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The physical properties of the rhizosphere are strongly influenced by root-exuded mucilage, and there is increasing evidence that mucilage affects the wettability of soils on drying. We introduce a conceptual model of mucilage deposition during soil drying and its impact on soil wettability. We hypothesized that as soil dries, water menisci recede and draw mucilage toward the contact region between particles. At low mucilage contents (milligrams per gram of soil), mucilage deposits have the shape of thin filaments that are bypassed by infiltrating water. At higher contents, mucilage deposits occupy a large fraction of the pore space and make the rhizosphere hydrophobic. This hypothesis was confirmed by microscope images and contact angle measurements. We measured the initial contact angle of quartz sand (0.5–0.63- and 0.125–0.2-mm diameter), silt (36–63-μm diameter), and glass beads (0.1–0.2-mm diameter) mixed with varying amounts of chia ( L.) seed mucilage (dry content range 0.2–19 mg g) using the sessile drop method. We observed a threshold-like occurrence of water repellency. At low mucilage contents, the water drop infiltrated within 300 ms. Above a critical mucilage content, the soil particle–mucilage mixture turned water repellent. The critical mucilage content decreased with increasing soil particle size. Above this critical content, mucilage deposits have the shape of hollow cylinders that occupy a large fraction of the pore space. Below the critical mucilage content, mucilage deposits have the shape of thin filaments. This study shows how the microscopic heterogeneity of mucilage distribution impacts the macroscopic wettability of mucilage-embedded soil particles.

Recent studies have drawn attention to the role of mucilage in shaping rhizosphere hydraulic properties and regulating root water uptake. During drying, mucilage keeps the rhizosphere wet and conductive, but on drying it turns hydrophobic, limiting root water uptake. In this study, we introduce the concept of , defined as additives that (i) rewet the rhizosphere and (ii) reduce mucilage swelling, thereby reducing the rhizosphere conductivity. We tested whether selected surfactants behaved as rhizoligands. We used neutron radiography to monitor water redistribution in the rhizosphere of lupine ( L. cv. Feodora) and maize ( L.) irrigated with water and rhizoligands. In a parallel experiment, we tested the effect of rhizoligands on the transpiration rate of lupine and maize subjected to repeated drying and wetting cycles. We also measured the effect of rhizoligands on the maximum swelling of mucilage and the saturated hydraulic conductivity of soil mixed with various mucilage concentrations. Rhizoligand treatment quickly and uniformly rewetted the rhizosphere of maize and lupine. Interestingly, rhizoligands also reduced transpiration during drying–wetting cycles. Our hypothesis is that the reduction in transpiration was triggered by the interaction between rhizoligand and mucilage exuded by roots. This hypothesis is supported by the fact that rhizoligand reduced the maximum swelling of mucilage, increased its viscosity, and decreased the hydraulic conductivity of soil–mucilage mixtures. The reduced conductivity of the rhizosphere induced a moderate stress to the plants, reducing transpiration. Rhizoligands increase the rhizosphere wetting kinetics and decrease the maximum swelling of mucilage. As a consequence, root rehydration following irrigation is faster, a larger volume of water is available to the plant, and this water is used more slowly. This slower water consumption would allow the plant to stay turgid during a prolonged drying period. We propose that by managing the hydraulic properties of the rhizosphere, we can improve plants’ adaptation to drought.

Mucilage secreted by roots alters hydraulic properties of soil close to the roots. Although existing models are able to mimic the effect of mucilage on soil hydraulic properties for specific soils, it has not yet been explored how the effects of mucilage on macroscopic soil hydraulic properties depend on soil particle size. We propose a conceptual model of how mechanistic pore-scale interactions of mucilage, water, and soil depend on pore size and mucilage concentration and how these pore-scale characteristics result in changes of macroscopic soil hydraulic properties. Water retention and saturated hydraulic conductivity of soils with different ranges of particle sizes mixed with various mucilage concentrations were measured and used to validate the conceptual model. We found that (i) at low mucilage concentrations, the saturated conductivity of a coarse sand was a few orders of magnitude higher than that of a silt, (ii) at an intermediate concentration, the hydraulic conductivity of a fine sand was lower than of a coarse sand or a silt, and (iii) at a high concentration, all soils had a hydraulic conductivity of the same magnitude. At low matric potentials, mucilage increased the water content in all soilsin all soils. In coarser soils, higher mucilage concentrations were needed to induce an increase in water content of >0.05 g g at low matric potentials. This study shows how pore-scale interactions between mucilage, water, and soil particles affect bulk soil hydraulic properties in a way that depends on soil particle size. Including such effects in quantitative models of root water uptake remains challenging.

