Vascular plants transport water under negative pressure without constantly creating gas bubbles that would disable their hydraulic system, the xylem. The recent discovery of lipid surfactants in angiosperm xylem raises questions about their functions, particularly how they might affect embolism formation under drought stress. Previous research on effects of artificial, soluble surfactants on xylem embolism formation suggest that lowering surface tension in sap increases the vulnerability to embolism. On the other hand, insoluble lipid xylem surfactants may decrease the sizes of bubbles that form below the critical size under which they would expand to form embolisms; instead these small nanobubbles could dissolve. In either case, it would be expected that drought-adapted plants differ from plants from mesic environments in the amounts and possibly kinds of surfactants found in their xylem. To explore the relationship between these xylem lipids and drought stress we conducted a study of six woody angiosperms from desert, semi-arid woodland, Mediterranean shrubland, temperate forest, and tropical forest, to determine if they differed in the amounts and kinds of xylem lipids. The study was conducted by imaging xylem via transmission electron microscopy and confocal microscopy, as well as enzymatic assays and mass spectrometry of lipids in xylem sap.
Results/Conclusions
Xylem sap of all angiosperms tested was found to contain phospholipids (mainly phosphatidylcholine and phosphatidic acid) and galactolipids, including monogalactosyldiacylglycerol and digalactosyldiacylglycerol. The lipids were found in sap, in pores of inter-vessel pit membranes, and deposited on vessel wall surfaces, especially in pits. No clear differences in the amounts and kinds of xylem lipids were found between plants from different environments. Although much more research on the functions of these lipids is needed, we tentatively propose that they do not play a major role in drought tolerance and instead maintain water transport under normal negative pressure conditions by coating hydrophobic surfaces and thereby rendering them hydrophilic, reducing the chance of bubble nucleation. They may also function by reducing the sizes of any nanobubbles that do form, thereby keeping the bubbles below the critical size at which they would expand under negative pressure to form embolisms. Support for this hypothesis is provided by recent findings that surfactants can prevent bubble formation and reduce bubble size under a variety of conditions, including under mildly negative pressure. Xylem lipids may be crucial for transporting water by the cohesion-tension mechanism, but our findings so far do not support a major role in drought tolerance.