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1.  The accumulation pattern of ferruginol in the heartwood-forming Cryptomeria japonica xylem as determined by time-of-flight secondary ion mass spectrometry and quantity analysis 
Annals of Botany  2014;113(6):1029-1036.
Background and Aims
Heartwood formation is a unique phenomenon of tree species. Although the accumulation of heartwood substances is a well-known feature of the process, the accumulation mechanism remains unclear. The aim of this study was to determine the accumulation process of ferruginol, a predominant heartwood substance of Cryptomeria japonica, in heartwood-forming xylem.
Methods
The radial accumulation pattern of ferruginol was examined from sapwood and through the intermediate wood to the heartwood by direct mapping using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The data were compared with quantitative results obtained from a novel method of gas chromatography analysis using laser microdissection sampling and with water distribution obtained from cryo-scanning electron microscopy.
Key Results
Ferruginol initially accumulated in the middle of the intermediate wood, in the earlywood near the annual ring boundary. It accumulated throughout the entire earlywood in the inner intermediate wood, and in both the earlywood and the latewood in the heartwood. The process of ferruginol accumulation continued for more than eight annual rings. Ferruginol concentration peaked at the border between the intermediate wood and heartwood, while the concentration was less in the latewood compared wiht the earlywood in each annual ring. Ferruginol tended to accumulate around the ray parenchyma cells. In addition, at the border between the intermediate wood and heartwood, the accumulation was higher in areas without water than in areas with water.
Conclusions
TOF-SIMS clearly revealed ferruginol distribution at the cellular level. Ferruginol accumulation begins in the middle of intermediate wood, initially in the earlywood near the annual ring boundary, then throughout the entire earlywood, and finally across to the whole annual ring in the heartwood. The heterogeneous timing of ferruginol accumulation could be related to the distribution of ray parenchyma cells and/or water in the heartwood-forming xylem.
doi:10.1093/aob/mcu028
PMCID: PMC3997644  PMID: 24651372
Wood formation; heartwood substances; intermediate wood; earlywood; extractives; ferruginol; Cryptomeria japonica; parenchyma cell; water; time-of-flight secondary ion mass spectrometry; TOF-SIMS; gas chromatography; GC; laser microdissection; LMD; cryo-scanning electron microscopy; cryo-SEM
2.  A rapid decrease in temperature induces latewood formation in artificially reactivated cambium of conifer stems 
Annals of Botany  2012;110(4):875-885.
Background and Aims
Latewood formation in conifers occurs during the later part of the growing season, when the cell division activity of the cambium declines. Changes in temperature might be important for wood formation in trees. Therefore, the effects of a rapid decrease in temperature on cellular morphology of tracheids were investigated in localized heating-induced cambial reactivation in Cryptomeria japonica trees and in Abies firma seedlings.
Methods
Electric heating tape and heating ribbon were wrapped on the stems of C. japonica trees and A. firma seedlings. Heating was discontinued when 11 or 12 and eight or nine radial files of differentiating and differentiated tracheids had been produced in C. japonica and A. firma stems, respectively. Tracheid diameter, cell wall thickness, percentage of cell wall area and percentage of lumen area were determined by image analysis of transverse sections and scanning electron microscopy.
Key Results
Localized heating induced earlier cambial reactivation and xylem differentiation in stems of C. japonica and A. firma as compared with non-heated stems. One week after cessation of heating, there were no obvious changes in the dimensions of the differentiating tracheids in the samples from adult C. japonica. In contrast, tracheids with a smaller diameter were observed in A. firma seedlings after 1 week of cessation of heating. Two or three weeks after cessation of heating, tracheids with reduced diameters and thickened cell walls were found. The results showed that the rapid decrease in temperature produced slender tracheids with obvious thickening of cell walls that resembled latewood cells.
Conclusions
The results suggest that a localized decrease in temperature of stems induces changes in the diameter and cell wall thickness of differentiating tracheids, indicating that cambium and its derivatives can respond directly to changes in temperature.
doi:10.1093/aob/mcs149
PMCID: PMC3423807  PMID: 22843340
Cambial activity; conifers; latewood formation; morphology of tracheids; rapid decrease in temperature
3.  Assessing Conifer Ray Parenchyma for Ecological Studies: Pitfalls and Guidelines 
Ray parenchyma is an essential tissue for tree functioning and survival. This living tissue plays a major role for storage and transport of water, nutrients, and non-structural carbohydrates (NSC), thus regulating xylem hydraulics and growth. However, despite the importance of rays for tree carbon and water relations, methodological challenges hamper knowledge about ray intra- and inter-tree variability and its ecological meaning. In this study we provide a methodological toolbox for soundly quantifying spatial and temporal variability of different ray features. Anatomical ray features were surveyed in different cutting planes (cross-sectional, tangential, and radial) using quantitative image analysis on stem-wood micro-sections sampled from 41 mature Scots pines (Pinus sylvestris). The percentage of ray surface (PERPAR), a proxy for ray volume, was compared among cutting planes and between early- and latewood to assess measurement-induced variability. Different tangential ray metrics were correlated to assess their similarities. The accuracy of cross-sectional and tangential measurements for PERPAR estimates as a function of number of samples and the measured wood surface was assessed using bootstrapping statistical technique. Tangential sections offered the best 3D insight of ray integration into the xylem and provided the most accurate estimates of PERPAR, with 10 samples of 4 mm2 showing an estimate within ±6.0% of the true mean PERPAR (relative 95% confidence interval, CI95), and 20 samples of 4 mm2 showing a CI95 of ±4.3%. Cross-sections were most efficient for establishment of time series, and facilitated comparisons with other widely used xylem anatomical features. Earlywood had significantly lower PERPAR (5.77 vs. 6.18%) and marginally fewer initiating rays than latewood. In comparison to tangential sections, PERPAR was systematically overestimated (6.50 vs. 4.92%) and required approximately twice the sample area for similar accuracy. Radial cuttings provided the least accurate PERPAR estimates. This evaluation of ray parenchyma in conifers and the presented guidelines regarding data accuracy as a function of measured wood surface and number of samples represent an important methodological reference for ray quantification, which will ultimately improve the understanding of the fundamental role of ray parenchyma tissue for the performance and survival of trees growing in stressed environments.
doi:10.3389/fpls.2015.01016
PMCID: PMC4649045  PMID: 26635842
cutting plane; measured wood surface; measurement accuracy; non-structural carbohydrates (NSC); number of samples; ray density; ray dimensions; ray volume
4.  Hydraulic and mechanical properties of young Norway spruce clones related to growth and wood structure 
Tree physiology  2007;27(8):1165-1178.
