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Steven Jansen - One of the best experts on this subject based on the ideXlab platform.
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within tree variability and sample storage effects of Bordered Pit membranes in xylem of acer pseudoplatanus
Trees-structure and Function, 2020Co-Authors: Rebecca Thom, Martyna M Kotowska, Ya Zhang, Jochen H Schenk, Steven JansenAbstract:Intervessel Pit membranes in xylem tissue of Acer pseudoplatanus differ in their thickness both within and across plant organs and may undergo considerable shrinkage during dehydration and sample preparation. Intervessel Pit membranes have been suggested to account for more than half of the total xylem hydraulic resistance in plants and play a major role in vulnerability to drought-induced hydraulic failure. While the thickness of intervessel Pit membranes was found to be associated with xylem embolism resistance at an interspecific level, variation in Pit membrane structure across different organs along the flow path within a single tree remains largely unknown. Based on transmission electron microscopy, we examined intra-tree variation of Bordered Pit and Pit membrane characteristics in xylem of roots, stems, branches, petioles, and leaf veins of Acer pseudoplatanus. Moreover, potential preparation artefacts on Pit membrane structure such as alcohol treatment and dehydration were tested. Our observations showed quantitative differences in Bordered Pits across organs, including variation in Pit membrane thickness within and across organs. Vessel size was weakly related to intervessel wall thickness, but not significantly linked to Pit membrane thickness. Gradual dehydration of wood samples resulted in irreversible shrinkage of Pit membranes, together with increased levels of aspiration. These findings are relevant to explore similarity in xylem embolism resistance across plant organs.
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The chemical identity of intervessel Pit membranes in Acer challenges hydrogel control of xylem hydraulic conductivity
Aob Plants, 2016Co-Authors: Matthias Klepsch, Marco Schmitt, J. Paul Knox, Steven JansenAbstract:: Ion-mediated enhancement of the hydraulic conductivity of xylem tissue (i.e. the ionic effect) has been reported for various angiosperm species. One explanation of the ionic effect is that it is caused by the swelling and shrinking of intervessel Pit membranes due to the presence of pectins and/or other cell-wall matrix polymers such as heteroxylans or arabinogalactan-proteins (AGPs) that may contain acidic sugars. Here, we examined the ionic effect for six Acer species and their Pit membrane chemistry using immunocytochemistry, including antibodies against glycoproteins. Moreover, anatomical features related to the Bordered Pit morphology and vessel dimensions were investigated using light and electron microscopy. The ionic effect varied from 18 % (± 9) to 32 % (± 13). EPitopes of homogalacturonan (LM18) and xylan (LM11) were not detected in intervessel Pit membranes. Negative results were also obtained for glycoproteins (extensin: LM1, JIM20; AGP glycan: LM2), although AGP (JIM13)-related ePitopes were detected in parenchyma cells. The mean vessel length was significantly correlated with the magnitude of the ionic effect, unlike other Pit or vessel-related characteristics. Our results suggest that intervessel Pit membranes of Acer are unlikely to contain pectic or other acidic polysaccharides. Therefore, alternative explanations should be tested to clarify the ionic effect.
