Gas exchange in the soil is determined by the size and connectivity of air-filled pores. Root mucilage can partially reduce air-filled pore connectivity and thus reduce gas diffusivity. However, it remains unclear to what extent mucilage affects soil pore connectivity and tortuosity. The aim of this study was to gain a better understanding of gas diffusion processes in the rhizosphere by explaining the geometric alterations of the soil pore space induced by mucilage. We quantified the effect of a root mucilage analogue collected from chia seeds without intrinsic respiratory activity on oxygen diffusion at different water contents during wetting-drying cycles in a diffusion chamber experiment. In addition, we used X-ray computed tomography (CT) imaging to visualize the distribution of air and water in the pore space, and quantified the connectivity of the gas phase. Furthermore, we used environmental scanning electron microscopy (ESEM) to visualize mucilage bridges in the dry soil samples. Quantification of oxygen diffusion showed that mucilage decreased the gas diffusion coefficient in dry soil without affecting air-filled porosity. Without mucilage, a hysteresis in gas diffusion coefficient during a drying-rewetting cycle could be observed for fine sandy soil as well as silt and clay soils. The effect diminished with increasing mucilage content. CT imaging indicated a hysteresis in the connectivity of the gas phase during a drying-rewetting cycle for samples without mucilage. This effect was attenuated with increasing mucilage content. Electron microscopy showed that mucilage forms membrane-like liquid bridges during drying. With increasing mucilage content cylindrical structured are created and at high content interconnected structures are observed throughout the pore space, thereby progressively reducing the connectivity of the gas phase. Our results suggest that the release of mucilage into the soil may be a plant adaptation strategy to balance soil oxygen availability and water content.

Gas exchange in the soil is determined by the size and connectivity of air-filled pores. Root mucilage reduces air-filled pore connectivity and thus gas diffusivity. It is unclear to what extent mucilage affects soil pore connectivity and tortuosity. The aim of this study was to gain a better understanding of gas diffusion processes in the rhizosphere by explaining the geometric alterations of the soil pore space induced by mucilage. We quantified the effect of a root mucilage analogue collected from chia seeds without intrinsic respiratory activity on oxygen diffusion at different water contents during drying-rewetting cycles in a diffusion chamber experiment. Quantification of oxygen diffusion showed that mucilage decreased the gas diffusion coefficient in dry soil without affecting air-filled porosity. Without mucilage, a hysteresis in gas diffusion coefficient during a drying-rewetting cycle was observed. The effect depended on particle size and diminished with increasing mucilage content. X-ray computed tomography imaging indicated a hysteresis in the connectivity of the gas phase during a drying-rewetting cycle for samples without mucilage. This effect was attenuated with increasing mucilage content. Furthermore, electron microscopy showed that mucilage structures formed in drying soil increase with mucilage content, thereby progressively reducing the connectivity of the gas phase. In conclusion, the effect of mucilage on soil gas diffusion highly depends on soil texture and mucilage content. The diminishing hysteresis with the addition of mucilage suggests that plant roots secrete mucilage to balance oxygen availability and water content, even under fluctuating moisture conditions.

