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Reduced blue light irradiance is known to enhance leaf elongation rate (LER) in grasses, but the mechanisms involved have not yet been elucidated. We investigated whether leaf elongation response to reduced blue light could be mediated by stomata-induced variations of plant transpiration. Two experiments were carried out on tall fescue in order to monitor LER and transpiration under reduced blue light irradiance. Additionally, LER dynamics were compared with those observed in the response to vapour pressure deficit (VPD)-induced variations of transpiration. Finally, we developed a model of water flow within a tiller to simulate the observed short-term response of LER to various transpiration regimes. LER dramatically increased in response to blue light reduction and then reached new steady states, which remained higher than the control. Reduced blue light triggered a simultaneous stomatal closure which induced an immediate decrease of leaf transpiration. The hydraulic model of leaf elongation accurately predicted the LER response to blue light and VPD, resulting from an increase in the growth-induced water potential gradient in the leaf growth zone. Our results suggest that the blue light signal is sensed by stomata of expanded leaves and transduced to the leaf growth zone through the hydraulic architecture of the tiller.

Backgrounds and Aims A major challenge when supporting the development of intercropping systems remains the design of efficient species mixtures. The ecological processes that sustain overyielding of legume-based mixtures compared to pure crops are well known, but their links to plant traits remain to be unravelled. A common assumption is that enhancing trait divergence among species for resource acquisition when assembling plant mixtures should increase species complementarity and improve community performance. Methods The Virtual Grassland model was used to assess how divergence in trait values between species on four physiological functions (namely light and mineral N acquisition, temporal development, and C–N use efficiency) affected overyielding and mixture stability in legume-based binary mixtures. A first step allowed us to identify the model parameters that were most important to interspecies competition. A second step involved testing the impact of convergent and divergent parameter (or trait) values between species on virtual mixture performance. Results Maximal overyielding was achieved in cases where trait values were divergent for the physiological functions controlling N acquisition and temporal development but convergent for light interception. It was also found that trait divergence should not affect competitive abilities of legume and non-legumes at random. Indeed, random trait combinations frequently led to reduced mixture yields when compared to a perfectly convergent neutral model. Combinations with the highest overyielding also tended to be associated with mixture instability and decreasing legume biomass proportion. Achieving both high overyielding and mixture stability was only found to be possible under low or moderate N levels, using combinations of traits adapted to each environment. Conclusions No simple assembly rule based on trait divergence could be confirmed. Plant models able to infer plant–plant interactions can be helpful for the identification of major interaction traits and the definition of ideotypes adapted to a targeted intercropping system.

International audience; Germplasm from perennial ryegrass (Lolium perenne L.) natural populations is useful for breeding because of its adaptation to a wide range of climates. Climate-adaptive genes can be detected from associations between genotype, phenotype and climate but an integrated framework for the analysis of these three sources of information is lacking. We used two approaches to identify adaptive loci in perennial ryegrass and their effect on phenotypic traits. First, we combined Genome-Environment Association (GEA) and GWAS analyses. Then, we implemented a new test based on a Canonical Correlation Analysis (CANCOR) to detect adaptive loci. Furthermore, we improved the previous perennial ryegrass gene set by de novo gene prediction and functional annotation of 39,967 genes. GEA-GWAS revealed eight outlier loci associated with both environmental variables and phenotypic traits. CANCOR retrieved 633 outlier loci associated with two climatic gradients, characterized by cold-dry winter versus mild-wet winter and long rainy season versus long summer, and pointed out traits putatively conferring adaptation at the extremes of these gradients. Our CANCOR test also revealed the presence of both polygenic and oligogenic climatic adaptations. Our gene annotation revealed that 374 of the CANCOR outlier loci were positioned within or close to a gene. Co-association networks of outlier loci revealed a potential utility of CANCOR for investigating the interaction of genes involved in polygenic adaptations. The CANCOR test provides an integrated framework to analyse adaptive genomic diversity and phenotypic responses to environmental selection pressures that could be used to facilitate the adaptation of plant species to climate change.

Aims Little is yet known about the component traits that control the deployment of root architecture in most grassland species. The aim of this study was to compare patterns of root growth in contrasting forage legumes and to analyse their dependency to shoot development. Methods The parameters of a dynamic simulation model were identified for seven contrasting species in a series of greenhouse experiments in order to characterize root elongation, root branching and taproot growth. The model allowed us to explore the interactions between root development and whole plant growth. Results Most of the root morphogenetic parameters studied displayed significant inter-specific variability. Several traits appeared to be correlated, indicating a close association between extreme root tip diameters, the branching ability of roots and their maximal elongation rate. The species differed along two PCA axes which accounted respectively for the branching ability of fine roots and taproot development. Quite remarkably, the group of species identified for root development did not coincide with those previously identified for shoots, suggesting a degree of independence between the morphogenetic traits that control shoot and root architecture. However, both plant parts remained tightly linked through the trophic relationship represented by root biomass allocation. Conclusions Our results suggest that the concomitant analysis of root and shoot development is necessary to properly decipher rooting strategies in forage legumes.

International audience; Functional–structural plant models are increasingly being used to analyse relationships between plant functioning and the topological and spatial organisation of their modular structure. In this study, the performance of an individualbased model accounting for the the architecture and population dynamics of forage legumes in multi-species grasslands was assessed. Morphogenetic shoot and root parameters were calibrated for seven widely used species. Other model parameters concerning C and N metabolism were obtained from the literature. The model was evaluated using a series of independent experiments combining the seven species in binary mixtures that were subject to regular defoliation. For all the species, the model could accurately simulate phytomer demography, leaf area dynamics, and root growth under conditions of weak competition. In addition, the plastic changes induced by competition for light and N in terms of plant development, leaf area, N uptake, and total plant biomass were correctly predicted. The different species displayed contrasting sensitivities to defoliation, and the model was able to predict the superior ability of creeping species to sustain regular defoliation. As a result of competition and management, the balance between species changed over time and was strongly dependent on the pair of species used. The model proved able to capture these differences in community dynamics. Overall, the results demonstrate that integrating the individual components of population dynamics in a process-based model can provide good predictive capacity regarding mixtures of cultivated species.