Summary
Stem segments of eight five-year-old Norway spruce (Picea abies (L.) Karst.) clones differing in growth characteristics were tested for maximum specific hydraulic conductivity (ks100), vulnerability to cavitation and behavior under mechanical stress. The vulnerability of the clones to cavitation was assessed by measuring the applied air pressure required to cause 12 and 50% loss of conductivity (Ψ12, Ψ50) and the percent loss of conductivity at 4 MPa applied air pressure (PLC4MPa). The bending strength and stiffness and the axial compression strength and stiffness of the same stem segments were measured to characterize wood mechanical properties. Growth ring width, wood density, latewood percentage, lumen diameter, cell wall thickness, tracheid length and pit dimensions of earlywood cells, spiral grain and microfibril angles were examined to identify structure–function relationships. High ks100 was strongly and positively related to spiral grain angle, which corresponded positively to tracheid length and pit dimensions. Spiral grain may reduce flow resistance of the bordered pits of the first earlywood tracheids, which are characterized by rounded tips and an equal distribution of pits along the entire length. Wood density was unrelated to hydraulic vulnerability parameters. Traits associated with higher hydraulic vulnerability were long tracheids, high latewood percentage and thick earlywood cell walls. The positive relationship between earlywood cell wall thickness and vulnerability to cavitation suggest that air seeding through the margo of bordered pits may occur in earlywood. There was a positive phenotypic and genotypic relationship between ks100 and PLC4MPa, and both parameters were positively related to tree growth rate. Variability in mechanical properties depended mostly on wood density, but also on the amount of compression wood. Accordingly, hydraulic conductivity and mechanical strength or stiffness showed no tradeoff.
PMCID: PMC3197722  PMID: 17472942
biomechanics; functional anatomy; hydraulic conductivity; Picea abies; tradeoffs; vulnerability to cavitation
5.  Transcriptome profiling of radiata pine branches reveals new insights into reaction wood formation with implications in plant gravitropism 
BMC Genomics  2013;14(1):768.
Background
Formation of compression (CW) and opposite wood (OW) in branches and bent trunks is an adaptive feature of conifer trees in response to various displacement forces, such as gravity, wind, snow and artificial bending. Several previous studies have characterized tracheids, wood and gene transcription in artificially or naturally bent conifer trunks. These studies have provided molecular basis of reaction wood formation in response to bending forces and gravity stimulus. However, little is known about reaction wood formation and gene transcription in conifer branches under gravity stress. In this study SilviScan® technology was used to characterize tracheid and wood traits in radiate pine (Pinus radiata D. Don) branches and genes differentially transcribed in CW and OW were investigated using cDNA microarrays.
Results
CW drastically differed from OW in tracheids and wood traits with increased growth, thicker tracheid walls, larger microfibril angle (MFA), higher density and lower stiffness. However, CW and OW tracheids had similar diameters in either radial or tangential direction. Thus, gravity stress largely influenced wood growth, secondary wall deposition, cellulose microfibril orientation and wood properties, but had little impact on primary wall expansion. Microarray gene transcription revealed about 29% of the xylem transcriptomes were significantly altered in CW and OW sampled in both spring and autumn, providing molecular evidence for the drastic variation in tracheid and wood traits. Genes involved in cell division, cellulose biosynthesis, lignin deposition, and microtubules were mostly up-regulated in CW, conferring its greater growth, thicker tracheid walls, higher density, larger MFA and lower stiffness. However, genes with roles in cell expansion and primary wall formation were differentially transcribed in CW and OW, respectively, implicating their similar diameters of tracheid walls and different tracheid lengths. Interestingly, many genes related to hormone and calcium signalling as well as various environmental stresses were exclusively up-regulated in CW, providing important clues for earlier molecular signatures of reaction wood formation under gravity stimulus.
Conclusions
The first comprehensive investigation of tracheid characteristics, wood properties and gene transcription in branches of a conifer species revealed more accurate and new insights into reaction wood formation in response to gravity stress. The identified differentially transcribed genes with diverse functions conferred or implicated drastic CW and OW variation observed in radiata pine branches. These genes are excellent candidates for further researches on the molecular mechanisms of reaction wood formation with a view to plant gravitropism.
Electronic supplementary material
The online version of this article (doi:10.1186/1471-2164-14-768) contains supplementary material, which is available to authorized users.
doi:10.1186/1471-2164-14-768
PMCID: PMC4046691  PMID: 24209714
Compression wood; Tracheid; Conifers; Transcriptome; Microarray; Plant gravitropism; Microfibril angle (MFA); Wood stiffness
6.  Effect of Local Heating and Cooling on Cambial Activity and Cell Differentiation in the Stem of Norway Spruce (Picea abies) 
Annals of Botany  2006;97(6):943-951.
• Background and Aims The effect of heating and cooling on cambial activity and cell differentiation in part of the stem of Norway spruce (Picea abies) was investigated.
• Methods A heating experiment (23–25 °C) was carried out in spring, before normal reactivation of the cambium, and cooling (9–11 °C) at the height of cambial activity in summer. The cambium, xylem and phloem were investigated by means of light- and transmission electron microscopy and UV-microspectrophotometry in tissues sampled from living trees.
• Key Results Localized heating for 10 d initiated cambial divisions on the phloem side and after 20 d also on the xylem side. In a control tree, regular cambial activity started after 30 d. In the heat-treated sample, up to 15 earlywood cells undergoing differentiation were found to be present. The response of the cambium to stem cooling was less pronounced, and no anatomical differences were detected between the control and cool-treated samples after 10 or 20 d. After 30 d, latewood started to form in the sample exposed to cooling. In addition, almost no radially expanding tracheids were observed and the cambium consisted of only five layers of cells. Low temperatures reduced cambial activity, as indicated by the decreased proportion of latewood. On the phloem side, no alterations were observed among cool-treated and non-treated samples.
