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Modern agriculture's goal of improving crop resource acquisition efficiency relies on the intricate relationship between the root system and the soil. Root and rhizosphere traits play a critical role in the efficient use of nutrients and water, especially under dynamic environments. This review emphasizes a holistic perspective, challenging the conventional separation of nutrient and water uptake processes and the necessity for an integrated approach. Anticipating climate change-induced increase in the likelihood of extreme weather events that result in fluctuations in soil moisture and nutrient availability, the study explores the adaptive potential of root and rhizosphere traits to mitigate stress. We emphasize the significance of root and rhizosphere characteristics that enable crops to rapidly respond to varying resource availabilities (i.e. the presence of water and mobile nutrients in the root zone) and their accessibility (i.e. the possibility to transport resources to the root surface). These traits encompass for example root hairs, mucilage and extracellular polymeric substance (EPS) exudation, rhizosheath formation and the expression of nutrient and water transporters. Moreover, we recognize the challenge of balancing carbon investments, especially under stress, where optimized traits must consider carbon-efficient strategies. To advance our understanding, the review calls for well-designed field experiments, recognizing the limitations of controlled environments. Non-destructive methods such as mini rhizotron assessments and in-situ stable isotope techniques, in combination with destructive approaches such as root exudation analysis, are proposed for assessing root and rhizosphere traits. The integration of modeling, experimentation, and plant breeding is essential for developing resilient crop genotypes capable of adapting to evolving resource limitation. [ABSTRACT FROM AUTHOR]

Wettability is an important parameter that significantly determines hydrology in porous media, and it especially controls the flow of water across the rhizosphere—the soil-plant interface. However, the influence of spatially heterogeneous distributions on the soil particles surfaces is scarcely known. Therefore, this study investigates the influence of spatially heterogeneous wettability distributions on infiltration into porous media. For this purpose, we utilize a two-phase flow model based on Lattice-Boltzmann to numerically simulate the infiltration in porous media with a simplified geometry and for various selected heterogeneous wettability coatings. Additionally, we simulated the rewetting of the dry rhizosphere of a sandy soil where dry hydrophobic mucilage depositions on the particle surface are represented via a locally increased contact angle. In particular, we can show that hydraulic dynamics and water repellency are determined by the specific location of wettability patterns within the pore space. When present at certain locations, tiny hydrophobic depositions can cause water repellency in an otherwise well-wettable soil. In this case, averaged, effective contact angle parameterizations such as the Cassie equation are unsuitable. At critical conditions, when the rhizosphere limits root water uptake, consideration of the specific microscale locations of exudate depositions may improve models of root water uptake. [ABSTRACT FROM AUTHOR]

Take home messageMucilage secreted by roots and EPS produced by microorganisms alter the physical properties of the soil solution and impact the water dynamics in the rhizosphere. The high viscosity of mucilage and EPS is responsible for the formation of thin filaments and interconnected thin lamellae that span throughout the soil matrix maintaining the continuity of the liquid phase across the pore space even during severe drying. The impact of these mechanisms on plant and microorganisms needs to be explored. [ABSTRACT FROM AUTHOR]

Plant roots exude a significant fraction of the carbon assimilated via photosynthesis into the soil. The mucilaginous fraction of root exudates affects the hydraulic properties of the soil near the roots, the so called rhizosphere, in a remarkable and dynamic way. After drying, mucilage becomes hydrophobic and limits the rewetting of the rhizosphere. Here, we aim to find a quantitative relation between rhizosphere rewetting, particle size, soil matric potential and mucilage concentration. We used a pore-network model in which mucilage was randomly distributed in a cubic lattice. The general idea was that the mucilage concentration per solid soil surface increases the contact angle between the liquid and solid phases consequently limiting the rewetting of pores covered with dry mucilage. We used the Young–Laplace equation to calculate the mucilage concentration at which pores are not wettable for varying particle sizes and matric potentials. Then, we simulated the percolation of water across a cubic lattice. Our simulations predicted that above a critical mucilage concentration water could not flow through the porous medium. The critical mucilage concentration decreased with increasing particle size and decreasing matric potential. The model was compared with experiments of capillary rise in soils of different particle size and mucilage concentration. The experiments confirmed the percolation behaviour of the rhizosphere rewetting. Mucilage turned hydrophobic at concentrations above 0.1 mg/cm 2 . The critical mucilage concentration at matric potential of −2.5 hPa was ca. 1% [g/g] for fine sand and 0.1 % [g/g] for coarse sand. Our conceptual model is a first step towards a better understanding of the water dynamics in the rhizosphere during rewetting and it can be used to predict in what soil textures rhizosphere water repellency becomes a critical issue for root water uptake. [ABSTRACT FROM AUTHOR]

Despite detailed investigations on the distinct biochemical properties of the rhizosphere and their effects on the availability of nutrients for plants, their biophysical aspects, particularly the effect of mucilage on the transport of water and nutrients are poorly understood. The aim of this study was to investigate the effect of mucilage on the diffusion coefficient of nutrients and consequently their transport in the soil and into the plant roots. Repeated phosphor imaging was used to monitor the temporospatial distribution of 137Cs (as an analog of K) within a modeled rhizosphere soil with and without mucilage (a sandy soil amended with mucilage extract from chia seed) under different soil water contents. The monitored profiles of activities were used to estimate the diffusion coefficient of soils with and without mucilage by solving a diffusion equation. Assuming an identical the effect of mucilage on diffusion coefficient of Cs and K, a diffusion-convection equation was numerically solved to predict the transport of K within the soil and its uptake by a single plant root during a soil drying cycle. To this end, the hydraulic and diffusive properties of the soil were parameterized based on the measured data and the K uptake and its concentration in soil were taken from literature data. The results of this study suggest that mucilage in the rhizosphere keeps the rhizosphere wetter and maintains the connectivity of the liquid phase during a soil drying cycle, and thereby could prevent a marked drop in the diffusion coefficient. The results of modeling of nutrient uptake by a single root showed that the presence of mucilage in the rhizosphere could favor nutrient uptake by the plant root. In the case of nutrients with low concentration in the soil solution, it prevents a marked concentration drop in the vicinity of the root as the soil dries and diffusion becomes restricted. This will delay the risk of nutrient deficiency to the plant root. In the case of nutrients with a high concentration in the soil solution, the presence of mucilage in the rhizosphere may delay the risk of salinity stress as the soil dries and the concentration of nutrients increases at the vicinity of the root surface. In conclusion, the results of this contribution show that mucilage may favor the transport of nutrient within the soil and their uptake by plant roots under drying soil condition. [ABSTRACT FROM AUTHOR]