International audience; Aim: Grasslands have been pivotal in the development of herbivore breeding since the Neolithic and still represent the most widespread agricultural land use across Europe. However, it remains unclear whether the current large‐scale genetic variation of plant species found in natural grasslands of Europe is the result of human activities or natural processes. Location: Europe. Taxon: Lolium perenne L. (perennial ryegrass). Methods: We reconstructed the phylogeographic history of L. perenne, a dominant grassland species, using 481 natural populations, including 11 populations of closely related taxa. We combined Genotyping‐by‐Sequencing (GBS) and pool‐Sequencing (pool‐Seq) to obtain high‐quality allele frequency calls of ~500 k SNP loci. We performed genetic structure analyses and demographic reconstructions based on the site frequency spectrum (SFS). We additionally used the same genotyping protocol to assess the genomic diversity of a set of 32 cultivars representative of the L. perenne cultivars widely used for forage purposes. Results: Expansion across Europe took place during the Würm glaciation (12–110 kya), a cooling period that decreased the dominance of trees in favour of grasses. Splits and admixtures in L. perenne fit historical climate changes in the Mediterranean basin. The development of agriculture in Europe (7–3.5 kya), that caused an increase in the abundance of grasslands, did not have an effect on the demographic patterns of L. perenne. We found that most modern cultivars are closely related to natural diversity from north-western Europe. Thus, modern cultivars do not represent the wide genetic variation found in natural populations. Main conclusions: Demographic events in L. perenne can be explained by the changing climatic conditions during the Pleistocene. Natural populations maintain a wide genomic variability at continental scale that has been minimally exploited by recent breeding activities. This variability constitutes valuable standing genetic variation for future adaptation of grasslands to climate change, safeguarding the agricultural services they provide.

International audience; The European Food Safety Authority concluded in February 2018 that "most uses of neonicotinoid insecticides represent a risk to wild bees and honeybees". In 2016, the French government passed a law banning the use of the five neonicotinoids previously authorized: clothianidin, imidacloprid, thiamethoxam, acetamiprid and thiacloprid. In the framework of an expert assessment conducted by the French Agency for Food, Environmental and Occupational Health and Safety to identify possible derogations, we performed a thorough assessment of the available alternatives to the five banned neonicotinoids. For each pest targeted by neonicotinoids use, we identified the main alternative pest management methods, which we then ranked for (i) efficacy for controlling the target pest, (ii) applicability (whether directly useable by farmers or in need of further research and development), (iii) durability (risk of resistance in targeted pests), and (iv) practicability (ease of implementation by farmers). We identified 152 authorized uses of neonicotinoids in France, encompassing 120 crops and 279 pest insect species (or genera). An effective alternative to neonicotinoids use was available in 96% of the 2968 case studies analyzed from the literature (single combinations of one alternative pest control method or product × one target crop plant × one target pest insect). The most common alternative to neonicotinoids (89% of cases) was the use of another chemical insecticide (mostly pyrethroids). However, in 78% of cases, at least one non-chemical alternative method could replace neonicotinoids (e.g. microorganisms, semiochemicals or surface coating). The relevance of non-chemical alternatives to neonicotinoids depends on pest feeding habits. Leaf and flower feeders are easier to control with non-chemical methods, whereas wood and root feeders are more difficult to manage by such methods. We also found that further field studies were required for many promising non-chemical methods before their introduction into routine use by farmers. Our findings, transmitted to policymakers, indicate that non-chemical alternatives to neonicotinoids do exist. Furthermore, they highlight the need to promote these methods through regulation and funding, with a view to reducing pesticide use in agriculture.

Temperature is one of the major factors controlling plant development, in particular seed germination. Alfalfa is a perennial pasture legume that holds an important place in cultivated grasslands. Breeding alfalfa cultivars adapted to new ranges of temperature could be necessary, requiring knowledge of the variability in response to temperature among different accessions. Six accessions of Medicago sativa subsp. sativa and one wild population of M. sativa subsp. falcata were evaluated for their germination temperature response. Seeds were tested for germination in the dark at eight constant temperatures, from 5 to 40°C. Significant differences (P < 0.01) were found. The optimum temperature for germination ranged between 11.7 and 21.1°C, whereas two varieties, ‘Demnate’ and ‘Luzelle’ had a very wide range of temperatures favouring maximum germination. The findings of this study suggest that the germination of alfalfa was little affected by low temperature (5°C) whereas germination at 40°C was lower and showed high variability. Our results revealed variability in the response to temperature for germination of M. sativa that gives room for breeding new varieties adapted to future environmental conditions induced by the global climate change.

Growing mixtures of annual arable crop species or genotypes is a promising way to improve crop production without increasing agricultural inputs. To design optimal crop mixtures, choices of species, genotypes, sowing proportion, plant arrangement, and sowing date need to be made but field experiments alone are not sufficient to explore such a large range of factors. Crop modeling allows to study, understand and ultimately design cropping systems and is an established method for sole crops. Recently, modeling started to be applied to annual crop mixtures as well. Here, we review to what extent crop simulation models and individual-based models are suitable to capture and predict the specificities of annual crop mixtures. We argued that: 1) The crop mixture spatio-temporal heterogeneity (influencing the occurrence of ecological processes) determines the choice of the modeling approach (plant or crop centered). 2) Only few crop models (adapted from sole crop models) and individual-based models currently exist to simulate annual crop mixtures. 3) Crop models are mainly used to address issues related to crop mixtures management and to the integration of crop mixtures into larger scales such as the rotation, whereas individual-based models are mainly used to identify plant traits involved in crop mixture performance and to quantify the relative contribution of the different ecological processes (niche complementarity, facilitation, competition, plasticity) to crop mixture functioning. This review highlights that modeling of annual crop mixtures is in its infancy and gives to model users some important keys to choose the model based on the questions they want to answer, with awareness of the strengths and weaknesses of each of the modeling approaches., Comment: 42 pages, 5 figures

International audience; We proposed a method to compute photosynthesis in response to environmental factors, applying in stadia environment. We managed to visualize heterogeneity of photosynthesis level for turfgrasses used in football pitch. We can not only bring out spatial variation of photosynthesis according to the position on the pitch, but also follow the temporal evolution throughout the year. The gross photosynthesis is evaluated with a model taking into account air temperature, daylength, intercepted irradiance and leaf area index. With a resolution of 7140 surface units, we can obtain daily gross photosynthesis on the whole pitch. This cartography highlights parts of pitch with a deficit of photosynthesis for the plant, and pitch managers could take more attention to these areas.