Recent studies have shown that rhizosphere hydraulic properties may differ from those of the bulk soil. Specifically, mucilage at the root-soil interface may increase the rhizosphere water holding capacity and hydraulic conductivity during drying. The goal of this study was to point out the implications of such altered rhizosphere hydraulic properties for soil-plant water relations. We addressed this problem through modeling based on a steady-rate approach. We calculated the water flow toward a single root assuming that the rhizosphere and bulk soil were two concentric cylinders having different hydraulic properties. Based on our previous experimental results, we assumed that the rhizosphere had higher water holding capacity and unsaturated conductivity than the bulk soil. The results showed that the water potential gradients in the rhizosphere were much smaller than in the bulk soil. The consequence is that the rhizosphere attenuated and delayed the drop in water potential in the vicinity of the root surface when the soil dried. This led to increased water availability to plants, as well as to higher effective conductivity under unsaturated conditions. The reasons were two: (i) thanks to the high unsaturated conductivity of the rhizosphere, the radius of water uptake was extended from the root to the rhizosphere surface; and (ii) thanks to the high soil water capacity of the rhizosphere, the water depletion in the bulk soil was compensated by water depletion in the rhizosphere. We conclude that under the assumed conditions, the rhizosphere works as an optimal hydraulic conductor and as a reservoir of water that can be taken up when water in the bulk soil becomes limiting.

It has been speculated that during periods of water deficit, roots may shrink and lose contact with the soil, with a consequent reduction in root water uptake. Due to the opaque nature of soil, however, this process has never been observed in situ for living plants. Through x-ray tomography and image analysis, we have demonstrated the formation and dynamics of air gaps around roots. The high spatial resolution required to image the soil–root gaps was achieved by combining tomography of the entire sample (field of view of 16 by 16 cm, pixel side 0.32 mm) with local tomography of the soil region around the roots (field of view of 5 by 5 cm, pixel side 0.09 mm). For a sandy soil, we found that when the soil dries to a water content of 0.025 m 3 m −3 , gaps occur around the taproot and the lateral roots of lupin ( Lupinus albus L.). Gaps were larger for the taproot than the laterals and were caused primarily by root shrinkage rather than by soil shrinkage. When the soil was irrigated again, the roots swelled, partially refilling the gaps; however, large gaps persisted in the more proximal, older part of the taproot. Gaps are expected to reduce water transfers between soil and roots. Opening and closing of gaps may help plants to prevent water loss when the soil dries, and to restore the soil–root continuity when water becomes available. The persistence of gaps in the more proximal parts is one reason why roots preferentially take up water from their more distal parts.

Aggregation of soil particles is crucial for water flow in the vadose zone. Recent studies demonstrated that the unsaturated water flow through an aggregate pair is controlled by the contacts between aggregates. In these studies, the hydraulic conductivity of an aggregate pair was calculated as the harmonic mean of the conductivities of one aggregate and one contact, and it was fitted increasing the tortuosity from 0.5 to 5. In the present study, we investigated whether the contacts between aggregates control the water flow also in large aggregate packings. Our hypothesis was that unsaturated water flow in aggregate packings is primarily a flow in series through aggregates and contacts, and therefore the hydraulic conductivity of the system is properly approximated with that of an aggregate pair. To verify this hypothesis, we used neutron radiography to image water infiltration and redistribution in a cylindrical packing of aggregates. Then we numerically simulated the experiment using the hydraulic properties obtained from the model of an aggregate pair—i.e., tortuosity equal to 5. The observed water flow showed that the water redistribution following infiltration was particularly slow and the wetting front was very sharp. These features are explained by the bottleneck effect of the contacts, which were rapidly drained and limited the flow. This flow behavior was well reproduced in the simulations with tortuosity equal to 5, while simulations with tortuosity equal to 0.5, which is the tortuosity obtained for the aggregates, could not reproduce the observations. This study shows that the contacts control the unsaturated water flow also in larger aggregate packings and the process can be in a first approximation modeled as a flow in series through aggregates and contacts.

Water infiltration into vegetated soils is affected by interactions between soil properties and plant activity. Water uptake by plant roots depends on the soil hydraulic properties and infiltration rate. In turn, roots are important water movers and induce nonuniform water content distributions with consequent impact on the infiltration rate. Our goal was to use neutron radiography to investigate root water uptake during rapid infiltration events and subsequent water redistribution. Neutron radiography is an imaging technique capable of measuring with high spatial and temporal resolution root density, soil structure, and water distributions in the vicinity of roots. We used time-series neutron radiography to image infiltration events in 0.17- by 0.15- by 0.013-m boxes filled with a sand mixture and that contained either lupine ( Lupinus L.) or maize ( Zea mays L.) plants. We measured four replications at two stages of plant growth. Neutron radiography can detect small variations in water content with high accuracy and high spatial and temporal resolution. Due to their high water content, roots were easily visible in the radiograms before infiltration. Following calibration, we quantified the water content in the sample during and after each infiltration event. Effects of soil heterogeneities such as thin soil layers were visible, as was water uptake by roots. We observed lower water contents along the main root of lupine during the early stage of infiltration, and slower and more diffuse uptake during the later stages of water redistribution, when the wetting front had propagated downward and the water content in the soil decreased. The observed root water uptake was not uniform but rather was localized in certain regions. For maize we observed a loss of water mainly in the upper part of the root system, closer to the stem rather than at root tips.

