Our vegetation is not just an essential part of the environment; we also expect its yields to feed the present and future world population, fuel renewable energy sources, and power vehicles. Especially in agriculture, plants need an adequate water supply to thrive. It is no secret that they absorb water through their roots, but many questions remain unanswered due to methodological difficulties: Where exactly do plants take up water? Can they actively control the absorption process? Can certain root secretions even change the soil’s properties to enable better access to stored water? It is clear that water must pass through the rhizosphere – the soil in the immediate vicinity of the roots – to get to the roots. It is now known that roots alter the rhizosphere soil not only mechanically but also chemically and biologically. Does this also apply to its hydraulic properties responsible for water movement in the ground? Novel imaging methods help here, because they are able to produce spatially and temporally high-resolution data, enabling a non-invasive, on-site examination of the root system and soil water distribution. Prof. Sascha Oswald is involved with the visualization of these processes.
A glimpse underground, thanks to magnetic resonance imaging
Sascha Oswald and his team are breaking new ground to gain insight into the visualization of “root-induced water uptake processes.” They even combine two imaging techniques: magnetic resonance imaging (MRI) – known primarily for medical applications – and neutron tomography (NT). As an environmental physicist, Oswald is interested in soil hydrology, soil conservation, and groundwater. For some time now, he has been collaborating with the Paul Scherrer Institute – Switzerland’s largest research institute for natural and engineering sciences – and the Helmholtz Zentrum Berlin, pursuing cutting-edge research in the fields of matter and material, people and health, and energy and the environment. Both institutes operate large-scale facilities with a variety of measurement techniques that can also be used by external researchers. In addition, these techniques include a method – similar to X-ray – that allows for the production of images with the help of neutrons. Neutrons are extremely sensitive to hydrogen, thus making water “visible,” a process that interests environmental scientist, geohydrologists, and soil physicists alike.
Oswald and his team began their investigations with lupine, which belongs to the same plant family as peas and chickpeas. In agriculture, lupine is cultivated primarily as a fodder plant. For studying plant water supply, the scientists used special containers that allowed plant roots to grow in natural soil. The material of the container is critical to the experimental setup. For experiments that explore only the root system and water distribution, experimenters use containers made of aluminum, because it is virtually “transparent” for neutrons. Using acrylic glass, which is nontransparent for neutrons, would produce a black image. Containers are now also made of a special glass that works for both neutron tomography and other methods such as MRI. Combining both methods allows the MRI to provide additional information on the pore structure and the manner in which the water is bound.
Water makes the roots visible
“The roots become clearly visible with our method, because they consist of at least 80 percent water,” says Oswald. If you let the plants grow and then dry up the soil, primary and secondary roots become visible on the images with good spatial resolution. The increase and decrease of water in the soil and its exact spatial distribution are studied over several days. The dry areas around the roots are where the roots took up water. The roots continue to grow, and the water absorption shifts. “The images show how and where the roots grow, as well as how the water moves without disturbing the system,” says Oswald. The images also show each plant’s “individuality.” They show how differently the root structures of individual plants develop and how this affects water distribution. Young and old, thick and thin roots do not take up the same amount of water. They transport it to varying degrees of effectiveness to the aboveground plant parts. “We have examined this in different variants, especially with agriculturally interesting plants such as corn, lupine, chickpeas, fava beans, and tomatoes.” These plants were also chosen because their thick roots and clear structures are conducive to this type of experiment. If the root system is too fine, the structures merge and impair the analysis of image data.
There are now increasingly more effective means of three-dimensionally visualizing root-induced water uptake. In neutron tomography, the object is placed on a revolving table and rotated incrementally within a range of at least 180°. A picture is taken at each angular step, and the resulting image series allows the researcher via computer algorithms to reconstruct the three-dimensional root system and the exact water distribution in the soil.
Plants change the soil around them
Such 3-D images suggest that the roots can also alter their immediate environment. They are thus better able to cope with a dilemma: Water absorption increasingly dries the surrounding soil, but the drier the soil, the more it inhibits further water movement, making it harder for the water to reach the roots. In other words, the plant aggravates its own water scarcity until it rains or until it is watered. The researchers found while taking their measurements, however, that this can be reversed in the rhizosphere. This zone then acts as a buffer that retains the water for longer and absorbs it more slowly after irrigation. This helps the plant to better withstand a critical drought. “We think that the plants produce a kind of mucilage gel or have microbes to produce it for them, thus creating this favorable characteristic of the rhizosphere,” Oswald explains. Scientists imagine it as a kind of diaper material, in which gel is able to bind a great deal of water. “It could have a substantial effect and adapt the plant to dry conditions.” Objectively, there is no larger quantity of water available, of course, but access to water is maintained in a wider surrounding area. Oswald estimates that these plants gain an advantage of half a day. Sometimes this is just enough to keep the plant from wilting before the next rainfall.
Too much water is problematic as well, because it cuts off the roots from the oxygen supply needed for the root cells to breathe. This can also be observed with a fluorescence-based method, which was developed by one of Oswald’s assistants. The method allows a two-dimensional visualization of both the oxygen concentration and the pH distribution. The latter is modified by the plant via root secretions in a way that allows the plant to better absorb nutrients. In addition, a change in pH also may change the functioning of the mucilage.
It is increasingly evident that plants actively influence the biochemical and hydraulic parameters at the interface of root and soil, creating temporally dynamic reactions to environmental conditions. Mapping these processes in the rhizosphere, which forms around the growing root system and is just a few millimeters thick, will continue to challenge researchers.
The Project
Visualization of Root-Induced Water Flows by Novel Combinations of “Magnetic Resonance Imaging” and Neutron Tomography
Participants: Prof. Sascha Oswald (University of Potsdam), RWTH Aachen University, Forschungszentrum Jülich GmbH
Duration: 2014–2017
The Researcher
Prof. Sascha Oswald studied physics at the University of Freiburg and the University of Heidelberg, and earned his doctorate in environmental sciences at ETH Zurich. Since 2009, he has been Professor for Subsurface Hydrology at the University of Potsdam.
Contact
Universität Potsdam
Institut für Erd- und Umweltwissenschaften
Karl-Liebknecht-Str. 24–25, 14476 Potsdam
Email: sascha.oswalduuni-potsdampde
Text: Dr. Barbara Eckardt
Translation: Susanne Voigt
Published online by: Agnetha Lang
Contact for the editorial office: onlineredaktionuuni-potsdampde