AimGrasslands have been pivotal in the development of herbivore breeding since the Neolithic and are still nowadays the most widespread agricultural land-use across Europe. However, it remains unclear whether the current large-scale genetic variation of plant species found in natural grasslands of Europe is the result of human activities or natural processes.LocationEurope.TaxonLolium perenne L (perennial ryegrass).MethodsWe reconstructed the phylogeographic history of L. perenne, a dominant grassland species, using 481 natural populations including 11 populations from closely related taxa. We combined the Genotyping-by-Sequencing (GBS) and Pool-sequencing (Pool-seq) methods to obtain high-quality allele frequency calls of ~ 500 k SNP loci. We performed genetic structure analyses and demographic reconstructions based on the site frequency spectrum (SFS). We additionally used the same genotyping protocol to assess the genomic diversity of a set of 32 cultivars representative of the L. perenne cultivars widely used for forage purposes.ResultsExpansion across Europe took place during the Würm glaciation (12-110 kya), a cooling period that decreased the dominance of trees in favour of grasses. Splits and admixtures in L. perenne fit historical sea level changes in the Mediterranean basin. The development of agriculture in Europe (7-3.5 kya), that caused an increase in the abundance of grasslands, did not have an effect on the demographic patterns of L. perenne. We found little differentiation between modern cultivars and certain natural variants. However, modern cultivars do not represent the wide genetic variation found in natural populations.Main conclusionsDemographic events in L. perenne can be explained by the changing climatic conditions during the Pleistocene. Natural populations maintain a wide genomic variability at continental scale that has been underused by recent breeding activities. This variability constitutes valuable standing genetic variation for future adaptation of grasslands to climate change, safeguarding the agricultural services they provide.

N uptake is positively correlated with photosynthetic capacity in ryegrass. The N sink strength of leaf meristems increased in response to defoliation. C and N reserves were remobilized only under severe defoliation., In grassland plant communities, the ability of individual plants to regrow after defoliation is of crucial importance since it allows the restoration of active photosynthesis and plant growth. The aim of this study was to evaluate the effects of increasing defoliation intensity (0, 25, 65, 84, and 100% of removed leaf area) on sugar remobilization and N uptake, remobilization, and allocation in roots, adult leaves, and growing leaves of ryegrass over 2 days, using a 15N tracer technique. Increasing defoliation intensity decreased plant N uptake in a correlative way and increased plant N remobilization, but independently. The relative contribution of N stored before defoliation to leaf growth increased when defoliation intensity was severe. In most conditions, root N reserves also contributed to leaf regrowth, but much less than adult leaves and irrespective of defoliation intensity. A threshold of defoliation intensity (65% leaf area removal) was identified below which C (glucose, fructose, sucrose, fructans), and N (amino acids, soluble proteins) storage compounds were not recruited for regrowth. By contrast, nitrate content increased in elongating leaf bases above this threshold. Wounding associated with defoliation is thus not the predominant signal that triggers storage remobilization and controls the priority of resource allocation to leaf meristems. A framework integrating the sequential events leading to the refoliation of grasses is proposed on the basis of current knowledge and on the findings of the present work.

Leaf length is a key factor in the economic value of different grass species and cultivars in forage production. It is also important for the survival of individual plants within a sward. The objective of this paper is to discuss the basis of within-species variation in leaf length. Selection for leaf length has been highly efficient, with moderate to high narrow sense heritability. Nevertheless, the genetic regulation of leaf length is complex because it involves many genes with small individual effects. This could explain the low stability of QTL found in different studies. Leaf length has a strong response to environmental conditions. However, when significant genotype × environment interactions have been identified, their effects have been smaller than the main effects. Recent modelling-based research suggests that many of the reported environmental effects on leaf length and genotype × environment interactions could be biased. Indeed, it has been shown that leaf length is an emergent property strongly affected by the architectural state of the plant during significant periods prior to leaf emergence. This approach could lead to improved understanding of the factors affecting leaf length, as well as better estimates of the main genetic effects.

A great variety of legume species are used for forage production and grown in multi-species grasslands. Despite their close phylogenetic relationship, they display a broad range of morphologies that markedly affect their competitive abilities and persistence in mixtures. Little is yet known about the component traits that control the deployment of plant architecture in most of these species. During the present study, we compared the patterns of shoot organogenesis and shoot organ growth in contrasting forage species belonging to the four morphogenetic groups previously identified in herbaceous legumes (i.e., stolon-formers, rhizome-formers, crown-formers tolerant to defoliation and crown-formers intolerant to defoliation). To achieve this, three greenhouse experiments were carried out using plant species from each group (namely alfalfa, birdsfoot trefoil, sainfoin, kura clover, red clover, and white clover) which were grown at low density under non-limiting water and soil nutrient availability. The potential morphogenesis of shoots characterized under these conditions showed that all the species shared a number of common morphogenetic features. All complied with a generalized classification of shoot axes into three types (main axis, primary and secondary axes). A common quantitative framework for vegetative growth and development involved: (i) the regular development of all shoot axes in thermal time and a deterministic branching pattern in the absence of stress; (ii) a temporal coordination of organ growth at the phytomer level that was highly conserved irrespective of phytomer position, and (iii) an identical allometry determining the surface area of all the leaves. The species differed in their architecture as a consequence of the values taken by component traits of morphogenesis. Assessing the relationships between the traits studied showed that these species were distinct from each other along two main PCA axes which explained 68% of total variance: the first axis captured a trade-off between maximum leaf size and the ability to produce numerous phytomers, while the second distinguished morphogenetic strategies reliant on either petiole or internode expansion to achieve space colonization. The consequences of this quantitative framework are discussed, along with its possible applications regarding plant phenotyping and modeling.