With ongoing climate change and the increase in extreme weather events, especially droughts, the challenge of maintaining food security is becoming ever greater. Locally adapted landraces of crops represent a valuable source of adaptation to stressful environments. In the light of future droughts-both by altered soil water supply and increasing atmospheric water demand (vapor pressure deficit [VPD])-plants need to improve their water efficiency. To do so, plants can enhance their access to soil water by improving rhizosphere hydraulic conductivity via the exudation of mucilage. Furthermore, plants can reduce transpirational water loss via stomatal regulation. Although the role of mucilage and stomata regulation on plant water management have been extensively studied, little is known about a possible coordination between root mucilage properties and stomatal sensitivity as well as abiotic drivers shaping the development of drought resistant trait suits within landraces. Mucilage properties and stomatal sensitivity of eight Mexican landraces of Zea mays in contrast with one inbred line were first quantified under controlled conditions and second related to water demand and supply at their respective site of origin. Mucilage physical properties-namely, viscosity, contact angle, and surface tension-differed between the investigated maize varieties. We found strong influences of precipitation seasonality, thus plant water availability, on mucilage production (R 2 = .88, p < .01) and mucilage viscosity (R 2 = .93, p < .01). Further, stomatal sensitivity to increased atmospheric water demand was related to mucilage viscosity and contact angle, both of which are crucial in determining mucilage's water repellent, thus maladaptive, behavior upon soil drying. The identification of landraces with pre-adapted suitable trait sets with regard to drought resistance is of utmost importance, for example, trait combinations such as exhibited in one of the here investigated landraces. Our results suggest a strong environmental selective force of seasonality in plant water availability on mucilage properties as well as regulatory stomatal effects to avoid mucilage's maladaptive potential upon drying and likely delay critical levels of hydraulic dysfunction. By this, landraces from highly seasonal climates may exhibit beneficial mucilage and stomatal traits to prolong plant functioning under edaphic drought. These findings may help breeders to efficiently screen for local landraces with pre-adaptations to drought to ultimately increase crop yield resistance under future climatic variability.

Mucilage, a polysaccharide-containing hydrogel, is hypothesized to play a key role in the rhizosphere as a self-organized system because it may vary its supramolecular structure with changes in the surrounding solution. However, there is currently limited research on how these changes are reflected in the physical properties of real mucilage. This study examines the role of solutes in maize root, wheat root, chia seed, and flax seed mucilage in relation to their physical properties. Two purification methods, dialysis and ethanol precipitation, were applied to determine the purification yield, cation content, pH, electrical conductivity, surface tension, viscosity, transverse 1 H relaxation time, and contact angle after drying of mucilage before and after purification. The two seed mucilage types contain more polar polymers that are connected to larger assemblies via multivalent cation crosslinks, resulting in a denser network. This is reflected in higher viscosity and water retention ability compared to root mucilage. Seed mucilage also contains fewer surfactants, making them better wettable after drying compared to the two root mucilage types. The root mucilage types, on the other hand, contain smaller polymers or polymer assemblies and become less wettable after drying. However, wettability not only depends on the amount of surfactants but also on their mobility, as well as the strength and mesh size of the network structure. The changes in physical properties and cation composition observed after ethanol precipitation and dialysis suggest that the polymer network of seed mucilage is more stable and specialized in protecting the seeds from unfavorable environmental conditions. In contrast, root mucilage is characterized by fewer cationic interactions and its network relies more on hydrophobic interactions. This allows root mucilage to be more flexible in responding to changing environmental conditions, facilitating nutrient and water exchange between root surfaces and the rhizosphere soil.

Aims High viscosity, low surface tension and hydrophobicity are specific properties of maize root mucilage which contribute to modulate the spatial configuration of the liquid phase in soil pores. Several processes in the rhizosphere, in particularly nutrient absorption, root exudation and microbial activity, may cause strong temporal variations in the chemistry of the soil solution of the rhizosphere. Although the physical properties of maize root mucilage have been repeatedly measured in the last years, their variation upon a changing chemical environment and understanding of the chemical mechanisms governing these properties remain unexplored. Methods We investigated how flow and surface properties of maize root mucilage varied by changes in pH, calcium chloride (CaCl 2 ) and lecithin concentrations. Results The physical properties of mucilage can strongly vary depending on the environmental conditions. Low surface tension of maize root mucilage at pH7 was increased by addition of calcium. Upon pH change and lecithin addition, hydrophobic mucilage turned hydrophilic. High Ca concentration above 0.83 mmol Ca (g dry mucilage) −1 , the addition of 167 μg lecithin (g dry mucilage) −1 and a pH rise to 9 decreased the viscosity of mucilage. Conclusion Such variations strongly suggest that the role of mucilage in hydraulic processes in the rhizosphere depends on changes of solutes concentration and composition, which themselves vary according to plant growth and soil water content. It seems that mucilage can best serve as a hydraulic bridge only under certain chemical environments, whose spatio-temporal occurrence in the changing rhizosphere remains to be defined.

