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International audience; Introduction Rhizodeposition, i.e. the release of organic materials by roots, represents a significant portion of plant’s carbon (C) budget, ranging from 5% to 15% of net photosynthesized C (Pausch and Kuzyakov, 2018). Various rhizodeposits can be released by roots, e.g. soluble exudates, secreted mucilage, sloughed cells, or volatile organic compounds. Despite their short lifetime, some of these products have been shown to favor plant growth, e.g. by increasing water and nutrient uptake. Among rhizodeposition processes, exudation has been suggested to depend on the concentration of carbohydrates inside the roots (Personeni et al., 2007). However, rhizodeposition not only depends on the availability of C in the roots, but also on the architecture of the root system, and many have shown that rhizodeposits are more concentrated in specific areas, such as root tips. Consequently, a Functional-Structural Plant Model (FSPM) would theoretically represent the best framework for simulating the spatial and temporal dynamics of rhizodeposition, as it can describe the evolution of both the metabolism and the architecture of the plant. The objective of this work is to create such a framework by coupling a whole-plant FSPM, a 3D root architectural model, and a new model simulating rhizodeposition.Modelling approachOur strategy has been to combine the FSPM CN-Wheat (Barillot et al., 2016), which describes the main processes of C and nitrogen (N) acquisition and transformation by an individual wheat plant and the 3D growth and development of its aerial organs, with the model ArchiSimple (Pagès et al., 2014) that simulates the development of the 3D root architecture for a range of plant species, and the new model RhizoDep, which calculates a full C balance in each part of a root system in order to simulate local rhizodeposition fluxes. The complementarity of the three models is illustrated in Figure 1: i) CN-Wheat is used to calculate the amount of C allocated from the shoots to the roots, ii) ArchiSimple provides the 3D structure of the root system, and iii) RhizoDep distributes the C provided by the shoots within the 3D root system and simulates the actual growth, respiration and rhizodeposition of each root element based on C availability. The major link between the three models lies in the exchange of C between aboveground and belowground tissues, which is driven by gradients of sucrose concentration in the different compartments of the plant.Preliminary results & short-term perspectivesThe coupling of the three models has been started using the OpenAlea platform and its Multiscale Tree Graph formalism (Pradal et al., 2008). First simulations were done using the allocation of C to the roots simulated by CN-Wheat as an input to the root model based on the effective coupling of ArchiSimple and RhizoDep. These simulations show how rhizodeposition is intrinsically dependent on the architecture of the root system and on the total amount of available C. For completing the coupling, several issues still need to be tackled, e.g. how N uptake and metabolism should be spatialized in a 3D root system, how it may be regulated by local C and N availability, and how rhizodeposition can modify soil N availability. However, this modelling approach has already led to a first prototype able to simulate rhizodeposition processes on a dynamic, 3D root system that is fully integrated within the functioning of the whole plant. Its refinement will offer unique opportunities to study the possible link between rhizodeposition and the environmental factors affecting plant growth, e.g. atmospheric CO2 concentration or soil N availability.

International audience

International audience

International audience

International audience

Plants are the main source of organic carbon in soils. Besides litter incorporation, most of plant carbon fluxes to soil occur belowground, through the release of organic compounds by roots and the decay of root tissues. Despite the importance of these processes for soil carbon sequestration and for soil biological functions, our understanding of such fluxes has been hampered by the difficulties associated to their measurement in actual soil environments and their integration to plant growth models. Our aim is to develop new modelling approaches in order to accurately describe trophic fluxes from roots to soil and their spatial and temporal evolution. Functional-structural plant models (FSPM), which take into account both plant physiology and plant architecture, may be well adapted to such a modelling strategy, but also bring new challenges in terms of processes coupling. This work will present our current strategy to simulate and integrate root exudation, mucilage emissions, root cells desquamation and root senescence into a common model, and will highlight some of the main knowledge gaps associated to the simulation of these processes.

International audience; The risk assessment of industrial contaminated soils is an important challenge in remediation process. Although chemical analyses reveal the typology of pollution in a given matrix, they do not give information about the real ecotoxic potential of the matrix, which takes into account the bioavailability of pollutants. This information requires the development of biological tests, and especially with plants. Plants present indeed a particular interest in ecotoxicology due to their (i) immobility, (ii) important roots network, and (iii) fundamental role in ecosystems as primary producers (Hock and Elstner, 2005). Ma (1999) described higher plants as the most sensitive organisms for the detection of mutagens and genotoxic effects of environmental pollutants. Although ecologically relevant for soil toxicity assessment, plants are surprisingly not the most commonly used organisms for genotoxicity tests (White and Claxton, 2004). This is the reason why genotoxicity tests with higher plants have been promoted (IPCS – United Nations Environment Programme, 1999).Genotoxicity - simply defined as the toxicity on the genome - is an indicator of dysfunctions appearing at sub-lethal concentrations. An easy endpoint to observe is the formation of micronuclei, that are small nuclei appearing whenever a chromosome fragment or a complete chromosome is not incorporated into the nuclei during mitosis. It therefore reveals a break of genetic material (clastogenic effect) or a dysfunction of mitotic spindles (aneugenic effect). This endpoint is very important to include in a battery of ecotoxicity tests for a better risk assessment of contaminated soils and of the impact of remediation techniques applied to them.Among techniques of in situ remediation, soil amendments with biochar, i.e. the solid product from biomass pyrolysis, have recently been investigated for decreasing the bioavailability of metals in industrial soils. Biochar has been shown to immobilize metals both by direct sorption at its surface and by an indirect effect through an increase of soil pH (Rees et al., 2014). The effect of biochar on the actual genotoxicity potential of metal-contaminated soils is however unknown.The aim of this study was to assess the genotoxic potential of a range of soils contaminated by heavy metals (Zn, Pb, Cd) and amended by a wood-derived biochar to create a gradient of metal availability. We recently obtained international standardization of the Vicia-micronucleus test (ISO 29200) and we performed it in this study by direct exposure of root tips to soils. The analysis of root morphology and root metal content completed the experiment. Results showed that these industrial soils induced genotoxicity, revealed by an increase of micronuclei frequency. Their genotoxic potential strongly decreased with the addition of biochar. Relationships between genotoxicity, soil metal extractability and root metal content will be discussed.