Evolutionary success in arid habitats: Morpho-anatomy of succulent leaves of Crassula species from southern Africa

Succulence is widely interpreted as an adaptation to drought, usually associated with CAM and xeromorphic features among arid-adapted plants. However, this syndrome can also be observed in species typical of mesic and even hydric environments. The leaf-succulent genus Crassula (Crassulaceae) occurs in contrasting habitats in all nine biomes of southern Africa. This study represents the first to compare leaf traits in Crassula species which in nature are confined to diverse habitats in southern Africa. To determine their potential adaptive significance, we investigated leaf succulence and several morpho-anatomical traits of five southern African Crassula species (C. ausensis, C. brevifolia, C. multicava, C. nudicaulis, C. tecta), which occur naturally in habitats of differing aridity; all plants were grown under glasshouse conditions. For each species, we recorded water content, leaf anatomy, and leaf surface structure and hydrophobicity. We found that water content is relatively consistent in Crassula regardless of natural habitat. In contrast, most leaf morpho-anatomical traits examined here are related to macroclimatic conditions. We hypothesize that differences in mesophyll traits in Crassula are potentially linked to water storage and CAM performance, while differences in leaf surface properties are more closely related to water conservation and probably also to water uptake through hydathodes.