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Model of the mechanical behaviour of Pit estimates Pit quality involvement in interspecific variability in vulnerability to cavitation
2013Co-Authors: Eric Badel, Nicole Brunel, Aude Tixier, HervÉ Cochard, Stephane Herbette, Steven JansenAbstract:Xylem sap is transported in vessels under tension through dead xylem conduits. During drought events, then tension level increases and cavitation of the water column may occur leading to air filled and non functional conduits. Since hydraulic failure impairs the wood productivity and the survival of the tree. In a context of changing climate expecting enhancement of frequency and severity of drought events, there is an increased interest in understanding the mechanism of cavitation phenomena in order to find the anatomical driver that could explain the interspecific variability of VC. Pits represent the most probable air entry point in a vessel. Within angiosperms, the Pit structure (Pit quality) and the Pit area per vessel (Pit quantity) are both suggested to explain variation in VC. Pit quality has generally been measured by quantifying the porosity and related thickness of the Pit membrane. Here, we propose a mechanical model integrating other morphological parameters of a Bordered Pit: Pit depth, chamber diameter and aperture diameter. These parameters allow us to estimate the quality of a Pit from a mechanistic point of view. The model was first used to explain variability in VC for four contrasting species, and was then extended with additional data from literature to perform a broader analysis. . Evidence of residual strains after embolism supports the hypothesis
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Plasmodesmatal pores in the torus of Bordered Pit membranes affect cavitation resistance of conifer xylem
Plant Cell and Environment, 2012Co-Authors: Steven Jansen, Peter Gasson, Jean-baptiste Lamy, Régis Burlett, HervÉ Cochard, Sylvain DelzonAbstract:: The Pit membrane in Bordered Pits of conifer tracheids is characterized by a porous margo and central thickening (torus), which is traditionally considered to function as an impermeable safety valve against air-seeding. However, electron microscopy based on 33 conifer species, including five families and 19 genera, reveals that pores occur in the torus of 13 of the species studied. The pores have a plasmodesmatal origin with an average diameter of 51 nm and grouped arrangement. Evidence for embolism spreading via pores in tori is supported by the pore sizes, which correspond relatively well with the pressure inducing cavitation. Predictions based on earlier correlations between Pit structure and cavitation resistance were only weakly supported for species with punctured tori. Moreover, species with punctured tori are significantly less resistant to cavitation than species with non-punctured tori. Nevertheless, absolute pore diameters must be treated with caution and correlations between theoretical and measured air-seeding pressures are weak. Because most pores appear not to traverse the torus but are limited to one torus pad, only complete pores would trigger air-seeding. Embolism spreading through a leaky torus is not universal across gymnosperms and unlikely to represent the only air-seeding mechanism.
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The phylogenetic significance of vestured Pits in Boraginaceae
Various articles, 2010Co-Authors: David Rabaey, Erik Smets, Frederic Lens, Steven JansenAbstract:The Bordered Pit structure in tracheary elements of 105 Boraginaceae species is studied using scanning electron microscopy to examine the systematic distribution of vestured Pits. Forty-three species out of 16 genera show a uniform presence of this feature throughout their secondary xylem. Most vestures are small, unbranched and associated with the outer Pit aperture of Bordered intervessel Pits. The feature is likely to have originated independently in the distantly related subfamilies Boraginoideae (tribe Lithospermeae) and Ehretioideae. The distribution of vestures in Ehretia agrees with recent molecular phylogenies: (1) species with vestured Pits characterise the Ehretia I group (incl. Rotula), and (2) species with non-vestured Pits belong to the Ehretia II group (incl. Carmona). The occurrence of vestured Pits in Hydrolea provides additional support for excluding this genus from Hydrophylloideae, since Hydrolea is the only species of this subfamily with vestured Pits. Functional advantages of vestured Pits promoting parallel evolution of this conservative feature are suggested. The hydraulic benefits of poorly developed vestures remain underinvestigated.
Maciej A Zwieniecki - One of the best experts on this subject based on the ideXlab platform.
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Ion induced changes in the structure of Bordered Pit membranes.
Frontiers in Plant Science, 2012Co-Authors: Noel Michele Holbrook, Maciej A ZwienieckiAbstract:Ion-mediated changes in xylem hydraulic resistance are hypothesized to result from hydrogel like properties of pectins located in the Bordered Pit membranes separating adjacent xylem vessels. Although the kinetics of the ion-mediated changes in hydraulic resistance are consistent with the swelling/deswelling behavior of pectins, there is no direct evidence of this activity. In this report we use atomic force microscopy (AFM) to investigate structural changes in Bordered Pit membranes associated with changes in the ionic concentration of the surrounding solution. When submerged in de-ionized water, AFM revealed Bordered Pit membranes as relatively smooth, soft, and lacking any sharp edges surface, in contrast to pictures from scanning electron microscope (SEM) or AFM performed on air-dry material. Exposure of the Bordered Pit membranes to 50 mM KCl solution resulted in significant changes in both surface physical properties and elevation features. Specifically, Bordered Pit membranes became harder and the fiber edges were clearly visible. In addition, the membrane contracted and appeared much rougher due to exposed microfibers. In neither solution was there any evidence of discrete pores through the membrane whose dimensions were altered in response to the ionic composition of the surrounding solution. Instead the variable hydraulic resistance appears to involve changes in the both the permeability and the thickness of the Pit membrane.
