Jessica Bertheloot

Dynamics of Light and Nitrogen Distribution during Grain Filling within Wheat Canopy

In monocarpic species, during the reproductive stage the growing grains represent a strong sink for nitrogen (N) and trigger N remobilization from the vegetative organs, which decreases canopy photosynthesis and accelerates leaf senescence. The spatiotemporal distribution of N in a reproductive canopy has not been described in detail. Here, we investigated the role of the local light environment on the spatiotemporal distribution of leaf lamina N mass per unit leaf area (SLN) during grain filling of field-grown wheat (Triticum aestivum). In addition, in order to provide some insight into the coordination of N depletion between the different vegetative organs, N dynamics were studied for individual leaf laminae, leaf sheaths, internodes, and chaff of the top fertile culms. At the canopy scale, SLN distribution paralleled the light gradient below the flag leaf collar until almost the end of grain filling. On the contrary, the significant light gradient along the flag leaf lamina was not associated with a SLN gradient. Within the top fertile culms, the time course of total (alive + necrotic tissues) N concentration of the different laminae and sheaths displayed a similar pattern. Another common pattern was observed for internodes and chaff. During the period of no root N uptake, N depletion of individual laminae and sheaths followed a first-order kinetics independent of leaf age, genotype, or N nutrition. The results presented here show that during grain filling, N dynamics are integrated at the culm scale and strongly depend on the local light conditions determined by the canopy structure.

Multiple pathways regulate shoot branching

Shoot branching patterns result from the spatio-temporal regulation of axillary bud outgrowth. Numerous endogenous, developmental and environmental factors are integrated at the bud and plant levels to determine numbers of growing shoots. Multiple pathways that converge to common integrators are most probably involved. We propose several pathways involving not only the classical hormones auxin, cytokinins and strigolactones, but also other signals with a strong influence on shoot branching such as gibberellins, sugars or molecular actors of plant phase transition. We also deal with recent findings about the molecular mechanisms and the pathway involved in the response to shade as an example of an environmental signal controlling branching. We propose the TEOSINTE BRANCHED1, CYCLOIDEA, PCF transcription factor TB1/BRC1 and the polar auxin transport stream in the stem as possible integrators of these pathways. We finally discuss how modeling can help to represent this highly dynamic system by articulating knowledges and hypothesis and calculating the phenotype properties they imply.

A process-based model to simulate nitrogen distribution in wheat (Triticum aestivum) during grain-filling

Nitrogen (N) distribution among plant organs plays a major role in crop production and, in general, plant fitness to the environment. In the present study, a process-based model simulating N distribution within a wheat (Triticum aestivum L.) culm during grain filling was developed using a functional-structural approach. A model of turnover of the photosynthetic apparatus was used to describe the fluxes between a common pool of mobile N and each leaf lamina. Grain N accumulation within a time-step was modelled as the minimum between the quantity calculated by a potential function and the N available in the common pool. Nitrogen dynamics in the other organs (i.e. stem, chaff, root N uptake and remobilisation) were accounted for by forced variables. Using a unique set of six parameters, the model was able to simulate the observed N kinetics of each lamina and of the grains under a wide range of crop N supplies and for three cultivars. The time-course of the vertical gradient of lamina N during grain filling was realistically simulated as an emerging property of the local processes defined at the lamina scale. The model described in the present study offers new insight into the interactions between N metabolism, plant architecture and productivity.

NEMA, a functional-structural model of nitrogen economy within wheat culms after flowering. I. Model description.

BACKGROUND AND AIMS Models simulating nitrogen use by plants are potentially efficient tools to optimize the use of fertilizers in agriculture. Most crop models assume that a target nitrogen concentration can be defined for plant tissues and formalize a demand for nitrogen, depending on the difference between the target and actual nitrogen concentrations. However, the teleonomic nature of the approach has been criticized. This paper proposes a mechanistic model of nitrogen economy, NEMA (Nitrogen Economy Model within plant Architecture), which links nitrogen fluxes to nitrogen concentration and physiological processes. METHODS A functional-structural approach is used: plant aerial parts are described in a botanically realistic way and physiological processes are expressed at the scale of each aerial organ or root compartment as a function of local conditions (light and resources). KEY RESULTS NEMA was developed for winter wheat (Triticum aestivum) after flowering. The model simulates the nitrogen (N) content of each photosynthetic organ as regulated by Rubisco turnover, which depends on intercepted light and a mobile N pool shared by all organs. This pool is enriched by N acquisition from the soil and N release from vegetative organs, and is depleted by grain uptake and protein synthesis in vegetative organs; NEMA accounts for the negative feedback from circulating N on N acquisition from the soil, which is supposed to follow the activities of nitrate transport systems. Organ N content and intercepted light determine dry matter production via photosynthesis, which is distributed between organs according to a demand-driven approach. CONCLUSIONS NEMA integrates the main feedbacks known to regulate plant N economy. Other novel features are the simulation of N for all photosynthetic tissues and the use of an explicit description of the plant that allows how the local environment of tissues regulates their N content to be taken into account. We believe this represents an appropriate frame for modelling nitrogen in functional-structural plant models. A companion paper will present model evaluation and analysis.

