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Wood characteristics of trees grown in agroforestry systems are little studied, even if growth conditions are different from conventional stands. This work aimed to determine the impact of the agroforestry system on the heartwood formation process of hybrid walnut (Juglans regia × nigra) trees, especially the resulting extractive contents. Ethanol and water extractions were successively performed on wood samples taken across the diameter of the trunk of agroforestry (AF) and forest (FC) walnut trees to get the radial distribution of the extractive contents. All the samples were analyzed by NIR-spectroscopy and NIR-hyperspectral imaging. Statistical discriminant models were developed to classify the samples from both different forestry systems, according to their chemical composition. The results indicated no significant differences between the values of extractive contents of AF and FC walnut woods, whatever the radial position. At the intra-tree scale, the quantity of extractives does not increase significantly with the radial position. However, partial least squares-discriminant analysis (PLS-DA) regression models, developed with NIRS measurements, showed that significant chemical differences exist between AF and FC trees, especially for extractives composition and lignin content. This allowed to classify wood specimens from both forestry systems. These results were confirmed by hyperspectral camera analyses.

Wood characteristics of trees grown in agroforestry systems are still little studied, while their growth conditions are different from conventional stands. This work focused on the impact of the agroforestry system on the lignin/cellulose ratio of hybrid poplar trees. One disk sample was harvested on 6 agroforestry (AF) and 6 forest control (FC) poplar trees, at breast height ground level (1.30m). Every disk was analyzed by NIR-Hyperspectral imaging using a Specim FX17 (Specim, Spectral Imaging Ltd.). Images from hyperspectral camera analyses corresponding to absorbance spectra were collected at the wavelength of 1450 nm, attributed to first overtone O-H stretching vibration of lignin/extractives compounds, in order to clearly observe the chemical difference between AF and FC poplar woods. The results indicated significant difference between the chemical composition, based on estimated lignin content, of AF and FC poplar woods. The lignin content appeared to be lower in AF poplar wood than in FC poplar wood. These results could be explained by the different tree growing conditions between the both systems. AF poplar tended to produce more tension wood and more juvenile wood than FC poplar, which resulted in a lower concentration in lignin.

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In conifers, xylogenesis produces during a growing season a very characteristic tree-ring structure: large thin-walled earlywood cells followed by narrow thick-walled latewood cells. Although many factors influence the dynamics of differentiation and the final dimensions of xylem cells, the associated patterns of variation remain very stable from one year to the next. While radial growth is characterised by an S-shaped curve, the widths of xylem differentiation zones exhibit characteristic skewed bell-shaped curves. These elements suggest a strong internal control of xylogenesis. It has long been hypothesised that much of this regulation relies on a morphogenetic gradient of auxin. However, recent modelling works have shown that while this hypothesis could account for the dynamics of stem radial growth and the zonation of the developing xylem, it failed to reproduce the characteristic tree-ring structure. Here we investigated the hypothesis of a regulation by a crosstalk between auxin and a second biochemical signal, using dynamical modelling. We found that, in conifers, such a crosstalk is sufficient to simulate the characteristic features of wood formation dynamics, as well as the resulting tree-ring structure. In this model, auxin controls cell enlargement rates while another signal (e.g., cytokinin, TDIF) drives cell division and auxin polar transport.HighlightA dynamical model proves that two interacting signals (auxin, plus a cytokinin or the TDIF peptide) can drive wood formation dynamics and tree-ring structure development in conifers.

International audience; Lycophytes are the earliest diverging extant lineage of vascular plants, sister to all other vas-cular plants. Given that most species are adapted to ever-wet environments, it has been hypothesized that lycophytes, and by extension the common ancestor of all vascular plants, have few adaptations to drought. We investigated the responses to drought of key fitness-related traits such as stomatal regulation , shoot hydraulic conductance (K shoot) and stem xylem embolism resistance in Selaginella haematodes and S. pulcherrima, both native to tropical understory. During drought stomata in both species were found to close before declines in K shoot , with a 50% loss of K shoot occurring at À1.7 and À2.5 MPa in S. haematodes and S. pulcherrima, respectively. Direct observational methods revealed that the xylem of both species was resistant to embolism formation, with 50% of embolized xylem area occurring at À3.0 and À4.6 MPa in S. haematodes and S. pulcherrima, respectively. X-ray microcomputed tomogra-phy images of stems revealed that the decline in K shoot occurred with the formation of an air-filled lacuna, disconnecting the central vascular cylinder from the cortex. We propose that embolism-resistant xylem and large capacitance, provided by collapsing inner cortical cells, is essential for Selaginella survival during water deficit.

International audience; Xylem hydraulic failure is a major driver of tree death during drought. However, to better understand mortality risk in trees, especially during hot-drought events, more information is required on both rates of residual water-loss from small branches (g(res)) after stomatal closure, as well as the phase transition temperature (T-p), beyond which g(res) significantly increases. Here, we describe and test a novel low-cost tool, the DroughtBox, for phenotyping g(res) and T-p across species. The system consists of a programmable climatically controlled chamber in which branches dehydrate and changes in the mass recorded. Test measurements show that the DroughtBox maintains stable temperature and relative humidity across a range of set points, a prerequisite for getting accurate g(res) and T-p values. Among a study group of four conifer and one angiosperm species, we observed a range of g(res) (0.44-1.64 mmol H2O m(-2) s(-1)) and T-p (39.4-43.8 degrees C) values. Furthermore, the measured time to hydraulic failure varied between two conifers species and was shortened in both species following a heatwave event. The DroughtBox is a reliable and customizable tool for phenotyping g(res) and T-p, as well as for testing models of time to hydraulic failure that will improve our ability to assess climate change impacts on plants.

How immune responses are activated and regulated is a central question in immunology. In addition to molecular signaling, recent work has shown that physical forces regulate the immune response of vertebrates by modifying transmembrane protein conformation and cell contact. Mechanical stress and strain produced by forces constitute physical cues perceived by cells instructing gene expression. Whether mechanical cues generated by pathogens during host colonization can trigger adaptive responses in plant cells remains elusive. We found that local and progressive variations of plant cell wall tension caused by fungal pathogen attacks are transmitted to neighboring healthy tissue around the infection site and trigger immunity in distal cells. This ‘thigmoimmunity’ process requires the reorganization of cortical microtubules and contributes strongly to Arabidopsis disease resistance.One-Sentence SummaryActivation of plants immunity depends on fluctuations of mechanical tension caused by a fungal pathogen.

International audience; Mediterranean forests face an intensification of droughts caused by ongoing climate change. To improve our understanding of tree and forest responses to increasing drought, we explored over nine years, the effects of a 30% rainfall exclusion experiment on the water potential, sap velocity and primary and secondary growth of two co-occurring species (Quercus ilex and Pinus halepensis) in a French Mediterranean forest. In addition, native embolism was measured after six and nine years of exclusion onset. Water potentials decreased earlier during summer drought for both species in the rainfall exclusion plot, and to a higher extent during the drought peak for Q. ilex, involving earlier stomatal closure and reduced sap velocity. Sap velocity reduction persisted throughout the years in the exclusion plot. Outside summer the water potential difference between predawn and midday was similar between treatment which indicate that reduced water transport efficiency may be linked to decrease hydraulic conductance. Such differences were neither related to differences in xylem embolism, that remained similar between treatments, nor to change in secondary growth. In contrast primary growth measurements indicate that P. halepensis trees, and Q. ilex to a lesser extent, experienced reduction in total leaf areas in response to the rainfall exclusion. Globally, our results suggest that increase drought lead, for both species, to a decrease in sap velocity not mediated by increase embolism but rather by a reduction in primary growth. So far little conclusion can be drawn regarding the competitive advantage of one species over the other in the context of increasing drought related to climate change.