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changes in Pit membrane porosity due to deflection and stretching the role of vestured Pits
Journal of Experimental Botany, 2004Co-Authors: Brendan Choat, Maciej A Zwieniecki, Erik Smets, Steven Jansen, Michelle N HolbrookAbstract:The effect of increasing pressure difference (deltaP) on intervessel Pit membrane porosity was studied in two angiosperm tree species with differing Pit architecture. Fraxinus americana L. possesses typical angiosperm Bordered Pit structure while Sophora japonica L. exhibits well-developed vestures in intervessel Pit chambers. It was hypothesized (a) that large deltaP across intervessel Pits would cause the deflection of Pit membranes in the stems of F. americana resulting in significant increases in porosity and thus lower cavitation thresholds, and (b) that the presence of vestures would prevent the deflection of Pit membranes in S. japonica. To determine if the porosity of Pit membranes increased under mechanical stress, suspensions of colloidal gold, 5 nm and 20 nm in diameter, were perfused across intervessel Pit membranes at deltaP ranging from 0.25 MPa to 6.0 MPa. The effect of increasing deltaP on membrane porosity was also tested by comparing air seeding thresholds (P(a)) in stems perfused with water or a solution with lower surface tension. Air seeding and colloidal gold experiments indicated that Pit membrane porosity increased significantly with deltaP in F. americana. In S. japonica, increases in permeability to colloidal gold with deltaP were small and maximum pore diameters predicted from P(a) were independent of deltaP, suggesting that vestures limited the degree to which the membrane can be deflected from the centre of the Pit cavity. This provides the first experimental evidence that vestures reduce the probability of air seeding through Pit membranes.
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Changes in Pit membrane porosity due to deflection and stretching: the role of vestured Pits
2004Co-Authors: Brendan Choat, Maciej A Zwieniecki, Erik Smets, Steven Jansen, Michele N HolbrookAbstract:The effect of increasing pressure difference (DP) on intervessel Pit membrane porosity was studied in two angiosperm tree species with differing Pit architecture. Fraxinus americana L. possesses typical angiosperm Bordered Pit structure while Sophora japonica L. exhib-its well-developed vestures in intervessel Pit chambers. It was hypothesized (a) that large DP across intervessel Pits would cause the deflection of Pit membranes in the stems of F. americana resulting in significant increases in porosity and thus lower cavitation thresholds, and (b) that the presence of vestures would prevent the de-flection of Pit membranes in S. japonica. To determine if the porosity of Pit membranes increased under me-chanical stress, suspensions of colloidal gold, 5 nm and 20 nm in diameter, were perfused across intervessel Pit membranes at DP ranging from 0.25 MPa to 6.0 MPa. The effect of increasing DP on membrane porosity was also tested by comparing air seeding thresholds (Pa) in stems perfused with water or a solution with lower surface tension. Air seeding and colloidal gold experi-ments indicated that Pit membrane porosity increased significantly with DP in F. americana. In S. japonica, increases in permeability to colloidal gold with DP were small and maximum pore diameters predicted from Pa were independent of DP, suggesting that vestures limited the degree to which the membrane can be deflected from the centre of the Pit cavity. This provides the first experimental evidence that vestures reduce the probability of air seeding through Pit membranes
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Vulnerability of xylem vessels to cavitation in sugar maple. Scaling from individual vessels to whole branches.
Plant physiology, 2003Co-Authors: Peter J Melcher, Maciej A Zwieniecki, N. Michele HolbrookAbstract:The relation between xylem vessel age and vulnerability to cavitation of sugar maple (Acer saccharum Marsh.) was quantified by measuring the pressure required to force air across Bordered Pit membranes separating individual xylem vessels. We found that the Bordered Pit membranes of vessels located in current year xylem could withstand greater applied gas pressures (3.8 MPa) compared with Bordered Pit membranes in vessels located in older annular rings (2.0 MPa). A longitudinal transect along 6-year-old branches indicated that the pressure required to push gas across Bordered Pit membranes of current year xylem did not vary with distance from the growing tip. To understand the contribution of age-related changes in vulnerability to the overall resistance to cavitation, we combined data on the pressure thresholds of individual xylem vessels with measurements of the relative flow rate through each annual ring. The annual ring of the current year contributed only 16% of the total flow measured on 10-cm-long segments cut from 6-year-old branches, but it contributed more than 70% of the total flow when measured through 6-year-old branches to the point of leaf attachment. The vulnerability curve calculated using relative flow rates measured on branch segments were similar to vulnerability curves measured on 6-year-old branches (pressure that reduces hydraulic conductance by 50% = 1.6-2.4 MPa), whereas the vulnerability curve calculated using relative flow rates measured on 6-year-old branches were similar to ones measured on the extension growth of the current year (pressure that reduces hydraulic conductance by 50% = 3.8 MPa). These data suggest that, in sugar maple, the xylem of the current year can withstand larger xylem tensions than older wood and dominates water delivery to leaves.