NEMA, a functional–structural model of nitrogen economy within wheat culms after flowering. II. Evaluation and sensitivity analysis

BACKGROUND AND AIMS Simulating nitrogen economy in crop plants requires formalizing the interactions between soil nitrogen availability, root nitrogen acquisition, distribution between vegetative organs and remobilization towards grains. This study evaluates and analyses the functional-structural and mechanistic model of nitrogen economy, NEMA (Nitrogen Economy Model within plant Architecture), developed for winter wheat (Triticum aestivum) after flowering. METHODS NEMA was calibrated for field plants under three nitrogen fertilization treatments at flowering. Model behaviour was investigated and sensitivity to parameter values was analysed. KEY RESULTS Nitrogen content of all photosynthetic organs and in particular nitrogen vertical distribution along the stem and remobilization patterns in response to fertilization were simulated accurately by the model, from Rubisco turnover modulated by light intercepted by the organ and a mobile nitrogen pool. This pool proved to be a reliable indicator of plant nitrogen status, allowing efficient regulation of nitrogen acquisition by roots, remobilization from vegetative organs and accumulation in grains in response to nitrogen treatments. In our simulations, root capacity to import carbon, rather than carbon availability, limited nitrogen acquisition and ultimately nitrogen accumulation in grains, while Rubisco turnover intensity mostly affected dry matter accumulation in grains. CONCLUSIONS NEMA enabled interpretation of several key patterns usually observed in field conditions and the identification of plausible processes limiting for grain yield, protein content and root nitrogen acquisition that could be targets for plant breeding; however, further understanding requires more mechanistic formalization of carbon metabolism. Its strong physiological basis and its realistic behaviour support its use to gain insights into nitrogen economy after flowering.

Rose bush leaf and internode expansion dynamics: analysis and development of a model capturing interplant variability

Rose bush architecture, among other factors, such as plant health, determines plant visual quality. The commercial product is the individual plant and interplant variability may be high within a crop. Thus, both mean plant architecture and interplant variability should be studied. Expansion is an important feature of architecture, but it has been little studied at the level of individual organs in rose bushes. We investigated the expansion kinetics of primary shoot organs, to develop a model reproducing the organ expansion of real crops from non-destructive input variables. We took interplant variability in expansion kinetics and the models ability to simulate this variability into account. Changes in leaflet and internode dimensions over thermal time were recorded for primary shoot expansion, on 83 plants from three crops grown in different climatic conditions and densities. An empirical model was developed, to reproduce organ expansion kinetics for individual plants of a real crop of rose bush primary shoots. Leaflet or internode length was simulated as a logistic function of thermal time. The model was evaluated by cross-validation. We found that differences in leaflet or internode expansion kinetics between phytomer positions and between plants at a given phytomer position were due mostly to large differences in time of organ expansion and expansion rate, rather than differences in expansion duration. Thus, in the model, the parameters linked to expansion duration were predicted by values common to all plants, whereas variability in final size and organ expansion time was captured by input data. The model accurately simulated leaflet and internode expansion for individual plants (RMSEP = 7.3 and 10.2% of final length, respectively). Thus, this study defines the measurements required to simulate expansion and provides the first model simulating organ expansion in rosebush to capture interplant variability.

Modelling Nitrogen Distribution in Virtual Plants, as Exemplified by Wheat Culm During Grain Filling

Nitrogen is fundamental for plant growth. In cereals, growing grains represent a strong sink that triggers nitrogen remobilisation from vegetative organs and results in plant death. A better understanding of this mechanism would help in optimizing crop productivity while reducing fertilization. This work presents an experimental analysis and a process–based model of the spatiotemporal nitrogen distribution during grain filling in winter wheat culms. Nitrogen was distributed homogeneously within individual laminae and sheaths, but a strong gradient existed between organs at successive positions along the culm. During grain filling, the changes in nitrogen content of individual laminae and sheaths showed identical patterns, differing only by a scale factor. Modelling N content of each lamina as the result of the turnover of photosynthetic nitrogen and supposing that all organs share a single pool of mobile nitrogen allowed predicting the observed patterns with high accuracy. This offers new insight for modelling plant nitrogen economy.

Nitrogen Acquisition and Utilization by Crops: Review of Different Approaches and Proposition of a Mechanistic Modeling

To avoid pollution by nitrogen fertilizers, nitrogen acquisition and utilization by plants should be optimized. Models are potentially efficient tools to study a priori new possible scenarios. This work reviews how models formalize nitrogen economy in plants. It appears that the formalization of the processes regulating nitrogen use in plants has remained largely empirical. This hampers models ability to simulate plant behavior in innovative scenarios. The functional-structural model developed by Bertheloot et al. (2008) demonstrates the possibility for a mechanistic modeling, based on the formalization of the turnover of the proteins of the photosynthetic apparatus. However, this model specifically addresses the distribution of nitrogen between laminae and the grains during the reproductive stage of wheat. Based on this approach, we propose here a more complete model, including the simulation of root N uptake, the dynamics of N in all plant components and the extension to the pre-flowering period, when vegetative structure is dynamic.

Sharing efforts for modelling plant systems: from publications to reusable software components

Plant models become increasingly complex and their implementation often implies the use of advanced techniques in computer science. This evolution has been accompanied by the production of dedicated plant modelling tools, such as simulation platforms, that facilitate research in this field. However, much less sharing is observed for plant models themselves, that is for computer programs produced by scientists to address their specific questions. Yet, these programs could be highly valuable for other researchers, to avoid redundant development of similar code or to help non-specialists to simulate parts of a complex system. Model descriptions found in academic publications, even combined with code sources, are generally not sufficient for model reuse. Most difficulties come from the heterogeneity of language used, the structure of the programs, the download and installation procedures, the accessibility to the source code of the model, and the availability of documentation. The OpenAlea initiative (http://openalea.gforge.inria.fr) has been launched to address these problems by providing plant modellers with collaborative tools and guidelines to increase software quality, hence re-usability of their models. The Alinea pilot project further tested these concepts in a sample community of ecophysiologists and biophysicists. Based on this experience, we illustrate pros and cons of the approach and discuss future direction of progress. We foresee three steps towards a better re-usability of models: a better interoperability of existing tools and simulation platforms, the emergence of design patterns for plant modelling, and the definition of standardised data structures.