As plant roots take up water and the soil dries, water depletion is expected to occur in the rhizosphere. However, recent experiments showed that the rhizosphere was wetter than the bulk soil during root water uptake. We hypothesise that the increased water content in the rhizosphere was caused by mucilage exuded by roots. It is probably that the higher water content in the rhizosphere results in higher hydraulic conductivity of the root–soil interface. In this case, mucilage exudation would favour the uptake of water in dry soils. To test this hypothesis, we covered a suction cup, referred to as an artificial root, with mucilage. We placed it in soil with a water content of 0.03 cm3 cm–3, and used the root pressure probe technique to measure the hydraulic conductivity of the root–soil continuum. The results were compared with measurements with roots not covered with mucilage. The root pressure relaxation curves were fitted with a model of root water uptake including rhizosphere dynamics. The results demonstrated that when mucilage is added to the root surface, it keeps the soil near the roots wet and hydraulically well conductive, facilitating the water flow from dry soils towards the root surface. Mucilage exudation seems to be an optimal plant trait that favours the capture of water when water is scarce.

Root hairs increase exudation and spatial rhizosphere extension, which probably enhance rhizosphere interactions and nutrient cycling in larger soil volumes. Root hairs may therefore be beneficial to plants under nutrient-limiting conditions. The greater C allocation below ground in the presence of root hairs may additionally foster C sequestration.

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 -1 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.Abbreviations: EPS, extracellular polymeric substances; REV, representative elementary volume.Mucilage secreted from plant roots and extracellular polymeric substances (EPS) produced by microorganisms have a strong impact on soil hydraulic properties (McCully and Boyer 1997;Or et al., 2007a;Carminati et al., 2010;Kroener et al., 2014;Volk et al., 2016). Extracellular polymeric substances have been suggested to buffer fast and strong oscillations in soil water content, maintaining the microenvironment where microorganisms live at a rather constant water content during drying and rewetting (Or et al., 2007b). Similarly, it has been proposed that mucilage maintains the rhizosphere wet and hydraulically conductive during drying, while it delays rewetting after irrigation and might temporarily limit root water uptake during a rewetting phase subsequent to severe soil drying. This hypothesis was based on observations of water content around plant roots (Carminati et al., 2010;Moradi et al., 2012;Zarebanadkouki et al., 2016).Experiments with a mucilage analog (from chia seeds [Salvia hispanica L.]) supported the hypothesis that mucilage plays a key role in shaping the hydraulic properties of the rhizosphere. Mucilage from chia seeds increases the capacity of soils to hold water against negative water potential. A sandy soil mixed with mucilage at a concentration of 1.25% (w/w dry gel/dry soil) had higher water...

Despite detailed images of water content, water fluxes and soil structure in the rhizosphere, a general understanding of how the rhizosphere affects root water uptake is still lacking. The missing elements of the puzzle are the gradient in water potential around roots. Measurements of the xylem water potential at varying soil water potentials and transpiration rates supported by numerical models of root water uptake would allow the estimation of the water potential across the rhizosphere. Such measurements are crucial to comprehend how water enters the roots.

The physical properties of the rhizosphere are strongly influenced by rootexuded 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-mm diameter), and glass beads (0.1-0.2-mm diameter) mixed with varying amounts of chia (Salvia hispanica L.) seed mucilage (dry content range 0.2-19 mg g −1 ) 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 mucilageembedded soil particles.With an extent of millimeters to a few centimeters, the rhizosphere is the part of soil actively modified by root growth and exudation (Gregory, 2006;Hinsinger et al., 2009;York et al., 2016;Roose et al., 2016). Its impact on soil hydrology might be profound, as about 40% of all terrestrial precipitation flows through the rhizosphere-plant-atmosphere continuum (Bengough, 2012). In view of this immense flow of water, Dakora and Phillips (2002) and Sposito (2013) proposed rhizosphere research as key for the sustainable management of water resources.One of the substances released by root tips is mucilage, a gel consisting mainly of polysaccharides and <1% lipids (Oades, 1978;Read et al., 2003). In combination with other sources of organic matter and root hairs, plant mucilage contributes to the formation of the rhizosheath, a region of interconnected soil particles bound to the root surface (Watt et al., 1993). The enhanced connection between roots and soil is supposed to have a major effect on microbial growth and plant nutrient uptake (Dakora and Phillips, 2002). Furthermore, mucilage is known to alter the hydraulic properties of the rhizosphere (Young, 1995;Hallett et al., 2003;Carminati et al., 2010;Moradi et al., 2012;Carminati, 2013;Zarebanadkouki et al., 2016). After a drying cycle, Carminati et al. (2010) found the rewett...

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Despite the importance of mucilage for soil-plant relations, little is known about the effect of soil drying on mucilage exudation. We introduce a method to collect mucilage from maize growing in wet and dry soils. Mucilage was collected from brace roots. The amount of mucilage exuded did not change with soil water content and transpiration rate. Mucilage exuded in dry soils had a higher degree of hydrophobicity, suggesting that the wetting properties of mucilage change in response to soil drying.

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