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Ionic control of the lateral exchange of water between vascular bundles in tomato
2003Co-Authors: Maciej A Zwieniecki, Peter J Melcher, Colin M Orians, Michele N HolbrookAbstract:Abstract Ions can enhance water¯ow through the xylem via changes in the hydraulic resistance at border Pit membranes. Because¯ow between adjacent xylem vessels occurs primarily via Bordered Pit ®elds, it is hypothesized that xylem sap ion concentrations would affect lateral movement of water more than longitudinal¯ow. Using tomato as a model system, evidence is presented for ion-mediated changes in xylem hydraulic resistance and the lateral transport of water. Water¯ow between adjacent xylem bundles increased by approximately 50% in the presence of ions while longitudinal¯ow only increased by approximately 20%. However, the enhancement of lateral exchange due to ions was magni®ed by the presence of a pressure difference between vascular bundles. These results indicate that the degree of nutrient-sharing among sectors of a plant may depend on both nutrient concentration and the availability of water in the root zone
Sylvain Delzon - One of the best experts on this subject based on the ideXlab platform.
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Plasmodesmatal pores in the torus of Bordered Pit membranes affect cavitation resistance of conifer xylem
Plant Cell and Environment, 2012Co-Authors: Steven Jansen, Peter Gasson, Jean-baptiste Lamy, Régis Burlett, HervÉ Cochard, Sylvain DelzonAbstract:: The Pit membrane in Bordered Pits of conifer tracheids is characterized by a porous margo and central thickening (torus), which is traditionally considered to function as an impermeable safety valve against air-seeding. However, electron microscopy based on 33 conifer species, including five families and 19 genera, reveals that pores occur in the torus of 13 of the species studied. The pores have a plasmodesmatal origin with an average diameter of 51 nm and grouped arrangement. Evidence for embolism spreading via pores in tori is supported by the pore sizes, which correspond relatively well with the pressure inducing cavitation. Predictions based on earlier correlations between Pit structure and cavitation resistance were only weakly supported for species with punctured tori. Moreover, species with punctured tori are significantly less resistant to cavitation than species with non-punctured tori. Nevertheless, absolute pore diameters must be treated with caution and correlations between theoretical and measured air-seeding pressures are weak. Because most pores appear not to traverse the torus but are limited to one torus pad, only complete pores would trigger air-seeding. Embolism spreading through a leaky torus is not universal across gymnosperms and unlikely to represent the only air-seeding mechanism.
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Plasmodesmatal pores in the torus of Bordered Pit membranes affect cavitation resistance of conifer xylem. Plant, cell & environment 35
2012Co-Authors: S. Jansen, Peter Gasson, Régis Burlett, -b. J. Lamy, H. Cochard, Sylvain DelzonAbstract:The Pit membrane in Bordered Pits of conifer tracheids is characterized by a porous margo and central thickening (torus), which is traditionally considered to function as an impermeable safety valve against air-seeding. However, electron microscopy based on 33 conifer species, including five families and 19 genera, reveals that pores occur in the torus of 13 of the species studied. The pores have a plas-modesmatal origin with an average diameter of 51 nm and grouped arrangement. Evidence for embolism spreading via pores in tori is supported by the pore sizes, which cor-respond relatively well with the pressure inducing cavita-tion. Predictions based on earlier correlations between Pit structure and cavitation resistance were only weakly sup-ported for species with punctured tori. Moreover, species with punctured tori are significantly less resistant to cavita-tion than species with non-punctured tori. Nevertheless, absolute pore diameters must be treated with caution and correlations between theoretical and measured air-seeding pressures are weak. Because most pores appear not to traverse the torus but are limited to one torus pad, only complete pores would trigger air-seeding. Embolism spreading through a leaky torus is not universal across gym-nosperms and unlikely to represent the only air-seeding mechanism. Key-words: air-seeding; conifer wood; torus-margo; trac-heid
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Mechanism of water-stress induced cavitation in conifers: Bordered Pit structure and function support the hypothesis of seal capillary-seeding
Plant Cell and Environment, 2010Co-Authors: Sylvain Delzon, Cyril Douthe, Anna Sala, HervÉ CochardAbstract:Resistance to water-stress induced cavitation is an important indicator of drought tolerance in woody species and is known to be intimately linked to the anatomy of the xylem. However, the actual mechanical properties of the Pit membrane are not well known and the exact mode of air-seeding by which cavitation occurs is still uncertain. We examined the relationship between cavitation resistance and Bordered Pit structure and function in 40 coniferous species. Xylem pressure inducing 50% loss of hydraulic conductance (P(50), a proxy for cavitation resistance) varied widely among species, from -2.9 to -11.3 MPa. The valve effect of the Pit membrane, measured as a function of margo flexibility and torus overlap, explained more variation in cavitation-resistance than simple anatomical traits such as Pit membrane, Pit aperture or torus size. Highly cavitation resistant species exhibited both a high flexibility of the margo and a large overlap between the torus and the Pit aperture, allowing the torus to tightly seal the Pit aperture. Our results support the hypothesis of seal capillary-seeding as the most likely mode of air-seeding, and suggest that the adhesion of the torus to the Pit border may be the main determinant of cavitation resistance in conifers.
