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

AbstractAlthough they account for many thousands of the world’s 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 (δ13C), and N, P and (Na+K) contents. As water-storage capacity is a function ofQvandz, this meansQ/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 withFaof 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 byQ/Aexceeding 1 mg water/mm2leaf 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.Wirelationships were essentially functions of variations inz(transpiration-resistance) and increased metabolic efficiency. We conclude thatzandQvare 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.