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Predicting tree frost tolerance is critical to select adapted species according to both the current and predicted future climate. The relative change in water to carbohydrate ratio is a relevant trait to predict frost acclimation in branches from many tree species. The objective of this study is to demonstrate the interspecific genericity of this approach across nine tree species. In the studied angiosperm species, frost hardiness dynamics were best correlated to a decrease in water content at the early stage of acclimation (summer and early autumn). Subsequently, frost hardiness dynamics were more tightly correlated to soluble carbohydrate contents until spring growth resumption. Based on different model formalisms, we predicted frost hardiness at different clade levels (angiosperms, family, genus and species) with high to moderate accuracy (1.5–6.0 °C root mean squared error (RMSE)) and robustness (2.8–6.1 °C prediction RMSE). The TOT model, taking all soluble carbohydrate and polyols into account, was more effective and adapted for large scale studies aiming to explore frost hardiness across a wide range of species. The ISC model taking the individual contribution of each soluble carbohydrate molecule into account was more efficient at finer scale such as family or species. The ISC model performance also suggests that the role of solutes cannot be reduced to a ‘bulk’ osmotic effect as could be computed if all of them were located in a single, common, compartment. This study provides sets of parameters to predict frost hardiness in a wide range of species, and clues for targeting specific carbohydrate molecules to improve frost hardiness.

The interaction between carbon and flows within the plant is at the center of most growth and developmental processes. Understanding how these fluxes influence each other, and how they respond to heterogeneous environmental conditions, is important to answer diverse questions in forest, agriculture and environmental sciences. However, due to the high complexity of the plant-environment system, specific tools are needed to perform such quantitative analyses.Here we present CPlantBox, full plant modelling framework based on the root system model CRootBox. CPlantbox is capable of simulating the growth and development of a variety of plant architectures (root and shoot). In addition, the flexibility of CPlantBox enables its coupling with external modeling tools. Here, we connected it to an existing mechanistic model of water and carbon flows in the plant, PiafMunch.The usefulness of the CPlantBox modelling framework is exemplified in four case studies. Firstly, we illustrate the range of plant structures that can be simulated using CPlantBox. In the second example, we simulated diurnal carbon and water flows, which corroborates published experimental data. In the third case study, we simulated impacts of heterogeneous environment on carbon and water flows. Finally, we showed that our modelling framework can be used to fit phloem pressure and flow speed to (published) experimental data.The CPlantBox modelling framework is open-source, highly accessible and flexible. Its aim is to provide a quantitative framework for the understanding of plant-environment interaction.

International audience; An important group of fungal diseases, such as apple scab on Malus x domestica leaves, perform infection only by remaining wetted by liquid water for a sufficient time, making leaf wetness duration (LWD) a crucial variable in epidemiological risk assessment. LWD is affected by the leaf wettability, i.e. the geometry of a liquid on a leaf as a consequence of the interaction between molecules of water and structural and physicochemical properties of the leaf surface. The variables describing wettability are the contact angles, which result from a genotype's physical and chemical properties. Accordingly, we assumed that wettability could be a direct expression of the genetic resistance of apple genotypes, thus influencing the physicochemical properties of the surface. In order to verify that hypothesis we measured static and dynamic contact angles on seven different Malus x domestica genotypes with various apple scab susceptibilities. The measured contact angle hysteresis was very high (from 58.8° to 82.6° among genotypes), leading to a high variability in possible values of static contact angles of water drops on leaf surface. The results obtained by mimicking both dew like and rain like water depositions during experiments, emphasize the importance of the water deposition process on the static contact angle of a water droplet on a leaf. A positive correlation between dew like droplets and the apple cultivar susceptibility to apple scab was observed and is discussed. This study is valuable for new LWD modeling approaches taking into account the leaf wetting properties.

Modeling is a fundamental part of quantitative science used to bring together several quantitative components, often developed though detailed reductionist approach on component parts, e.g., sucrose transport through a membrane osmotic relation. It is now generally accepted that phloem transport is the result of bulk solution flow generated by the difference in osmotic pressure between source and sink tissues. However, there is still little agreement on how different sink tissues compete for available carbohydrate. Furthermore, the impact of phloem pathway leakage (unloading) and reloading on source-to-sink carbon transport remains unclear. Moreover, it is debated to what degree the interactions between phloem and xylem flows influence carbohydrate source-sink relations. These aspects are extremely difficult to research by a reductionist approach, with modeling being an important tool to examine the consequences of proposed mechanisms, which can then be tested on whole plants.Phloem/xylem modeling has been at the limits of quantitative modeling, especially when dynamic models are needed to explain tracer studies. Advances in computing now enable more realistic modeling, which are utilized by the PiafMunch approach described here. This model enables a high level of mechanistic detail to be incorporated and the observable effect of it to be tested. In the most recent version of the software with the introduction of tracer dynamics, it can now predict the effects of specific phloem mechanisms upon the shape of evolving tracer profiles.

Plant growth and development are limited by the available resources. The carbon and water flows are both important content and indicators of resource translocations. Modelling is helpful to increase the spatial and temporal resolutions of carbon and water reallocations in plants. However, the mechanism of carbon and water translocations has not been coupled into a whole Functional-Structural Plant Model (FSPM) yet. Here we developed a FSPM called CPlantBox which could (1) simulate the growth and development of the full plant structure (both root and shoot); (2) connect to a mechanistic model of water and carbon flow (PiafMunch). Our results demonstrate how carbon and water are flowing inside a plant which has three sources (leaf) and two sinks (root). We anticipate the model can be used as a tool to explore the variabilities and possibilities of plant behavior. Furthermore, several tool sets will be developed to visualize the morphological and physiological attributes of plants, which are helpful to deepen our understanding of plants and produce more with less.

It is now accepted that the transport phloem, linking major sources and sinks, is leaky, and this leakage can be considerable. Hence for phloem transport to function over the long distances observed, a large fraction of this unloaded photosynthate must be reloaded. A fraction of this unloaded solute is used to maintain tissues surrounding the phloem, as well as being stored. Also, pathway unloading/reloading acts as a short-term buffer to source and sink changes. In this work we present the first attempt to include both pathway unloading and reloading of carbohydrate in the modelling of pressure driven flow to determine if this has any significant effect upon source–sink dynamics. Our results indicated that the flow does not follow Poiseuille dynamics, and that pathway unloading alters the solute concentration and hydrostatic pressure profiles. Hence, measurement of either of these without considerable other detail tells us very little about the flow mechanisms. With adequate reloading along the pathway, the effects of pathway unloading can completely compensate for, making the entire system look like one with no pathway unloading.