Purpose Mucilage plays crucial roles in root-soil interactions. Collection systems for maize ( Zea mays L.) use primary and seminal roots of aeroponically-grown seedlings (CS A ), or brace roots of soil-grown plants (CS B ). While each method represents specific plant developmental stages, and root types growing in specific (micro-)environments, these factors are rarely considered. It is unclear whether mucilage exhibits distinct physico-chemical properties related to collection system-inherent factors. Methods Mucilage of maize genotype B73 was collected from systems CS A and CS B . Chemical composition was assessed by pH, nutrient contents, neutral sugar composition, and polysaccharide polymer length. Viscosity, surface tension and contact angle represented physical properties. Results The share of hexoses among total polysaccharides was 11% higher in CS B than in CS A , whereas pentoses were predominant in CS A , together with higher nutrient concentrations and pH values. Mannose was detected only in CS B , which also exhibited higher surface tension, viscosity and contact angle compared to CS A . Conclusions Physico-chemical differences between the two mucilages are related to root type functions, environmental root growth conditions, and plant developmental state. Higher fractions of pentoses in CS A mucilage seem related to semi-sterile system conditions. Higher viscosity of CS B mucilage might reflect the need for enhanced water holding capacity of brace roots growing in drier conditions. A strong influence of environmental factors on mucilage properties even for a single genotype might play additional roles e.g. in the attraction of microbiomes. These aspects are relevant when assessing the role of mucilage in the rhizosphere, or when developing models of rhizosphere processes.

Abstract Compared with bulk soil, rhizosphere has different properties because of the existence of root mucilage, which affects physical, chemical, and microbial processes. The slow response of rhizosphere to changes in water potential buffers water content changes and leads the rhizosphere to be wetter than bulk soil during drying. By affecting connectivity of the liquid and gas phases, mucilage can also influence solute transport and gas diffusion. Overview of the literature and previous models shows the lack of a model that describes the connectivity between different phases in the rhizosphere pore space during wetting and drying processes. A major challenge is that mucilage shows a complex behavior, which at low concentrations is more like a liquid, whereas at higher concentration, dry mucilage becomes a solid. In between, a viscoelastic state is observed where mucilage can be considered as a hydrogel. In this study a three‐dimensional pore‐scale model based on the lattice spring method is introduced and used to simulate drying of mucilage between two soil particles. The model is capable of reproducing spider‐web‐like structures that are specific for mucilage. This three‐dimensional mucilage drying model is qualitatively validated via environmental scanning electron microscopy (ESEM) images of dry mucilage between glass beads. The proposed model may provide us with a new perspective on hydrodynamic processes within the pore space of the rhizosphere. In addition, the model may help to better understand further important processes that strongly depend on rhizosphere hydraulic dynamics, such as solute transport, connectivity of the liquid phase, root penetration resistance, rhizosheath formation, and microbial activity.

Abstract High viscosity, low surface tension and hydrophobicity are specific properties of maize root mucilage which contribute to modulate the spatial configuration of the liquid phase in soil pores. Being a hotspot for nutrient absorption, root exudation and microbial activity, the rhizosphere soil solution is suspected to chemically vary strongly upon time. Although the physical properties of maize root mucilage have been repeatedly measured in the last years, their variation upon a changing chemical environment and understanding of the chemical mechanisms governing these properties remain unexplored. Therefore, we investigated how flow and surface properties of maize root mucilage varied by changes in pH, CaCl2 and lecithin concentrations. Results reveal that the physical properties of mucilage can strongly vary depending on the environmental conditions. Low surface tension of maize root mucilage at pH7 was increased by addition of calcium. Upon pH change and lecithin addition, hydrophobic mucilage turned hydrophilic. Viscosity of mucilage decreased with increasing Ca concentration above 2.5 mM, the addition of 0.5 mg/L lecithin and a pH rise to 9. Such variations strongly suggest that the role of mucilage in hydraulic processes in the rhizosphere depends on changes of solutes concentration and composition, which themselves vary according to plant growth and soil water content. It seems that mucilage can best serve as a hydraulic bridge only under certain chemical environments, whose spatio-temporal occurrence in the changing rhizosphere remains to be defined.