A three-dimensional study of sub-foliar condensation in desert rhubarb (Rheum palaestinum, Polygonaceae)
The rare perennial plant species Rheum palaestinum Feinbr. (Polygonaceae), commonly known as desert rhubarb, is endemic to Jordan and southern Israel and grows in areas with low annual rainfall (Al-Eisawi 1998). It has an underground woody stem and grows mostly in shallow ravines of stonysandy terrains during the winter and early spring in years with above-normal precipitation. It produces one to four rounded leaves, 20–60 cm in diameter, with a wrinkled surface (fig. 1). The large leaf size of this species is atypical within arid ecosystems; in the presence of abundant sun, small leaf size is considered as one of the most common patterns of desert plants to decrease transpiration rates and increase water-use efficiency (Gibson 1996). The annual precipitation in Jordan’s desert is less than 75 mm, and January is the wettest month (Tarawneh & Kadıoğlu 2002). A preliminary study of the relationship between precipitation patterns and leaf growth suggested complex and under-studied water-absorption strategies for plants growing in arid environments. Our personal observations showed some unique morphologies and timing patterns that perennial plants in the tableland desert of Jordan have developed as survival strategies. A previous study showed that the large size and surface morphology of rhubarb leaves help to create a self-irrigation system and increase rainwater harvesting by 16-fold compared to other desert plants (Lev-Yadun et al. 2008). A simple leaf morphology can effectively drain rainwater without the need for wrinkles that dramatically increase leaf transpiration area, as has been shown for the lotus leaf (Barthlott & Neinhuis 1996). The leaf morphology of rhubarb may not play a crucial role in water collection, as rainfall is limited during its maximum growth period (March–April). Additionally, the leaves are tightly attached to the ground and the margins are bent downward to contact the soil; these morphological features favour the function of a vapour-trapping system under the leaf rather than water drainage on the upper surface. Rainwater collection is related to the leaf horizontal catchment area and not to the surface area maximized by the wrinkles. Because the water absorption rate needs to be higher than the transpiration rate, rhubarb leaf wrinkles increase the leaf surface area relative to the leaf footprint and