Convergent tapering of xylem conduits in different woody species

A recent theoretical model (the West, Brown and Enquist, WBE model) hypothesized that plants have evolved a network of xylem conduits with a tapered structure (narrower conduits distally) which should minimize the cost of water transport from roots to leaves. Specific measurements are required to test the model predictions. We sampled both angiosperms and gymnosperms (50 trees) growing in different environments with heights ranging from 0.5 to 44.4 m, measuring variations of the xylem-conduit diameter from tree top to stem base. In all trees measured, mean hydraulically weighted conduit diameters (Dh) at the tree top were narrower than those at the stem base. In actively growing trees, the longitudinal variation of Dh showed a degree of tapering in agreement with WBE predictions, while trees close to their maximum height showed slightly lower conduit tapering. Comparing different species, a very good correlation was observed between degree of xylem tapering and tree height (r2 = 0.88; P < 0.0001) independently of any other variable (age, site, altitude, etc.). As predicted by WBE, sampled trees seemed to converge towards similar xylem conduit tapering. However, trees approaching their maximum height had a nonoptimal tapering which appeared insufficient to compensate for the progressive increase in tree height.     

Comparative axial widening of phloem and xylem conduits in small woody plants

Key message

Along the stem axis phloem’s sieve elements increase in diameter basally at rates comparable to those of xylem conduits and in agreement with principles of hydraulic optimization.

Abstract

Plant physiology relies on the efficiency of the two long-distance transport systems of xylem and phloem. Xylem architecture comprises conduits of small dimensions towards the stem apex, where transpiration-induced tensions are the highest along the root-to-leaves hydraulic pathway, and widen basally to minimize the path length resistance to water flow. Instead, information on phloem anatomy and allometry is extremely scarce, although potentially relevant for the efficiency of sugar transportation. We measured the hydraulic diameter (Dh) of both xylem conduits and phloem sieve elements in parallel at different heights along the stem of a small tree of Picea abies, Fraxinus excelsior and Salix eleagnos. Dh increased from the stem apex to base in both xylem and phloem, with a higher scaling exponent (b) of sieve elements than that of tracheids in the conifer (0.19 vs. 0.14) and lower than that of vessels in the angiosperms (0.14–0.22 vs. 0.19–0.40). In addition, sieve elements were larger than tracheids in P. abies and narrower than angiosperms vessels at any height along the stem. In conclusion, axial conduit widening would seem to be a key feature of both xylem and phloem long-distance transport architectures.     

云南哀牢山6种常绿阔叶树木质部解剖特征的轴向和径向变化

West、Brown和Enquist提出的树木水分传导的分形网络模型(简称WBE模型)认为,树木连续分枝之间的导管或管胞直径按照一定的比率均匀变细,其总的水力阻力与水分传导的路径长度无关,从而使不同部位叶片获得基本相当的水分供应。该模型对树木高生长的水力限制假说提出了置疑。为了验证WBE模型中树木导管或管胞均匀变细的假说,该文研究了云南哀牢山中湿性常绿阔叶林中6种常绿阔叶树, 腾冲栲(Castanopsis wattii)、景东石砾(Lithocarpus chintungensis)、木果石砾(L. xylocarpus)、长尾青冈(Cyclobalanopsis stewardiana)、滇木荷(Schima noronhae)和舟柄茶(Hartia sinensis)木质部解剖特征随树高和年龄的变化。对这6个树种共14株样木进行了不同高度树干圆盘和边材生长轮取样,样木的高度为15~25 m,按照常规木材解剖的处理和分析方法,在显微镜下测定木材切片的导管直径和密度等特征。结果表明:在14株样木中,有4株树木导管直径随树木高度增加呈线性减小, 1株没有明显变化,其它9株树木导管直径在树冠以下的树干部分变化幅度较小或没有明显变化,而从树冠基部往上直到树木顶端导管直径显著减小。同一植株随着高度的增加,导管密度增加并且在树冠内增加更显著。有三分之一的树木导管占边材面积的比例随树高增加没有明显变化,其余树木导管占边材面积比在树冠以上有所减小。多数树木理论比导率在树冠以下没有明显变化而在树冠基部往上显著降低。在从髓芯开始往外的20~40个年轮范围内导管直径增加显著,但大部分植株导管直径在40个年轮后趋于稳定。不同高度圆盘导管直径随形成层发育时间的变化呈相似的趋势,并且相同发育年龄的导管直径没有明显差异。该文的研究结果说明,导管直径的轴向和径向变化一定程度上补偿了水分运输阻力随树木个体增大而增加的缺陷,但是6种常绿阔叶树树干的导管基本不按一定比率均匀变细,不支持WBE模型。     