Adequate radial water transport between elastic bark tissue and xylem is crucial in trees, because it smoothens abrupt changes in xylem water potential, greatly reducing the likelihood of suffering dangerous levels of embolism. The radial hydraulic conductance involved is generally thought to be constant. Evidence collected about variable root and leaf hydraulic conductance led us to speculate that radial hydraulic conductance in stem/branches might also be variable and possibly modulated by putative aquaporins. We therefore correlated diameter changes in walnut (Juglans regia L.) with changes in water potential, altered by perfusion of twig samples with d-mannitol solutions having different osmotic potentials. Temperature and cycloheximide (CHX; a protein synthesis inhibitor) treatments were performed. The temperature response and diameter change inhibition found in CHX-treated twigs underpinned our hypothesis that radial hydraulic conductance is variable and likely mediated by a putative aquaporin abundance and/or activity. Our data demonstrate that radial water transport in stem/branches can take two routes in parallel: an apoplastic and a cell-to-cell route. The contribution of either route depends on the hydraulic demand and is closely linked to a boost of putative aquaporins, causing radial conductance to be variable. This variability should be considered when interpreting and modelling diameter changes.

As observed for most stresses, tree frost resistance can be split into two main processes: avoidance and tolerance. Avoidance of freezing is achieved by introducing species only in the climatic context in which the probability of freezing events is very low for the sensitive stages of buds or stems; i.e., when good synchronism exists between the annual cycle and the critical climatic periods. Buds become able to grow only after chilling requirements have been satisfied (endodormancy released) during winter; they subsequently break after heat requirements have been completed (end of ecodormancy) in early spring. Actually, this period is often subject to more or less severe freezing events. Trees are also able to adjust their freezing tolerance by increasing their capacity of extracellular freezing and decreasing the possibility of intracellular freezing through the process of frost acclimation. Both freezing resistance processes (avoidance and tolerance) are environmentally driven (by photoperiod and temperature), but there are also genotypic effects among species or cultivars. Here, we evaluated the degree to which differences in dormancy release and frost acclimation were related to environmental and genetic influences by comparing trees growing in common garden conditions. This investigation was carried out for two winters in lowland and mountain locations on different walnut genotypes differing significantly for budburst dates. Chilling requirement for endodormancy release and heat requirement during ecodormancy were evaluated in all situations. In addition, frost acclimation was assessed by the electrolyte leakage method on stems from the same trees before leaf fall through budburst. No significant differences were observed in chilling requirements among genotypes. Moreover, frost acclimation dynamics were similar between genotypes or locations when expressed depending on chilling units accumulated since 15 September as a time basis instead of Julian day. The only exception was for maximal frost hardiness observed during winter with the timber-oriented being significantly more resistant than fruit-oriented genotypes. Heat requirement was significantly different among genotypes. Thus, growth was significantly faster in fruit-oriented than in wood-oriented genotypes. Furthermore, among wood-oriented genotypes, differences in growth rate were observed only at cold temperatures. Frost acclimation changes differed significantly between fruit- and wood- walnuts from January through budburst. In conclusion, from September through January, the acclimation dynamic was driven mainly by environmental factors whereas from January through budburst a significant genotype effect was identified in both frost tolerance and avoidance processes.

When the current level of carbohydrates produced by photosynthesis is not enough to meet the C demand for maintenance, growth or metabolism, trees use stored carbohydrates. In rubber trees (Hevea brasiliensis Muell. Arg.), however, a previous study (Silpi U., A. Lacointe, P. Kasemsap, S. Thanisawanyangkura, P. Chantuma, E. Gohet, N. Musigamart, A. Clement, T. Ameglio and P. Thaler. 2007. Carbohydrate reserves as a competing sink: evidence from tapping the rubber tree. Tree Physiol. 27: 881-889) showed that the additional sink created by latex tapping results not in a decrease, but in an increase in the non-structural carbohydrate (NSC) storage in trunk wood. In this study, the response of NSC storage to latex tapping was further investigated to better understand the trade-off between latex regeneration, biomass and storage. Three tapping systems were compared to the untapped Control for 2 years. Soluble sugars and starch were analyzed in bark and wood on both sides of the trunk, from 50 to 200 cm from the ground. The results confirmed over the 2 years that tapped trees stored more NSC, mainly starch, than untapped Control. Moreover, a double cut alternative tapping system, which produced a higher latex yield than conventional systems, led to even higher NSC concentrations. In all tapped trees, the increase in storage occurred together with a reduction in trunk radial growth. This was interpreted as a shift in carbon allocation toward the creation of reserves, at the expense of growth, to cover the increased risk induced by tapping (repeated wounding and loss of C in latex). Starch was lower in bark than in wood, whereas it was the contrary for soluble sugars. The resulting NSC was twice as low and less variable in bark than in wood. Although latex regeneration occurs in the bark, changes related to latex tapping were more marked in wood than in bark. From seasonal dynamics and differences between the two sides of the trunk in response to tapping, we concluded that starch in wood behaved as the long-term reserve compartment at the whole trunk level, whereas starch in bark was a local buffer. Soluble sugars behaved like an intermediate, ready-to-use compartment in both wood and bark. Finally, the dynamics of carbohydrate reserves appears a relevant parameter to assess the long-term performance of latex tapping systems.

Water yam (Dioscorea alata L.) is one of the most Popular tuber crops in the tropics. Although it is well known that photoperiod greatly affects yam development, little information is available on the effect of temperature on different developmental phases, and there has been no attempt to model the effect of both environmental factors. The aim of this study was to assess the combined effect of photoperiod and temperature on the development of two early maturing varieties of yam with similar growth pattern. For this We used a model proposed for potatoes grown under tropical conditions. Experimental information was obtained from 15 field experiments carried out in Guadeloupe (French West Indies) covering a wide range of planting dates. Two yam phases were analyzed: from emergence (EM) to tuber initiation (TI), and from TI to harvest (HA). The EM-HA period varied from 3 to 6 months, with the longer cycles corresponding to early planting dates (e.g. April). On average, EM-TI represented one-third of the EM-HA period, and was mainly affected by photoperiod and to a lesser extent by temperature. Both factors also affected the duration of TI-HA but their effects were less noticeable. The observed mean temperature during TI-HA was near the estimated optimum and its effect was less than that of photoperiod. The variation of the phase duration was higher for EM-TI (CV 45%) than for TI-HA (8%), which was satisfactorily explained by the model, For EM-TI there was a positive interaction between both environmental factors which, together with the greater influence of climatic conditions, resulted in much variability in its duration. For TI-HA, a small influence of environmental factors, coupled with a compensatory effect between environmental factors, resulted in a relatively smaller variation in the duration of this phase. The model estimated satisfactorily the dates and the duration of each phase. The root mean square error (RMSE) was 7.4 d which corresponded to 13% and 7% of the observed mean duration of EM-TI and TI-HA, respectively. Our results showed that small changes in photoperiod and temperature, which are very usual in the tropics, have a big effect on the tested yam varieties. We also showed that the model applied in this study can be a useful tool to predict yam development for management purposes as well as for the modeling of yam growth. (C) 2009 Elsevier B.V. All rights reserved.

