Leaf margin organisation and the existence of vaterite-producing hydathodes in the alpine plant Saxifraga scardica

Some sections of species within the genus Saxifraga deposit a chalky crust on the surface of their leaves, originating from the guttation medium produced by the sunken hydathodes found generally at or near the leaf margin. The organisation of the hydathode tissues, that of the rest of the leaf and the physiology of the crust is poorly understood. We have used cryo-SEM and cryo-fracture to study leaf tissue organisation and structure in Saxifraga scardica and compared it to the imaging data with the previously characterised Saxifraga cochlearis. We find S. scardica contains a transparent and tapered leaf margin containing thick walled cylindrical cells that may serve to deflect light to the adjacent palisade mesophyll tissue. Raman microscopy reveals the S. scardica leaf crust contains the rare and metastable calcium carbonate polymorph vaterite whereas the crust from S. cochlearis contains only calcite. Vaterite-crust is also observed on the leaves of some species within the section Porphyrion but is not found on members of the section Ligulatae. The implications of these findings are discussed.