• Conclusions Heating and cooling can influence cambial activity and cell differentiation in Norway spruce. However, at the ultrastructural and topochemical levels, no changes were observed in the pattern of secondary cell-wall formation and lignification or in lignin structure, respectively.
doi:10.1093/aob/mcl050
PMCID: PMC2803384  PMID: 16613904
Norway spruce; Picea abies; cambium; xylem; phloem; cell differentiation; heating; cooling; light microscopy; transmission electron microscopy; UV-microspectrophotometry
7.  Shrinkage processes in standard-size Norway spruce wood specimens with different vulnerability to cavitation 
Tree physiology  2009;29(11):1419-1431.
Summary
The aim of this study was to observe the radial shrinkage of Norway spruce [Picea abies (L. Karst.)] trunkwood specimens with different hydraulic vulnerability to cavitation from the fully saturated state until the overall shrinkage reaches a stable value, and to relate wood shrinkage and recovery from shrinkage to cavitations of the water column inside the tracheids. Radial shrinkage processes in standard-size sapwood specimens (6 mm × 6 mm × 100 mm; radial, tangential and longitudinal) obtained at different positions within the trunk, representing different ages of the cambium, were compared. Cavitation events were assessed by acoustic emission (AE) testing, hydraulic vulnerability by the AE feature analysis and shrinkage was calculated from the changes in contact pressure between the 150 kHz AE transducer and the wood specimen. Two shrinkage processes were observed in both juvenile (annual rings 1 and 2) and mature wood (annual rings 17–19), the first one termed tension shrinkage and the second one cell wall shrinkage process, which started when most of the tracheids reached relative water contents below fiber saturation. Maximum tension shrinkage coincided with high-energy AEs, and the periods of shrinkage recovery could be traced to tension release due to cavitation. Juvenile wood, which was less sensitive to cavitation, had lower earlywood tracheid diameters and was less prone to deformation due to tensile strain than mature wood, showed a lower cell wall shrinkage, and thus total shrinkage. Earlywood lumen diameters and maximum tension shrinkage were strongly positively related to each other, meaning that bigger tracheids are more prone to deformation at the same water tension than the smaller tracheids.
doi:10.1093/treephys/tpp077
PMCID: PMC3196842  PMID: 19797244
acoustic emission testing; conduit reinforcement; tensile strain; wood shrinkage
8.  A new Late Devonian genus with seed plant affinities 
Background
Many ovules of Late Devonian (Famennian) seed plants have been well studied. However, because few taxa occur with anatomically preserved stems and/or petioles, the vascular system of these earliest spermatophytes is little understood and available data come mostly from Euramerica. There remains great controversy over the anatomical differentiation of Late Devonian and Carboniferous seed plant groups of Buteoxylonales, Calamopityales and Lyginopteridales. Protostele evolution of these early spermatophytes needs more research.
Results
A new taxon Yiduxylon trilobum gen. et sp. nov. with seed plant affinities has been discovered in the Upper Devonian (Famennian) Tizikou Formation of Hubei Province, China. It is represented by stems, helically arranged and bifurcate fronds with two orders of pinnae and planate pinnules. Both secondary pinnae and pinnules are borne alternately. Stems contain a small protostele with three primary xylem ribs possessing a single peripheral protoxylem strand. Thick secondary xylem displays multiseriate bordered pitting on the tangential and radial walls of the tracheids, and has biseriate to multiseriate and high rays. A narrow cortex consists of inner cortex without sclerotic nests and sparganum-type outer cortex with peripheral bands of vertically aligned sclerenchyma cells. Two leaf traces successively arise tangentially from each primary xylem rib and they divide once to produce four circular-oval traces in the stem cortex. Four vascular bundles occur in two C-shaped groups at each petiole base with ground tissue and peripheral bands of sclerenchyma cells.
Conclusions
Yiduxylon justifies the assignment to a new genus mainly because of the protostele with protoxylem strands only near the periphery of primary xylem ribs, leaf trace origination and petiolar vascular supply structure. It shares many definitive characters with Calamopityales and Lyginopteridales, further underscoring the anatomical similarities among early seed plants. The primary vascular system, pycnoxylic-manoxylic secondary xylem with bordered pits on both tangential and radial walls of a tracheid and leaf trace divergence of Yiduxylon suggest transitional features between the early spermatophytes and ancestral aneurophyte progymnosperms.
Electronic supplementary material
The online version of this article (doi:10.1186/s12862-015-0292-6) contains supplementary material, which is available to authorized users.
doi:10.1186/s12862-015-0292-6
PMCID: PMC4341886  PMID: 25887625
China; Early seed plant; Famennian; Late devonian; Leaf trace; Petiole; Protostele; Tizikou formation; Yiduxylon trilobum
9.  Hydraulic efficiency compromises compression strength perpendicular to the grain in Norway spruce trunkwood 
Trees (Berlin, Germany : West)  2011;25(2):289-299.
The aim of this study was to investigate bending stiffness and compression strength perpendicular to the grain of Norway spruce (Picea abies (L.) Karst.) trunkwood with different anatomical and hydraulic properties. Hydraulically less safe mature sapwood had bigger hydraulic lumen diameters and higher specific hydraulic conductivities than hydraulically safer juvenile wood. Bending stiffness (MOE) was higher, whereas radial compression strength lower in mature than in juvenile wood. A density-based tradeoff between MOE and hydraulic efficiency was apparent in mature wood only. Across cambial age, bending stiffness did not compromise hydraulic efficiency due to variation in latewood percent and because of the structural demands of the tree top (e.g. high flexibility). Radial compression strength compromised, however, hydraulic efficiency because it was extremely dependent on the characteristics of the “weakest” wood part, the highly conductive earlywood. An increase in conduit wall reinforcement of earlywood tracheids would be too costly for the tree. Increasing radial compression strength by modification of microfibril angles or ray cell number could result in a decrease of MOE, which would negatively affect the trunk’s capability to support the crown. We propose that radial compression strength could be an easily assessable and highly predictive parameter for the resistance against implosion or vulnerability to cavitation across conifer species, which should be topic of further studies.
doi:10.1007/s00468-010-0505-y
PMCID: PMC3207224  PMID: 22058609
Compression strength perpendicular to the grain; Conduit wall reinforcement; Hydraulic efficiency; Modulus of elasticity in bending; Norway spruce; Structure–function relationships; Vulnerability to cavitation; Wood shrinkage
10.  Cavitation in dehydrating xylem of Picea abies: energy properties of ultrasonic emissions reflect tracheid dimensions 
Tree physiology  2011;31(1):59-67.