International audience; Key message: Using several experimental approaches, we have demonstrated that electrical resistivity tomography (ERT) is a reliable nondestructive tool for estimating the moisture content of heartwood in situ. ERT measurements show that water pockets in heartwood (wetwood) are present in a large majority (90%) of silver fir (Abies alba Mill.) trunks. Context: For wood professionals, the presence of wetwood in wood logs leads to an increase in costs, especially during the drying process. Assessing these internal properties in situ with a nondestructive method will provide reliable information for improved management of respective forests. Aims: The objective of this study was to evaluate the efficiency of the electrical resistivity tomography (ERT) tool to detect wetwood in standing trees and to estimate the mean moisture content (MC) of silver fir trunks. Methods: The study was carried out in 3 forests located in the region "Massif Central" in France. We selected 58 silver fir trees, visually healthy and without visible default. Each tree has been subject to regular ERT measurements for more than a year. At the same time, one to three cores were taken from each tree in order to measure the actual MC of the wood. Results: 90% of the silver fir trees showed the presence of wetwood in their heartwood. Our results showed a significant correlation between the mean heartwood MC measured on cores and the mean electrical resistivity (ER) obtained with ERT. Conclusion: (i) The presence of wetwood occurs in a high proportion of the silver fir trees studied, and (ii) ERT can be used to estimate the average MC of the heartwood of standing trees. However, the data provided by ERT vary seasonally and do not allow the precise location of wetwood.

Under natural conditions, plants experience external mechanical stresses such as wind and touch that impact their growth. A remarkable feature of this mechanically induced growth response is that it may occur at distance from the stimulation site, suggesting the existence of a signal propagating through the plant. In this study, we investigated the electrical response of poplar trees to a transient controlled bending stimulation of the stem that mimics the mechanical effect of wind. Stem bending was found to cause an electrical response that we called ‘gradual’ potential, similar in shape to an action potential. However, this signal distinguished from the well-known plant action potential by its propagation up to 20 cm along the stem and its strong dumping in velocity and amplitude. Two hypotheses regarding the mode of propagation of the ‘gradual’ potential are discussed.One sentence summaryPoplar stem bending induces an electrical response with high speed and strong decrement.

Hydraulic failure has been extensively studied during drought-induced plant dieback, but its role in plant-pathogen interactions is under debate. During esca, a grapevine (Vitis vinifera) disease, symptomatic leaves are prone to irreversible hydraulic dysfunctions but little is known about the hydraulic integrity of perennial organs over the short- and long-term. We investigated the effects of esca on stem hydraulic integrity in naturally infected plants within a single season and across season(s). We coupled direct (ks) and indirect (kth) hydraulic conductivity measurements, and tylose and vascular pathogen detection with in vivo X-ray microtomography visualizations. Xylem occlusions (tyloses) and subsequent loss of stem hydraulic conductivity (ks) occurred in all shoots with severe symptoms (apoplexy) and in more than 60% of shoots with moderate symptoms (tiger-stripe), with no tyloses in asymptomatic shoots. In vivo stem observations demonstrated that tyloses occurred only when leaf symptoms appeared, and resulted in more than 50% loss of hydraulic conductance in 40% of symptomatic stems, unrelated to symptom age. The impact of esca on xylem integrity was only seasonal, with no long-term impact of disease history. Our study demonstrated how and to what extent a vascular disease such as esca, affecting xylem integrity, could amplify plant mortality through hydraulic failure.

Knowledge on variations of drought resistance traits are needed to predict the potential of trees to acclimate to coming severe drought events. Xylem vulnerability to embolism is a key parameter related to such droughts, and its phenotypic variability relies mainly on environmental plasticity. We investigated the structural determinants controlling the plasticity of vulnerability to embolism, focusing on the key elements involved in the air bubble entry in vessels, especially the intervessel pits. Poplar saplings (Populus tremula x alba (Aiton) Sm., 1804) grown in contrasted water availability or light exposure exhibited differences in the vulnerability to embolism (P50) in a range of 0.76 MPa. We then characterized the structural changes in features related to pit quantity and pit structure, from the pit ultrastructure to the organization of xylem vessels, using different microscopy techniques (transmission electron microscopy, scanning electron microscopy, light microscopy). A multispectral combination of X-ray microtomography and light microscopy analysis allowed measuring the vulnerability of each single vessel and testing some of the relationships between structural traits and vulnerability to embolism inside the xylem. The pit ultrastructure did not change, whereas the vessel dimensions increased with the vulnerability to embolism and the grouping index and fraction of intervessel cell wall both decreased with the vulnerability to embolism. These findings hold when comparing between trees or between the vessels inside the xylem of an individual tree. These results evidenced that plasticity of vulnerability to embolism in hybrid poplar occurs through changes in the pit quantity properties such as pit area and vessel grouping rather than changes on the pit structure.

International audience; Abstract Global climatic models predict an increment in the frequency and intensity of drought events, which have important consequences on forest dieback. However, the mechanisms leading to tree mortality under drought conditions and the physiological thresholds for recovery are not totally understood yet. This study aimed to identify what are the key physiological traits that determine the tree capacity to recover from drought. Individuals of a conifer (Pseudotsuga menziesii M.) and an angiosperm (Prunus lusitanica L.) species were exposed to drought and their ability to recover after rehydration monitored. Results showed that the actual thresholds used for recovery from drought based on percentage loss of conductance (PLC) (i.e. 50% for conifers, 88% for angiosperms) do not provide accurate insights about the tree capacity for surviving extreme drought events. On the contrary, differences in stem relative water content (RWCStem) and the level of electrolytes leakage (EL) were directly related to the capacity of the trees to recover from drought. This was the case for the conifer species, P. menziesii, for which higher RWCStem and lower EL values were related to the recovery capacity. Even if results showed a similar trend for the angiosperm P. lusitanica as for the conifers, differences between the two traits were much more subtle and did not allow an accurate differentiation between trees able to recover and those that were not. RWCStem and EL could work as indicators of tree capacity to recover from drought for conifers but more studies are required to confirm this observation for angiosperms.

International audience; Shoot morphogenetic plasticity is crucial to the adaptation of plants to their fluctuating environments. Major insights on shoot morphogenesis have been gathered studying meristems, especially the Shoot Apical Meristem (SAM), through a methodological effort in multiscale systems biology and biophysics. However, morphogenesis at the SAM is robust to environmental changes. Plasticity emerges later on during post-SAM development. The purpose of this review is to show that multiscale systems biology and biophysics is insightful for the shaping of the whole plant as well. More specifically, we review the shaping of axes and crowns through tropisms and elasticity, combining the recent advances on the morphogenetic control by physical cues and by genes. We focus mostly on land angiosperms, but with growth habits ranging from small herbs to big trees. We show that generic (universal) morphogenetic processes have been identified, revealing feed-forward and feedback effects of the global shape on the local morphogenetic process. In parallel, major advances have been made in the analysis the major genes involved in the shaping of axes and crowns, revealing conserved genic networks among angiosperms. Then, we show that these two approaches are now starting to converge, revealing exciting perspectives.