Plant roots exude approximately 10% of the carbon assimilated through photosynthesis into the soil, a process referred to as rhizodeposition. Here, we show that the mucilaginous fraction of the rhizodeposits, referred to as mucilage, plays a crucial role on soil–plant water relation and it has the potential to increase plant drought tolerance. Mucilage is a gel that can absorb large volumes of water, altering the physical properties of the rhizosphere and maintaining the rhizosphere wet and conductive when the soil dries. It is hypothesized that mucilage acts as a hydraulic bridge between roots and the soil, facilitating root water uptake and maintaining transpiration in dry soils. By employing a simplified model of root water uptake coupled with mucilage dynamics, we found that in a sandy soil the benefit of mucilage in maintaining root water uptake commenced to manifest when the soil matric potential dropped below approximately −0.8 MPa. This critical matric potential varied with transpiration rate, root length, and exudation rate. Below the critical potential, mucilage maintained photosynthesis and resulted in a net gain of carbon. In summary, rhizodeposition modifies the physical soil environment and has an impact on transpiration and photosynthesis. In other words: water for carbon, but also carbon for water.

The rhizosphere has a controlling role in the flow of water and nutrients from soil to plant roots; however, its hydraulic properties are not well understood. As roots grow, they change the pore size distribution of the surrounding soil. Roots release polymeric substances such as mucilage into their rhizosphere. Microorganisms living in the rhizosphere feed on these organic materials and release other polymeric substances into the rhizosphere. The presence of these organic materials might affect the water retention properties and the hydraulic conductivity of the rhizosphere soil during drying and rewetting. We used neutron radiography to monitor the dynamics of water distribution in the rhizosphere of lupin ( Lupinus albus L.) plants during a period of drying and rewetting. The rhizosphere was shown to have a higher water content than the bulk soil during the drying period but a lower one during the subsequent rewetting. We evaluated the wettability of the bulk soil and the rhizosphere soil by measuring the contact angle of water in the soil. We found significantly higher contact angles for the rhizosphere soil than the bulk soil after drying, which indicates slight water repellency in the rhizosphere. This explains the lower soil water content in the rhizosphere than the bulk soil after rewetting. Our results suggest that the water holding capacity of the rhizosphere is dynamic and might shift toward higher or lower values than those of the surrounding bulk soil, not affected by roots, depending on the history of drying and rewetting cycles.

Water flow from soil to roots is controlled by the hydraulic behavior of the soil near the roots, the rhizosphere. Recent experiments showed that during drying, the rhizosphere held more water than the bulk soil. After irrigation, the rhizosphere remained temporarily dry, and it slowly rewetted after a few days. The hypothesis is that the observed hydraulic behavior was caused by mucilage exuded by roots. Mucilage is a polymeric material that is capable of holding a large amount of water but that contains also lipids that makes it hydrophobic when it dries. A model of root water uptake coupled with swelling and shrinking of mucilage id proposed. Water flow is modeled solving the Richards equation in a domain composed of two concentric regions, bulk soil and rhizosphere, with the root in the center as a boundary condition. It is assumed that during drying, mucilage is in equilibrium with the bulk water. After irrigation, mucilage does not rehydrate immediately, and water content and water potential in the rhizosphere are decoupled. The rhizosphere rewetting rate is assumed to be proportional to the difference between the water potential in the rhizosphere and the potential that the rhizosphere would have if it was in equilibrium at the actual water content. The proportional coefficient depends on mucilage properties and pore characteristics. The model reproduced well the observed water dynamics in the rhizosphere. According to this model, the rhizosphere conductivity is not a unique function of soil water potential, but it is variable and depends on the drying and wetting history. The study illustrates the dynamic and interacting nature of water flow to roots.

Knowledge of local water fluxes across the soil–root interface is essential to understand and model root water uptake. Despite its importance, there is a lack of direct methods to measure the location of water uptake along roots. The aim of this study was to develop a technique to quantify local flux of water from soil to the roots of living plants. To this end, we used neutron radiography to trace the transport of deuterated water (D 2 O) into individual roots. We grew lupin ( Lupinus albus L.) in 30- by 25- by 1-cm containers filled with a sandy soil, which was partitioned into different compartments using 1-cm-thick layers of coarse sand. We locally injected D 2 O into a selected soil compartment near the roots of 18-d-old lupin during the day (transpiring) and night (not transpiring). The transport of D 2 O into roots was then monitored using time-series neutron radiography. The results show that: (i) the transport of D 2 O into roots was faster during the day than at night; and (ii) during the day, D 2 O was quickly transported along the roots toward the shoots, while at night this transport was insignificant. The differences between day and night measurements were explained by convective transport of D 2 O into the roots driven by transpiration. To quantify the local transport of D 2 O into roots, we developed a simple convection–diffusion model that assumed the endodermis as the main resistance to water transport. The D 2 O uptake predicted by the model was in agreement with the axial flow within the roots as derived from D 2 O transport behind the capillary barrier. This new method allows quantification of local water uptake in different parts of the root system.