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New Insights into the Mechanisms of Water-Stress-Induced Cavitation in Conifers
Plant Physiology, 2009Co-Authors: HervÉ Cochard, Stephane Herbette, Sylvain Delzon, Teemu Holtta, Maurizio MencucciniAbstract:Cavitation resistance is a key parameter to understand tree drought tolerance but little is known about the mechanisms of air entry into xylem conduits. For conifers three mechanisms have been proposed: (1) a rupture of Pit margo microfibrils, (2) a displacement of the Pit torus from its normal sealing position over the Pit aperture, and (3) a rupture of an air-water menisci in a pore of the Pit margo. In this article, we report experimental results on three coniferous species suggesting additional mechanisms. First, when xylem segments were injected with a fluid at a pressure sufficient to aspirate Pit tori and well above the pressure for cavitation induction we failed to detect the increase in sample conductance that should have been caused by torus displacement from blocking the Pit aperture or by membrane rupture. Second, by injecting xylem samples with different surfactant solutions, we found a linear relation between sample vulnerability to cavitation and fluid surface tension. This suggests that cavitation in conifers could also be provoked by the capillary failure of an air-water meniscus in coherence with the prediction of Young-Laplace's equation. Within the Bordered Pit membrane, the exact position of this capillary seeding is unknown. The possible Achilles' heel could be the seal between tori and Pit walls or holes in the torus. The mechanism of water-stress-induced cavitation in conifers could then be relatively similar to the one currently proposed for angiosperms.
Holger Militz - One of the best experts on this subject based on the ideXlab platform.
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A new approach for the study of the chemical composition of Bordered Pit membranes: 4Pi and confocal laser scanning microscopy
American Journal of Botany, 2013Co-Authors: Daniela Maschek, Mark D. Lessard, Barry Goodell, Jody Jellison, Holger MilitzAbstract: Premise of the study: Coniferous Bordered Pits are some of the most unique and fascinating microstructures of the lignifi ed cell wall. The Pit membrane consists of a margo and a torus region, hence facilitating both xylary water transport and also limiting air intrusion by Pit aspiration. Additionally, Bordered Pits have been reported to play a decisive role in the control of rapid liquid fl ow via the shrinkage and swelling of pectin. The study of the nanostructural chemical composition of Pit membranes has been diffi cult with common imaging/chemical techniques, which involve drying and/or coating of the samples. Methods: Using fl uorescent tagging and antibodies specifi c to pectin, and a His-tagged cellulose-binding module that reacts with crystalline cellulose, in combination with confocal laser scanning microscopy (CLSM) and 4Pi microscopy, we generated three-dimensional images of intact Pit membranes. Key results: With enhanced resolution in the z -direction of the 4Pi microscope, it was possible to distinguish cellulose in the torus and the margo strands of Pinus strobus . The torus was surrounded by pectin, and a pectin ring was found at the margin of the torus. We also found differences in the structure of the Pit membrane between aspirated and unaspirated Pits, with a displacement of pectin to form a ring-like structure, the collapse of a void in the interior of the torus, and an apparent change in the chemical structure of cellulosic components, during the aspiration process. Conclusions: The 4Pi microscope is well suited to scanning Pit membranes to discover previously undescribed anatomical features in Bordered Pits and can provide information on chemical composition when used in combination with appropriate probes.