Hydraulic failure has been extensively studied during drought-induced plant dieback, but its role in plant-pathogen interactions is under debate. During esca, a grapevine (Vitis vinifera) disease, symptomatic leaves are prone to irreversible hydraulic dysfunctions but little is known about the hydraulic integrity of perennial organs over the short- and long-term. We investigated the effects of esca on stem hydraulic integrity in naturally infected plants within a single season and across season(s). We coupled direct (ks) and indirect (kth) hydraulic conductivity measurements, and tylose and vascular pathogen detection with in vivo X-ray microtomography visualizations. We found xylem occlusions (tyloses), and subsequent loss of stemks, in all of the shoots with severe symptoms (apoplexy) and in more than 60% of the shoots with moderate symptoms (tiger-stripe), and no tyloses in shoots that were currently asymptomatic. In vivo stem observations demonstrated that tyloses were observed only when leaf symptoms appeared, and resulted in more than 50% PLC in 40% of symptomatic stems, unrelated to symptom age. The impact of esca on xylem integrity was only seasonal and no long-term impact of disease history was recorded. Our study demonstrated how and to what extent a vascular disease such as esca, affecting xylem integrity, could amplify plant mortality by hydraulic failure.HighlightOur study reveals that esca can critically affect xylem water movement in grapevine perennial organs, by the presence of plant-derived tyloses.

Knowledge on variations of drought resistance traits are needed to predict the potential of trees to acclimate to coming severe drought events. Xylem vulnerability to embolism is a key parameter related to such droughts, and its phenotypic variability relies mainly on environmental plasticity. We investigated the structural determinants controlling the plasticity of vulnerability to embolism, focusing on the key elements involved in the air bubble entry in a vessel, especially the inter-vessel pits. Poplar saplings (Populus tremula x alba) grown in contrasted water availability or light exposure exhibited differences in vulnerability to embolism in a range of 0.76 MPa. We then characterized the structural changes related to qualitative and quantitative pit characteristics, from the pit structure to the organization of xylem vessels, using different microscopy techniques (TEM, SEM, light). X-ray microtomography analysis allowed observing the vessel vulnerability and testing some of the relationships between structural traits and vulnerability to embolism inside the xylem. The pit ultrastructure did not change, whereas the vessel dimensions increased with vulnerability to embolism and the grouping index and fraction of inter-vessel cell wall decreased with vulnerability to embolism. These findings holds when comparing trees or when comparing vessels inside the xylem. These results evidenced that plasticity of vulnerability to embolism occurs through changes in the quantitative pit properties such as pit area and vessel grouping rather than on the pit structure.

International audience; & Key message Mechanical acclimation of young poplars (Populus tremula × Populus alba, INRA 717-1B4) submitted to periodic stem bending is mainly driven by compressive strains. Flexure wood and compressive flexure wood exhibit higher mechanical resilience and lower mechanical damage. & Context It is well known that thigmomorphogenesis modulates tree growth and the anatomical structure of wood. However, nothing is known about the mechanical behaviour of the tissues of fresh wood formed under mechanical stimulation. & Aims We investigated the elastic and plastic properties of the fresh wood of young poplar trees (Populus tremula × Populus alba, INRA 717-1B4) submitted to periodic controlled stem bending that mimics the mechanical effect of wind. & Methods For a set of trees, we applied symmetrical bending treatments, which led to the formation of "flexure wood". For another set of trees, asymmetrical bending treatments, including compression (or tension) only, were applied and generated specific wood formation: "compressive flexure wood" and "tensile flexure wood". We investigated the elastic and plastic properties of these woods at the stem and at the local tissue levels. & Results The results revealed that fresh wood formed under compressive treatments is more resistant to damage (damage reduced by 44%) and a higher mechanical resilience (+ 33%), suggesting that this tissue is able to withstand higher mechanical strains than "normal wood". This improvement could explain the higher mechanical strength of the stem to bending (+ 42%). & Conclusion When trees experience repetitive mechanical stimulations, they adjust the plastic plastic behaviour of their wood in a way that improves the mechanical safety. This demonstrates the adaptive benefit of the mechanical acclimation of trees.

Xylem vulnerability to drought-induced embolism did not differ between stems and petioles of four woody species ( Betula pendula , Liriodendron tulipifera , Populus tremula and Olea europaea ). Our results, together with data compiled from published literature, indicate that hydraulic segmentation during drought stress is not consistently driven by difference in vulnerability to embolism between stem and terminal organs. Hydraulic failure and disconnection of distal organs during protracted drought stress is thought to protect large branches or trunks by reducing water loss and restricting the spread of embolism. Hydraulic segmentation and preferential sacrifice of distal organs such as leaves can be driven by two mechanisms: more negative water potentials at the terminal section of the hydraulic pathway and/or by higher vulnerability to xylem embolism of distal organs. Although vulnerability segmentation has been reported in the literature, the generality of this phenomenon is unclear, in part due to the methodological limitations related to direct measurement of xylem vulnerability to embolism in intact plants. The objective of this study was to evaluate vulnerability segmentation between petioles and stems using non-invasive micro computed tomography (microCT). Vulnerability to embolism was measured in leaf petioles and subtending stems of four woody species (Betula pendula R., Liriodendron tulipifera L., Populus tremula L. and Olea europaea L.) with contrasting drought tolerances. In addition, previously published vulnerability data for petioles and stems were compiled from the literature to investigate the commonality of hydraulic segmentation across a wide range of woody species, with the vulnerability curve methodology distinguished. Using non-invasive imaging on intact plants, we found no evidence of hydraulic segmentation between petioles and stems of four angiosperm tree species, regardless of mechanism. Moreover, the literature dataset indicated that little or no difference in vulnerability to embolism is present between petioles and stems when vulnerability curves were constructed using methods specifically measuring the dynamics of xylem tissue during dehydration (e.g. optical visualization, MicroCT). Our results suggest that vulnerability segmentation between stems and distal organs (petioles and leaves) is limited when only xylem tissue is considered. Large differences in vulnerability between stems and leaves are likely to be driven by extra-xylary components, rather than xylem embolism.

International audience; Climate change threatens food security, and plant science researchers have investigated methods of sustaining crop yield under drought. One approach has been to overproduce abscisic acid (ABA) to enhance water use efficiency. However, the concomitant effects of ABA overproduction on plant vascular system functioning are critical as it influences vulnerability to xylem hydraulic failure. We investigated these effects by comparing physiological and hydraulic responses to water deficit between a tomato (Solanum lycopersicum) wild type control (WT) and a transgenic line overproducing ABA (sp12). Under well-watered conditions, the sp12 line displayed similar growth rate and greater water use efficiency by operating at lower maximum stomatal conductance. X-ray microtomography revealed that sp12 was significantly more vulnerable to xylem embolism, resulting in a reduced hydraulic safety margin. We also observed a significant ontogenic effect on vulnerability to xylem embolism for both WT and sp12. This study demonstrates that the greater water use efficiency in the tomato ABA overproducing line is associated with higher vulnerability of the vascular system to embolism and a higher risk of hydraulic failure. Integrating hydraulic traits into breeding programmes represents a critical step for effectively managing a crop's ability to maintain hydraulic conductivity and productivity under water deficit.