UMR AGAP - équipe AFEF - Architecture et fonctionnement des espèces fruitières; International audience; The onset of the growing season of trees has been earlier by 2.3 days per decade during the last 40 years in temperate Europe because of global warming. The effect of temperature on plant phenology is, however, not linear because temperature has a dual effect on bud development. On one hand, low temperatures are necessary to break bud endodormancy, and, on the other hand, higher temperatures are necessary to promote bud cell growth afterward. Different process-based models have been developed in the last decades to predict the date of budbreak of woody species. They predict that global warming should delay or compromise endodormancy break at the species equatorward range limits leading to a delay or even impossibility to flower or set new leaves. These models are classically parameterized with flowering or budbreak dates only, with no information on the endodormancy break date because this information is very scarce. Here, we evaluated the efficiency of a set of phenological models to accurately predict the endodormancy break dates of three fruit trees. Our results show that models calibrated solely with budbreak dates usually do not accurately predict the endodormancy break date. Providing endodormancy break date for the model parameterization results in much more accurate prediction of this latter, with, however, a higher error than that on budbreak dates. Most importantly, we show that models not calibrated with endodormancy break dates can generate large discrepancies in forecasted budbreak dates when using climate scenarios as compared to models calibrated with endodormancy break dates. This discrepancy increases with mean annual temperature and is therefore the strongest after 2050 in the southernmost regions. Our results claim for the urgent need of massive measurements of endodormancy break dates in forest and fruit trees to yield more robust projections of phenological changes in a near future.

Biomass equations are a helpful tool to estimate the tree and stand biomass production and standing stock. Such estimations are of great interest for science but also of great importance for global reports on the carbon cycle and the global climate system. Even though there are various collections and generic meta-analyses available with biomass equations for mature trees, reports on biomass equations for juvenile trees (seedlings and saplings) are mainly missing. Against the background of an increasing amount of reforestation and afforestation projects and forests in young successional stages, such equations are required. In this study we have collected data from various studies on the aboveground woody biomass of 19 common tree species growing in Europe. The aim of this paper was to calculate species-specific biomass equations for the aboveground woody biomass of single trees in dependence of root-collar-diameter (RCD), height (H) and the combination of the two (RCD2 H). Next to calculating species-specific biomass equations for the species available in the dataset, we also calculated generic biomass equations for all broadleaved species and all conifer species. The biomass equations should be a contribution to the pool of published biomass equations, whereas the novelty is here that the equations were exclusively derived for young trees. We thank the national research project “Ecosystem Services of Natural Forests at Forestry and Climate Policy (FKZ 3511 84 0200),” from the Federal Agency for Nature Conservation (BfN) of the Federal Ministry for the Environment Nature Conservation and Nuclear Safety (BMU) for funding this project.

To improve source–sink relationship based carbon-allocation models, the basic proportional model was extended to account for a well-known effect of individual source to sink distances: among different sinks of similar characteristics, the more distant from the source, the lower the allocation coefficient. This was achieved through multiplication of the sink strength value by a coefficient that is proportional to a decreasing, simple function of distance, f; the power form was chosen for both simplicity and theoretical reasons. The resulting model was parameterized and evaluated on the empirical allocation matrix of the ECOPHYS model, after grouping together several individual, small sinks of similar nature and close location to remove any phyllotaxy-related bias. Both goodness of fit and predictive value were significantly improved compared with the basic proportional model (f = constant). The f-extended model yielded even better results if segments of different nature or age on the source to sink pathway were assigned different weights in the expression of distance, whereas the default expression of f, with an exponent of –1 and no additive constant, was optimal with no further parameter required. Thus, only 7 parameters (3 for pathway segment weights and 4 for sink strength values) were sufficient to retrieve the original 68 independent experimental allocation coefficients with a reasonable degree of accuracy. Pathway segment weights likely reflect both intrinsic transport pathway properties and situation within the plant architecture; this is discussed in relation to the possibilities of generalization and practical use of the model.

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.

Summary In winter and spring, walnut trees exhibit variations of sugar content in the vascular sap. According to their location, the vessel-associated cells (VACs, also called contact cells) could control nutrient exchanges between the storage parenchyma and the xylem vessels. According to the literature, the recovery of sap (influx) occurs at the VAC/vessel interface via an H+/sugar symport that depends on the transmembrane pH gradient generated by the plasma membrane H+-ATPase. The aim of this study was to investigate the ATPase activity, using a perfusion technique that allows the use of several effectors: carbonyl cyanide m-chlorophenylhydrazone (CCCP) and fusicoccin (FC). During winter, the uncoupler CCCP revealed a low pH gradient between the xylem vessels and the vessel-associated cells. Under these conditions, FC, an activator of the H+-ATPase, had no effect on the pH of the perfusion solution, suggesting that the enzyme could be lightly active. In contrast, close to bud break, a high pH gradient was revealed by the use of CCCP, and an acidification of the perfusion solution was observed in the presence of FC. Moreover, cytochemical investigation showed high activity of two respiratory enzymes located in mitochondria: NAD-dependent isocitrate dehydrogenase and succinate dehydrogenase. The hypothesis is that in spring this high respiratory activity of VACs provides a consequent increase in available ATP that can be utilized by the plasma membrane H+-ATPase.

International audience; SIMWAL (SIMulated WALnut) is a structural-functional tree model developed for single young walnut tree (Juglans sp.). It simulates the 3D structure dynamics of the tree, and biomass partitioning among its different organs, for a period ranging from a few months to several years, according to climatic conditions (temperature, radiation and air CO$_2$ concentration) and pruning. The aerial part of the tree is represented by axes split into growth units, inter-nodes, buds and leaves. The root system is described very coarsely by three compartments (taproot, coarse root and fine root). Only carbon-related physiological processes, i.e., radiation interception, photosynthesis, respiration, photosynthate allocation, and reserve storage and mobilisation are taken into account. Water and mineral nutrients are assumed to be optimal. We describe the model, and present preliminary tests of its ability to simulate tree architecture dynamics and carbon balance compared with field observations. Data requirements, and limits and improvements of the model are discussed.; SIMWAL : un modèle d'arbre structure-fonction simulant la croissance d'un noyer en fonction du climat et de la taille. SIMWAL est un modèle d'arbre structure-fonction développé pour le jeune noyer (Juglans sp.). Il simule l'évolution de la structure 3D de l'arbre et la répartition de la biomasse entre ses différents organes, pour des périodes allant de quelques mois à quelques années, en fonction des conditions climatiques (température, radiation, concentration dans l'air du CO$_2$) et des opérations de taille. La partie aérienne de l'arbre est représentée par des axes, eux-mêmes divisés en unités de croissance, entrenœuds, bourgeons et feuilles. Le système racinaire est plus grossièrement décrit en trois compartiments : pivot, racines moyennes et racines fines. Seuls les processus physiologiques relatifs au carbone sont pris en compte : interception lumineuse, photosynthèse, respiration, répartition carbonée, et stockage et mobilisation des réserves. L'eau et les éléments minéraux sont considérés à l'optimum. Nous décrivons en détail le modèle et présentons quelques tests préliminaires visant à vérifier sa capacité à simuler l'évolution de l'architecture d'un arbre et son bilan de carbone en comparaison à des observations de terrain. Le manque de certaines connaissances, les limites et les améliorations possibles du modèle sont discutés.