Wood diameter indicates diurnal and long-term patterns of xylem water potential in Norway spruce

The stem diameter of adult Norway spruce trees was measured to see whether changes in xylem water potential lead to detectable radial deformation of the wood. The dendrometers used in these experiments measured only the dimensional changes of the woody cylinder (sap- and heartwood). Wood diameter was measured close to the ground and just below the living crown. After correction for thermal expansion of dendrometers and wood, diurnal variation of wood diameter ranged between 50 and 180 µm. Psychrometric measurements showed that xylem water potential varied in parallel to wood diameter. Diameter changes were always more pronounced at the higher stem position and exhibited a clear diurnal pattern. During the day, wood diameter decreased with increasing vapor pressure deficit and transpiration rate and with decreasing twig water potential. At night, the wood re-expanded but did not always reach the dimension of the previous day. Pre-dawn wood diameter decreased during periods of soil drought, a process which rapidly stopped and reversed after rain events. On several days, oscillation in wood diameter was observed during the mid-day hours. The oscillation had a period of approximately 50 min and showed a phase shift between different stem heights. All observed patterns of wood shrinkage and expansion were consistent with the hypothesis that xylem water tension leads to an elastic contraction of xylem conduits. The results demonstrate that xylem diameter is more suitable than whole-stem diameter for monitoring changes in xylem water potential.     

On the Cytochemistry of Cell Wall Formation in Poplar Trees

Abstract: The ultrastructure of cell walls and the mechanisms of cell wall formation are still not fully understood. The objective of our study was therefore to obtain additional fine structural details on the deposition of cell wall components during the differentiation of xylem cells in hybrid aspen ( Populus tremula L. × P. tremuloides Michx.) we used as a model tree. At the electron microscope level, PATAg staining revealed a successive deposition of polysaccharides with increasing distance from the cambium. Staining with potassium permanganate and UV microspectrophotometry showed that the cell walls were lignified, with some delay to the deposition of polysaccharides. Immunogold labelling of three lignin types in developing cell walls varied with progressive deposition of cell wall layers. Condensed lignin subunits were localized in corners of cells adjacent to the cambium prior to S1 formation, whereas non-condensed lignin subunits became labelled only in later stages - in secondary walls near cell corners and simultaneously with the completion of S1 formation. As S2 polysaccharide deposition progressed, the labelling extended towards the lumen. Labelling of peroxidases revealed their presence in cell corner regions of young xylem cells, still lacking a secondary wall, implying that peroxidases are incorporated into the developing cell wall at early developmental stages. A weak labelling of middle lamella regions and secondary walls could also be seen at later stages. The results are discussed in relation to current knowledge on the succession of polysaccharide and lignin deposition in woody cell walls.     

