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International audience; Plant transpiration is a major component of water fluxes in the critical zone, which needs to be better characterized to improve our ability to understand and model the hydrological cycle. In water-limited ecosystems such as those encountered on karst environments, climate-induced changes in transpiration are expected to be strongly influenced by the ability of the vegetation cover to resist or adapt to drought. However, because of the high heterogeneity of karst environments, the amount of water available for trees can change within a stand, which may lead to significant differences in drought vulnerability resistance between trees of the same species. So far it is not known if soil and subsoil environment influence the magnitude of deep water extraction, at the intra-specific scale. Here, we investigate the variability in deep water extraction for six individual Quercus ilex trees growing on a karst substrate in a Mediterranean forest. We combined three approaches: (i) electrical resistivity tomography to determine the variability of soil/subsoil characteristics, (ii) isotope tracing to determine the origin of water transpired by plants, and (iii) predawn and midday leaf water potential (Ψ) to assess the trees’ water stress and transpiration regulation. Along the summer season, deep water extraction increased with drought intensity. Deep water use varies between individuals and according to drought intensity. At moderate water stress levels, we found no significant relationship between the origin of xylem water and soil/subsoil characteristics or individual stress level. However, at the peak of the drought (average predawn Ψ < −2 MPa), individuals that had the least total available water in soil/subsoil (0–2 m) relied more on deep water and were also subject to less water stress. These results suggest that trees with less favorable soil/subsoil conditions (i.e. low water retention capacity) in the near surface (0–2 m) adapt their root systems to exploit deep water reserves more intensively so as to enhance their drought tolerance, while trees with more favorable surface conditions exhibit greater water stress and may be more vulnerable to extreme droughts because of a lower root development in deeper horizons.

Background and Aims Sugar concentration is a key determinant of fruit quality. Soluble sugars and starch concentrations in fruits vary greatly from one species to another. The aim of this study was to investigate similarities and differences in sugar accumulation strategies across ten contrasting fruit species using a modelling approach. Methods We developed a coarse-grained model of primary metabolism based on the description of the main metabolic and hydraulic processes (synthesis of compounds other than sugar and starch, synthesis and hydrolysis of starch, and water dilution) involved in the accumulation of soluble sugars during fruit development. Key Results Statistical analyses based on metabolic rates separated the species into six groups according to the rate of synthesis of compounds other than sugar and starch. Herbaceous species (cucumber, tomato, eggplant, pepper and strawberry) were characterized by a higher synthesis rate than woody species (apple, nectarine, clementine, grape and kiwifruit). Inspection of the dynamics of the processes involved in sugar accumulation revealed that net sugar importation, metabolism and dilution processes were remarkably synchronous in most herbaceous plants, whereas in kiwifruit, apple and nectarine, processes related to starch metabolism were temporally separated from other processes. Strawberry, clementine and grape showed a distinct dynamic compared with all other species. Conclusions Overall, these results provide fresh insights into species-specific regulatory strategies and into the role of starch metabolism in the accumulation of soluble sugars in fleshy fruits. In particular, inter-specific differences in development period shape the co-ordination of metabolic processes and affect priorities for carbon allocation across species. The six metabolic groups identified by our analysis do not show a clear separation into climacteric and non-climacteric species, possibly suggesting that the metabolic processes related to sugar concentration are not greatly affected by ethylene-associated events.

Background One of the key goals of fruit biology is to understand the factors that influence fruit growth and quality, ultimately with a view to manipulating them for improvement of fruit traits. Scope Primary metabolism, which is not only essential for growth but is also a major component of fruit quality, is an obvious target for improvement. However, metabolism is a moving target that undergoes marked changes throughout fruit growth and ripening. Conclusions Agricultural practice and breeding have successfully improved fruit metabolic traits, but both face the complexity of the interplay between development, metabolism and the environment. Thus, more fundamental knowledge is needed to identify further strategies for the manipulation of fruit metabolism. Nearly two decades of post-genomics approaches involving transcriptomics, proteomics and/or metabolomics have generated a lot of information about the behaviour of fruit metabolic networks. Today, the emergence of modelling tools is providing the opportunity to turn this information into a mechanistic understanding of fruits, and ultimately to design better fruits. Since high-quality data are a key requirement in modelling, a range of must-have parameters and variables is proposed.

sous-presse; International audience; Karst environments are unusual because their dry, stony and shallow soils seem to be unfavorable to vegetation, and yet they are often covered with forests. How can trees survive in these environments ? Where do they find the water that allows them to survive? This study uses midday and predawn water potentials and xylem water isotopes of branches to assess tree water status and the origin of transpired water. Monitoring was conducted during the summers of 2014 and 2015 in two dissimilar plots of Mediterranean forest located in karst environments. The results show that the three monitored tree species (Abies alba Mill, Fagus sylvatica L, and Quercus ilex L.) use deep water resources present in the karst vadose zone (unsaturated zone) more intensively during drier years. Quercus ilex, a species welladapted to water stress, which grows at the drier site, uses the deep water resource very early in the summer season. Conversely, the two other species exploit the deep water resource only during severe drought. These results open up new perspectives to a better understanding of ecohydrological equilibrium and to improved water balance modeling in karst forest settings.

Inter-individual variability of tree drought responses within a stand has received little attention. Here we explore whether the spatial variations in soil/subsoil properties assessed through electrical resistivity tomography (ERT) could explain variations in drought response traits among trees. We used ERT to compute the percent variation in resistivity (PVR) between dry and wet conditions as an indicator of spatial variability in total available water content. PVR was computed in two different depth ranges (0-2 and 2-5 m) for eleven Quercus ilex stools in a Mediterranean forest stand. PVR values were compared to biological traits, including tree water status (predawn water potential (Ψ)), leaf traits (δ 13 C, leaf mass area (LMA)), and canopy defoliation measured after intense drought. We found significant correlations between PVR and biological variables. For Ψ , the nature and strength of the correlations vary according to the level of drought intensity. The correlation between Ψ and PVR was positive during well-watered conditions in the upper layer (0-2 m) and during water-limited conditions in the deeper layer (2-5 m). During most severe droughts, however, the Ψ was negatively correlated with PVR in the upper layer. Trees with lower PVR in the upper layer were also associated with water use efficiency (higher δ 13 C), higher LMA, and a lower level of defoliation after extreme drought. Overall, our results indicate that local differences in soil/subsoil properties affect tree response to drought and suggest that less favorable soil/subsoil conditions (lower PVR) can lead to lower water stress during the driest period and to lower defoliation after extreme drought. Plausible explanations for this better acclimation include higher stomatal regulation and improved deep soil and subsoil water exploration by trees located in more adverse conditions. We encourage the development of ERT in ecological studies to further explore the interrelated relationships between soil/subsoil, climate, and tree functioning.