Tree Physiol. ISI Document Delivery No.: 271EP Times Cited: 0 Cited Reference Count: 61 Cited References: Aitken SN, 1996, FOREST ECOL MANAG, V80, P187, DOI 10.1016/0378-1127(95)03609-1 Améglio T., 2000, Dormancy in plants: from whole plant behaviour to cellular control., P109, DOI 10.1079/9780851994475.0109 AMEGLIO T, 1995, CR ACAD SCI III-VIE, V318, P351 Ameglio T, 2002, TREE PHYSIOL, V22, P1211 Ameglio T, 2001, J EXP BOT, V52, P2135 Ameglio T, 2004, TREE PHYSIOL, V24, P785 Andergassen S, 2002, PLANT ECOL, V161, P207, DOI 10.1023/A:1020365422879 Aronsson A, 1975, STUD FOREST SUECICA, V128, P1 Bertrand A, 1997, TREE PHYSIOL, V17, P31 BIGRAS FJ, 1994, CAN J FOREST RES, V24, P1039 Bryant G, 2001, SEED SCI RES, V11, P17, DOI 10.1079/SSR200056 CALME S, 1994, TREE PHYSIOL, V14, P1313 Cavender-Bares Jeannine, 2005, P401, DOI 10.1016/B978-012088457-5/50021-6 Charrier G, 2011, INT J BIOMETEOROL, V55, P763, DOI 10.1007/s00484-011-0470-1 Charrier G, 2011, ENVIRON EXP BOT, V72, P351, DOI 10.1016/j.envexpbot.2010.12.019 Christensen JH, 2007, CLIMATIC CHANGE, V81, P7, DOI 10.1007/s10584-006-9210-7 COTTIGNIES A, 1990, CR ACAD SCI III-VIE, V310, P211 Crowe LM, 2002, COMP BIOCHEM PHYS A, V131, P505, DOI 10.1016/S1095-6433(01)00503-7 El Zein R, 2011, TREE PHYSIOL, V31, P843, DOI 10.1093/treephys/tpr074 Ewers FW, 2001, TREE PHYSIOL, V21, P1123 Greer DH, 2000, TREE PHYSIOL, V20, P107 GREER DH, 1982, AUST J PLANT PHYSIOL, V9, P333 GUSTA LV, 1975, PLANT PHYSIOL, V56, P707, DOI 10.1104/pp.56.5.707 Gusta LV, 2004, PLANT PHYSIOL, V135, P1642, DOI 10.1104/pp.103.028308 HANSEN J, 1988, BOT ACTA, V101, P76 JUNTTILA O, 1983, CAN J BOT, V61, P164 Klotke J, 2004, PLANT CELL ENVIRON, V27, P1395, DOI 10.1111/j.1365-3040.2004.01242.x Kreyling J, 2012, ENVIRON EXP BOT, V78, P99, DOI 10.1016/j.envexpbot.2011.12.026 LARCHER W., 1987, ECOL STUD, V62, P321 Lee SH, 2012, PLANT PHYSIOL, V159, P479, DOI 10.1104/pp.112.194506 Leinonen I, 1996, ANN BOT-LONDON, V78, P687, DOI 10.1006/anbo.1996.0178 Lenne T, 2007, BBA-BIOMEMBRANES, V1768, P1019, DOI 10.1016/j.bbamem.2007.01.008 Lenne T, 2009, J PHYS CHEM B, V113, P2486, DOI 10.1021/jp808670t Levitt J, 1980, RESPONSES PLANTS ENV, V1 Luoranen J, 2004, CAN J FOREST RES, V34, P1108, DOI 10.1139/X03-285 Morin X, 2007, TREE PHYSIOL, V27, P817 Neuner G, 2010, PROTOPLASMA, V243, P145, DOI 10.1007/s00709-009-0052-9 Ogren E, 1997, TREE PHYSIOL, V17, P47 Pavel EW, 1998, PHYSIOL PLANTARUM, V104, P525, DOI 10.1034/j.1399-3054.1998.1040402.x POGOSIAN K. S., 1969, Low Temperature Science, V27, P125 Poirier M, 2010, TREE PHYSIOL, V30, P1555, DOI 10.1093/treephys/tpq087 Poirier M, 2008, THESIS U BLAISE PASC, P307 R Development Core Team, 2005, R LANG ENV STAT COMP Repo T, 1989, SCAND J FOREST RES, V4, P67, DOI 10.1080/02827588909382547 SAKAI A, 1968, CRYOBIOLOGY, V5, P160, DOI 10.1016/S0011-2240(68)80161-0 SAKAI AKIRA, 1962, CONTRIB INST LOW TEMP SCI HOKKAIDO UNIV SER B, V11, P1 SCHWARZ W, 1970, Flora (Jena), V159, P258 Stattin E, 2012, NEW FOREST, V43, P665, DOI 10.1007/s11056-012-9320-1 Stattin E, 2000, CAN J FOREST RES, V30, P964, DOI 10.1139/cjfr-30-6-964 SUTINEN ML, 1992, TREE PHYSIOL, V11, P241 TANINO K, 1990, PLANT PHYSIOL, V93, P460, DOI 10.1104/pp.93.2.460 Thomas FM, 2004, ANN FOREST SCI, V61, P455, DOI 10.1051/forest:2004039 Tinus RW, 2000, TREE PHYSIOL, V20, P1097 Turcotte A, 2009, AGR FOREST METEOROL, V149, P1403, DOI 10.1016/j.agrformet.2009.03.010 Wan XC, 2001, TREE PHYSIOL, V21, P691 Weiser CJ, 1970, COLD HARDINESS DORMA, P403 WITT W, 1994, J PLANT PHYSIOL, V143, P625 Wolfe J, 2002, CRYOLETTERS, V23, P157 ZHANG MIN, 1987, CAN J BOT, V65, P710 Zuther E, 2012, PLANT CELL ENVIRON, V35, P1860, DOI 10.1111/j.1365-3040.2012.02522.x Zweifel R, 2000, AGR FOREST METEOROL, V102, P213, DOI 10.1016/S0168-1923(00)00135-0 Charrier, Guillaume Poirier, Magalie Bonhomme, Marc Lacointe, Andre Ameglio, Thierry French Ministry of Research This work was funded by a doctoral grant from the French Ministry of Research to G.C. Oxford univ press Oxford; In the literature, frost hardiness (FH) studies in trees have often been restricted to one organ (buds, leaves, needles or twigs). To extend our knowledge and gain a unified view, FH differences between organs and tissues or throughout the life of the tree have to be characterized in relation to physiological changes. In this study, different organs and tissues of young potted and mature orchard walnut trees (Juglans regia L.) were compared for seasonal changes in FH during different years. FH was assessed using the electrolyte leakage method. Physiological parameters were concomitantly monitored focusing on two significant traits: water content (WC) and carbohydrate content (glucose + fructose + sucrose, GFS). No seasonal variation in FH was observed in the root system, but acclimation and deacclimation were observed aboveground. Among organs and tissues, cold sensitivity levels were different in deep winter, with buds most sensitive and bark most resistant, but acclimation/deacclimation dynamics followed similar patterns. Physiological variation was also similar among organs: FH increased when WC decreased and/or soluble carbohydrates increased. Based on these results, relations between soluble carbohydrate content, WC and FH were calculated independently or in interaction. The key results were that: (i) the relationship between FH and physiological parameters (GFS and WC), which had previously been shown for branches only, could be generalized to all aboveground organs; (ii) lower WC increased the cryoprotective effect of GFS, showing a synergic effect of the two factors; (iii) the best fit was a non-linear function of WC and GFS, yielding a predictive model with an root mean square error of 5.07 degrees C on an independent dataset and 2.59 degrees C for the most sensitive stages; and (iv) the same parameters used for all organs yielded a unified model of FH depending on physiology, although the variability of GFS or WC was wide. The model should be of value for predicting FH in walnut independently of previous growing conditions (i.e., after sublethal stress accumulation).