Growth Rings,Phenology, Hurricane Disturbance and Climate in Cyrilla racemiflora L., a Rain Forest Tree of the Luquillo Mountains,Puerto Rico1

The growth phenology of Cyrilla racemiflora L., the dominant tree species of the montane rain forest, (subtropical lower montane rain forest, sensu Holdridge) of the Luquillo Mountains of Puerto Rico was studied intensively during 1989, and then semiannually through mid-1993 to determine the periodicity of changes in xylem structure. Four trees at 770 m were monitored for flowering, branch elongation, leaf litterfall, and xylem cell growth and differentiation in the lower stem, and these events were related to local seasonal patterns of rainfall and temperature. Hurricane Hugo defoliated study trees in September, 1989. Bud-break and branch elongation in March, 1989 were followed by earlywood xylem cell production in the lower stem in April and the onset of flowering in May. Leaf litterfall was greatest between April and June, coinciding with peak branch growth and new leaf formation. Latewood xylem was produced in December. The general phenological pattern was synchronized between trees and over study years. Vessel diameter and density were monitored along with thickness of earlywood and latewood and the former converted to vessel lumen area, a measure of xylem conductance capacity. Annual growth rings were formed with periods of earlywood and latewood production coinciding with traditional summer (rainy) and winter (dry) seasons, respectively, in the Luquillo Mountains. Hurricane defoliation was followed by heavy flowering in 1990, a year of reduced branch elongation and annual xylem ring width, and was associated with maximum vessel lumen area, as was flowering in 1989, prior to the hurricane. Hurricane Hugo provided a perturbation that, through its elicited stress response, allowed for the demonstration of the interplay between flowering, branching, structural growth of xylem, and xylem function.     

Method to classify xylem cell types using cross sectionsof 1-year-old branches in Mediterranean woody species

Measurements of anatomical parameters of wood are of great interest both for eco-physiological purposes and for technological applications. The aim of this paper is to describe a new method for classifying and measuring cell lumen of xylem, analysing cross sections under the light microscope. The proposed method is based on the application of digital image analysis on images of the cross sections of xylem in combination with graphical and statistical methods. The methodology was tested on 1-year-old branches of several woody species, both trees and shrubs, occurring in a Mediterranean natural ecosystem in southern Italy. The development of the procedure was based on statistical comparison between data collected according to four procedures: (a) manual identification and measurement of lumen diameter of conduits on longitudinal sections; (b) manual identification and measurement of lumen diameter of conduits on cross sections; (c) manual identification and measurement of lumen area of conduits on cross sections; and (d) automatic measurement of lumen area of conduits on cross sections. The influence of image resolution and that of the position of the selected area on cell classification and measurements were ascertained. The proposed method was proved to be specific to woody structures and allowed the construction of a model-graph that is species-specific. Interpretation of the model-graphs allows classification and hence measurement of identified cells.     

Hormonal signals involved in the regulation of cambial activity, xylogenesis and vessel patterning in trees

The radial growth of plant stem is based on the development of cribro-vascular cambium tissues. It affects the transport efficiency of water, mineral nutrients and photoassimilates and, ultimately, also plant height. The rate of cambial cell divisions for the assembly of new xylem and phloem tissue primordia and the rate of differentiation of the primordia into mature tissues determine the amount of biomass produced and, in the case of woody species, the wood quality. These complex physiological processes proceed at a rate which depends on several factors, acting at various levels: growth regulators, resource availability and environmental factors. Several hormonal signals and, more recently, further regulatory molecules, have been shown to be involved in the induction and maintenance of cambium and the formation of secondary vascular tissues. The control of xylem cell patterning is of particular interest, because it determines the diameter of xylem vessels, which is central to the efficiency of water and nutrient transport from roots to leaves through the stem and may strongly influence the growth in height of the tree. Increasing scientific evidence have proved the role of other hormones in cambial cell activities and the study of the hormonal signals and their crosstalking in cambial cells may foster our understanding of the dynamics of xylogenesis and of the mechanism of vessel size control along the stem. In this article, the role of the hormonal signals involved in the control of cambium and xylem development in trees and their crosstalking are reviewed.     

Cambial activity and intra-annual xylem formation in roots and stems of Abies balsamea and Picea mariana

Background and Aims

Studies on xylogenesis focus essentially on the stem, whereas there is basically no information about the intra-annual growth of other parts of the tree. As roots strongly influence carbon allocation and tree development, knowledge of the dynamics of xylem production and maturation in roots at a short time scale is required for a better understanding of the phenomenon of tree growth. This study compared cambial activity and xylem formation in stem and roots in two conifers of the boreal forest in Canada.

