Claude Doussan

Plant root growth, architecture and function.

Without roots there would be no rhizosphere and no rhizodeposition to fuel microbial activity. Although micro-organisms may view roots merely as a source of carbon supply this belies the fascinating complexity and diversity of root systems that occurs despite their common function. Here, we examine the physiological and genetic determinants of root growth and the complex, yet varied and flexible, root architecture that results. The main functions of root systems are also explored including how roots cope with nutrient acquisition from the heterogeneous soil environment and their ability to form mutualistic associations with key soil micro-organisms (such as nitrogen fixing bacteria and mycorrhizal fungi) to aid them in their quest for nutrients. Finally, some key biotic and abiotic constraints on root development and function in the soil environment are examined and some of the adaptations roots have evolved to counter such stresses discussed.

Water uptake by plant roots: II - Modelling of water transfer in the soil root-system with explicit account of flow within the root system : Comparison with experiments

Soil water uptake by plant roots results from the complex interplay between plant and soil which modulates and determines transport processes at a range of spatial and temporal scales: at small scales, uptake rates are determined by local soil and root hydraulic properties but, at the plant scale, local processes interact within the root system and are integrated through the hydraulic architecture of the root system and plant transpiration. However, because of the inherent complexity of the root system (both structural and functional), plant roots are commonly account for with synthetic but over-simplifying descriptors, valid at a given spatial scale. In this article, we present a model describing both soil and plant processes involved in water uptake at the scale of the whole root system with explicit account of individual roots. This is achieved through the unifying concepts of root system architecture and hydraulic continuity between the soil and plant. The model is based on a combination of architectural, root system hydraulic and soil water transfer modelling. The model can reproduce qualitatively and quantitatively laboratory experimental data obtained from imaging of water uptake by light transmission (cf. Garrigues et al., Water uptake by plant roots: I-Formation and propagation of a water extraction front in mature root systems as evidenced by 2D light transmission imaging. Plant and soil (2006, this issue) or X-ray imaging for two soil types (a sand/clay mix and a sandy clay loam) and different narrow-leaf lupin root systems (taprooted and fibrous), using independently measured soil–plant parameters. Results of the experiments and modelling reported in this paper concur to show that a water extraction front formed on the root system. This uptake front’s spatial extension and propagation were closely related to the local dependence between root and soil hydraulic properties and root axial conductance. Hence, a sharp front formed in the sand/clay mix but was much more attenuated in the sandy loam. Comparison between taprooted and fibrous root systems grown in a sand/clay mix, show that the taprooted architecture induced a more spatially concentrated uptake zone (near the soil surface) with higher flux rates, but with xylem water potential at the base of the root system twice as low than in the fibrous architecture. Modelling provided evidence that hydraulic lift might have occurred when transpiration declined, particularly in soil prone to abrupt variations in soil water potential (sand/clay mix). Finally, such a model, explicitly coupling root system-soil water transfers, can be useful to study water uptake in relation with root architectural traits, distribution of root hydraulic conductance or influence of heterogeneous conditions (localised irrigation, root clumping).

River bank filtration: modelling of the changes in water chemistry with emphasis on nitrogen species

Bank-filtrated water is an important component of the drinking water production in many countries. The changes in the water chemistry during the transfer from the river to the aquifer have important implications for the quality of the produced water. In this paper, we first describe certain features of the evolution of the water chemistry during bank-filtration in the case of an experimental site, part of a large well field (Seine river, France). Here, bank-filtration leads to highly reducing conditions in the aquifer. A conceptual and numerical macroscopic model of this evolution, focusing on nitrogen compounds, is then presented. The model is designed to simulate organic matter mineralization and redox reactions catalyzed by bacteria in the river bed sediments where water infiltrates. Growth and decay of bacteria are explicitly accounted for and a numerical solution is found with an operator splitting technique. The model is able to reproduce column experiments by von Gunten and Zobrist (1993) designed to simulate infiltration of organically polluted river water into an aquifer. A model application to the characteristics of the experimental site is also presented. Results of a sensitivity analysis highlight the importance of. (1) the flow rate of water infiltrating river bed sediments; and (2) the organic carbon content of these sediments, for the evolution of the water quality during transfer from the river to the aquifer.

