On the air permeability of Populus pit

Sap hydrodynamics in vascular cells of trees seems to be controlled by small membranes called pits. Understanding how the pit junctions regulate the sap flow and stop embolism by cavitation is today a challenging issue. The hypothesis that the pit porosity adjusts the flow under negative pressure and stops the air bubble diffusion need to be validated. In this talk, we will present the experimental results on Populus trees that support the idea that pits operate "passively" in a biological point of view. This work is based on atomic force microscope (AFM) experiments, which have been realised to measure quantitatively the mechanical properties of pits at the nanoscale

Couplage fluide-structure dans l’embolie gazeuse du peuplier

Les arbres irriguent leurs organes par un système microfluidique complexe qui permet le transfert d’eau sous des pressions hydrostatiques négatives allant de -0.1 à -13 MPa. Dans ces conditions de métastabilité, les arbres vivent avec le risque d’une vaporisation soudaine de leur sève, qui conduit à l’embolie. Après cavitation, la poche de gaz créée croît ensuite par différence de pression et diffuse dans les vaisseaux du xylème à travers des membranes poreuses appelées “ponctuations”. Ces membranes permettent d’assurer le transfert hydrique vasculaire et de lutter contre la propagation de l’embolie gazeuse pouvant conduire à un état létal de la plante. Pour comprendre les mécanismes d’embolie d’air dans le xylème, nous avons pu étudier la structure des ponctuations du peuplier et caractériser par microscopie à force atomique (AFM) leurs propriétés mécaniques. Nous avons fait des expériences de nano-indentation et de flexion sur des échantillons secs et saturés en eau. Les premières expériences semblent montrer que les propriétés mécaniques sont peu affectées par la sorption d’eau (le module d’Young de la membrane primaire est E 0:40 GPa). Nous avons pu établir que le module des ponctuations était inférieur à celui de la paroi vaisseaux du xylème (E 8 GPa). Des expériences de micromoulages ont montré que les membranes des ponctuations se déforment sous l’effet d’une différence de pression. En tenant compte de la présence des parois secondaires, l’analyse par éléments finis de la déformation des ponctuations permet de calculer le module d’Young des membranes qui est identique à celui déterminé en AFM. Nous avons pu mettre en évidence que les mâchoires constituées par les parois secondaires limitent la déformation de la membrane au niveau de l’encastrement près du bord. Des expériences d’injection d’air ont permis de déterminer la pression critique (Pc = 1:8 MPa) et les diamètres critiques des pores présents sur les valves capillaires (dpore 160 nm). La taille des pores estimée est cohérente avec les données de la littérature. Nous avons proposé une première modélisation de la propagation d’une embolie sur la base de l’écoulement de Darcy dans la membrane. L’ensemble des résultats semble montrer que la diffusion de gaz est rendu possible par l’effet conjoint de la déformation de la membrane qui génère l’ouverture des pores diminuant ainsi la pression critique de passage du gaz et de la rupture des ponts capillaires présents dans les pores de la membrane. ABSTRACT : The xylem vessels of trees constitute a model natural microfluidic system. In this work, we have studied the mechanism of air flow in the Populus xylem. The vessel microstructure was characterized by optical microscopy, transmission electronic microscopy (TEM), and atomic force microscopy (AFM) at different length scales. The xylem vessels have length ≈ 15 cm and diameter ≈ 20 μm. Flow from one vessel to the next occurs through ∼ 10 2 pits, which are grouped together at the ends of the vessels. The pits contain a thin, porous pit membrane with a thickness of 310 nm. We have measured the Young’s moduli of the vessel wall and of the pits (both water-saturated and after drying) by specific nanoindentation and nanoflexion experiments with AFM. We found that both the dried and water-saturated pit membranes have Young’s modulus around 0.4 MPa, in agreement with values obtained by micromolding of pits deformed by an applied pressure difference. Air injection experiments reveal that air flows through the xylem vessels when the differential pressure across a sample is larger than a critical value ∆P c ≈ 1.8 MPa. In order to model the air flow rate for ∆P ≥ ∆P c , we assumed the pit membrane to be a porous medium that is strained by the applied pressure difference. Water menisci in the pit pores play the role of capillary valves, which open at ∆P = ∆P c . From the point of view of the plant physiology, this work presents a basic understanding of the physics of bordered pits

One-dimension visco-elastic modelling of wood in the process of formation to clarify the Hygrothermal Recovery behavior of tension wood

International audienceWood production on stem by deposit of concentric layers on its periphery are going along with the setting up of growth stress. Growth stress has two origins: (1) loading due to weight of the structure is applied progressively when the tree is growing; (2) cell maturation, which happened at the end of the deposit of a new layer, causes an expansion, called maturation deformation, which can’t happen freely due to the previous layer and lead to the creation of initial growth stress [1]. The growth stress can be released during cutting and also during hygrothermal treatment (HT), it can be called Hygrothermal Recovery (HTR) [2]

On the air permability of Populus pit

Sap hydrodynamics in vascular cells of trees seems to be controlled by small membranes called pits. Understanding how the pit junctions regulate the sap flow and stop embolism by cavitation is today a challenging issue. The hypothesis that the pit porosity adjusts the flow under negative pressure and stops the air bubble diffusion need to be validated. In this talk, we will present the experimental results on Populus trees that support the idea that pits operate ‘passively’ in a biological point of view. This work is based on atomic force microscope (AFM) experiments, which have been realised to measure quantitatively the mechanical properties of pits at the nanoscale

Modelling the mechanical behaviour of pit membranes in bordered pits with respect to cavitation resistance in angiosperms

Background and Aims Various correlations have been identified between anatomical features of bordered pits in angiosperm xylem and vulnerability to cavitation, suggesting that the mechanical behaviour of the pits may play a role. Theoretical modelling of the membrane behaviour has been undertaken, but it requires input of parameters at the nanoscale level. However, to date, no experimental data have indicated clearly that pit membranes experience strain at high levels during cavitation events. Methods Transmission electron microscopy (TEM) was used in order to quantify the pit micromorphology of four tree species that show contrasting differences in vulnerability to cavitation, namely Sorbus aria, Carpinus betulus, Fagus sylvatica and Populus tremula. This allowed anatomical characters to be included in a mechanical model that was based on the Kirchhoff–Love thin plate theory. A mechanistic model was developed that included the geometric features of the pits that could be measured, with the purpose of evaluating the pit membrane strain that results from a pressure difference being applied across the membrane. This approach allowed an assessment to be made of the impact of the geometry of a pit on its mechanical behaviour, and provided an estimate of the impact on air-seeding resistance. Key Results The TEM observations showed evidence of residual strains on the pit membranes, thus demonstrating that this membrane may experience a large degree of strain during cavitation. The mechanical modelling revealed the interspecific variability of the strains experienced by the pit membrane, which varied according to the pit geometry and the pressure experienced. The modelling output combined with the TEM observations suggests that cavitation occurs after the pit membrane has been deflected against the pit border. Interspecific variability of the strains experienced was correlated with vulnerability to cavitation. Assuming that air-seeding occurs at a given pit membrane strain, the pressure predicted by the model to achieve this mechanical state corresponds to experimental values of cavitation sensitivity (P50). Conclusions The results provide a functional understanding of the importance of pit geometry and pit membrane structure in air-seeding, and thus in vulnerability to cavitation