Developmental responses to temperature are critical to yield formation in crops and perennial grassland species. However, their characterisation over a broad range of temperatures relevant to climate change studies has been limited in these species. The present study sought to determine the non-linear developmental responses to temperature of two major grassland species, tall fescue (Festuca arundinacea) and alfalfa (Medicago sativa), and to assess i) whether a coordinated response occurred between different developmental processes, ii) if genotypes from contrasting origins differed in their responses, and iii) to quantify how a lack of coordination and genetic variability might affect estimates of thermal time progression under present and future climate scenarios. Two series of experiments were carried out under controlled conditions with eight constant temperatures ranging from 5 degrees C to 40 degrees C applied to seedlings and mature plants. Different growth and developmental processes were characterized, including shoot development and radicle, leaf and stem growth. Once normalized to a temperature of 20 degrees C, the temperature responses of the different processes displayed no significant variability between temperate and Mediterranean genotypes. On the other hand, not all developmental processes within a genotype displayed a coordinated response to temperature. Significant departures from the response of shoot development in mature plants were observed regarding several processes, particularly at low and supra-optimal temperatures (up to +/- 5 degrees C and +/- 2 degrees C for minimal and maximal temperature estimates, respectively). Differences were particularly marked relative to node and stem elongation responses. Overall, when using the temperature dependencies of different processes to estimate cumulative thermal time, significant bias was observed in alfalfa when considering stem elongation by comparison with other processes. The consequences for improvements to forage crop models, and for the meta-analyses often used to calibrate them, are discussed.

Uncertainties remain regarding the fate of atmospheric tritium after it has been assimilated in grasslands (ryegrass) in the form of TFWT (Tissue Free Water Tritium) or OBT (Organically Bound Tritium). One such uncertainty relates to the tritium forms discrimination during transfer from TFWT to OBT resulting from photosynthesis (OBTphoto), corresponding to the OBTphoto/TFWT ratio. In this study, the OBT/TFWT ratio is determined by experiments in the laboratory using a ryegrass model and hydroponic cultures, with constant activity of tritium in the form of tritiated water (denoted as HTO) in the “water” compartment (liquid HTO) and “air” compartment (HTO vapour in the air). The OBTphoto/TFWT ratio and the exchangeable OBT fraction are measured for three parts of the plant: the leaf, seed and root. Plant growth is modelled using dehydrated biomass measurements taken over time in the laboratory and integrating physiological functions of the plant during the first ten days after germination. The results suggest that there is no measurable discrimination of tritium in the plant organic matter produced by photosynthesis.

Rhagoletis completa Cresson (Diptera: Tephritidae) is native to North America and invaded Western Europe in the late 1980s, causing important damage to its principal host, walnut (Juglans spp.). In this review, we summarize the important elements of R. completa's biology, phytosanitary status and methods used in Europe for its control, and then present the main conclusions associated with a completed risk analysis performed in 2014 to evaluate the dispersion and establishment potential of R. completa in Europe. The walnut husk fly was initially identified in Switzerland (1988) and Italy (1991), from where it spread to at least seven additional countries: France, Spain, Germany, Austria, Croatia, Slovenia and Hungary. R. completa has not reached the limits of its potential distribution. The main dissemination pathways within Europe include: (1) natural adult dissemination; (2) adult hitchhiker behaviour; and, to a lesser extent, (3) transportation of larvae in fresh fruits. R. completa host plants are widely distributed in Europe, either as isolated wild trees or in orchards, favouring the probability of fly establishment in currently fly-free areas. In addition, the European territories where Juglans species are present share biogeographic similarities. In orchards where R. completa is present and uncontrolled, 100% of walnut trees can be infested, causing losses in walnut yields of up to 80%. The negative effect is low (< 10% yield loss) under phytosanitary control, although additional costs must also be considered to support specific monitoring for R. completa. The information presented here underlines a strong need for better walnut husk fly monitoring across European countries, as well as for increasing efforts to develop biological methods to control this emerging pest.

Chez les espèces monocarpiques telles que le maïs, la sénescence des feuilles est coordonnée avec celle de la plante entière et sous l’influence du fonctionnement reproductif. L’objectif de ce travail était de décrire l’évolution de la sénescence du maïs selon deux axes : (1) l’axe de la plante entière, où les différents étages foliaires sont considérés chacun comme une entité, et (2) l’axe foliaire, où la dynamique de trois symptômes macroscopiques de la sénescence sur chaque feuille prise individuellement, selon des zones allant de la pointe vers la base du limbe de la feuille, a été prise en considération. Sur une culture au champ, des pieds et des feuilles de maïs ont été suivis ou échantillonnés de la floraison à la récolte. Les dynamiques des teneurs en eau, en matière sèche et en chlorophylle sont différentes, aussi bien le long d’une feuille qu’entre les rangs de feuilles. De plus, la teneur en chlorophylle dans la plante suit une dynamique plus complexe que celles de l’eau et de la matière sèche. À partir du suivi de la décoloration le long des feuilles de 15 plantes sur toute leur hauteur, nous avons établi des relations fonctionnelles décrivant la progression du front de sénescence pour chaque rang des feuilles 7 à terminales (15 ou 16). Cette description quantitative peut être utilisée directement pour compléter des modèles de culture et des modèles de type structure–fonction. Cela permettrait d’estimer l’impact de la sénescence des feuilles sur l’interception du rayonnement et donc sur la photosynthèse et le rendement. Nos résultats peuvent servir aussi à l’interprétation des mesures de réflectance des couverts de maïs faites par télédétection.