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a new approach for the study of the chemical composition of Bordered Pit membranes 4pi and confocal laser scanning microscopy
American Journal of Botany, 2013Co-Authors: Daniela Maschek, Mark D. Lessard, Barry Goodell, Jody Jellison, Holger MilitzAbstract:UNLABELLED PREMISE OF THE STUDY Coniferous Bordered Pits are some of the most unique and fascinating microstructures of the lignified cell wall. The Pit membrane consists of a margo and a torus region, hence facilitating both xylary water transport and also limiting air intrusion by Pit aspiration. Additionally, Bordered Pits have been reported to play a decisive role in the control of rapid liquid flow via the shrinkage and swelling of pectin. The study of the nanostructural chemical composition of Pit membranes has been difficult with common imaging/chemical techniques, which involve drying and/or coating of the samples. • METHODS Using fluorescent tagging and antibodies specific to pectin, and a His-tagged cellulose-binding module that reacts with crystalline cellulose, in combination with confocal laser scanning microscopy (CLSM) and 4Pi microscopy, we generated three-dimensional images of intact Pit membranes. • KEY RESULTS With enhanced resolution in the z-direction of the 4Pi microscope, it was possible to distinguish cellulose in the torus and the margo strands of Pinus strobus. The torus was surrounded by pectin, and a pectin ring was found at the margin of the torus. We also found differences in the structure of the Pit membrane between aspirated and unaspirated Pits, with a displacement of pectin to form a ring-like structure, the collapse of a void in the interior of the torus, and an apparent change in the chemical structure of cellulosic components, during the aspiration process. • CONCLUSIONS The 4Pi microscope is well suited to scanning Pit membranes to discover previously undescribed anatomical features in Bordered Pits and can provide information on chemical composition when used in combination with appropriate probes.
HervÉ Cochard - One of the best experts on this subject based on the ideXlab platform.
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Model of the mechanical behaviour of Pit estimates Pit quality involvement in interspecific variability in vulnerability to cavitation
2013Co-Authors: Eric Badel, Nicole Brunel, Aude Tixier, HervÉ Cochard, Stephane Herbette, Steven JansenAbstract:Xylem sap is transported in vessels under tension through dead xylem conduits. During drought events, then tension level increases and cavitation of the water column may occur leading to air filled and non functional conduits. Since hydraulic failure impairs the wood productivity and the survival of the tree. In a context of changing climate expecting enhancement of frequency and severity of drought events, there is an increased interest in understanding the mechanism of cavitation phenomena in order to find the anatomical driver that could explain the interspecific variability of VC. Pits represent the most probable air entry point in a vessel. Within angiosperms, the Pit structure (Pit quality) and the Pit area per vessel (Pit quantity) are both suggested to explain variation in VC. Pit quality has generally been measured by quantifying the porosity and related thickness of the Pit membrane. Here, we propose a mechanical model integrating other morphological parameters of a Bordered Pit: Pit depth, chamber diameter and aperture diameter. These parameters allow us to estimate the quality of a Pit from a mechanistic point of view. The model was first used to explain variability in VC for four contrasting species, and was then extended with additional data from literature to perform a broader analysis. . Evidence of residual strains after embolism supports the hypothesis
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Plasmodesmatal pores in the torus of Bordered Pit membranes affect cavitation resistance of conifer xylem
Plant Cell and Environment, 2012Co-Authors: Steven Jansen, Peter Gasson, Jean-baptiste Lamy, Régis Burlett, HervÉ Cochard, Sylvain DelzonAbstract:: The Pit membrane in Bordered Pits of conifer tracheids is characterized by a porous margo and central thickening (torus), which is traditionally considered to function as an impermeable safety valve against air-seeding. However, electron microscopy based on 33 conifer species, including five families and 19 genera, reveals that pores occur in the torus of 13 of the species studied. The pores have a plasmodesmatal origin with an average diameter of 51 nm and grouped arrangement. Evidence for embolism spreading via pores in tori is supported by the pore sizes, which correspond relatively well with the pressure inducing cavitation. Predictions based on earlier correlations between Pit structure and cavitation resistance were only weakly supported for species with punctured tori. Moreover, species with punctured tori are significantly less resistant to cavitation than species with non-punctured tori. Nevertheless, absolute pore diameters must be treated with caution and correlations between theoretical and measured air-seeding pressures are weak. Because most pores appear not to traverse the torus but are limited to one torus pad, only complete pores would trigger air-seeding. Embolism spreading through a leaky torus is not universal across gymnosperms and unlikely to represent the only air-seeding mechanism.