International audience; Lignocellulosic by-products are frequently disposed by means of combustion. This study investigates an alternative route for corn cob and Kraft lignin resources in order to support circular economy. The respective plant-based fibres and filler were compounded for the first time together with a poly(lactic acid) (PLA) matrix. Consecutively, seven different biocomposites were processed by injection-moulding and further characterized. The biocomposite containing a mixture of Kraft lignin and corn cob (12 wt% in total) exhibited the highest flexural strength (84 MPa). A proper wetting of PLA onto the corn cob particles demonstrated a good compatibility at matrix/fibre interface. PLA molecular structure changed in presence of 20 wt% lignin filler, with effect on the glass transition temperature and on the composite mechanical strength. The fibres moderately influenced composites surface tension, while Kraft lignin contributed to a slight increase of surface hydrophobicity. Surface energy (σsTotal) of composites have been estimated at 27.6, 28.7 and 27.8 mN/m for PLA/KL-20, PLA/CC-10 and PLA/KL-15/CC-5 respectively. While the polar component (σsPolar) have been estimated at 17.8, 20.0 and 18.7 mN/m for PLA/KL-20, PLA/CC-10 and PLA/KL-15/CC-5 respectively. Unlike the PLA/corn cob composite, those containing Kraft lignin were entirely biodegraded within 2 months in industrial composting conditions study. The materials could be utilized for end-use products thanks to their good mechanical and thermal properties. By adding wood-lignin and corn by-products, materials cost and carbon footprint shall decrease in comparison to pure PLA, while being a biodegradable and sustainable replacement of polyolefins.

Bio-based composites with lignosulfonates as a matrix and corn cob particles as reinforcement were developed. Their mechanical strength in compression mode was evaluated with determination of Young modulus (E) and ultimate compressive strength (σ max). The influence of fiber content, fibers particle size, and compaction pressure applied on the composite during processing was investigated with a design of experiments approach. The model enabled to calculate an optimum for σ max equal to 18 MPa for a corresponding Young modulus of 0.27 GPa. Particle size was found to be the most influent parameter on mechanical properties responses. In order to understand this dependency, an automated optical particle analysis was performed to measure size and shape of corn cob particles. Distribution leading to the highest σ max was the one with the less smooth surface of particles (low convexity shape parameter), providing a better mechanical adhesion between fiber and matrix. Increasing compaction pressure and fiber content was in favor of compressive strength. Interface between lignosulfonates and corn cob inside the composites was observed with X-ray microtomography technique. It was noticed that interlocking occurred between the two phases.

While the xylem hydraulic properties, such as vulnerability to cavitation (VC), are of paramount importance in drought resistance, their genetic determinants remain unexplored. There is evidence that pectins and their methylation pattern are involved, but the detail of their involvement and the corresponding genes need to be clarified. We analysed the hydraulic properties of the 35S::PME1 transgenic aspen that ectopically under- or over-express a xylem-abundant pectin methyl esterase, PtxtPME1. We also produced and analyzed 4CL1::PGII transgenic poplars expressing a fungal polygalacturonase, AnPGII, under the control of the Ptxa4CL1 promoter that is active in the developing xylem after xylem cell expansion. Both the 35S::PME1 under and over-expressing aspen lines developed xylem with lower specific hydraulic conductivity and lower VC, while the 4CL1::PGII plants developed xylem with a higher VC. These xylem hydraulic changes were associated with modifications in xylem structure or in intervessel pit structure that can result in changes in mechanical behaviour of the pit membrane. This study shows that homogalacturonans and their methylation pattern influence xylem hydraulic properties, through its effect on xylem cell expansion and on intervessel pit properties and it show a role for PtxtPME1 in the xylem hydraulic properties.

Xylem resistance to cavitation is a key parameter in drought resistance of trees. Indeed, water stress tolerant species are less vulnerable to cavitation than water stress intolerant species. At the within species level, phenotypic variability was reported for the xylem cavitation resistance, mainly based on plasticity among environmental growth conditions. This raises the question of genetic and structural determinants of this plasticity. The objective of the present work is to identify the structural determinants controlling plasticity of xylem resistance to cavitation. Young poplars trees were grown in contrasted water availability or light exposure in order to induce acclimation in hydraulic properties. We then characterized the structural modifications of their acclimated xylem at three levels: xylem organization, vessels dimensions and pits shape. We used different microscopy techniques including Transmission Electron Microscope (T.E.M.), Scanning Electron Microscope (M.E.B.) and Light Microscope – on the same individual trees. We specially investigated the intervessel pits that provide hydraulic connections between vessels while they prevent air embolism spreading. Moreover, a local approach using direct observation of embolism spreading at the cellular level via X-ray micro-CT allowed us to explore the structural determinants of vessel resistance to cavitation.

International audience; Key messagePeriodic bending of young poplars increase the wood production whatever their hydric status; especially in the most highly stressed zones; improving the mechanical behaviour of the stem.AbstractThe ability of trees to acclimate the building of their structures to windy conditions under various hydric conditions is essential in the context of the predicted climate changes. In this study, we investigated the biomechanical responses of young poplar trees to periodic controlled bending stimulations that mimic the mechanical effect of trees growing under windy conditions. This treatment was conducted for 5months in well-watered conditions or under hydric stress. Results demonstrate the high impact of thigmomorphogenesis on growth processes, even under the water shortage. While axial growth was reduced by mechanical stimulations and hydric stress, radial growth was strongly increased by the periodic stem bending. The secondary growth was preferentially increased in the direction of highest longitudinal strains leading to the ovalisation of the cross-section. This ovalisation yielded 16%, regardless the hydric condition and generated a huge increase of the bending rigidity of the trees (+212%). Further, we observed a differential growth between the side growing under tension and the side growing under compression. A Finite Element model was built to investigate the mechanical benefits of the anisotropic cross-section shapes. This FE model enlightened the modulation of the spatial stress distribution that lead to a reduction of the stress in the weakest zones of the trunk; suggesting an improvement of the mechanical safety margin of wood. Thigmomorphogenesis acclimation appears as a complex and costly, but necessary process for the long-term mechanical support of the trees, even under hydric stress conditions.