International audience; Twenty-seven individual tree growth models are reviewed. The models take into account the same main physiological processes involved in carbon metabolism (photosynthate production, respiration, reserve dynamics, allocation of assimilates and growth) and share common rationales that are discussed. It is shown that the spatial resolution and representation of tree architecture used mainly depend on model objectives. Beyond common rationales, the models reviewed exhibit very different treatments of each process involved in carbon metabolism. The treatments of all these processes are presented and discussed in terms of formulation simplicity, ability to account for response to environment, and explanatory or predictive capacities. Representation of photosynthetic carbon gain ranges from merely empirical relationships that provide annual photosynthate production, to mechanistic models of instantaneous leaf photosynthesis that explicitly account for the effects of the major environmental variables. Respiration is often described empirically as the sum of two functional components (maintenance and growth). Maintenance demand is described by using temperature-dependent coefficients, while growth efficiency is described by using temperature-independent conversion coefficients. Carbohydrate reserve pools are generally represented as black boxes and their dynamics is rarely addressed. Storage and reserve mobilisation are often treated as passive phenomena, and reserve pools are assumed to behave like buffers that absorb the residual, excessive carbohydrate on a daily or seasonal basis. Various approaches to modelling carbon allocation have been applied, such as the use of empirical partitioning coefficients, balanced growth considerations and optimality principles, resistance mass-flow models, or the source-sink approach. The outputs of carbon-based models of individual tree growth are reviewed, and their implications for forestry and ecology are discussed. Three critical issues for these models to date are identified: (i) the representation of carbon allocation and of the effects of architecture on tree growth is Achilles' heel of most of tree growth models; (ii) reserve dynamics is always poorly accounted for; (iii) the representation of below ground processes and tree nutrient economy is lacking in most of the models reviewed. Addressing these critical issues could greatly enhance the reliability and predictive capacity of individual tree growth models in the near future.; Les modèles de croissance d'individus arbres basés sur le fonctionnement carboné : une évaluation critique. Vingt-sept modèles simulant la croissance d'arbres à l'échelle individuelle sont évalués. Ces modèles prennent en compte les principaux processus impliqués dans le métabolisme carboné (assimilation photosynthétique, respiration, dynamique des réserves, allocation des assimilats et croissance). Les concepts communs à tous ces modèles sont discutés. Il est montré que l'échelle d'espace et la représentation de l'architecture utilisées dépendent principalement des objectifs du modèle. Au-delà de concepts communs, les modèles évalués utilisent des représentations très différentes pour chacun des processus impliqués dans le métabolisme carboné. Les différentes représentations de ces processus sont présentées et discutées en termes de simplicité de formulation, de capacité à prendre en compte la réponse aux variables environnementales, et de capacités prédictives. La représentation des gains de carbone va de relations purement empirique calculant la production annuelle de photosynthétats jusqu'à des modèles de photosynthèse foliaire à bases mécanistes prenant explicitement en compte les effets des principales variables environnementales. La respiration est souvent décrite de façon empirique comme la somme de deux composantes (maintenance et croissance). La demande de maintenance est calculée à partir de coefficients dépendant de la température, alors que l'efficience de croissance est calculée à partir de coefficients de conversion indépendant de la température. Les réserves carbonées sont généralement représentées comme des boîtes noires, et leur dynamique est rarement prise en compte. La mise en réserve et l'utilisation des réserves sont souvent traitées comme des processus passifs, les réserves servant souvent de compartiment tampon absorbant les assimilats produits en excès sur une base journalière ou saisonnière. De nombreuses approches ont été utilisées pour modéliser l'allocation de carbone, telles que l'utilisation de coefficients d'allocation empiriques, l'application des principes de l'équilibre fonctionnel et d'optimisation, l'utilisation de schémas flux-résistance, ou des approches sources-puits. Les sorties des modèles simulant le bilan carboné et la croissance de plantes ligneuses à l'échelle individuelle sont présentées, et leurs implications en foresterie et en écologie sont discutées. Trois points particulièrement critiques actuellement pour ces modèles sont identifiés : (i) la représentation de l'allocation du carbone et des effets de l'architecture sur la croissance de l'arbre est le talon d'Achille de la majorité de ces modèles ; (ii) la dynamique des réserves est toujours faiblement représentée ; (iii) la représentation du fonctionnement racinaire et de la gestion des nutriments dans l'arbre est absente dans presque tous les modèles évalués. Une meilleure prise en compte de ces points critiques devrait fortement améliorer la fiabilité et les capacités prédictives des modèles de croissance d'arbres à l'échelle individuelle dans le futur.

International audience; Pea is the main legume species grown in European agricultural systems although its productivity is limited due to foliar diseases or lodging. Architectural plant models therefore appeared to be suitable tools for studying the behaviour of contrasting pea architecture against environmental factors. We characterized the growth of six contrasting pea genotypes in order to analyse their developmental rules that are needed to build up such functional-structural models. Virtual plant models were reconstructed from digitization of the pea cultivars along their growing cycle, resulting in a database of 950 mock-ups. Pea cultivars exhibited contrasting architectures in terms of green area, plant height and growth rates. Three categories of growth habits were identified among the experimented cultivars which were distinguished according to their earliness, leaf type and axis development. The present study highlights the strong genotypic variability of pea architecture and also provides a quantitative description of pea morphogenesis that can be used to build functional-structural models. Moreover, these contrasting architectures also allowed us to assess the behaviour of different pea cultivar towards environmental factors.

Light sharing within virtual wheat-pea mixtures was influenced by the variability of pea’s architectural parameters affecting LAI and height. Light capture was affected by the development of leaflets, number of branches and phytomers and internode length., Background and aims Light interception is a key factor driving the functioning of wheat–pea intercrops. The sharing of light is related to the canopy structure, which results from the architectural parameters of the mixed species. In the present study, we characterized six contrasting pea genotypes and identified architectural parameters whose range of variability leads to various levels of light sharing within virtual wheat–pea mixtures. Methodology Virtual plants were derived from magnetic digitizations performed during the growing cycle in a greenhouse experiment. Plant mock-ups were used as inputs of a radiative transfer model in order to estimate light interception in virtual wheat–pea mixtures. The turbid medium approach, extended to well-mixed canopies, was used as a framework for assessing the effects of leaf area index (LAI) and mean leaf inclination on light sharing. Principal results Three groups of pea genotypes were distinguished: (i) early and leafy cultivars, (ii) late semi-leafless cultivars and (iii) low-development semi-leafless cultivars. Within open canopies, light sharing was well described by the turbid medium approach and was therefore determined by the architectural parameters that composed LAI and foliage inclination. When canopy closure started, the turbid medium approach was unable to properly infer light partitioning because of the vertical structure of the canopy. This was related to the architectural parameters that determine the height of pea genotypes. Light capture was therefore affected by the development of leaflets, number of branches and phytomers, as well as internode length. Conclusions This study provides information on pea architecture and identifies parameters whose variability can be used to drive light sharing within wheat–pea mixtures. These results could be used to build up the architecture of pea ideotypes adapted to multi-specific stands towards light competition.

International audience; The morphology of forage grass, in particular ryegrass, emerges from the combination of many entangled dynamical processes. Although a plant seems to possess an intrinsic (i.e., genetically determined) architecture, its morphogenesis also displays very high plasticity with respect to environmental conditions. This plasticity could be partly mediated by a self-regulatory process. Indeed, the architecture of a shoot can affect subsequent architectural development recursively. For instance, leaf length is directly influenced by the length of the preceding leaves. We present a functional-structural 3D model of ryegrass shoots based on the hypothesis that their morphogenesis obeys such self-regulatory processes. The behaviour and emergent properties of the model are highly compatible with observations regarding plant morphology development, genetic variability and plasticity. Experimental and simulation results therefore suggest that the architectural development of ryegrass could emerge from the collaborative association between genetic programmability and self-organisation, instead of resulting from a centralised control of each architectural trait.