Methods

Wood microcores were collected weekly in stem and roots of ten Abies balsamea and ten Picea mariana during the 2004–2006 growing seasons. Cross-sections were cut using a rotary microtome, stained with cresyl violet acetate and observed under visible and polarized light. The number of cells in the cambial zone and in differentiation, plus the number of mature cells, was counted along the developing xylem.

Key Results

Xylem formation lasted from the end of May to the end of September, with no difference between stem and roots in 2004–2005. On the contrary, in 2006 a 1-week earlier beginning of cell differentiation was observed in the stem, with cell wall thickening and lignification in roots ending up to 22 d later than in the stem. Cell production in the stem was concentrated early in the season, in June, while most cell divisions in roots occurred 1 month later.

Conclusions

The intra-annual dynamics of growth observed in stem and roots could be related to the different amount of cells produced by the cambium and the patterns of air and soil temperature occurring in spring.Key words: Abies balsamea, boreal forest, cambium, cell differentiation, cell wall thickening, lignification, Picea mariana, root, stem, xylem     

A Radial Concentration Gradient of Indole-3-Acetic Acid Is Related to Secondary Xylem Development in Hybrid Aspen

The radial distribution pattern of indole-3-acetic acid (IAA) was determined across the developing tissues of the cambial region in the stem of hybrid aspen (Populus tremula L. x Populus tremuloides Michx). IAA content was measured in consecutive tangential cryo-sections using a microscale mass spectrometry technique. Analysis was performed with wild-type and transgenic trees with an ectopic expression of Agrobacterium tumefaciens IAA-biosynthetic genes. In all tested trees IAA was distributed as a steep concentration gradient across the developing tissues of the cambial region. The peak level of IAA was within the cambial zone, where cell division takes place. Low levels were reached in the region where secondary wall formation was initiated. The transgenic trees displayed a lower peak level and a wider radial gradient of IAA compared with the wild type. This alteration was related to a lower rate of cambial cell division and a longer duration of xylem cell expansion in the transgenic trees, resulting in a decreased xylem production and a larger fiber lumen area. The results indicate that IAA has a role in regulating not only the rate of physiological processes such as cell division, but also the duration of developmental processes such as xylem fiber expansion, suggesting that IAA functions as a morphogen, conveying positional information during xylem development.     

Ethylene effects on cambial activity and cell wall formation in hypocotyls of Picea abies seedlings

Ethylene regulation of cell division in the vascular cambium and cell wall formation was studied in hypocotyls of Norway spruce ( Picea abies [L.] Karst.) seedlings. Cuttings from 6-week-old seedlings were placed in water culture to which compounds affecting the synthesis and action of ethylene were added. After a 3-week treatment period, growth, ethylene production, morphology and cell wall composition of the hypocotyls were determined. Addition of high concentrations of the potent ethylene releasing agent 2-chloroethylphosphonic acid (ethrel), which increased ethylene emission by more than twice compared to control plants, inhibited the expansion of xylem cells while stimulating the incorporation of cell wall material, especially cellulose. Addition of small amounts of ethrel, which slightly stimulated ethylene emission, led to increases in the size of xylem cells, the amount of phloem tissue and the number of intercellular spaces in the cortex, and thus to increased hypocotyl diameter. However, no significant change in cell wall composition was detected. When ethylene production was decreased by adding Co²⁺ to the nutrient solution, differentiation of new xylem was disturbed, but the rate of cell division was not affected. Although the incorporation of cell wall material was inhibited, the proportions of lignin and cellulose in the wall appeared to remain unchanged. Silver ions stimulated the expansion of both xylem and cortex cells, but had no significant effect on cell wall formation. We conclude that ethylene has a role in regulating the incorporation of wall carbohydrates.     

