Abstract
Lipids may play an important role in preventing gas embolisms by coating nanobubbles in xylem sap. Few studies on xylem sap lipids have been reported for temperate plants, and it remain unclear whether sap lipids have adaptational significance in tropical plants. In this study, we quantify the lipid composition of xylem sap for angiosperm species from a tropical savanna (seven species) and a seasonal rainforest (five species) using mass spectrometry. We found that all twelve species studied contained lipids in their xylem sap, including galactolipids, phospholipids and triacylglycerol, with a total lipid concentration ranging from 0.09 to 0.26 nmol/L. There was no difference in lipid concentration or composition between plants from the two sites, and the lipid concentration was negatively related to species' open vessel volume. Furthermore, savanna species showed little variation in lipid composition between the dry and the rainy season. These results support the hypothesis that xylem sap lipids are derived from the cytoplasm of individual conduit cells, remain trapped inside individual conduits, and undergo few changes in composition over consecutive seasons. A xylem sap lipidomic data set, which includes 12 tropical tree species from this study and 11 temperate tree species from literature, revealed no phylogenetic signals in lipid composition for these species. This study fills a knowledge gap in the lipid content of xylem sap in tropical trees and provides additional support for their common distribution in xylem sap of woody angiosperms. It appears that xylem sap lipids have no adaptive significance.