International audience; The objective of this work was to describe the dynamics of senescence of field-grown maize from silking to harvest, at both leaf and plant level. At the leaf level, the dynamics of symptoms of senescence were considered on each leaf taken individually and described according to zones from the tip to the base of the leaf lamina. At the whole plant level, foliar ranks were each considered as an entity to describe the time course of senescence. To this end, we use a database built-up from field trials conducted during three consecutive years (1994-1996) and thus undergoing variable meteorological and soil water conditions. Leaf chlorophyll content was estimated using two different methods based on the optical properties of the leaves. In Trial I, chlorophyll estimates were based on leaf optical density measured with a MacBeth TR-924 densitometer. In Trial II and Trial III, chlorophyll estimations were based on measurement using a hand-held Minolta SPAD-502 device. In Trial II and Trial III, chlorophyll fluorescence analyses under filed conditions were performed. These analyses focused on the photosystem II (PSI!) maximum efficiency (Fv'/Fm') parameter in the light-adapted state. Measurements were done with a Waltz PAM-2000 portable fluorometer. We report detailed descriptions of the spatio-temporal dynamics of these indicators of senescence. We found that, after silking, a strong relationship exists between available water and leaf chlorophyll content. Further, the maximum efficiency of PSII decreased faster in maize plants undergoing low available soil water than in irrigated plants. The rank of a leaf is determinant of the time for the beginning of the decline in both chlorophyll content and maximum efficiency of PSII. At plant and leaf level, the onset of senescence was marked by a decrease in chlorophyll content that was not concomitant with a dramatic decrease in the maximum efficiency of PSII. Our analyses suggest that a non-linear functional relationship could exist between these two parameters during monocarpic senescence. In the mean time, the results presented in this paper could be used to refine the senescence related modules in plant and crop models.

Functional-structural plant models (FSPMs) explore and integrate relationships between a plant's structure and processes that underlie its growth and development. In recent years, the range of topics being addressed by scientists interested in functional-structural plant modelling has expanded greatly. FSPM techniques are now being used to dynamically simulate growth and development occurring at the microscopic scale involving cell division in plant meristems to the macroscopic scales of whole plants and plant communities. The plant types studied also cover a broad spectrum from algae to trees. FSPM is highly interdisciplinary and involves scientists with backgrounds in plant physiology, plant anatomy, plant morphology, mathematics, computer science, cellular biology, ecology and agronomy. This special issue of Annals of Botany features selected papers that provide examples of comprehensive functional-structural models, models of key processes such as partitioning of resources, software for modelling plants and plant environments, data acquisition and processing techniques and applications of functional-structural plant models for agronomic purposes.

Publication Inra prise en compte dans l'analyse bibliométrique des publications scientifiques mondiales sur les Fruits, les Légumes et la Pomme de terre. Période 2000-2012. http://prodinra.inra.fr/record/256699; International audience; Background and Aims Most studies dealing with light partitioning in intercropping systems have used statistical models based on the turbid medium approach, thus assuming homogeneous canopies. However, these models could not be directly validated although spatial heterogeneities could arise in such canopies. The aim of the present study was to assess the ability of the turbid medium approach to accurately estimate light partitioning within grass-legume mixed canopies. Methods Three contrasted mixtures of wheat-pea, tall fescue-alfalfa and tall fescue-clover were sown according to various patterns and densities. Three-dimensional plant mock-ups were derived from magnetic digitizations carried out at different stages of development. The benchmarks for light interception efficiency (LIE) estimates were provided by the combination of a light projective model and plant mock-ups, which also provided the inputs of a turbid medium model (SIRASCA), i.e. leaf area index and inclination. SIRASCA was set to gradually account for vertical heterogeneity of the foliage, i.e. the canopy was described as one, two or ten horizontal layers of leaves. Key Results Mixtures exhibited various and heterogeneous profiles of foliar distribution, leaf inclination and component species height. Nevertheless, most of the LIE was satisfactorily predicted by SIRASCA. Biased estimations were, however, observed for (1) grass species and (2) tall fescue-alfalfa mixtures grown at high density. Most of the discrepancies were due to vertical heterogeneities and were corrected by increasing the vertical description of canopies although, in practice, this would require time-consuming measurements. Conclusions The turbid medium analogy could be successfully used in a wide range of canopies. However, a more detailed description of the canopy is required for mixtures exhibiting vertical stratifications and inter-/intra-species foliage overlapping. Architectural models remain a relevant tool for studying light partitioning in intercropping systems that exhibit strong vertical heterogeneities. Moreover, these models offer the possibility to integrate the effects of microclimate variations on plant growth.

Background Association studies are of great interest to identify genes explaining trait variation since they deal with more than just a few alleles like classical QTL analyses. They are usually performed using collections representing a wide range of variability but which could present a genetic substructure. The aim of this paper is to demonstrate that association studies can be performed using synthetic varieties obtained after several panmictic generations. This demonstration is based on an example of association between the gibberellic acid insensitive gene (GAI) polymorphism and leaf length polymorphism in 'Herbie', a synthetic variety of perennial ryegrass. Methods Leaf growth parameters, consisted of leaf length, maximum leaf elongation rate (LERmax) and leaf elongation duration (LED), were evaluated in spring and autumn on 216 plants of Herbie with three replicates. For each plant, a sequence of 370 bp in GAI was analysed for polymorphism. Results Genetic effect was highly significant for all traits. Broad sense heritabilities were higher for leaf length and LERmax with about 0.7 in each period and 0.5 considering both periods than for LED with about 0.4 in each period and 0.3 considering both periods. GAI was highly polymorphic with an average of 12 bp between two consecutive SNPs and 39 haplotypes in which 9 were more frequent. Linkage disequilibrium declined rapidly with distance with r 2 values lower than 0.2 beyond 150 bp. Sequence polymorphism of GAI explained 8-14% of leaf growth parameter variation. A single SNP explained 4% of the phenotypic variance of leaf length in both periods which represents a difference of 33 mm on an average of 300 mm. Conclusions Synthetic varieties in which linkage disequilibrium declines rapidly with distance are suitable for association studies using the "candidate gene" approach. GAI polymorphism was found to be associated with leaf length polymorphism which was more correlated to LERmax than to LED in Herbie. It is a good candidate to explain leaf length variation in other plant material.

