Service Communautaire d'Information sur la Recherche et le Développement - CORDIS

Final Activity Report Summary - WATER WATCH (Non-invasive imaging of the water dynamics in a soil plant groundwater system)

How does water flow to roots? Water flows from soil to roots following a negative gradient in water potential, that comprises of gravity and capillarity forces. Roots work as pumps that take up water from the soil in its vicinity, the rhizosphere. Water that is removed from the rhizosphere is then replaced by water flowing from more distant soil. In other words, water moves from wetter and more distant soil to the relatively dryer rhizosphere. This concept is well known and is included in models of root water-uptake with increasing complexity. However the results of this project showed that the story is a bit different.

We used the technique of neutron radiography to observe distribution of water in the root zone of living plants. Instead of observing water depletion around the roots, we observed an increase in water towards the roots even though water was flowing to roots. Now, if the soil around roots was really homogeneous our results would be physically impossible. It would be like a ball rolling up-hill against its potential. Another surprise arrived when we irrigated the samples after a drying period. We observed that the rhizosphere remained dry for the subsequent 2 days although the bulk soil was wet. It was unlikely that such huge and continuous water depletion was caused by root uptake. "Your plants are wrong" was a common comment after presentation of these results. Instead, searching in the literature, we found similar results in studies about bacteria. Bacteria are surrounded by extracellular polymeric substances (EPS) that act as a protecting layer against dessication and fast rewetting.

EPS has a very high water holding capacity and it has a slow rehydration rate compared to soils. We concluded that mucilage exuded by roots is a very reasonable hypothesis to explain our observations. Mucilage alters the hydraulic properties of the rhizosphere. The resulting heterogeneity of the rhizosphere compared to the bulk soil determines the water dynamics in the rhizosphere. The distinct properties of the rhizosphere very likely affect the gradients of water potential close to the roots. We investigated this hypothesis by a modelling study. We found that the rhizosphere, with its high water holding capacity, restrained the water depletion next to roots and significantly weakened the drop in water potential as the soil dried. Evidently, this favours water availability to plants during drought, helping plants to survive in arid regions. In other words, plants learned to modify their soil in order to facilitate water uptake.

This study shed a new light on the specific and dynamic properties of the root-soil interface. Including the rhizosphere's dynamics in irrigation policy may help to increase water storage in the root-zone, decrease water leaching, and improve water-use efficiency. These scientific findings are the results of intensive experimental and modelling work performed by the two fellows together with a pool of scientists of the host institute. Part of the findings has already been published in international journals and part of them is still in preparation and will bring to additional publications.

The results have also been presented by the fellows at several international conferences. Additionally, the training activities and the scientific research performed by the fellows during the 4 years has opened the doors for new work for both, the institute and the two fellows, demonstrating an effective transfer of knowledge between the partners.

Reported by