Vascular pathogens cause disease in a large spectrum of perennial plants, with leaf scorch being one of the most conspicuous symptoms. Esca in grapevine (Vitis vinifera) is a vascular disease with huge negative effects on grape yield and the wine industry. One prominent hypothesis suggests that vascular disease leaf scorch is caused by fungal pathogen-derived elicitors and toxins. Another hypothesis suggests that leaf scorch is caused by hydraulic failure due to air embolism, the pathogen itself, and/or plant-derived tyloses and gels. In this study, we transplanted mature, naturally infected esca symptomatic vines from the field into pots, allowing us to explore xylem integrity in leaves (i.e. leaf midveins and petioles) using synchrotron-based in vivo x-ray microcomputed tomography and light microscopy. Our results demonstrated that symptomatic leaves are not associated with air embolism. In contrast, symptomatic leaves presented significantly more nonfunctional vessels resulting from the presence of nongaseous embolisms (i.e. tyloses and gels) than control leaves, but there was no significant correlation with disease severity. Using quantitative PCR, we determined that two vascular pathogen species associated with esca necrosis in the trunk were not found in leaves where occlusions were observed. Together, these results demonstrate that symptom development is associated with the disruption of vessel integrity and suggest that symptoms are elicited at a distance from the trunk where fungal infections occur. These findings open new perspectives on esca symptom expression where the hydraulic failure and elicitor/toxin hypotheses are not necessarily mutually exclusive.

Leaf hydraulic conductance (Kleaf) and capacitance (Cleaf) are among the key parameters in plant-water regulation. Understanding the responses of these hydraulic traits to drought conditions remains a challenge for describing comprehensive plant-water relationships. The ability of an organism to resist and/or tolerate embolism events, which may occur at high negative pressure caused by hydric stress, relies on how well it can sustain a hydraulic system in a dynamic equilibrium. Populus deltoides is a water-saving tree species with a stomatal conductance that declines rapidly with reduced water availability. Under unfavorable conditions, the stomatal control of transpiration is known to be closely coordinated with a loss of plant hydraulic functioning that can ultimately result in hydraulic failure through xylem embolism, notably in leaves. The effects of drought on leaf hydraulics are also related to regulation in water permeases such as the aquaporins. To describe the responses linked to leaf hydraulics under severe drought and rewatering conditions, water-stressed poplars were monitored daily on an ecophysiological and a molecular scale. A structural and expression analysis on a set of aquaporins was carried out in parallel by in situ hybridization analysis and quantitative PCR. In complement, water distribution in water-challenged leaves was investigated using X-ray microtomography. A general depression of leaf hydraulic conductance and relative water content occurred under drought, but was reversed when plants were rewatered. More interestingly, (i) extreme leaf water deficiency led to marked xylem and lamina embolism, but a degree of hydric integrity in the midrib extraxylem territories and the bundle sheath of the minor veins was maintained, and (ii) the sub-tissue water allocation correlated well with an over-accumulation of several PIP and TIP aquaporins. Our multi-facet molecular ecophysiological approach revealed that leaves were able to secure a certain level of hydric status, in particular in cell territories near the “living ribs”, which provided rapid hydric adjustment responses once favorable conditions were restored. These findings contribute to an integrated approach to leaf hydraulics, thus favoring a better understanding of the cell mechanisms involved in tree vulnerability to climate changes.

Centrifuge-based techniques to assess xylem vulnerability to embolism are increasingly being used, although we are yet to reach a consensus on the nature and extent of artefactual embolism observed in some angiosperm species. In particular, there is disagreement over whether these artefacts influence both the spin (Cavitron) and static versions of the centrifuge technique equally. We tested two methods for inducing embolism: bench dehydration and centrifugation. We used three methods to measure the resulting loss of conductivity: gravimetric flow measured in bench-dehydrated and centrifuged samples (static centrifuge), in situ flow measured under tension during spinning in the centrifuge (Cavitron) and direct imaging using X-ray computed microtomography (microCT) observations in stems of two species of Hakea that differ in vessel length. Both centrifuge techniques were prone to artefactual embolism in samples with maximum vessel length longer than, or similar to, the centrifuge rotor diameter. Observations with microCT indicated that this artefactual embolism occurred in the outermost portions of samples. The artefact was largely eliminated if flow was measured in an excised central part of the segment in the static centrifuge or starting measurements with the Cavitron at pressures lower than the threshold of embolism formation in open vessels. The simulations of loss of conductivity in centrifuged samples with a new model, CAVITOPEN, confirmed that the impact of open vessels on the vulnerability to embolism curve was higher when vessels were long, samples short and when embolism is formed in open vessels at less negative pressures. This model also offers a robust and quantitative tool to test and correct for artefactual embolism at low xylem tensions.

The frequencies of free oscillations of plants, or plant parts, depend on their geometries, stiffnesses, and masses. Besides direct biomechanical interest, free frequencies also provide insights into plant properties that can usually only be measured destructively or with low-throughput techniques (e.g., change in mass, tissue density, or stiffness over development or with stresses). We propose here a new high-throughput method based on the noncontact measurements of the free frequencies of the standing plant. The plant is excited by short air pulses (typically 100 ms). The resulting motion is recorded by a high speed video camera (100 fps) and processed using fast space and time correlation algorithms. In less than a minute the mechanical behavior of the plant is tested over several directions. The performance and versatility of this method has been tested in three contrasted species: tobacco (Nicotiana benthamian), wheat (Triticum aestivum L.), and poplar (Populus sp.), for a total of more than 4000 data points. In tobacco we show that water stress decreased the free frequency by 15%. In wheat we could detect variations of less than 1 g in the mass of spikes. In poplar we could measure frequencies of both the whole stem and leaves. The work provides insight into new potential directions for development of phenotyping.

Xylem embolism is one of the main processes involved in drought-related plant mortality. Although its consequences for plant physiology are already well described, embolism formation and spread are poorly evaluated and modelled, especially for tracheid-based species. The aim of this study was to assess the embolism formation and spread in Pinus sylvestris as a case study using X-ray microtomography and hydraulics methods. We also evaluated the potential effects of cavitation fatigue on vulnerability to embolism and the micro-morphology of the bordered pits using scanning electron microscopy (SEM) to test for possible links between xylem anatomy and embolism spread. Finally, a novel model was developed to simulate the spread of embolism in a 2D anisotropic cellular structure. Results showed a large variability in the formation and spread of embolism within a ring despite no differences being observed in intertracheid pit membrane anatomical traits. Simulations from the model showed a highly anisotropic tracheid-to-tracheid embolism spreading pattern, which confirms the major role of tracheid-to-tracheid air seeding to explain how embolism spreads in Scots pine. The results also showed that prior embolism removal from the samples reduced the resistance to embolism of the xylem and could result in overestimates of vulnerability to embolism.

Wood ensures the mechanical stability of the trunk and branches. Its formation is impacted by windy environments: living cells sense mechanical strains and modify the formation of wood accordingly in terms of quantity and quality: these biological responses are called “Thigmomorphogenesis”. We bent young poplars three times a day, five days per week during six months. When a stem is bent, wood endures compressive and/or tensile stresses. Our objective is to determine the impact of these growing conditions on the mechanical properties of wood.

