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

当考虑到水通过植物的流动对我们的生物圈以及人类的功能至关重要时，大多数人都会非常惊讶地得知，植物究竟是如何完成长距离水运输的。虽然有大量的证据表明在负压下植物木质部中的水运输，但长期以来一直存在一个问题，为什么这个系统不会因为吸入气泡而不断失败。该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有边界的坑水平处干旱诱导的栓塞形成中的作用。特别地，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，并且最有可能是不溶性脂质（包括磷脂、半乳糖脂和蛋白质）;（2）由于它们的纤维介质和表面活性剂的存在，导管间凹坑膜可以用作纳米气泡发生器，预期其具有动态的气泡发生特性。（而不是静态的）表面张力，和（3）木质部汁液表面活性剂可以通过保持气泡小而稳定而有助于负压下的水运输。这些假设将通过应用木质部汁液的化学分析和光和透射电子显微镜染色方法对三种被子植物和两种裸子植物物种进行测试。测量将集中在动态表面张力的木质部汁液表面活性剂，孔隙率的管间纹孔膜，润湿性的液压通路，和表面活性剂涂层的纳米气泡在木质部汁液的发生。实验工作将与建模方法相结合，以预测在多孔纤维素网络的坑膜的空气-水弯月面的行为，并通过气泡折断事件的纳米气泡的形成。我们还将对木质部汁液进行额外的实验，以验证不溶性表面活性剂使气泡保持小而稳定的想法。这一创新项目将有助于我们了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，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耐旱性的理解，这与当前对气候变化的担忧特别相关。

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.