Creeping bentgrass roots Photo by Shawn Askew
In this month’s Verdure, we’re going full “sciencey-nerd” and reviewing the biophysics of the rhizosphere. Biophysics refers to biological, chemical and physical properties. The rhizosphere is defined as the zone or vicinity of
soil immediately surrounding a plant root where the biology, chemistry and physical properties of the soil are influenced by the root.
Andrea Carminati, Ph.D., (Federal Institute of Technology in Zurich) and colleagues provided a review of current research into the rhizosphere and how roots take up water. This is possible due to advances in imaging techniques in botanical research that
include neutron radiography, MRI, light transmission imaging and X-ray computed tomography designed to measure water properties and soil structure specifically at the root-soil interface. As a result, these techniques produce high-resolution, three-dimensional
images of water distribution in the soil and root zone.
Detailed images of soil pore spaces and water content around roots confirm the heterogeneity of the rhizosphere compared to the soil farther from the roots. In other words, the water content immediately surrounding roots is not evenly distributed. MRI
imaging revealed a depletion of water around roots where the root hairs are located. Healthy, functional root hairs are vital, as they act like a sponge and absorb water and nutrients.
In 1904, the German plant physiologist Lorenz Hiltner first described the rhizosphere as the area around a plant root that is inhabited by a unique population of microorganisms influenced, he postulated, by the chemicals released from plant roots. Today
we know these chemicals as root exudates or mucilage. These substances are exuded from the root tip and shrink or swell as the soil dries and is rewetted. Mucilage is partially degraded by soil microorganisms, and some of those microorganisms also
produce extracellular polymeric substances, which are similar to mucilage.
Mucilage is essentially a gel that is capable of absorbing water; however, it becomes hydrophobic (or water repellent) after dying. Mucilage helps stabilize soil moisture in the rhizosphere during periods of drought stress (i.e., low soil water content)
and possibly maintains the “hydraulic connection” between roots and soil. High-resolution tomography of water distribution around roots showed that during a drying-down period, the soil around roots remained wetter than the adjacent bulk
soil. In other words, mucilage is helping those root hairs function to access water and nutrients. However, with severe drying-down of the soil root zone, mucilage becomes viscous and hydrophobic, which can delay rewetting of the root zone when water
is applied by rain or irrigation. Does this sound familiar with extreme drying-down of sand-based putting greens?
Recent research into the rhizosphere has revealed more insight about the role of mucilage exuded by roots. As plant transpiration increases during the day, roots shrink and lose some of their connectivity with the soil. However, rhizosheaths limit the
loss of contact between roots and soil. The rhizosheath is defined as the soil that “sticks” to the roots. Rhizosheaths are formed by the combination of root hairs and root exudates (mucilage) that essentially connect those glued soil
particles to roots. Besides their influence on water relations in the rhizosphere, root exudates help to stabilize the soil around roots. Thus, roots modify soil root zone properties.
How does all this “rhizosphere stuff” relate to turfgrass? Those basic and sustainable agronomic practices of mowing, fertilizing (plant and soil health) and irrigating (water management) and maintaining consistent and uniform soil moisture
are the best approaches to help those roots function and remain connected within the rhizosphere and thereby produce desirable playing surfaces. Significant advances in research have begun to unravel the relationships among, soil, water, nutrients
and those vital plant-microbial-soil interactions within the rhizosphere.
So, to quote our colleagues working on these connectivity interactions, let’s “stay in touch.” Or, as Stan Kostka, Ph.D., says in an informative webinar, “Watch this space” (http://tinyurl.com/n2u3afxj).
Sources
- Carminati, A., M. Zarebanadkouki, E. Kroener, M.A. Ahmed and M. Holz. 2016. Biophysical rhizosphere processes affecting root water uptake. Annals of Botany 118(4):561-571 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5055629/).
- Affortit, P., M.A. Ahmed, A. Grondin, S. Delzon, A. Carminati and L. Laplaze. 2023. Keep in touch: The soil-root hydraulic continuum and its role in drought resistance in crops. Journal of Experimental Botany (https://doi.org/10.1093/jxb/erad312).
Mike Fidanza, Ph.D., is a professor of plant and soil science in the Division of Science, Berks Campus, at Pennsylvania State University in Reading, Pa. He is a 22-year member of GCSAA.