During root progression in soil, root cap cells are the first to encounter obstacles. The root cap is known to sense environmental cues, making it a relevant candidate for a mechanosensing site. An original two-layer medium was developed in order to study root responses to growth medium strength and the importance of the root cap in the establishment of these responses. Root growth and trajectory of primary roots of Arabidopsis thaliana seedlings were investigated using in vivo image analysis. After contact with the harder layer, the root either penetrated it or underwent rapid curvature, enabling reorientation of the root primary growth. The role of the root cap in tip reorientation was investigated by analyzing the responses of Arabidopsis mutant roots with altered caps. The primary root of fez-2 mutant lines, which has fewer root cap cell layers than wild-type roots, showed impaired penetration ability. Conversely, smb-3 roots of mutant lines, which display a higher number of root cap cells, showed enhanced penetration abilities. This work highlights that alterations in root cap shape and size affect the root responses to medium strength.HighlightThe analysis of the growth and orientation of Arabidopsis thaliana mutant roots affected in root cap size and shape showed that properly formed root cap is required to trigger the root responses to medium strength.AbbreviationsCOLcolumella;LRCLateral Root Cap;SISharpness Index;SMBSOMBRERO.

International audience; The plant ability to orient their growth with respect to environmental stimuli such as gravity, is a key factor for survival and acclimation to their environment. In heterogeneous soils, plant roots modulate their growth towards gravity, allowing soil exploration and uptake of water and nutrients. Gravity sensing in roots occurs in specialized cells called statocytes, located in the central root cap, and which contain starch-filled plastids called statoliths. Being denser than the cytoplasm, statolith position depends on the inclination of roots. Root growth reorientation, triggered by statoliths displacement, is based on auxin redistribution in the root apex, inducing differential growth between the root upward and downward sides. At the cell scale, signaling events starting from statoliths displacement and leading to auxin redistribution remain poorly documented. Some signaling molecules such as calcium, reactive oxygen species, nitric oxide and inositol 1,4,5-triphosphate are serious candidates previously shown to be involved within minutes before modification of the expression of auxin-related genes. In order to investigate the effects of statoliths location on the triggering of signaling pathways, a space experiment called GRAVI-2 was set up aboard the ISS in 2014. During the experiment, lentil roots were grown in microgravity conditions in the European modular cultivation system (EMCS) and were then submitted to a period of gravity (9h at 0.01 g or 15 min at 2 g). The analysis of statolith location showed that statoliths are clustered in microgravity. This clusterization is not affected by 0.01g. On the contrary, a period of 5 min at 2g induces a change in statolith location. Thanks to these changes in statolith positioning, cytological analysis were coupled with RNA sequencing of root apex. Transcriptomic analyses clearly show a regulation in gene expression. Our data are among the first involving global transcriptomic analyses of root material grown in microgravity and of root submitted to a gravity-short period after a growth in microgravity. The effects of the statolith location on signaling mechanisms will be discussed.

Background and aims Trees constantly experience wind, perceive resulting mechanical cues, and modify their growth and development accordingly. Previous studies have demonstrated that multiple bending treatments trigger ovalization of the stem and the formation of flexure wood in gymnosperms, but ovalization and flexure wood have rarely been studied in angiosperms, and none of the experiments conducted so far has used multidirectional bending treatments at controlled intensities. Assuming that bending involves tensile and compressive strain, we hypothesized that different local strains may generate specific growth and wood differentiation responses. Methods Basal parts of young poplar stems were subjected to multiple transient controlled unidirectional bending treatments during 8 weeks, which enabled a distinction to be made between the wood formed under tensile or compressive flexural strains. This set-up enabled a local analysis of poplar stem responses to multiple stem bending treatments at growth, anatomical, biochemical and molecular levels. Key results In response to multiple unidirectional bending treatments, poplar stems developed significant cross-sectional ovalization. At the tissue level, some aspects of wood differentiation were similarly modulated in the compressed and stretched zones (vessel frequency and diameter of fibres without a G-layer), whereas other anatomical traits (vessel diameter, G-layer formation, diameter of fibres with a G-layer and microfibril angle) and the expression of fasciclin-encoding genes were differentially modulated in the two zones. Conclusions This work leads us to propose new terminologies to distinguish the 'flexure wood' produced in response to multiple bidirectional bending treatments from wood produced under transient tensile strain (tensile flexure wood; TFW) or under transient compressive strain (compressive flexure wood; CFW). By highlighting similarities and differences between tension wood and TFW and by demonstrating that plants could have the ability to discriminate positive strains from negative strains, this work provides new insight into the mechanisms of mechanosensitivity in plants.

Key message: Direct, non-invasive X-ray microtomography and optical technique observations applied in stems and leaves of intact seedlings revealed that laurel is highly resistant to drought-induced xylem embolism. Contrary to what has been brought forward, daily cycles of embolism formation and refilling are unlikely to occur in this species and to explain how it copes with drought. Context: There has been considerable controversy regarding xylem embolism resistance for long-vesselled angiosperm species and particularly for the model species for refilling (Laurus nobilis L.). Aims: The purpose of this study was to resolve the hydraulic properties of this species by documenting vulnerability curves of different organs in intact plants. Methods: Here, we applied a direct, non-invasive method to visualize xylem embolism in stems and leaves of intact laurel seedlings up to 2-m tall using X-ray microtomography (microCT) observations and the optical vulnerability technique. These approaches were coupled with complementary centrifugation measurements performed on 1-m long branches sampled from adult trees and compared with additional microCT analyses carried out on 80-cm cut branches. Results: Direct observations of embolism spread during desiccation of intact laurels revealed that 50% loss of xylem conductivity (Psi(50)) was reached at - 7.9 +/- 0.5 and - 8.4 +/- 0.3 MPa in sterns and leaves, respectively, while the minimum xylem water potentials measured in the field were - 4.2 MPa during a moderate drought season. Those findings reveal that embolism formation is not routine in Laurus nobilis contrary to what has been previously reported. These Psi(50) values were close to those based on the flow-centrifuge technique (- 9.2 +/- 0.2 MPa), but at odds with microCT observations of cut branches (- 4.0 +/- 0.5 MPa). Conclusion: In summary, independent methods converge toward the same conclusion that laurel is highly resistant to xylem embolism regardless its development stage. Under typical growth conditions without extreme drought events, this species maintains positive hydraulic safety margin, while daily cycles of embolism formation and refilling are unlikely to occur in this species.

Plants have developed different tropisms: in particular, they re-orient the growth of their branches towards light (phototropism) or upwards (gravitropism). How these tropisms affect the shape of a tree crown remains unanswered. We address this question by developing a propagating front model of tree growth. This model being length-free, it leads to self-similar solutions, independent of the initial conditions, at long time. Varying the intensities of each tropism, different self-similar shapes emerge, including singular ones. Interestingly, these shapes bear similarities with existing tree species. It is concluded that the core of specific crown shapes in trees relies on the balance between tropisms., 11 pages, 7 figures

Plants can be highly segmented organisms with an independently redundant design of organs. In the context of plant hydraulics, leaves may be less embolism resistant than stems, allowing hydraulic failure to be restricted to distal organs that can be readily replaced. We quantified drought-induced embolism in needles and stems of Pinus pinaster using high-resolution computed tomography (HRCT). HRCT observations of needles were compared with the rehydration kinetics method to estimate the contribution of extra-xylary pathways to declining hydraulic conductance. High-resolution computed tomography images indicated that the pressure inducing 50% of embolized tracheids was similar between needle and stem xylem (P50 needle xylem = -3.62 MPa, P50 stem xylem = -3.88 MPa). Tracheids in both organs showed no difference in torus overlap of bordered pits. However, estimations of the pressure inducing 50% loss of hydraulic conductance at the whole needle level by the rehydration kinetics method were significantly higher (P50 needle = -1.71 MPa) than P50 needle xylem derived from HRCT. The vulnerability segmentation hypothesis appears to be valid only when considering hydraulic failure at the entire needle level, including extra-xylary pathways. Our findings suggest that native embolism in needles is limited and highlight the importance of imaging techniques for vulnerability curves.