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Mechanism of water-stress induced cavitation in conifers: Bordered Pit structure and function support the hypothesis of seal capillary-seeding
Plant Cell and Environment, 2010Co-Authors: Sylvain Delzon, Cyril Douthe, Anna Sala, HervÉ CochardAbstract:Resistance to water-stress induced cavitation is an important indicator of drought tolerance in woody species and is known to be intimately linked to the anatomy of the xylem. However, the actual mechanical properties of the Pit membrane are not well known and the exact mode of air-seeding by which cavitation occurs is still uncertain. We examined the relationship between cavitation resistance and Bordered Pit structure and function in 40 coniferous species. Xylem pressure inducing 50% loss of hydraulic conductance (P(50), a proxy for cavitation resistance) varied widely among species, from -2.9 to -11.3 MPa. The valve effect of the Pit membrane, measured as a function of margo flexibility and torus overlap, explained more variation in cavitation-resistance than simple anatomical traits such as Pit membrane, Pit aperture or torus size. Highly cavitation resistant species exhibited both a high flexibility of the margo and a large overlap between the torus and the Pit aperture, allowing the torus to tightly seal the Pit aperture. Our results support the hypothesis of seal capillary-seeding as the most likely mode of air-seeding, and suggest that the adhesion of the torus to the Pit border may be the main determinant of cavitation resistance in conifers.
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Poplar vulnerability to xylem cavitation acclimates to drier soil conditions
Physiologia Plantarum, 2010Co-Authors: Hosam Mohammed Awad, HervÉ Cochard, Eric Badel, Tete Severien Barigah, Stephane HerbetteAbstract:Xylem vulnerability to cavitation differs between tree species according to their drought resistance, more xerophilous species being more resistant to xylem cavitation. Variability in xylem vulnerability to cavitation is also found within species, especially between in situ populations. The origin of this variability has not been clearly identified. Here we analyzed the response of xylem hydraulic traits of Populus tremula xPopulus alba trees to three different soil water regimes. Stem xylem vulnerability was scored as the xylem water potential causing 12, 50 and 88% loss of conductivity (P(12), P(50) and P(88)). Vulnerability to cavitation was found to acclimate to growing conditions under different levels of soil water content, with P(50) values of -1.82, -2.03 and -2.45 MPa in well-watered, moderately water-stressed and severely water-stressed poplars, respectively. The value of P(12), the xylem tension at which cavitation begins, was correlated with the lowest value of midday leaf water potential (epsilon m) experienced by each plant, the difference between the two parameters being approximately 0.5 MPa, consistent with the absence of any difference in embolism level between the different water treatments. These results support the hypothesis that vulnerability to cavitation is a critical trait for resistance to drought. The decrease in vulnerability to cavitation under growing conditions of soil drought was correlated with decreased vessel diameter, increased vessel wall thickness and a stronger Bordered Pit field (t/b)2. The links between these parameters are discussed.
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New Insights into the Mechanisms of Water-Stress-Induced Cavitation in Conifers
Plant Physiology, 2009Co-Authors: HervÉ Cochard, Stephane Herbette, Sylvain Delzon, Teemu Holtta, Maurizio MencucciniAbstract:Cavitation resistance is a key parameter to understand tree drought tolerance but little is known about the mechanisms of air entry into xylem conduits. For conifers three mechanisms have been proposed: (1) a rupture of Pit margo microfibrils, (2) a displacement of the Pit torus from its normal sealing position over the Pit aperture, and (3) a rupture of an air-water menisci in a pore of the Pit margo. In this article, we report experimental results on three coniferous species suggesting additional mechanisms. First, when xylem segments were injected with a fluid at a pressure sufficient to aspirate Pit tori and well above the pressure for cavitation induction we failed to detect the increase in sample conductance that should have been caused by torus displacement from blocking the Pit aperture or by membrane rupture. Second, by injecting xylem samples with different surfactant solutions, we found a linear relation between sample vulnerability to cavitation and fluid surface tension. This suggests that cavitation in conifers could also be provoked by the capillary failure of an air-water meniscus in coherence with the prediction of Young-Laplace's equation. Within the Bordered Pit membrane, the exact position of this capillary seeding is unknown. The possible Achilles' heel could be the seal between tori and Pit walls or holes in the torus. The mechanism of water-stress-induced cavitation in conifers could then be relatively similar to the one currently proposed for angiosperms.