Streaming current generation in two‐phase flow conditions

Self-potential (SP) signals that are generated under two-phase flow conditions could be used to study vadose zone dynamics and to monitor petroleum production. These streaming-potentials may also act as an error source in SP monitoring of vulcanological activity and in magnetotelluric studies. We propose a two-phase flow SP theory that predicts streaming currents as a function of the pore water velocity, the excess of charge in the pore water, and the porosity. The source currents that create the SP signals are given by the divergence of the streaming currents, and contributions are likely to be located at infiltration fronts, at the water table, or at geological boundaries. Our theory was implemented in a hydrogeological modeling code to calculate the SP distribution during primary drainage. Forward and inverse modeling of a well-calibrated 1D drainage experiment suggest that our theory can predict streaming potentials in the vadose zone.

Architectural analysis and synthesis of the plum tree root system in an orchard using a quantitative modelling approach

A dynamic 3D representation of the root system architecture of plum is proposed by gathering quantitative and morphological observations of the tree root system in a model. The model includes two information levels: (i) a typology of root axes, based on morphological and developmental characteristics; (ii) a set of basic processes (axial and radial growth, ramification and reiteration, decay). The basic processes are qualitatively identical in space and time. An original approach was used to investigate these processes and to formalize them in the model. Concerning the main roots, a mechanism of reiteration is described that has a substantial influence on the structuring of the root system. Root mortality is assessed using the variation in branching density along the root axes. Radial growth is calculated from the ramification of root axes, using root section conservation properties. This model enables a link between static field observations and a dynamic simulation of the root system architecture. The architectural model allows examination of the global consequences of the basic processes at the level of the root system. The simulations provide useful output, from a simple root depth profile to a simulation of the dynamic 3D root system architecture, to investigate plant functioning and especially water and nutrient uptake.

Modeling soil-root water transport with non-uniform water supply and heterogeneous root distribution

A 2D physically based framework is proposed to analyze the effect of a non-uniform water supply at the soil surface generated by rainfall interception and stemflow on soil-root water transport in the case of heterogeneous distribution of the roots in the soil profile. To model soil-root water transport, the root water potential of two plants placed in two adjacent rows was simulated so as to minimize the difference between the evaporative demand and the amount of water taken up by each plant. A characterization of the throughfall to incident rainfall, soil hydrodynamic properties, soil-root contacts, and maize evapotranspiration, was carried out during a 10-day experiment with a leaf area index of about 4 to 5 m2 m−2. Mean rainfall interception percentages were in the [47.4%–52.6%] range at half the distance between two adjacent rows, whereas an interception percentage higher than 80% was found near the stems along the rows. As a result, the mean estimated stemflow was 1 L per plant per 16.4 mm water supply above the canopy. Good agreement was found between the measured and predicted transpiration values. As the soil started to moisten, the predicted root water potential rapidly increased, in line with the predicted number of active roots that rapidly decreased. Effects due to stemflow during infiltration disappeared progressively when drying was in progress. The proposed approach could be useful for analyzing soil-root water transport and possible pollution when solutes move with water under various realistic conditions where non-uniform water supply is involved.

The Water Cycle, a Potential Source of the Bacterial Pathogen Bacillus cereus.

The behaviour of the sporulating soil-dwelling Bacillus cereus sensu lato (B. cereus sl) which includes foodborne pathogenic strains has been extensively studied in relation to its various animal hosts. The aim of this environmental study was to investigate the water compartments (rain and soil water, as well as groundwater) closely linked to the primary B. cereus sl reservoir, for which available data are limited. B. cereus sl was present, primarily as spores, in all of the tested compartments of an agricultural site, including water from rain to groundwater through soil. During rain events, leachates collected after transfer through the soil eventually reached the groundwater and were loaded with B. cereus sl. In groundwater samples, newly introduced spores of a B. cereus model strain were able to germinate, and vegetative cells arising from this event were detected for up to 50 days. This first B. cereus sl investigation in the various types of interrelated environments suggests that the consideration of the aquatic compartment linked to soil and to climatic events should provide a better understanding of B. cereus sl ecology and thus be relevant for a more accurate risk assessment of food poisoning caused by B. cereus sl pathogenic strains.