International audience; Hydraulic failure induced by xylem embolism is one of the primary mechanisms of plant dieback during drought. However, many of the methods used to evaluate the vulnerability of different species to drought-induced embolism are indirect and invasive, increasing the possibility that measurement artifacts may occur. Here, we utilize x-ray computed microtomography (microCT) to directly visualize embolism formation in the xylem of living, intact plants with contrasting wood anatomy (Quercus robur, Populus tremula × Populus alba, and Pinus pinaster). These observations were compared with widely used centrifuge techniques that require destructive sampling. MicroCT imaging provided detailed spatial information regarding the dimensions and functional status of xylem conduits during dehydration. Vulnerability curves based on microCT observations of intact plants closely matched curves based on the centrifuge technique for species with short vessels (P. tremula × P. alba) or tracheids (P. pinaster). For ring porous Q. robur, the centrifuge technique significantly overestimated vulnerability to embolism, indicating that caution should be used when applying this technique to species with long vessels. These findings confirm that microCT can be used to assess the vulnerability to embolism on intact plants by direct visualization.

Torres-Ruiz, J.M. et al... .--11 páginas.-- 6 figuras.-- 1 tabla.-- 63 referencias, Xylem vulnerability to embolism represents an essential trait for the evaluation of the impact of hydraulics in plant function and ecology. The standard centrifuge technique is widely used for the construction of vulnerability curves, although its accuracy when applied to species with long vessels remains under debate. We developed a simple diagnostic test to determine whether the open-vessel artefact influences centrifuge estimates of embolism resistance. Xylem samples from three species with differing vessel lengths were exposed to less negative xylem pressures via centrifugation than the minimum pressure the sample had previously experienced. Additional calibration was obtained from non-invasive measurement of embolism on intact olive plants by X-ray microtomography. Results showed artefactual decreases in hydraulic conductance (k) for samples with open vessels when exposed to a less negative xylem pressure than the minimum pressure they had previously experienced. X-Ray microtomography indicated that most of the embolism formation in olive occurs at xylem pressures below -4.0 MPa, reaching 50% loss of hydraulic conductivity at -5.3 MPa. The artefactual reductions in k induced by centrifugation underestimate embolism resistance data of species with long vessels. A simple test is suggested to avoid this open vessel artefact and to ensure the reliability of this technique in future studies, Was supported by a scholarship from the Spanish Ministry of Economy and Competitiveness (EEBB-I-15-09191). We thank the PSICHE beamline (SOLEIL synchrotron facility, project 20141229). This work was supported by the programme ‘Investments for the Future’ (ANR-10-EQPX-16, XYLOFOREST) from the French National Agency for Research. B.C. was supported by the Australian Research Council (Future Fellowship grant no. FT130101115). R.L. was supported by a Marie Curie fellowship (no. ) of the EU FP7 Programme.

A significant increase in drought events frequency is predicted for the next decades induced by climate change, potentially affecting plant species mortality rates and distributions worldwide. The main trigger of plant mortality is xylem hydraulic failure due to embolism and induced by the low pressures at which water is transported through xylem. As the Mediterranean basin will be severely affected by climate change, the aim of this study was to provide novel information about drought resistance and tolerance of one of its most widely distributed and common genera as a case study: the genus Cistus. Different functional and anatomical traits were evaluated in four co-occurring Cistus species in the Mediterranean Montado ecosystem. Soil water availability for each species was also assessed to evaluate if they show different ecological niches within the area. Results showed physiological and xylem anatomical differences between the four co-occurring species, as well as in the soil water availability of the sites they occupy. Despite the significant differences in embolism resistance across species, no trade-off between hydraulic safety and efficiency was observed. Interestingly, species with narrower vessels showed lower resistance to embolism than those with higher proportions of large conduits. No correlation, however, was observed between resistance to embolism and wood density. The four species showed different water-use and drought-tolerance strategies, occupying different ecological niches that would make them cope differently with drought. These results will allow us to improve the predictions about the expected changes in vegetation dynamics in this area due to ongoing climate change.

We document xylem structure and hydraulic properties in the earliest woody plant Armoricaphyton chateaupannense gen. nov. & sp. nov. based on c. 407-million-year-old fossils from the Armorican Massif, western France. The plant was small, and the woody axes were narrow and permineralized in pyrite (FeS2). We used standard palaeobotanical methods and employed propagation phase contrast X-ray synchrotron microtomography (PPC-SRμCT) to create three-dimensional images of the wood and to evaluate its properties. The xylem comprised tracheids and rays, which developed from a cambium. Tracheids possessed an early extinct type of scalariform bordered pitting known as P-type. Our observations indicate that wood evolved initially in plants of small stature that were members of Euphyllophytina, a clade that includes living seed plants, horsetails and ferns. Hydraulic properties were calculated from measurements taken from the PPC-SRμCT images. The specific hydraulic conductivity of the xylem area was calculated as 8.7 kg m−1 s−1 and the mean cell thickness-to-span ratio (t/b)2 of tracheids was 0.0372. The results show that the wood was suited to high conductive performance with low mechanical resistance to hydraulic tension. We argue that axis rigidity in the earliest woody plants initially evolved through the development of low-density woods

International audience; The level of stresses of tension wood changes during the gravitropic movement. These changes are induced by the perception of strains experienced by the tree during reorientation to the upright position.In most hardwood species, tension wood is produced to ensure tropic movements in radially growing organs. Tension wood exhibits internal tensional forces (autostresses) greater than those of normal wood, which enable the trunk to restore its verticality. During the gravitropic response, there is a first phase when the trunk curves upwards and a second phase when the trunk decurves to reach a final vertical and straight shape. Tension wood appears to be of varying strength, but the source of these variations remains partly undefined. We set out to assess the involvement of mechanosensing in the regulation of the strength of tension wood. Autostress levels characterise the strength of tension wood and can be indirectly estimated by measuring the associated residual longitudinal maturation strains (rlms) after the autostresses release. The higher the tension, the higher the measured associated shrinkage. To look for the involvement of mechanosensing in the regulation of tension wood strength, rlms were measured in different types of experiments in which the trunk mechanical state was modified. Results showed that (1) bigger trees exhibited higher levels of rlms, (2) there was a quantitative relationship between the rlms and the sum of strains experienced by the trunk, (3) artificial curving induced an increase in rlms and (4) in tilted staked trees, rlms increased towards negative values for 3 weeks and then remained constant. These findings are consistent evidence for the regulation of rlms values by mechanosensing. This brings new insight into gravitropism.