Ultrasonic emission (UE) testing is used to analyse the vulnerability of xylem to embolism, but the number of UEs often does not sufficiently reflect effects on hydraulic conductivity. We monitored the absolute energy of UE signals in dehydrating xylem samples hypothesizing that (i) conduit diameter is correlated with UE energy and (ii) monitoring of UE energy may enhance the utility of this technique for analysis of xylem vulnerability. Split xylem samples were prepared from trunk wood of Picea abies, and four categories of samples, derived from mature (I: earlywood, II: 30–50% latewood, III: >50% latewood) or juvenile wood (IV: earlywood) were used. Ultrasonic emissions during dehydration were registered and anatomical parameters (tracheid lumen area, number per area) were analysed from cross-sections. Attenuation of UE energy was measured on a dehydrating wood beam by repeated lead breaks. Vulnerability to drought-induced embolism was analysed on dehydrating branches by hydraulic, UE number or UE energy measurements. In split samples, the cumulative number of UEs increased linearly with the number of tracheids per cross-section, and UE energy was positively correlated with the mean lumen area. Ultrasonic emission energies of earlywood samples (I and IV), which showed normally distributed tracheid lumen areas, increased during dehydration, whereas samples with latewood (II and III) exhibited a right-skewed distribution of lumina and UE energies. Ultrasonic emission energy was hardly influenced by moisture content until ~40% moisture loss, and decreased exponentially thereafter. Dehydrating branches showed a 50% loss of conductivity at −3.6 MPa in hydraulic measurements and at −3.9 and −3.5 MPa in UE analysis based on cumulative number or energy of signals, respectively. Ultrasonic emission energy emitted by cavitating conduits is determined by the xylem water potential and by the size of element. Energy patterns during dehydration are thus influenced by the vulnerability to cavitation, conduit size distribution as well as attenuation properties. Measurements of UE energy may be used as an alternative to the number of UEs in vulnerability analysis.
doi:10.1093/treephys/tpq099
PMCID: PMC3199436  PMID: 21389002
earlywood; latewood; Picea abies; signal energy; tracheid dimension; ultrasonic emission; vulnerability to xylem embolism
11.  Climatic influences on wood anatomy and tree-ring features of Great Basin conifers at a new mountain observatory1 
Applications in Plant Sciences  2014;2(10):apps.1400054.
• Premise of the study: A network of mountain observing stations has been installed in the Great Basin of North America. NevCAN (Nevada Climate-ecohydrological Assessment Network), which spans a latitudinal range of 2.5° and two elevation ranges of about 2000 m each, enabled us to investigate tree growth in relation to climate.
• Methods: We analyzed wood anatomy and tree-ring characteristics of four conifer species in response to different levels of water availability by comparing a low- and a high-elevation population. Chronologies of earlywood and latewood widths, as well as cellular parameters, were developed from the year 2000 to 2012.
• Results: At the southern (drier and warmer) sites, Pinus monophylla had smaller cell lumen, tracheid diameter, and cell wall thickness. Pinus monophylla and P. flexilis showed bigger cellular elements at the higher elevations, whereas the opposite pattern was found in Picea engelmannii and Pinus longaeva. When all species and sites were pooled together, stem diameter was positively related with earlywood anatomical parameters.
• Discussion: We have provided a glimpse of the applications that NevCAN, as a new scientific tool, could allow in the general field of botany. In particular, we were able to investigate how differences in water stress related to elevation lead to changes in xylem anatomy.
doi:10.3732/apps.1400054
PMCID: PMC4189497  PMID: 25309838
elevation-latitude gradients; NevCAN; Picea engelmannii; Pinus flexilis; Pinus longaeva; Pinus monophylla; tracheid size
12.  Changes in the localization and levels of starch and lipids in cambium and phloem during cambial reactivation by artificial heating of main stems of Cryptomeria japonica trees 
Annals of Botany  2010;106(6):885-895.
Background and Aims
Cambial reactivation in trees occurs from late winter to early spring when photosynthesis is minimal or almost non-existent. Reserve materials might be important for wood formation in trees. The localization and approximate levels of starch and lipids (as droplets) and number of starch granules in cambium and phloem were examined from cambial dormancy to the start of xylem differentiation in locally heated stems of Cryptomeria japonica trees in winter.
Methods
Electric heating tape was wrapped on one side of the stem of Cryptomeria japonica trees at breast height in winter. The localization and approximate levels of starch and lipids (as droplets) and number of starch granules were determined by image analysis of optical digital images obtained by confocal laser scanning microscopy.
Key Results
Localized heating induced earlier cambial reactivation and xylem differentiation in stems of Cryptomeria japonica, as compared with non-heated stems. There were clear changes in the respective localizations and levels of starch and lipids (as droplets) determined in terms of relative areas on images, from cambial dormancy to the start of xylem differentiation in heated stems. In heated stems, the levels and number of starch granules fell from cambial reactivation to the start of xylem differentiation. There was a significant decrease in the relative area occupied by lipid droplets in the cambium from cambial reactivation to the start of xylem differentiation in heated stems.
Conclusions
The results showed clearly that the levels and number of storage starch granules in cambium and phloem cells and levels of lipids (as droplets) in the cambium decreased from cambial reactivation to the start of xylem differentiation in heated stems during the winter. The observations suggest that starch and lipid droplets might be needed as sources of energy for the initiation of cambial cell division and the differentiation of xylem in Cryptomeria japonica.
doi:10.1093/aob/mcq185
PMCID: PMC2990657  PMID: 21037242
Cambial reactivation; confocal laser scanning microscopy; Cryptomeria japonica; lipid; starch; xylem differentiation
13.  Widening of xylem conduits in a conifer tree depends on the longer time of cell expansion downwards along the stem 
Journal of Experimental Botany  2011;63(2):837-845.