Soil type determines how root and rhizosphere traits relate to phosphorus acquisition in field-grown maize genotypes

AimsPhosphorus (P) is frequently limiting crop production in agroecosystems. Large progress was achieved in understanding root traits associated with P acquisition efficiency (PAE, i.e. P uptake achieved under low P conditions). Most former studies were performed in controlled environments, and avoided the complexity of soil-root interactions. This may lead to an oversimplification of the root-soil relations. The aim of the present study was, therefore, to identify the dominant root and rhizosphere-related traits determining PAE, in contrasting soil conditions in the field.MethodsTwenty-three maize hybrids were grown at two contrasting P levels of a long-term P-fertilizer trial in two adjacent soil types: alkaline and neutral. Bulk soil, rhizosphere and root parameters were studied in relation to plant P acquisition.ResultsSoil type had robust effect on PAE. Hybrids’ performance in one soil type was not related to that in the other soil type. In the neutral soil, roots exhibited higher specific root length, higher root/shoot ratio but lower PAE. Best performing hybrids in the neutral soil were characterized by top soil exploration, i.e., greater root surface and topsoil foraging. In contrast, in the alkaline soil, PAE and foraging traits were not correlated, P availability in the rhizosphere was greater than the bulk soil and phosphatase activity was higher, suggesting a ‘mining strategy’ in that case (i.e. traits that facilitate elevated P availability).ConclusionsThese results indicate the key role of environmental factors for roots traits determining high PAE. The study highlights the need to consider soil properties when breeding for high PAE, as various soil types are likely to require different crop ideotypes.

Effects of corn (Zea mays L.) on the local and overall root development of young rubber tree (Hevea brasiliensis Muel. Arg).

Understanding better the interactions between root systems in associated crops is significant for basic knowledge in plant science and to help designing cropping systems. Current research on inter-specific root interactions concentrates on static descriptions of the horizontal extension of root systems or on the dynamics of provoked root encounters. This study considers detailed observations of the dynamics of inter-specific root interactions, in the vertical plane, at both the whole root system and the individual root levels. Corn and young rubber trees were grown in association in artificial conditions that excluded the possibility of competition for resources, using rhizoboxes, i.e. thin containers with a transparent wall. The paper presents novel approaches, such as the study of root system growth trajectories, to document root system development in terms of overall growth rate, colonization of soil space and individual root growth patterns. It was found that (i) corn roots developed towards rubber roots until a contact was established, (ii) rubber roots expanded faster and more vertically in association with corn, (iii) the expansion rates of both root systems varied concomitantly and (iv) inter-specific root encounters resulted in reduced elongation rates in both species. Implications of these results for corn/rubber inter-cropping are discussed. This work advocates in favour of a better understanding of under-ground facilitative effects between species. If understood enough to be manipulated, such knowledge might become a powerful tool for the design of more sustainable and efficient cropping systems.

Magnetic Resonance Imaging and Relaxometry as Tools to Investigate Water Distribution in Soils

Relaxation times and two imaging sequences (spin echo and single point imaging) were performed onto repacked soil samples to study respectively water distribution within the porosity and to measure water content profiles, distinguishing water contained in large pores from water contained in the whole porosity. These methods were applied to 25 samples of the same soil that was prepared to obtain aggregates of three different size, then repacked to five bulk densities. Samples were then equilibrated with water at five matric potentials. We found that T1 and T2 measurements present similar time distributions with essentially four peaks. We attributed the two shortest times to textural pores, and the two longest times to structural pores. The water profile measured with spin echo sequence was attributed to water contained in structural pores.