Leaf epidermal hydathodes and the ecophysiological consequences of foliar water uptake in species of Crassula from the Namib Desert in southern Africa

Epidermal hydathodes were found on leaves of 46 of 48 species of Crassula c ollected from the Namib Desert in southern Africa. The possibility that the se structures might allow the absorption of surface water was investigated in 27 species (including subspecies). The presence of hydathodes on leaf ep idermi correlated, in most cases, with increases in leaf thickness and enha nced rates of nocturnal, and sometimes diurnal, CO2 up-take following wetti ng of the leaves during the night. The precise nature of these responses va ried depending on the species. In addition, wetting only the older leaves o n the lower portion of the shoot of C. tetragona ssp. acutifolia not only r esulted in increased thickness of these leaves, but also effected an increa se in leaf thickness and stimulation of CO2 uptake rates in the distal, you nger portion of the shoot that was not wetted. Overall, foliar hydathodes w ere implicated in the absorption of surface water in many species of Crassu la such that the ecophysiology of these desert succulents was positively af fected. Although rainfall in the Namib Desert is infrequent, surface wettin g of the leaves is a more common occurrence as a result of nighttime dew or fog deposition. Presumably, species with hydathodes benefit directly from this source of moisture. These findings have important implications in unde rstanding a relatively unexplored adaptation of some xerophytes to an extre mely arid environment.