Lipids have been observed attached to lumen-facing surfaces of mature xylem conduits of several plant species, but there has been little research on their functions or effects on water transport, and only one lipidomic study of the xylem apoplast. Therefore, we conducted lipidomic analyses of xylem sap from woody stems of seven plants representing six major angiosperm clades, including basal magnoliids, monocots, and eudicots, to characterize and quantify phospholipids, galactolipids, and sulfolipids in sap using mass spectrometry. Locations of lipids in vessels of Laurus nobilis were imaged using TEM and confocal microscopy. Xylem sap contained the galactolipids di- and mono-galactosyldiacylglycerol (DGDG and MGDG), as well as all common plant phospholipids, but only traces of sulfolipids, with total lipid concentrations in extracted sap ranging from 0.18 to 0.63 nmol / mL across all seven species. Contamination of extracted sap from lipids in cut living cells was found to be negligible. Lipid composition of sap was compared to wood in two species and was largely similar, suggesting that sap lipids, including galactolipids, originate from cell content of living vessels. Seasonal changes in lipid composition of sap were observed for one species. Lipid layers coated all lumen-facing vessel surfaces of Laurus nobilis, and lipids were highly concentrated in inter-vessel pits. The findings suggest that apoplastic, amphiphilic xylem lipids are a universal feature of angiosperms. The findings require a reinterpretation of the cohesion-tension theory of water transport to account for the effects of apoplastic lipids on dynamic surface tension and hydraulic conductance in xylem.

中文翻译：

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整个被子植物系统发育的木本植物木质部汁液中的脂质

已经观察到脂质附着在几种植物的成熟木质部导管的面向内腔的表面上，但是对它们的功能或对水运输的影响的研究很少，只有对木质部质外体的脂质组学研究。因此，我们对代表6个主要被子植物进化枝的7种植物的木质茎木质部汁液进行了脂质组学分析，包括基底木兰，单子叶植物和双子叶植物，以利用质谱法表征和定量了汁液中的磷脂，半乳糖脂和硫脂。使用透射电镜和共聚焦显微镜对月桂树血管中脂质的位置进行成像。木质部汁液含有半乳糖脂二和单半乳糖基二甘油甘油（DGDG和MGDG），以及所有常见的植物磷脂，但仅含有微量的脂脂，所有七个物种的提取液中的总脂质浓度为0.18至0.63 nmol / mL。发现切出的活细胞中从脂质中提取的汁液的污染可以忽略不计。在两个物种中，树液的脂质组成与木材进行了比较，并且在很大程度上相似，这表明树液脂质（包括半乳糖脂）起源于活体血管的细胞含量。观察到一种物种的汁液脂质组成的季节性变化。脂质层覆盖了月桂树的所有面向内腔的血管表面，脂质在血管间窝中高度浓缩。这些发现表明，质外两亲的木质部脂质是被子植物的普遍特征。