One of the key functions of wood is hydraulic conductivity, and the general physical properties controlling this are well characterized in living plants. Modern species capture only a fraction of the known diversity of wood, which is well preserved in a fossil record that extends back over 400 million years to the origin of the vascular plants. Early fossil woods are known to differ in many key respects from woods of modern gymnosperms (e.g., tracheid size, secondary wall thickenings, lignin chemistry, cambium development) and recent discoveries are shedding new light on the earliest stages of wood evolution, raising questions about the performance of these systems and their functions. We provide an overview of the early fossil record focusing on tracheid morphology in the earliest primary and secondary xylem and on cambial development. The fossil record clearly shows that wood evolved in small stature plants prior to the evolution of a distinctive leaf-stem-root organography. The hydraulic properties of fossil woods cannot be measured directly, but with the development of mathematical models it is becoming increasingly feasible to make inferences and quantify performance, enabling comparison with modern woods. Perhaps the most difficult aspect of hydraulic conductance to quantify is the resistance of pits and other highly distinctive and unique secondary wall features in the earliest tracheids. New analytical methods, in particular X-ray synchrotron microtomography (PPC-SRμCT), open up the possibility of creating dynamic, three-dimensional models of permineralized woods facilitating the analysis of hydraulic and biomechanical properties.

GRAVI-2 space experiment: investigating statoliths displacement and location effects on early stages of gravity perception pathways in plant roots. ASGSR meeting

International audience; Angiosperm trees are able to reorient their axes thanks to their capacities to differentiate tension wood on the upper side of stems and branches. In poplar, tension wood fibres develop an extra cell wall layer, named G-layer, responsible for the peculiar mechanical properties of tension wood. G-layer is composed of highly crystalline cellulose microfibrils embedded in a polysaccharide/glycoprotein matrix devoid of lignin. During the G-layer deposition, cellulose microfibrils get oriented parallel to the fibre axis and placed under the state of high tensile stress. Chitinase-like proteins (CTL) have been proposed to play different roles in plant growth and development. Genes encoding CTL have been associated to cellulose synthesis and their mutation causes ectopic deposition of lignin. Transcripts of CTL2 gene were shown to be highly abundant in differentiating tension wood (Déjardin et al., 2004). Its homologue in Arabidopsis is specifically expressed in stems and co-expressed with the CesA genes involved in cellulose synthesis in secondary cell walls (Persson et al., 2005). In our study, we investigated potential function of CTL2 in the formation and mechanical properties of tension wood. Expression of CTL2 was quantified in different tissues and the effects of CTL2 down-regulation have been characterized in GM poplar trees. CTL2 gene appeared highly expressed in xylem with no difference in expression between tension and opposite wood. Downregulation of CTL2 did not seem to impact growth and development of trees in greenhouse conditions. However, stems appeared to be highly breakable in the transverse direction. Anatomical observations of xylem showed altered formation of Glayers in vessel-surrounding fibres in transgenic poplars. Further analyses indicated a substantial increase in wood lignin content. Cellulose crystallinity, microfibril angle and stem mechanical properties are currently assessed. Results will be presented and discussed in the light of present hypothesis regarding CTL2 function in plant development.

In recent years, the validity of embolism quantification methods has been questioned, especially for long-vesseled plants. Some studies have suggested that cutting xylem while under tension, even under water, might generate artificial cavitation. Accordingly, a rehydration procedure prior to hydraulic measurements has been recommended to avoid this artefact. On the other hand, concerns have been raised that xylem refilling might occur when samples are rehydrated. Here, we explore the potential biases affecting embolism quantification for grapevine (Vitis vinifera L.) petioles harvested under tension or after xylem relaxation. We employ direct visualization of embolism through X-ray micro-computed tomography (microCT) to test for the occurrence of fast refilling (artifactually low per cent loss of conductivity (PLC) due to rehydration prior to sample harvest) as well as excision-induced embolism (artifactually high embolism due to air introduction during harvest). Additionally, we compared the response functions of both stomatal regulation and xylem embolism to xylem pressure (Psix). Short-time (20 min) xylem tension relaxation prior to the hydraulic measurement resulted in a lower degree of embolism than found in samples harvested under native tensions, and yielded xylem vulnerability curves similar to the ones obtained using direct microCT visualization. Much longer periods of hydration (overnight) were required before xylem refilling was observed to occur. In field-grown vines, over 85% of stomatal closure occurred at less negative Psix than that required to induce 12% PLC. Our results demonstrate that relaxation of xylem tension prior to hydraulic measurement allows for the reliable quantification of native embolism in grapevine petioles. Furthermore, we find that stomatal regulation is sufficiently conservative to avoid transpiration-induced cavitation. These results suggest that grapevines have evolved a strategy of cavitation resistance, rather than one of cavitation tolerance (diurnal cycles of embolism and repair).

Virot et al. [E. Virot et al., Phys. Rev. E 93, 023001 (2016)] assert that the critical wind speed at which \ensuremath{\geqslant}50% of all trees in a population break is \ensuremath{\approx}42 m/s, regardless of tree characteristics. We show that empirical data do not support this assertion, and that the assumptions underlying the theory used by Virot et al. are inconsistent with the biomechanics of trees.

The vascular system of grapevine has been reported as being highly vulnerable, even though grapevine regularly experiences seasonal drought. Stomata would consequently remain open below water potentials that would generate a high loss of stem hydraulic conductivity via xylem embolism. This situation would necessitate daily cycles of embolism repair to restore hydraulic function.. However, a more parsimonious explanation is that some hydraulic techniques are prone to artifacts in species with long vessels, leading to overestimation of vulnerability. The aim of this study was to provide an unbiased assessment of (i) the vulnerability to drought-induced embolism in perennial and annual organs, and (ii) the ability to refill embolized vessels in two Vitis species. X-ray micro-CT observations on intact plants indicated that both V. vinifera and V. riparia were relatively vulnerable, with the pressure inducing 50% loss of stem hydraulic conductivity (Ψ50Stem) = -1.7 and -1.3MPa, respectively. In V. vinifera, both the stem and petiole had similar sigmoidal vulnerability curves, but differed in Ψ50 (-1.7 and -1.0MPa for stem and petiole, respectively). Refilling was not observed as long as bulk xylem pressure remained negative (e.g. at the apical part of the plants): P=-0.11{plus minus}0.02MPa; ∆PLC=0.02{plus minus}0.01%). However, positive xylem pressure was observed at the basal part of the plant (P=0.04{plus minus}0.01MPa), leading to recovered conductance (∆PLC=-0.24{plus minus}0.12%). Our findings provide evidence that grapevine is unable to repair embolized xylem vessels under negative pressure, but its hydraulic vulnerability segmentation provides a significant protection of the perennial stem.

The 2D deformation of plain weave ready-to-wear clothing made of bleached cotton was measured during adsorption and desorption steps. A digital X-ray imaging system was coupled with a climatic chamber to control temperature and relative humidity during continuous cycles. A simplex method process was used to compare the acquired images and to obtain the 2D strain. The variations of the 2D deformation in the warp and the weft directions are explained by geometrical consideration of the structure at microscopic and macroscopic levels. The isotropy between the two main directions is explained by weave symmetry and the similar yarns properties. The shear deformation is due to the cohesion by twist between fibers. The influence of the ironing process was studied. The ironing effect allows the yarns to be flattened and increases their friction, which amplifies the deformation in the first adsorption cycle. This effect is cancelled, however, at the beginning of the second adsorption cycle. The structure returns t...