A phenology-based approach to the analysis of conifers intra-annual xylem anatomy in water-limited environments

Xylem structure and cambial phenology (i.e. onset and cessation of cambial cell division) of conifers growing under severe water-limitations can change dramatically in relation to moisture availability. In hyperarid environments, analytical tools commonly used to investigate intra-annual variability of xylem anatomy (i.e. tracheidograms), may fail to capture the complexity of tree phenological responses to environmental conditions. This greatly limits our ability to accurately date the onset of intra-annual density variations, including the transition between earlywood and latewood. I present a new approach for developing phenological tracheidograms (“pheno-tracheidograms”) calibrated to account for the seasonal variations in cell division rates. Pheno-tracheidograms were developed for a population of Pinus ponderosa in the Mojave Desert (Nevada, USA) during the period 2015–2016 in order 1) to determine the onset date of latewood formation and 2) to investigate relationships between environmental conditions, lumen diameter, and cell wall thickness targeting specific climatic windows for each tracheid. Pheno-tracheidograms were standardized at the tree-level, showing more flexibility compared to tracheidograms standardized according to a pre-determined number of cells. By displaying cellular parameters with respect to the date of formation of the tracheid to which each measurement is associated, pheno-tracheidograms allowed to determine the beginning of latewood formation with daily resolution. Lumen diameter was significantly correlated with the onset date of cellular enlargement, while cell wall thickness showed a weaker relationship with the beginning of secondary wall deposition. Soil moisture positively affected the duration of cell enlargement and tracheid lumen diameter, particularly in the earlywood, while cell wall thickness was not significantly influenced by environmental conditions. Pheno-tracheidograms represent an empirical, yet effective way to date intra-annual xylem structures and to investigate high-resolution climate-anatomy relationships in conifer species from arid environments characterized by high phenological plasticity.     

Regeneration of Tissues in Wounded Stems: A Quantitative Study

When a dicotyledonous stem is wounded by longitudinally splittinga young internode into halves, cells near the cut surface proliferateto form a callus within which vascular tissues differentiateand tend to restore a vascular cylinder in each half. Threephases of regeneration after wounding were identified and quantifiedin stems of three Solanaceous species. (1) In an initial ‘lag’phase, lasting about 2 d, neither cell division nor enlargementwere detected, but mitotic figures were observed within about300 µm of the cut surface. (2) Throughout a second, ‘division’phase, from about days 2–10, cell division and enlargementoccurred. Both were initiated mainly in the two cell layersnearest the surface. A mass of callus formed, with new cellwalls mostly parallel to the surface. Cell enlargement laggedbehind cell division for the first few days, so that mean radialcell diameter decreased until day 6, thereafter remaining almostconstant at 30–40 µm. Towards the end of this phase,mitoses ceased within the callus except in the positions ofthe future vascular and cork cambia, where radial cell diameterfell towards a constant 15–20 µm. (3) During a third,‘differentiation’ phase, cell division was restrictedto the cambial zones, and derivatives differentiated into cork,phloem or xylem according to position. The rate of increasein cell number per transect was 1.5–2.0 cells d^–1,of which more than half was xylem. Capsicum annuum L., sweet pepper, Lycopersicon esculentum Mill., tomato, cambium, cell division, differentiation, regeneration, wounding of stems, xylem     

Xylem phenology and wood production: resolving the chicken‐or‐egg dilemma

Delays in the start of the growing season reduce the period available for growth and the amount of xylem production. However, a higher number of developing tracheids could prolong cell differentiation and, consequently, lengthen the growing season. The relationship between the amount and duration of cell production in the xylem remains an unresolved issue. The aim of this study was to resolve the chicken‐or‐egg causality dilemma about duration of growth and cell production through simple‐ and double‐cause models. This was achieved by (1) analysing the intra‐annual growth dynamics of the xylem in Picea mariana (Mill.) BSP during 2006–2009 in two contrasting sites of the boreal forest of Quebec, Canada, and (2) extracting the dates of onset and ending of xylem formation and the number of radial cells along the tree ring. A higher number of cells was linked to an earlier onset (r = 0.74) and later ending (r = 0.61) of cell differentiation. The absence of a relationship between the residuals of the onset and ending of xylogenesis (r_p = ?0.06) indicated that cell production influenced the correlation between the two phenophases of the xylem. These results demonstrated that a higher number of cells produced delay the ending of xylem maturation, so extending the duration of wood formation.     

