the dynamic surface tension of xylem sap surfactants, the porosity of intervessel pit membranes, the wettability of the hydraulic pathway, and the occurrence of surfactant coated nanobubbles in xylem sap. Experimental work will be combined with a modelling approach to predict the behaviour of an air-water meniscus in the porous cellulose network of pit membranes, and the formation of nanobubbles via bubble snap-off events. Additional experiments with xylem sap will be conducted to test the idea that insoluble surfactants keep air-bubbles small and stable.This innovative project will contribute to our understanding of surfactants in xylem sap, which has been neglected for many years. By better understanding how plants transport water under negative pressure, this proposal will have implications for biomimetic applications, and will increase our understanding of plant water use and drought tolerance, which is especially relevant given current concerns about climate change.