The desert rhubarb grows in the arid mountains of Jordan and Israel, where average annual rainfall is only 75 millimeters. In an average rainfall episode, up to two thirds of rainwater can evaporate before penetrating the soil. Because of this, most desert plants have shallow roots that collect the remaining surface water before it evaporates further.
The desert rhubarb sets itself apart by having a sophisticated water collection system that transports and absorbs water deep in the ground. First, rain water collects on the surface of the rhubarb’s leaves. The rhubarb has one to four meter long leaves with a series of successively wider, hydrophobic (meaning “water-fearing”) grooves embedded into its sides. In a sleek system, the grooves funnel rain water down the leaf similar to a system of rivers and creeks down a mountain.
Next, the collected rainwater pools on the soil at the base of the plant. This small area of soil becomes saturated, allowing water to seep deeper into the soil. In an average rainfall, water penetrates desert soil between one to three centimeters. Pooling around the rhubarb, however, helps water to penetrate over ten centimeters into the ground. This is a comparatively high volume of water collected. In addition, the deeper the water penetrates the soil, the less exposed it is to the sun’s heat, and the less it will evaporate.
Lastly, the water is absorbed. The rhubarb has a single, long root that extends down into the desert ground. Compared to the thin shallow root system of its neighbors, the rhubarb root absorbs up to three times as much rainwater. While the single root is still technically collecting surface water, the pooling enabled by the grooved leaves has already brought the water much deeper into the soil.
This summary was contributed by Allison Miller.Edit Summary
“The rare plant Rheum palaestinum (Polygonaceae) is a perennial hemicryptophyte that grows during the rainy winter in desert mountainous areas in Israel and Jordan that receive an average annual rainfall of ca. 75 mm. It produces between one and four large round leaves that are tightly attached to the ground and form large rosettes of up to 1 m2. These leaves differ markedly from the typical small leaves of most desert plants. Moreover, they have a unique 3D morphology resembling a scaled-down mountainous area with well-developed steep drainage systems, raising the question which selective agents were involved in their evolution. We propose that the large leaves collect rainwater that then infiltrates the soil surrounding the root. We measured the seasonal course of leaf growth, examined the area of wet soil surrounding the root after actual and simulated rain, and modeled the water harvesting capacity using the plant leaf area and the weekly precipitation. We show that even in the slightest rains, water flows above the veins to the leaf’s base where it irrigates the vertical root. A typical plant harvests more than 4,100 cm3 of water per year, and enjoys a water regime of about 427 mm/year, equivalent to the water supply in a Mediterranean climate. This is the first example of self-irrigation by large leaves in a desert plant, creating a leaf-made mini oasis.” (Lev-Yadun et al. 2009:393)