Carbon (C) and nitrogen (N) autotrophy was studied in walnut seedlings (Juglans regia L.) grown in controlled conditions (22 o C, 12 h, 90% RH) during the first two months of development. Seedlings were submitted to simultaneous 13 CO 2 (1.3 13 C atom %) and 13 NO 3 (2.0 15 N atom %) long-term labelling, to quantify the assimilation and partitioning of recent C and N in growing organs compared to the mobilization and use of C and N derived from the seed reserves. The first organ to develop, at day 8, was the taproot, which built its biomass essentially during the first month with the help of C and N reserves of the seed

Leaf Wetness Duration (LWD) is the main parameter involved in the apple scab development in apple orchards. LWD values can be inferred from weather conditions by solving a non-linear energy balance equation. Such energy balance equation is usually linearized leading to the Penman-Monteith equations. However this approximation leads to significant errors in the evaporative term and by consequence in the LWD computation. In this work, consequences of the use of the Penman-Monteith equations instead of the exact solution were firstly investigated on single droplets and compared to experimental values obtained in controlled and constant conditions. For these studies, the LWD is overestimated by 12% when Penman-Monteith equations were used. The ability of LWD estimate from the linearized energy balance equation to forecast risk infection of a leaf by V. inaequalis conidia was investigated. For some conditions, the error in LWD estimate by Penman-Monteith approached 6 h, leading to an incorrect risk assessment of apple scab development.

A mechanistic model of carbon partitioning, based on the Münch hypothesis of phloem transport and implemented with PIAF-Münch modelling platform (Lacointe and Minchin 2008), was tested for an architecture more complex than any tested previously. Using 11C to label photosynthate, responses in transport of photosynthate within a heavily pruned dwarf bean plant (Phaseolus vulgaris L.) to changes in source and sink activities were compared with model predictions. The observed treatment responses were successfully predicted. However, the observations could not be completely explained if the modelled stem contained only one phloem pathway: tracer from a labelled leaf was always detected in both shoot apex and root, whichever of the two leaves was labelled. This shows that bidirectional flow occurred within the stem, with solute moving simultaneously in both directions. Nevertheless, a model architecture with very little more complexity could incorporate such bidirectional flow. We concluded that the model could explain the observations, and that the PIAF-Münch model platform can be expected to describe partitioning in even more complex architectures.

It has been hypothesized that the increase in temperature in this century could lead to an increase in frost damage to plant tissues. Several models have been proposed to describe the development of cold hardiness, but never taking into account extreme climatic and/or physiological events. Our results on walnut tree (Juglans regia L.) show that cold hardiness was best correlated with average daily temperatures minimal temperatures over the last 15 days before sampling ((T) over bar (min 15 days)), indicating that the freezing tolerance depended on the tree's climatic history. Moreover, this study also shows that the accumulation of sucrose and the water content (WC) decrease are an essential step towards cold hardiness. Thus, a simple linear model based on climatic ((T) over bar (min 15 days)) and physiological (soluble sugars, WC) explanatory variables was developed to predict the cold hardiness level in walnut stem at any time during the leafless period. Each of the three input variables can be assigned a specific role contributing to the simulated function, cold hardiness. The extent and robustness of this relation was assessed on extreme physiological events on walnut trees bearing three main branches. On each tree, one branch was defoliated to limit the local carbohydrate and transpiration, one was girdled to increase local carbohydrate and prevent carbohydrate export and the third one was kept untreated as control. As expected, these treatments impacted both local carbon reserves and WC in the stems born by each main branch in comparison with the control on the same tree. The impact of these treatments on stem's freezing tolerance, as evaluated by an electrolyte leakage method (LT(50)), confirmed the direct impact of soluble sugar and WC on cold hardiness over a wide range of carbohydrate and WC. This is discussed in relation to the branch autonomy theory for carbon but also for water during summer growth and winter periods. The present study demonstrates the importance of physiological parameters in the prediction of cold hardiness and proposes a way to model cold hardiness with extreme climatic and/or physiological events.

