Contrasting leaf thickness & saturated water content explain diverse structural/physiological properties of arid species

Although they account for many thousands of the worlds flowering plants, little is known about the physical/chemical properties of leaf succulents. Eight species in the Namib Desert, South Africa were assessed for leaf area (A) and thickness (z), saturated (Q) and dry mass, relative volume of air (Fa), intrinsic water-us efficiency ({delta}13C), and N, P and (Na+K) contents. As water-storage capacity is a function of Qv and z, this means Q/A (= Qv *z) is an ideal index of succulence compared with specific-leaf-area and other indices that highlight mass rather than volume. Specific gravities have a different relationship with Fa of sclerophyll-mesophylls: rising succulence infers decreasing air content replaced by water rather than dry matter. The trend among succulent species from Argentina/Spain added to our data was characterized by Q/A exceeding 1 mg water/mm2 leaf whose overall slope was ten times that for co-occurring sclerophyll-mesophyll species. (Na+K), N and P concentrations varied on a dry-matter, but not water-volume, basis. Wi relationships were essentially functions of variations in z (transpiration-resistance) and increased metabolic efficiency. We conclude that z and Qv are keys to the special physiological properties of succulent leaves. Including succulents would force many current monotonic relationships to dichotomize. HighlightThe key to understanding leaf succulents is their high volumetric water content and thick leaves. These explain their superior water-use-efficiency and contrasting relationships with other variables compared with temperate species.