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

当考虑到植物体内的水流对我们的生物圈和人类的功能至关重要时，大多数人会非常惊讶地发现，植物究竟是如何完成长距离水运的。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是通过吸入气泡而不断失败，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输送系统中的功能含义，特别是它们在相邻管道之间的边界坑水平干旱诱导栓塞形成中的作用。具体来说，将检验以下三种假设：(1)表面活性剂存在于种子植物木质部汁液中，很可能是不溶性脂类（包括磷脂、半乳糖脂和蛋白质）；(2)管间坑膜由于其纤维介质和表面活性剂的存在，可能具有纳米气泡产生器的功能，这些表面活性剂预计具有动态（而不是静态）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，有助于负压下的水输送。这些假设将通过对三种被子植物和两种裸子植物进行木质部汁液的化学分析和光学和透射电镜染色来验证。测量将集中在木质部汁液表面活性剂的动态表面张力，容器间坑膜的孔隙度，水力途径的润湿性，以及木质部汁液中表面活性剂涂层纳米气泡的发生。实验工作将与建模方法相结合，以预测多孔孔膜纤维素网络中空气-水半月板的行为，以及通过气泡断裂事件形成纳米气泡。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观点。这个创新的项目将有助于我们对木质部汁液中表面活性剂的理解，这是多年来被忽视的。通过更好地了解植物在负压下如何输送水分，这一建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，这在当前对气候变化的担忧中尤为重要。

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.