International audience; Background and Aims: In rain forests, sapling survival is highly dependent on the regulation of trunk slenderness (height/diameter ratio): shade-intolerant species have to grow in height as fast as possible to reach the canopy but also have to withstand mechanical loadings (wind and their own weight) to avoid buckling. Recent studies suggest that mechanosensing is essential to control tree dimensions and stability-related morphogenesis. Differences in species slenderness have been observed among rainforest trees; the present study thus investigates whether species with different slenderness and growth habits exhibit differences in mechanosensitivity. Methods: Recent studies have led to a model of mechanosensing (sum-of-strains model) that predicts a quantitative relationship between the applied sum of longitudinal strains and the plant's responses in the case of a single bending. Saplings of five different neotropical species (Eperua falcata, E. grandiflora, Tachigali melinonii, Symphonia globulifera and Bauhinia guianensis) were subjected to a regimen of controlled mechanical loading phases (bending) alternating with still phases over a period of 2 months. Mechanical loading was controlled in terms of strains and the five species were subjected to the same range of sum of strains. The application of the sum-of-strain model led to a dose-response curve for each species. Dose-response curves were then compared between tested species. Key Results: The model of mechanosensing (sum-of-strain model) applied in the case of multiple bending as long as the bending frequency was low. A comparison of dose-response curves for each species demonstrated differences in the stimulus threshold, suggesting two groups of responses among the species. Interestingly, the liana species B. guianensis exhibited a higher threshold than other Leguminosae species tested. Conclusions: This study provides a conceptual framework to study variability in plant mechanosensing and demonstrated interspecific variability in mechanosensing.

Evaluation of endodormancy release models in temperate fruit tree species has generally been indirect, as a component of the global evaluation of bud break-date models that include an endodormancy release module. Due to the lack of data, their direct evaluation based on the knowledge of the dynamics of bud dormancy states has been very succinct. Consequently, these models have not been optimized, except for the 'amount of chill units required to break endodormancy' taken as the genotype specific parameter. As part of different studies on bud state during the rest period, near 30 yearly dynamics of the dormancy state of 'Redhaven' peach vegetative buds under natural conditions have been recorded, based on the biological 'one node cuttings' test under standard conditions (25C). These data were used to optimize classic models (Weinberger-W; Utah-U, etc.) of endodormancy release in fruit tree species with temperature as the only input variable, in order to compare their goodness of fit and their predictive capacity. The fitted parameters were the 'amount of chill units required to break endodormancy', those of the temperature functions f(T) describing the rate of endodormancy development and, when relevant, the starting date of endodormancy release. The results and practical conclusions for predicting dormancy release date are presented. The optimization method used was the minimization of the mean square error (MSE) between the observed (biological test) and the computed dates of endodormancy release and was carried out using the nls module of R software. As main results: the optimization of the W model resulted in an increase of the standard threshold temperature; the endodormancy release effect of negative temperatures (up to freezing injury) in the optimized form of U model that was not taken into account in the standard form; the sharp transition from positive to negative f(T) values for the temperatures above 12C, in the optimized U model; and the fact that the model of Weinberger is a particular form of smoothed Utah model.

Bud break pattern is a key determinant of tree architecture. The mechanisms leading to the precedence of certain buds over the others are not yet fully explained, but the availability of soluble sugars may play a significant role, especially those in the xylem sap at the onset of the growing period. Here, we measured carbon availability in the different tissues (bud, xylem and bark). To assess the capacity of buds to use the xylem sap carbohydrates, the fluxes between xylem vessels and parenchyma cells, bark and buds of walnut (Juglans regia cv 'Franquette') were measured during the rest period until bud break. This uptake capacity varies according to the temperature, the sugar and the position on the branch of the fragment studied. Between December and March, in xylem tissues, the active component of sucrose uptake was predominant compared with diffusion (90% of the total uptake), whereas the active component accounted for more moderate amounts in buds (50% of the uptake). The active uptake of hexoses took place belatedly (April) in xylem. The flow rates between xylem vessels and buds increased 1 month before bud break and reached 2000 microg sucrose h(-)(1) g DW(-)(1). Fluxes seemed to depend on bud position on the branch. However, this study strongly suggests that they were mainly dependent on the sink strength of the buds and on the sink competition between bud, xylem parenchyma and bark.

† Background and Aims The major economic product of Hevea brasiliensis is a rubber-containing cytoplasm (latex), which flows out of laticifers (latex cells) when the bark is tapped. The latex yield is stimulated by ethylene. Sucrose, the unique precursor of rubber synthesis, must cross the plasma membrane through specific sucrose transporters before being metabolized in the laticifers. The relative importance of sucrose transporters in determining latex yield is unknown. Here, the effects of ethylene (by application of Ethrel w ) on sucrose transporter gene expression in the inner bark tissues and latex cells of H. brasiliensis are described. † Methods Experiments, including cloning sucrose transporters, real time RT-PCR and in situ hybridization, were carried out on virgin (untapped) trees, treated or untreated with the latex yield stimulant Ethrel. † Key Results Seven putative full-length cDNAs of sucrose transporters were cloned from a latex-specific cDNA library. These transporters belong to all SUT (sucrose transporter) groups and differ by their basal gene expression in latex and inner soft bark, with a predominance of HbSUT1A and HbSUT1B. Of these sucrose transporters, only HbSUT1A and HbSUT2A were distinctly increased by ethylene. Moreover, this increase was shown to be specific to laticifers and to ethylene application. † Conclusion The data and all previous information on sucrose transport show that HbSUT1A and HbSUT2A are related to the increase in sucrose import into laticifers, required for the stimulation of latex yield by ethylene in virgin trees.

The function of the plant’s vasculature, incorporating both phloem and xylem, is of fundamental importance to the survival of all higher plants. Although the physiological mechanism involved in these two transport pathways has been known for some time, quantitative modelling of this has been slow to develop. 1-D continuous models have shown that the proposed mechanisms are quantitatively plausible (Thompson and Holbrook 2003) but more complex geometries (architectures) have remained out of reach because of mathematical difficulties. In this work, we extend the alternative modular approach by Daudet et al. (2002) using recently developed numerical tools which allow us to model complex architectures. After a full description of the extended model, we first show that it efficiently reproduces the results of the continuous approach when applied to the same simple configurations. The model is then applied to a more complex configuration with two sinks, confirming that sink priority is an emergent property of the Münch flow as earlier found with a minimalist model (Minchin et al. 1993). It is further shown how source leaf transpiration can change the relative carbon allocation rates among sinks.

International audience; The carbon economy of young walnut grafted trees in their second growing year, was investigated through carbohydrate analysis and 14C-labelling experiments. In October, 3 weeks before leaf-fall, starch and soluble sugars represented up to 25% of the total dry matter, with starch highly predominant among them. Sucrose was more abundant in the bark than in the wood, while the starch content was similar in both compartments. The reserves were located in the wood and in the innermost pith. Between October and January, the minor roots (ø < 15mm) underwent a significant depletion of their August-labelled reserves, whereas the tap-root was the main organ depleted between January and May (early growth resumption). Both labelling experiments, and root respiration monitoring, provided some information about the timing of translocation processes and the early metabolism of the translocation compounds.