The diameter of vascular conduits increases towards the stem base. It has been suggested that this profile is an efficient anatomical feature for reducing the hydraulic resistance when trees grow taller. However, the mechanism that controls the cell diameter along the plant is not fully understood. The timing of cell differentiation along the stem was investigated. Cambial activity and cell differentiation were investigated in a Picea abies tree (11.5 m in height) collecting microsamples at nine different heights (from 1 to 9 m) along the stem with a 4 d time interval. Wood sections (8–12 μm thick) were stained and observed under a light microscope with polarized light to differentiate the developing xylem cells. Cell wall lignification was detected using cresyl violet acetate. The first enlarging cells appeared almost simultaneously along the tree axis indicating that cambium activation is not height-dependent. A significant increase in the duration of the cell expansion phase was observed towards the tree base: at 9 m from the ground, xylem cells expanded for 7 d, at 6 m for 14 d, and at 3 m for 19 d. The duration of the expansion phase is positively correlated with the lumen area of the tracheids (r2=0.68, P < 0.01) at the same height. By contrast, thickness of the cell wall of the earlywood did not show any trend with height. The lumen area of the conduits down the stem appeared linearly dependent on time during which differentiating cells remained in the expansion phase. However, the inductive signal of such long-distance patterned differentiation remains to be identified.
doi:10.1093/jxb/err309
PMCID: PMC3254684  PMID: 22016427
Auxin; cambium; cell differentiation; conduit tapering; Picea abies polar pattern growth
14.  Transcriptome profiling of Pinus radiata juvenile wood with contrasting stiffness identifies putative candidate genes involved in microfibril orientation and cell wall mechanics 
BMC Genomics  2011;12:480.
Background
The mechanical properties of wood are largely determined by the orientation of cellulose microfibrils in secondary cell walls. Several genes and their allelic variants have previously been found to affect microfibril angle (MFA) and wood stiffness; however, the molecular mechanisms controlling microfibril orientation and mechanical strength are largely uncharacterised. In the present study, cDNA microarrays were used to compare gene expression in developing xylem with contrasting stiffness and MFA in juvenile Pinus radiata trees in order to gain further insights into the molecular mechanisms underlying microfibril orientation and cell wall mechanics.
Results
Juvenile radiata pine trees with higher stiffness (HS) had lower MFA in the earlywood and latewood of each ring compared to low stiffness (LS) trees. Approximately 3.4 to 14.5% out of 3, 320 xylem unigenes on cDNA microarrays were differentially regulated in juvenile wood with contrasting stiffness and MFA. Greater variation in MFA and stiffness was observed in earlywood compared to latewood, suggesting earlywood contributes most to differences in stiffness; however, 3-4 times more genes were differentially regulated in latewood than in earlywood. A total of 108 xylem unigenes were differentially regulated in juvenile wood with HS and LS in at least two seasons, including 43 unigenes with unknown functions. Many genes involved in cytoskeleton development and secondary wall formation (cellulose and lignin biosynthesis) were preferentially transcribed in wood with HS and low MFA. In contrast, several genes involved in cell division and primary wall synthesis were more abundantly transcribed in LS wood with high MFA.
Conclusions
Microarray expression profiles in Pinus radiata juvenile wood with contrasting stiffness has shed more light on the transcriptional control of microfibril orientation and the mechanical properties of wood. The identified candidate genes provide an invaluable resource for further gene function and association genetics studies aimed at deepening our understanding of cell wall biomechanics with a view to improving the mechanical properties of wood.
doi:10.1186/1471-2164-12-480
PMCID: PMC3224210  PMID: 21962175
15.  Differentiation of Terminal Latewood Tracheids in Silver Fir Trees During Autumn 
Annals of Botany  2005;95(6):959-965.
• Background and Aims The differentiation of terminal latewood tracheids of silver fir (Abies alba) trees grown in Slovenia was investigated in autumn/winter 2001/2002.
• Methods The experimental trees were divided into three groups: one with narrow annual rings, width less than 1 mm; one with annual ring widths between 1 and 4 mm; and one group with broad rings larger than 4 mm. The differentiation of terminal latewood tracheids was investigated by light-, electron- and UV-microscopy in tissues sampled in October and November 2001 and March 2002.
• Key Results In the middle of October, cambial divisions did not occur any more in any of the trees. In trees with narrow annual rings, cell wall deposition as well as lignification were completed in terminal latewood tracheids at this date, whereas in trees with annual ring widths of more than 1 mm these processes still continued. Electron microscopy as well as UV microscopy revealed an unlignified inner S2 layer and the absence of S3 and warty layers. With increasing distance from the cambium, wall formation and lignification gradually appeared to be completed. Samples of all trees taken in the middle of November only contained differentiated terminal latewood tracheids. At the structural and lignin topochemical level, November and March samples showed completed differentiation of walls of terminal latewood tracheids.
• Conclusions In trees with broader annual rings, the final steps of differentiation of the youngest latewood tracheids near the cambium still continued during autumn, but were finished prior to winter. It was concluded from structural observations that duration of cambial activity is longer in trees with broad annual rings than in trees with narrow rings.
doi:10.1093/aob/mci112
PMCID: PMC4246759  PMID: 15760912
Silver fir (Abies alba); latewood tracheids; cell wall structure; autumn differentiation; lignification; light microscopy; transmission electron microscopy; UV-microspectrophotometry
16.  Impact of drought on the temporal dynamics of wood formation in Pinus sylvestris 
Tree physiology  2010;30(4):490-501.
Summary
We determined the temporal dynamics of cambial activity and xylem cell differentiation of Scots pine (Pinus sylvestris L.) within a dry inner Alpine valley (750 m asl, Tyrol, Austria), where radial growth is strongly limited by drought in spring. Repeated micro-sampling of the developing tree ring of mature trees was carried out during 2 contrasting years at two study plots that differ in soil water availability (xeric and dry-mesic site).
