Leaves and Root Maximize Water Collection

a small bush of thick light green stalks with small red and green flowers stand in the desert with large dark green curving leaves around their base

Biological Strategy

Desert rhubarb

AskNature Team

Image: Gideon Pisanty / Wikimedia Commons / CC BY - Creative Commons Attribution alone

Capture, Absorb, or Filter Liquids

The most common liquid used by living systems is water, which they require to survive. But there are many other liquids that provide nourishment, play a role in defense mechanisms, or serve other purposes. Water varies in its availability; it is sometimes plentiful and sometimes very scarce or only available as fog or mist. To minimize the energy required to capture, absorb, or filter liquids, living systems have strategies that take advantage of the unique properties of the given liquid. For example, water moves from a gaseous to liquid state when it encounters a surface colder than the air. Plants in forests that experience fog and clouds more than rain have strategies that condense liquid water from moist air.

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Distribute Liquids

Liquids include water, as well as body fluids such as blood, gastric juices, nutrient-laden liquids, and more. To survive, many living systems must move such liquids within themselves or between locations. Because of their properties, liquids tend to disperse unless they are confined in some way. To address this, living systems have strategies to confine fluids for transport, and to overcome barriers such as gravity, friction, and other forces. Some of these same barriers also provide opportunities. Trees and giraffes face the same challenge: how to move fluids (water and blood, respectively) upward against gravity. But their strategies are quite different. The tree moves water using capillary action and evaporation, possibly due to water’s properties of polarity and adhesion. The giraffe’s tight skin provides pressure to assist in blood circulation. and keep blood from pooling in the legs.

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Flowering Plants

Clade Angiosperms (“receptacle seed”): Dandelions, oaks, grasses, cacti, apples

With 416 families containing some 300,000 known species, angiosperms are the most diverse group of plants, and they can be found around the globe in a wide variety of habitats. They are characterized by seeds that grow enclosed in ovaries, which are enclosed in flowers. The floral organs then develop into fruits of myriad kinds and dimensions, from simple seed casings on maples to elaborate fleshy growths like papayas. The oldest flower known from fossils, Montsechia vidalii, appeared during the Jurassic Period 130 million years ago. They are the primary food source for herbivorous animals, which in turn makes them the indirect food source for carnivores as well.

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The leaves and root of the desert rhubarb maximize water collection by directing rainwater to the plant's base and absorbing water from saturated ground.

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.

A science graphic illustrating what is described in the text.

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.

Image: Gideon Pisanty / CC BY - Creative Commons Attribution alone

Last Updated July 19, 2017

References

“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)

Journal article