International audience; Light quality and, in particular, its content of blue light is involved in plant functioning and morphogenesis. Blue light variation frequently occurs within a stand as shaded zones are characterized by a simultaneous decrease of PAR and blue light levels which both affect plant functioning, for example, gas exchange. However, little is known about the effects of low blue light itself on gas exchange. The aims of the present study were (i) to characterize stomatal behaviour in Festuca arundinacea leaves through leaf gas exchange measurements in response to a sudden reduction in blue light, and (ii) to test the putative role of Ci on blue light gas exchange responses. An infrared gas analyser (IRGA) was used with light transmission filters to study stomatal conductance (gs), transpiration (Tr), assimilation (A), and intercellular concentration of CO(2) (Ci) responses to blueless PAR (1.80 mu mol m(-2) s(-1)). The results were compared with those obtained under a neutral filter supplying a similar photosynthetic efficiency to the blueless PAR filter. It was shown that the reduction of blue light triggered a drastic and instantaneous decrease of gs by 43.2% and of Tr by 40.0%, but a gradual stomatal reopening began 20 min after the start of the low blue light treatment, thus leading to new steady-states. This new stomatal equilibrium was supposed to be related to Ci. The results were confirmed in more developed plants although they exhibited delayed and less marked responses. It is concluded that stomatal responses to blue light could play a key role in photomorphogenetic mechanisms through their effect on transpiration.

International audience; Although it is well known that plant morphogenesis is regulated by light quality, most plant modelling is blind to photomorphogenesis. Variability in light quality within plant canopy result first from geometrical interaction between incident light and plant phytoelements and second from optical properties of these elements. In a previous study, we established a series of functional relationships between photosynthetically active radiation (PAR) or energy and morphogenetically active radiation (MAR) within a sorghum canopy. In this study, we applied these relationships to walnut trees within which light spectra were measured at different sites. We estimated the variables of the MAR such as the quantity of blue radiation, the ratio red: far red and the phytochrome photoequilibrium from the proportion of transmitted PAR. The results showed good agreement between estimated and measured MAR variables within both trees canopies. Although differences between trees and sorghum architecture exist, MAR variables could be estimated accurately using the functional relationships.

International audience

Light quality, drives several photomorphogenetic responses in plants such as leaf growth. A decrease in blue irradiance in the incident light leads to an increase in leaf elongation rate and final leaf length. The effects of blue light could be mediated by water flux into the growing zone since blue light has a direct effect on stomata aperture and consequently on plant transpiration. Indeed, several studies have demonstrated the effects of changes in transpiration rate on leaf elongation rate but little is known about the control of leaf elongation by water flux under low blue light conditions.

Model Light quality plays a key role in higher plants' morphogenesis. In most plant models, light is considered as a consumable resource and plants are assumed blind to light signals. However, prior to any effort for modelling photomorphogenetic mechanisms, it is necessary to characterise the spatial distribution of the Morphogenetically Active Radiation (MAR) over and within plant canopies. Measurements of local photosynthetic photon flux density (PPFD) and broadband irradiance (Es) are easy to carry out by using small sensors. Thus, the distribution of the MAR within a canopy can be estimated whenever the functional relationships between these measurements and the photon flux within any spectral band are known. The objective of this work was to determine these functional relationships from the light spectra received above and at various positions around a target plant within a growing sorghum crop. The MAR components considered in this work are related either to photon flux densities in various wavebands between 330 and 950 nm or to the ratio between two photon flux densities. A part of the photon flux-related variables is strictly included in the PAR band and might be estimated from PPFD measurements using linear relationships. The other variables are related to both PPFD and Es by multiple linear relationships. The phytochrome photoequilibrium and the red to far-red ratios were related to the relative transmitted PPFD and to the ratio PPFD/Es within the canopy using a non-linear model. Models were validated against an independent set of data. We demonstrate that the MAR components within a sorghum crop can be accurately estimated with the functional relationships presented in this paper.

http://www-sop.inria.fr/virtualplants/Publications/2009/FPCBLRCCBMSVEAG09/Alinea@Glasgow.pdf; International audience; 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.

20 ref.; International audience; Quantifying and simulating the relationships between crop growth, total‐nitrogen (total‐N) and nitrate‐N (NO3–‐N) concentration can improve crop nutritional husbandry. In this study, the relationship between shoot relative growth rate (RGR) and shoot total‐N, organic‐N and NO3–‐N concentration of hydroponically‐grown lettuce (Lactuca sativa var. capitata L. cv. Kennedy) was described and simulated. Plants were grown hydroponically for up to 74 d. Nitrogen was supplied throughout (control; T1), or removed at 35 d (T2) and 54 d (T3), respectively, after sowing. The organic‐N and NO3–‐N concentration declined in the shoots of control plants with growth, until commercial maturity approached when organic‐N and NO3–‐N concentration increased. There were sub‐linear relationships between both total‐N and organic‐N concen tration, and shoot RGR, in the N‐limited treatments, i.e. shoot RGR approached an asymptote at high shoot N concentration. The proportional effects of total‐N and organic‐N concentration on shoot RGR were independent of plant age. A dynamic simulation model (‘Nicolet’), derived previously under different conditions, was used to simulate the growth, dry matter content, organic‐N, and NO3–‐N concentration of lettuce grown under the extreme N‐stress conditions experienced by the plants. In view of the largely successful fitting of the model to experimental data, the model was used to interpret the results. Suggestions for model improvement are made.

An extensive literature reports that Cs(+), an environmental contaminant, enters plant cells through K(+) transport systems. Several recently identified plant K(+) transport systems are permeable to Cs(+). Permeation models indicate that most Cs(+) uptake into plant roots under typical soil ionic conditions will be mediated by voltage-insensitive cation (VIC) channels in the plasma membrane and not by the inward rectifying K(+) (KIR) channels implicated in plant K nutrition. Cation fluxes through KIR channels are blocked by Cs(+). This paper tests directly the hypothesis that the dominant KIR channel in plant roots (AKT1) does not contribute significantly to Cs(+) uptake by comparing Cs(+) uptake into wild-type and the akt1 knockout mutant of Arabidopsis thaliana (L.) Heynh. Wild-type and akt1 plants were grown to comparable size and K(+) content on agar containing 10 mM K(+). Both Cs(+) influx to roots of intact plants and Cs(+) accumulation in roots and shoots were identical in wild-type and akt1 plants. These data indicate that AKT1 is unlikely to contribute significantly to Cs(+) uptake by wild-type Arabidopsis from 'single-salt' solutions. The influx of Cs(+) to roots of intact wild-type and akt1 plants was inhibited by 1 mM Ba(2+), Ca(2+) and La(3+), but not by 10 microM Br-cAMP. This pharmacology resembles that of VIC channels and is consistent with the hypothesis that VIC channels mediate most Cs(+) influx under 'single-salt' conditions.