Carbohydrate reserve storage in trees is usually considered a passive function, essentially buffering temporary discrepancies between carbon availability and demand in the annual cycle. Recently, however, the concept has emerged that storage might be a process that competes with other active sinks for assimilate. We tested the validity of this concept in Hevea brasiliensis Mull. Arg. (rubber) trees, a species in which carbon availability can be manipulated by tapping, which induces latex regeneration, a high carbon-cost activity. The annual dynamics of carbohydrate reserves were followed during three situations of decreasing carbon availability: control (no tapping), tapped and tapped with Ethephon stimulation. In untapped control trees, starch and sucrose were the main carbohydrate compounds. Total nonstructural carbohydrates (TNC), particularly starch, were depleted following bud break and re-foliation, resulting in an acropetal gradient of decreasing starch concentration in the stem wood. During the vegetative season, TNC concentration increased. At the end of the vegetative season, there were almost no differences in TNC concentration along the trunk. In tapped trees, the vertical gradient of starch concentration was locally disturbed by the presence of the tapping cut. However, the main effect of tapping was a dramatic increase in TNC concentration, particularly starch, throughout the trunk and in the root. The difference in TNC concentration between tapped and untapped trees was highest when latex production was highest (October); the difference was noticeable even in areas of the trees that are unlikely to be directly involved in latex regeneration, and it was enhanced by Ethephon stimulation, which is known to increase latex metabolism and flow duration. Thus, contrary to what could be expected if reserves serve as a passive buffer, a decrease in carbohydrate availability resulted in a net increase in carbohydrate reserves at the trunk scale. Such behavior supports the view that trees tend to adjust the amount of carbohydrate reserves stored to the level of metabolic demand, at the possible expense of growth.

The variations of stem diameter as they can be accurately measured by Linear Variable Differential Transformer (LVDT) reflect the addition of four components: irreversible radial growth, reversible living-cell dehydration/rehydration, thermal expansion and contraction, and expansion of dead conducting elements due to the increase and relaxation of internal tensions. The correct interpretation of LVDT signals, with respect to the practical applications, should make an exact distinction between these four components. This paper describes a set of two experiments with potted hybrid walnut trees. Double girdling, water stress, and duration of the day versus night periods were used in the phytotron as experimental factors to induce variations of the carbon and water status of plant tissues. The latter were assessed, respectively, by water potential and transpiration, and by local stem respiration and carbohydrate content. The results are interpreted in terms of carbon or water limitation effects on stem diameter variations where radial growth and tissue elasticity could be distinguished. Moreover, they suggest no or very low involvement of CO2 originating from a distance, i.e. carried by the transpirational flux of xylem sap, in the total stem CO2 efflux rate.

Endogenous carbohydrate contents and availability in structures near the meristematic zone (scales, cushion, part of stem) during winter with and without sufficient chilling were investigated. Insufficient chilling is known to provoke an abnormal pattern of budbreak and development in temperate zone fruit trees cultivated in warm climates. Mild temperatures applied during winter to strictly floral buds, as is the case in peach, are known to provoke the necrosis of flower primordia in the buds. One of the hypotheses to explain this is that carbohydrate fluxes to the buds are impeded and/or that internal reserves are completely used up. Carbohydrate availability and bud survival were compared between peach trees under natural temperate conditions and under complete deprivation of cold, by keeping trees in a greenhouse with temperature constantly above 15 °C from early October to the first sign of necrosis in flower buds. Concentrations and carbohydrate contents were compared during a little more than 2 months (beginning mid-December) before the first signs of necrosis in flower buds. Results showed that cold deprivation deeply disturbed carbohydrate dynamics, blocking the import of carbohydrates into vegetative and floral primordia, even though sucrose and sorbitol concentrations within these structures remained high. Conversely, soluble carbohydrates accumulated strongly in bud scales and in the cushion beneath the primordia. This suggests a strong diversion towards structures close to the primordia under cold deprivation. However, since sucrose and sorbitol concentrations in the floral primordia remained high, necrosis could be the consequence of the inability to use sucrose and sorbitol reserves rather than the result of a lack of available carbohydrates.

The impact of a heterogeneous within-crown light environment on carbon allocation was investigated on young walnut trees trained on two branches: one left in full sunlight, the other shaded until leaf fall resulting in 67% reduction in photosynthetically active radiation. In September, the two branches were separately labelled with 14CO2 and 13CO2, respectively, so that the photosynthates from each branch could be traced independently at the same time. Although some carbon movements could be detected within 5 d in both directions (including from the shaded branch to the sun branch), between-branch carbon movements were very limited: approximately 1% of the diurnal net assimilation of a branch. At this time of the year branch autonomy was nearly total, leading to increased relative respiratory losses and a moderate growth deficit in the shaded branch. The ratio of growth to reserve storage rate was only slightly affected, indicating that reserves acted not as a mere buffer for excess C but as an active sink for assimilates. In winter, branch autonomy was more questionable, as significant amounts of carbon were imported into both branches, possibly representing up to 10% of total branch reserves. Further within-plant carbon transfers occurred in spring, which totally abolished plant autonomy, as new shoots sprouted on each branch received significantly more C mobilized from tree-wide reserves than from local, mother-branch located reserves. This allowed great flexibility of tree response to environment changes at the yearly time scale. As phloem is considered not functional in winter, it is suggested that xylem is involved as the pathway for carbohydrate movements at this time of the year. This is in agreement with other results regarding sugar exchanges between the xylem vessels and the neighbouring reserve parenchyma tissues.

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A model of within-plant carbon allocation is proposed which makes a generalized use of the Münch mechanism to integrate carbon and water functions and their involvement in growth limitations. The plant is envisioned as a branched network of resistive pathways (phloem and xylem) with nodal organs acting as sources and sinks for sucrose. Four elementary organs (leaf, stem, fruit, root) are described with their particular sink functions and hydraulic attributes. Given the rates of photosynthesis and transpiration and the hydraulic properties of the network as inputs, the model calculates the internal fluxes of water and sucrose. Xylem water potential (Psi), phloem sucrose concentration (C) and turgor pressure (P) are calculated everywhere in the network accounting for osmotic equilibrium between apoplasm and symplasm and coupled functioning of xylem and phloem. The fluxes of phloem and xylem saps are driven by the gradients of P and Psi, respectively. The fruit growth rate is assumed as turgor pressure dependent. To demonstrate its ability to address within-plant competition, the model is run with a simple-branched structure gathering three leaves, eight stem segments, three competing growing fruits and one root. The model was programmed with P-Spice, a software specifically designed for simulating electrical circuits but easily adaptable to physiology. Simulations of internal water fluxes, sucrose concentrations and fruit growth rates are given for different conditions of soil water availability and hydraulic resistances (sensitivity analysis). The discussion focuses on the potential interest of this approach in functional--structural plant models to address water stress-induced effects.