Effect of local heating and cooling on cambial activity and cell differentiation in the stem of Norway spruce (Picea abies)

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 degrees C) was carried out in spring, before normal reactivation of the cambium, and cooling (9-11 degrees 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.     

AutoCellRow (ACR) – A new tool for the automatic quantification of cell radial files in conifer images

Quantitative wood anatomy (QWA) is a growing field of dendrochronology that allows obtaining a large number of parameters as the number, size and spatial arrangement of cellular elements, elements that highlight the adjustments of trees to their environment. In this work, we presented the free/libre open-source software AutoCellRow (ACR), a ready-to-use tool for automatic QWA in conifers. The ACR analyzes radial files of cells on cross-sections views of tree rings and provides automatic measurements of different cell parameters (e.g., lumen radial diameter, double cell wall thickness and cell radial diameter) for each cell along the selected radial file. The ACR measurements are based on high performed image analysis of xylem cells. The accuracy of the software measurements was tested in cross-sections of five conifer species from a semi-arid area of southern Siberia, and compared with measurements obtained by a semiautomatic tool. Our results suggested high accuracy in the ACR cell traits measurements, facilitating and speeding the analysis of quantitative wood anatomy in conifers over radial files of cells.     

Changes in the arrangement of cellulose microfibrils associated with the cessation of cell expansion in tracheids

 The relationship between the cessation of cell expansion and formation of the secondary wall was investigated in the early-wood tracheids of Abies sachalinensis Masters by image analysis and field emission scanning electron microscopy. The area of the lumen and the length of the perimeter of the lumen of differentiating tracheids increased from the cambium towards the xylem. These increases had just ceased in the case of tracheids closest to the cambium in which birefringence was first detected by observations with a polarizing light microscope. Cellulose microfibrils (MFs) deposited on the innermost surfaces of radial walls were not well ordered during the expansion of cells, but well ordered MFs were deposited at the subsequent stage of cell wall formation. The first well ordered MFs were oriented in an S-helix. The well ordered MFs had already been deposited at the tracheids where birefringence was first detected under the polarizing light microscope. These results indicate that the deposition of the well ordered MFs, namely, the formation of the secondary wall, begins before the cessation of cell expansion of tracheids. Therefore, it seems that the expansion of tracheids is restricted by the deposition of the secondary wall because the cell walls become rigid simultaneously with the development of the secondary wall and, therefore, the yield point of cell walls exceeds the turgor pressure of the cell. Received: 3 July 1996 / Accepted: 24 September 1996     

Vertical and radial profiles in tracheid characteristics along the trunk of Douglas-fir trees with implications for water transport

The main stems of three young Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirbel) Franco) trees were dissected to obtain samples of secondary xylem from internodes axially along the trunk and radially within each internode. From these samples, measurements were obtained of tracheid diameter, length, the number of inter-tracheid pits per tracheid, and the diameter of the pit membranes. In addition, samples were obtained along the trunks of three old growth trees and also a small sample of roots for measurement of tracheid diameter. A gradient was apparent in all measured anatomical characters vertically along a sequence among the outer growth rings. These gradients arose not because of a gradient vertically along the internodes, but because of the strong gradients present at each internode among growth rings out from the pith. Tracheid characteristics were correlated: wider and longer tracheids had more numerous pits and wider pits, such that total pit area was about 6% of tracheid wall area independent of tracheid size. A stem model combining growth rings in parallel and internodes in series allowed for estimates of whole trunk conductance as a function of tree age. Conductance of the stem (xylem area specific conductivity) declined during the early growth of the trees, but appeared to approach a stable value as the trees aged.