In 2007, when air temperature at the beginning of the growing season in April exceeded the long-term mean by 6.4 °C, cambial cell division started in early April at both study plots. A delayed onset of cambial activity of c. 2 wk was found in 2008, when average climate conditions prevailed in spring, indicating that resumption of cambial cell division after winter dormancy is temperature-controlled. Cambial cell division consistently ended about the end of June/early July in both study years. Radial enlargement of tracheids started almost 3 wk earlier in 2007 compared with 2008 at both study plots. At the xeric site, the maximum rate of tracheid production in 2007 and 2008 was reached in early and mid-May, respectively, and c. 2 wk later, at the dry-mesic site. Since in both study years, more favorable growing conditions (i.e., an increase in soil water content) were recorded during summer, we suggest a strong sink competition for carbohydrates to mycorrhizal root and shoot growth. Wood formation stopped c. 4 wk earlier at the xeric compared with the dry-mesic site in both years, indicating a strong influence of drought stress on cell differentiation. This is supported by radial widths of earlywood cells, which were found to be significantly narrower at the xeric than at the dry-mesic site (P < 0.05).
Repeated cellular analyses during the two growing seasons revealed that, although spatial variability in the dynamics and duration of cell differentiation processes in Pinus sylvestris exposed to drought is strongly influenced by water availability, the onset of cambial activity and cell differentiation is controlled by temperature.
doi:10.1093/treephys/tpq003
PMCID: PMC3046340  PMID: 20197285
Cambium; dry inner Alpine valley; intra-annual growth; Scots pine; tracheid production; xylogenesis
17.  Plasticity in variation of xylem and phloem cell characteristics of Norway spruce under different local conditions 
There is limited information on intra-annual plasticity of secondary tissues of tree species growing under different environmental conditions. To increase the knowledge about the plasticity of secondary growth, which allows trees to adapt to specific local climatic regimes, we examined climate–radial growth relationships of Norway spruce [Picea abies (L.) H. Karst.] from three contrasting locations in the temperate climatic zone by analyzing tree-ring widths for the period 1932–2010, and cell characteristics in xylem and phloem increments formed in the years 2009–2011. Variation in the structure of xylem and phloem increments clearly shows that plasticity in seasonal dynamics of cambial cell production and cell differentiation exists on xylem and phloem sides. Anatomical characteristics of xylem and phloem cells are predominantly site-specific characteristics, because they varied among sites but were fairly uniform among years in trees from the same site. Xylem and phloem tissues formed in the first part of the growing season seemed to be more stable in structure, indicating their priority over latewood and late phloem for tree performance. Long-term climate and radial growth analyses revealed that growth was in general less dependent on precipitation than on temperature; however, growth sensitivity to local conditions differed among the sites. Only partial dependence of radial growth of spruce on climatic factors on the selected sites confirms its strategy to adapt the structure of wood and phloem increments to function optimally in local conditions.
doi:10.3389/fpls.2015.00730
PMCID: PMC4564692  PMID: 26442044
cambium; growth/climate relation; Picea abies; tracheid; tracheidogram; cell differentiation; sieve cell; phloemogram
18.  Changes in tracheid and ray traits in fire scars of North American conifers and their ecophysiological implications 
Annals of Botany  2014;114(2):223-232.
Background and Aims
Fire scars have been widely used as proxies for the reconstruction of fire history; however, little is known about the impact of fire injury on wood anatomy. This study investigates changes in tracheid and ray traits in fire scars of Douglas fir (Pseudotsuga menziesii), western larch (Larix occidentalis) and ponderosa pine (Pinus ponderosa), and discusses their ecophysiological implications for tree recovery from fire.
Methods
Transverse and tangential microsections were prepared for light microscopy and image analysis. Measurements of tracheids and rays were made in the three spatial dimensions: axially (at different section heights), radially (in different rings) and tangentially (with increasing distance from the wound margin).
Key Results
Changes were strongest in the first year after fire injury, with a decrease in tracheid size (by 25–30 %) and an increase in tracheid density (by 21–53 %) for the three species. In addition, an increase in ray size (by 5–27 %) and an increase in ray density (by 19–36 %) were found in P. menziesii and L. occidentalis. Changes were comparable along the fire-injured stem and were often most marked close to the fire scar.
Conclusions
The differentiation after fire injury of narrower and more numerous tracheids expresses a trade-off between hydraulic safety and hydraulic efficiency, while that of larger and more numerous rays serves compartmentalization and wound closure, mechanical strength and defence responses. Pinus ponderosa does not generally produce more ray tissue after fire injury and thus appears to be more adapted to fire.
doi:10.1093/aob/mcu112
PMCID: PMC4111399  PMID: 24941999
Ecophysiology; conifer; fire scar; Larix occidentalis; western larch; Pinus ponderosa; ponderosa pine; Pseudotsuga menziesii; Douglas fir; ray; tracheid; wood anatomy
19.  Quantitative proteomics reveals protein profiles underlying major transitions in aspen wood development 
BMC Genomics  2016;17:119.
Background
Wood development is of outstanding interest both to basic research and industry due to the associated cellulose and lignin biomass production. Efforts to elucidate wood formation (which is essential for numerous aspects of both pure and applied plant science) have been made using transcriptomic analyses and/or low-resolution sampling. However, transcriptomic data do not correlate perfectly with levels of expressed proteins due to effects of post-translational modifications and variations in turnover rates. In addition, high-resolution analysis is needed to characterize key transitions. In order to identify protein profiles across the developmental region of wood formation, an in-depth and tissue specific sampling was performed.