International audience; A model was developed that simulated photosynthesis, growth and allocation in tree seedlings. The model was parameterized with data from experiments on seedlings of sycamore (Acer pseudoplatanus L.), Sitka spruce (Picea sitchensis (Bong) Carr.) and young birch trees (Betula pendula Roth.). In these experiments, CO2 concentration ([CO2]) and nutrient addition rate were varied. Parameters quantifying nutrient uptake, translocation and starch synthesis were fitted, bared on data from control treatments. Elevated [CO2] and low-nutrient treatments were then used to test the predicted response of growth and allocation against observations. The model accurately predicted total seedling growth in the elevated [CO2] treatments. A response of growth to elevated [CO2] was seen in the birch and sycamore experiments, but not in the Sitka spruce, because of photosynthetic down-regulation. Predictions of allocation were reasonably accurate in the birch and Sitka spruce experiments, but were notably poorer in the sycamore experiments, possibly because of differences in sink strength between root and shoot. In the birch and sycamore experiments, little change in allocation with elevated [CO2] was observed or predicted. This was ascribed to the relative values of K-Tc and K-Tn, the translocation coefficients that determine the sensitivity of allocation to carbon and nitrogen uptake rates, respectively. Growth and allocation in the low-nutrient treatments were poorly predicted by the model. In Sitka spruce, it was suspected that the photosynthetic parameters measured in August 1994 had been higher earlier in the season, before nutrients became depleted. In sycamore, the discrepancies were thought to relate to differences in sink strength between root and shoot that could not be described by the model.

International audience; Photosynthesis, metabolic carbon partitioning and the contribution of sorbitol to the osmotic potential of mature peach (Prunus persica (L.) Batsch) leaves and phloem sap were examined in plants undergoing two levels of a short term drought stress. The relationship between osmotic potential at full turgor and water potential showed that neither mild nor severe drought stress induced a significant active osmotic adjustment in mature leaves. The osmotic potential of leaves at full turgor was -1.9 MPa; sorbitol was the major organic component (20%). Leaf sucrose and starch contents were significantly reduced by drought. The partitioning of newly-fixed carbon was also affected by stress. These changes appeared to originate from the inhibition of photosynthesis induced by drought stress. At low photosynthetic rates, the turnover of organic ions was low, and sorbitol synthesis was favoured over that of sucrose. Water stress did not affect the in vitro activity of sucrose phosphate synthase (EC 2.4.1.14), the key enzyme in sucrose synthesis. The in vitro activity of aldose-6-phosphate reductase (EC 1.1.1.200), the key enzyme in sorbitol synthesis, tended to increase linearly in response to drought stress. It is concluded that, contrary to some other polyol-synthesising species, peach did not seem to benefit from sorbitol synthesis during short term drought stress for active osmotic adjustment in mature leaves. However, in phloem sap, increases in sucrose and especially sorbitol concentration were observed in stressed plants.

A deterministic and dynamic model of carbon allocation in walnut seedlings is described. Two experimental data sets were used to calibrate and validate the model. These data included: time course of the carbon content, chemical and isotope composition (12C and 13C) of the kernel and growing organs (roots, stem, leaves), and gas exchange rates during the first 55 days of the life of the plant with continuous 13CO2 feeding. The plant is modelled as a network with nodal organs acting as sources or sinks for carbohydrates. In a sink organ the demand for carbon is the sum of four elementary demands: maintenance respiration, structural growth, growth-associated respiration and carbon storage. The organs of the plant are assumed to be in exponential growth phase. The supply of carbon readily accessible for the organs is the store of soluble sugars present in a local reservoir. Carbon flow in the network is determined by the source/sink activities of the organs and local levels of demand and supply. Two carbon sources are considered: soluble sugars from the kernel, and gross photosynthesis. The rate of synthesis of soluble sugars in the kernel, and measured photosynthesis in the leaves are inputs for the plant model. The outputs are the predicted fluxes of carbon within the seedling; 13C composition,carbohydrate allocation to the growing organs, starch and soluble sugars accumulation, and respiration. The mathematical equations were translated into PSPICE software instructions. After optimisation of the parameter values, the model provided an accurate description of experimental observations in the seed-plant system during the critical transition from heterotrophy to autotrophy, especially C allocation to organs and C partitioning between storage, structural growth and respiration in each organ. The growth of the young plant is supply-limited, except at the earliest stages. A sensitivity analysis suggests that intense competition for carbohydrates dominates the relations among and within organs. Copyright 1998 Academic Press Limited

40 ref.; International audience; The characteristics of sorbitol and sucrose export out of mature leaves in seedlings of peach (Prunus persica L. Batsch cv CF 305) were investigated by simulating carbon fluxes through the leaf. Three treatments were employed: a control treatment and two treatments modifying leaf export, the latter using either shading or girdling. Photosynthesis and 14C partitioning into sorbitol and sucrose were measured during carbohydrate pool buildup at the beginning of the photoperiod, and the export rate of sorbitol and sucrose was modeled using a PSPICE (Simulation Program with lntegrated Circuit Emphasis) simulator. The simulation allowed predidion of the resulting sorbitol and sucrose contents, which were compared to experimental carbohydrate contents. The apparent Km, for sorbitol and sucrose phloem loading, estimated by carbon flux modeling, was 6.6 and 4 mol m(-3), respedively. The predided export capacity of the leaf, charaderized by the estimated Vmax, values for phloem loading of sorbitol and sucrose, was similar to the photosynthetic carbon flux measured under the leaf growth conditions. This export capacity was enhanced in plants in which all leaves except those studied were shaded. The mature leaf had a higher storage capacity for sorbitol than for sucrose in control plants, especially in the girdled treatment. Sucrose content appears to be tightly regulated.