Functional and evolutionary significance of the unique ‘lime-secreting’ hydathodes and amphistomatic leaves in Saxifraga (Saxifragaceae)

The genus Saxifraga comprises about 480 species primarily found in the mountains of the Northern Hemisphere. About 24% of these species have lime-encrusted leaves caused by unique ‘lime-secreting glands’, which contain epithem hydathodes that are connected to the xylem. These ‘glands’ release guttation fluid presumably containing dissolved calcium hydrogen carbonate. We examined their structure in 81 representative species and subspecies using light and scanning electron microscopy (SEM). Lime incrustations are confined to two of the fifteen Saxifraga sections, which typically inhabit calcareous or base-rich substrates. Thus, the ‘lime secretion’ may contribute to regulating internal Ca²⁺ concentrations, among other functions. Phylogenetic analysis of the entire genus Saxifraga shows that the ability to produce lime incrustations evolved once in the last common ancestor (LCA) of the sections Ligulatae and Porphyrion . However, this ability was subsequently lost in two lineages: sections Gymnopera and Trachyphyllum . The peculiar sunken hydathodes on the adaxial leaf surface and their association with camptodromous leaf vascularization support this scenario because they suggest that ‘lime secretion’ is a complex anatomical and physiological syndrome that is unlikely to have evolved twice. While this syndrome plays a role in ecological adaptation and biogeography, it does not appear to be a classic evolutionary ‘key innovation.’ Most species also have amphistomatic leaves with notable interspecific differences in stomatal arrangement related to hydathode arrangement. Amphistomaty is believed to enhance photosynthetic rates, which is consistent with these saxifrages’ adaptation to sunlit, high-elevation environments and their drought-tolerant, partly succulent, xerophytic growth forms.