Results
We examined protein profiles, using an ultra-performance liquid chromatography/quadrupole time of flight mass spectrometry system, in high-resolution tangential sections spanning all wood development zones in Populus tremula from undifferentiated cambium to mature phloem and xylem, including cell expansion and cell death zones. In total, we analyzed 482 sections, 20–160 μm thick, from four 47-year-old trees growing wild in Sweden. We obtained high quality expression profiles for 3,082 proteins exhibiting consistency across the replicates, considering that the trees were growing in an uncontrolled environment. A combination of Principal Component Analysis (PCA), Orthogonal Projections to Latent Structures (OPLS) modeling and an enhanced stepwise linear modeling approach identified several major transitions in global protein expression profiles, pinpointing (for example) locations of the cambial division leading to phloem and xylem cells, and secondary cell wall formation zones. We also identified key proteins and associated pathways underlying these developmental landmarks. For example, many of the lignocellulosic related proteins were upregulated in the expansion to the early developmental xylem zone, and for laccases with a rapid decrease in early xylem zones. We observed upregulation of two forms of xylem cysteine protease (Potri.002G005700.1 and Potri.005G256000.2; Pt-XCP2.1) in early xylem and their downregulation in late maturing xylem. Our data also show that Pt-KOR1.3 (Potri.003G151700.2) exhibits an expression pattern that supports the hypothesis put forward in previous studies that this is a key xyloglucanase involved in cellulose biosynthesis in primary cell walls and reduction of cellulose crystallinity in secondary walls.
Conclusion
Our novel multivariate approach highlights important processes and provides confirmatory insights into the molecular foundations of wood development.
Electronic supplementary material
The online version of this article (doi:10.1186/s12864-016-2458-z) contains supplementary material, which is available to authorized users.
doi:10.1186/s12864-016-2458-z
PMCID: PMC4758094  PMID: 26887814
Quantitative proteomics; Stepwise linear modelling; Aspen wood formation
20.  Spatial Patterns in Hyphal Growth and Substrate Exploitation within Norway Spruce Stems Colonized by the Pathogenic White-Rot Fungus Heterobasidion parviporum▿ †  
Applied and Environmental Microbiology  2009;75(12):4069-4078.
In Norway spruce, a fungistatic reaction zone with a high pH and enrichment of phenolics is formed in the sapwood facing heartwood colonized by the white-rot fungus Heterobasidion parviporum. Fungal penetration of the reaction zone eventually results in expansion of this xylem defense. To obtain information about mechanisms operating upon heartwood and reaction zone colonization by the pathogen, hyphal growth and wood degradation were investigated using real-time PCR, microscopy, and comparative wood density analysis of naturally colonized trees with extensive stem decay. The hyphae associated with delignified wood at stump level were devoid of any extracellular matrix, whereas incipient decay at the top of decay columns was characterized by a carbohydrate-rich hyphal sheath attaching hyphae to tracheid walls. The amount of pathogen DNA peaked in aniline wood, a narrow darkened tissue at the colony border apparently representing a compromised region of the reaction zone. Vigorous production of pathogen conidiophores occurred in this region. Colonization of aniline wood was characterized by hyphal growth within polyphenolic lumen deposits in tracheids and rays, and the hyphae were fully encased in a carbohydrate-rich extracellular matrix. Together, these data indicate that the interaction of the fungus with the reaction zone involves a local concentration of fungal biomass that forms an efficient translocation channel for nutrients. Finally, the enhanced production of the hyphal sheath may be instrumental in lateral expansion of the decay column beyond the reaction zone boundary.
doi:10.1128/AEM.02392-08
PMCID: PMC2698336  PMID: 19376909
21.  Season-associated modifications in symplasmic organization of the cambium in Populus nigra 
Annals of Botany  2010;105(3):375-387.
Background and Aims
Alterations of plasmodesma (PD) connectivity are likely to be very important for plant development. Here, the repetitive division pattern of cambial initials in Populus nigra ‘italica’ was studied to follow the development of the PD network during maturation. Furthermore, seasonal changes were investigated in order to trace indications for developmental and functional adaptations.
Methods
Cambium samples of P. nigra twigs, collected in summer, autumn and spring, were chemically fixed for transmission electron microscopy. The parameters, PD density (number of PDs per square micrometre cell-wall area) and PD frequency (total number of PDs per average cell-wall area), were determined for radial and tangential cell interfaces deposited in chronological order.
Key Results
Data sets, presented in plasmodesmograms, show a strong variability in the PD network throughout the year. In summer, high PD numbers occur at the division wall which, after PD doubling by longitudinal fission, decline with further development both at the xylem and the phloem side. In autumn, the number of PDs at the division wall is low as they are in subsequent tangential interfaces. In spring, the first cell division coincides with a massive increase in PD numbers, in particular at the division wall. Only the radial walls between initials maintain their PD equipment throughout the year. This feature can be exploited for identification of the initial layer.
Conclusions
PD networks in the cambium go through a strict developmental programme depending on the season, which is associated with changing functional requirements. For instance, PD numbers correlate with proliferative activity and potential pathways for intercellular signalling. Increases in PD numbers are ascribed to longitudinal fission as a major mechanism, whereas the decline in older derivatives is ascribed to PD degradation.
doi:10.1093/aob/mcp300
PMCID: PMC2826250  PMID: 20045870
Cambium; meristem initials; plasmodesmata; Populus nigra ‘italica’; seasonal conditions; ultrastructure
22.  Complete tylosis formation in a latest Permian conifer stem 
Annals of Botany  2013;111(6):1075-1081.
Background and Aims
Our knowledge of tylosis formation is mainly based on observations of extant plants; however, its developmental and functional significance are less well understood in fossil plants. This study, for the first time, describes a complete tylosis formation in a fossil woody conifer and discusses its ecophysiological implications.
Methods
The permineralized stem of Shenoxylon mirabile was collected from the upper Permian (Changhsingian) Sunjiagou Formation of Shitanjing coalfield, northern China. Samples from different portions of the stem were prepared by using the standard thin-sectioning technique and studied in transmitted light.
Key Results
The outgrowth of ray parenchyma cells protruded into adjacent tracheids through pits initially forming small pyriform or balloon-shaped structures, which became globular or slightly elongated when they reached their maximum size. The tracheid luminae were gradually occluded by densely spaced tyloses. The host tracheids are arranged in distinct concentric zones representing different growth phases of tylosis formation within a single growth ring.
Conclusions
The extensive development of tyloses from the innermost heartwood (metaxylem) tracheids to the outermost sapwood tracheids suggests that the plant was highly vulnerable and reacted strongly to environmental stress. Based on the evidence available, the tyloses were probably not produced in response to wound reaction or pathogenic infection, since evidence of wood traumatic events or fungal invasion are not recognizable. Rather, they may represent an ecophysiological response to the constant environmental stimuli.
doi:10.1093/aob/mct060
PMCID: PMC3662507  PMID: 23532049
Shenoxylon mirabile; tylose; fossil plant; conifer wood; ecophysiological response; late Permian; China
23.  Generation and analysis of expressed sequence tags from six developing xylem libraries in Pinus radiata D. Don 
BMC Genomics  2009;10:41.