In south-central France, Walnut exhibited freezing tolerance by acclimation in the fall and deacclimation in the spring. This involved a sequence of active processes that result in well-defined changes in cellular composition (starch level in particular). At the same time, the response to freezing by dehydration of cells showed that Walnut trees present also freezing avoidance. We developed a new simple tool for testing cold hardiness and compared it with the classical LT 50 test (electrical leakage conductivity). The new test gave similar results. We observed also similar results for both tests when applied to colddeprived trees (in greenhouse >15°C). Frost hardiness was correlated with carbohydrate reserve status in the both cases (cold-deprived trees and orchard trees).

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International audience; Carbon assimilates flow from "source" areas such as leaves to "sink" areas where they are taken up and used. The assimilate fluxes from sources to sinks are mainly dependent on the source-sink distances and on the respective abilities of the different sinks to take up and use the assimilates that are available to them. The widely accepted, basic mechanism of assimilate movement by mass-flow, although conceptually simple, has so far proved too complex for practical modeling purposes in whole tree systems. Four main modeling approaches can be found in current models: (i) models involving empirically determined allocation coefficients; (ii) models based on growth rules, including functional balance or "goal-seeking" principles; (iii) transport-resistance models; (iv) models based on relative sink strength, with two main sub-classes: "hierarchical" and "proportional" models. These different model classes can be conceptually closer to each other than is readily apparent. They are presented in relation to their generality and ability to account for complex architectures or responses to environmental changes. The feedback relationship of allocation to growth is pointed out.; La répartition du carbone entre organes chez les arbres : processus de base et représentation dans les modèles "structure-fonction". Les assimilats carbonés circulent de zones "sources" telles les feuilles, vers des "puits" où ils sont prélevés et utilisés. Ces flux d'assimilats dépendent principalement des distances entre sources et puits ainsi que des capacités respectives des différents puits à prélever et utiliser les assimilats disponibles. Il est largement admis aujourd'hui que le mécanisme de base de la translocation est un flux de masse. Mais malgré sa simplicité conceptuelle, la simulation de ce processus implique des calculs trop complexes pour une modélisation pratique. On trouve actuellement quatre approches modélisatrices principales : (i) l'utilisation de coefficients d'allocation empiriques ; (ii) la mise en œuvre de règles de croissance, notamment des équilibres fonctionnels et autres principes téléonomiques ; (iii) l'analogie électrique avec des résistances ; (iv) des règles de répartition basées sur les "forces" relatives des différents puits (modèles hiérarchiques et modèles proportionnels). Ces diverses classes de modèles sont parfois conceptuellement moins éloignées les unes des autres qu'il n'y paraît. Elles sont présentées en relation avec leur généralité et leur capacité à rendre compte d'une architecture complexe ou des influences de l'environnement. On souligne l'importance des rétroactions entre allocation et croissance.

International audience; An ecophysiological growth process model, called INCA, for simulating the growth and development of a young walnut tree (Juglans regia L.) during three or four years, is presented. This tool, currently under development, aims at integrating architectural and physiological knowledge of the processes involved, in order to give a more rational understanding of the pruning operation. The model describes a simple three-dimensional representation of tree crown, solar radiation interception, photosynthesis, respiration, growth and partitioning of assimilates to leaves, stems, branches and roots. It supports the hypothesis that the tree grows as a collection of semiautonomous, interacting organs that compete for resources, based on daily sink strengths and proximity to sources. The actual growth rate of organs is not predetermined by empirical data, but reflects the pattern of available resources. The major driving variables are solar radiation, temperature, topological, geometrical and physiological factors. Outputs are hourly and daily photosynthate production and respiration, daily dimensional growth, starch sorage, biomass production and total number of different types of organ. The user can interact or override any or all of the input variables to examine the effects of such changes on photosynthate production and growth. Within INCA, the tree entities and the surrounding environment are structured in a frame-based representation whereas the processes are coded in a rule-based language. The simulation mechanism is primarilybased on the rule chaining capabilities of an inference engine.; L'article présente un modèle écophysiologique de croissance et développement du noyer. INCA vise à intégrer les connaissances architecturales et physiologiques que l'on a sur le sujet, de manière à optimiser les opérations de taille.

We studied seasonal variation in xylem sap pH of Juglans regia L. Our main objectives were to (1) test the effect of temperature on seasonal changes in xylem sap pH and (2) study the involvement of plasma membrane H+-ATPase of vessel-associated cells in the control of sap pH. For this purpose, orchard-grown trees were compared with trees grown in a heated (> or = 15 degrees C) greenhouse. During autumn, sap pH was not directly influenced by temperature. A seasonal change in H+-ATPase activity resulting from seasonal variation in the amount of protein was measured in orchard-grown trees, whereas no significant seasonal changes were recorded in greenhouse-grown trees. Our data suggest that H+-ATPase does not regulate xylem sap pH directly by donating protons to the xylem, but by facilitating secondary active H+/sugar transport, among other mechanisms.

Rubber tree (Hevea brasiliensis Mull. Arg.) radial growth dynamics were monitored with displacement sensors, together with latex production, to investigate three aspects of the dual production of latex and wood: (1) the usefulness of fine-scale dendrometric measurements as a physiological tool to detect water shortage through radial growth; (2) the dynamic aspects, both at the seasonal and at the multi-year scale, of the competition between latex and wood production; and (3) the spatial distribution of radial growth rates around the tapping cut. Radial growth of untapped control trees started with the onset of the rainy season and lasted until the onset of the dry season, ceasing completely during the driest period. Displacement sensors provided a sensitive means of detecting water shortage, with a clear correlation between diameter variations and changes in water availability (both daily evapotranspiration and monthly rainfall) over the whole annual cycle. However, the correlation was significantly disturbed in tapped trees. After resumption of tapping, the radial growth rate dropped sharply within two weeks and the effect persisted throughout the whole season, so that the cumulative growth of tapped trees was about half that of untapped trees, with the cumulative growth deficit reaching 80% for the period from mid-June to November. This long-known negative impact of tapping on growth was much stronger in the second year of tapping than in the first, whereas latex production increased significantly between the first and second year of tapping. The increased latex production, which could not be ascribed to climatic conditions, shows that the establishment of an artificial latex sink is a progressive, long-term process likely involving many aspects of metabolism. As expected, ethylene significantly increased latex production in both years; however, ethylene had no effect on the growth rates of tapped trees. Radial growth was differentially affected at different locations around the tapping cut, with growth rates significantly lower in the tapped panel than in the untapped panel, and higher above the cut than below the cut. Thus, caution is needed when deriving whole stem wood production from girth measurements at one location on the stem, especially from girth measurements made close to the tapping cut. This also provides new evidence for the location of the latex regeneration area in the tapped panel, below the cut.