Background
Wood is a major renewable natural resource for the timber, fibre and bioenergy industry. Pinus radiata D. Don is the most important commercial plantation tree species in Australia and several other countries; however, genomic resources for this species are very limited in public databases. Our primary objective was to sequence a large number of expressed sequence tags (ESTs) from genes involved in wood formation in radiata pine.
Results
Six developing xylem cDNA libraries were constructed from earlywood and latewood tissues sampled at juvenile (7 yrs), transition (11 yrs) and mature (30 yrs) ages, respectively. These xylem tissues represent six typical development stages in a rotation period of radiata pine. A total of 6,389 high quality ESTs were collected from 5,952 cDNA clones. Assembly of 5,952 ESTs from 5' end sequences generated 3,304 unigenes including 952 contigs and 2,352 singletons. About 97.0% of the 5,952 ESTs and 96.1% of the unigenes have matches in the UniProt and TIGR databases. Of the 3,174 unigenes with matches, 42.9% were not assigned GO (Gene Ontology) terms and their functions are unknown or unclassified. More than half (52.1%) of the 5,952 ESTs have matches in the Pfam database and represent 772 known protein families. About 18.0% of the 5,952 ESTs matched cell wall related genes in the MAIZEWALL database, representing all 18 categories, 91 of all 174 families and possibly 557 genes. Fifteen cell wall-related genes are ranked in the 30 most abundant genes, including CesA, tubulin, AGP, SAMS, actin, laccase, CCoAMT, MetE, phytocyanin, pectate lyase, cellulase, SuSy, expansin, chitinase and UDP-glucose dehydrogenase. Based on the PlantTFDB database 41 of the 64 transcription factor families in the poplar genome were identified as being involved in radiata pine wood formation. Comparative analysis of GO term abundance revealed a distinct transcriptome in juvenile earlywood formation compared to other stages of wood development.
Conclusion
The first large scale genomic resource in radiata pine was generated from six developing xylem cDNA libraries. Cell wall-related genes and transcription factors were identified. Juvenile earlywood has a distinct transcriptome, which is likely to contribute to the undesirable properties of juvenile wood in radiata pine. The publicly available resource of radiata pine will also be valuable for gene function studies and comparative genomics in forest trees.
doi:10.1186/1471-2164-10-41
PMCID: PMC2636829  PMID: 19159482
24.  Radial shrinkage and ultrasound acoustic emissions of fresh versus pre-dried Norway spruce sapwood 
Trees (Berlin, Germany : West)  2010;24(5):931-940.
Acoustic emission (AE) and radial shrinkage were compared between fully saturated fresh and pre-dried Norway spruce sapwood during dehydration at ambient temperature. Hydraulic conductivity measurements, anatomical investigations on bordered pits and X-ray computed tomography (CT) scans were done to search for possible AE sources other than the breakage of the water columns inside the tracheids. Both fresh and pre-dried specimens showed radial shrinkage due to drying surface layers right from the beginning of dehydration, which induced almost no AE. Whereas no dimensional changes occurred in pre-dried wood thereafter, fresh wood showed a rapid shrinkage increase starting at 25% relative water loss. This dimensional change ceased when further moisture got lost and was even partially reversed. AE of fresh wood showed much higher activity and energy, which is a waveform feature that describes the strength of the acoustic signal. Extremely high single AE energy events were detected at this critical stage of dehydration. After partial recovery from shrinkage, neither dimensional changes nor AE activity showed differences between fresh and pre-dried wood after more than 80% relative moisture loss. Our results suggested that fresh sapwood is more prone to dehydration stresses than pre-dried sapwood. Differences in AE and shrinkage behavior might be due to the weakening or distortion of the pit membranes (cavitation fatigue), pit aspiration, structural changes of the cell walls and micro-checks, which occurred during the first dehydration cycle.
doi:10.1007/s00468-010-0464-3
PMCID: PMC3207202  PMID: 22064842
Bordered pits; Cavitation fatigue; Functional wood anatomy; Hydraulic conductance; Norway spruce (Picea abies); Acoustic emission testing; Wood shrinkage; X-ray computed tomography
25.  Radial shrinkage and ultrasound acoustic emissions of fresh versus pre-dried Norway spruce sapwood 
Trees (Berlin, Germany : West)  2010;24(5):931-940.
Acoustic emission (AE) and radial shrinkage were compared between fully saturated fresh and pre-dried Norway spruce sapwood during dehydration at ambient temperature. Hydraulic conductivity measurements, anatomical investigations on bordered pits and X-ray computed tomography (CT) scans were done to search for possible AE sources other than the breakage of the water columns inside the tracheids. Both fresh and pre-dried specimens showed radial shrinkage due to drying surface layers right from the beginning of dehydration, which induced almost no AE. Whereas no dimensional changes occurred in pre-dried wood thereafter, fresh wood showed a rapid shrinkage increase starting at 25% relative water loss. This dimensional change ceased when further moisture got lost and was even partially reversed. AE of fresh wood showed much higher activity and energy, which is a waveform feature that describes the strength of the acoustic signal. Extremely high single AE energy events were detected at this critical stage of dehydration. After partial recovery from shrinkage, neither dimensional changes nor AE activity showed differences between fresh and pre-dried wood after more than 80% relative moisture loss. Our results suggested that fresh sapwood is more prone to dehydration stresses than pre-dried sapwood. Differences in AE and shrinkage behavior might be due to the weakening or distortion of the pit membranes (cavitation fatigue), pit aspiration, structural changes of the cell walls and micro-checks, which occurred during the first dehydration cycle.
doi:10.1007/s00468-010-0464-3
PMCID: PMC3207202  PMID: 22064842
Bordered pits; Cavitation fatigue; Functional wood anatomy; Hydraulic conductance; Norway spruce (Picea abies); Acoustic emission testing; Wood shrinkage; X-ray computed tomography

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