Radial Cell Walls Research Articles

Morphology, ultrastructure and typology of orbicules in Loganiaceae s.l. and related genera, in relation to systematics

Minute granules of sporopollenin, called orbicules or Ubisch bodies, can be observed on the innermost tangential and/or radial walls of secretory tapetum cells. Orbicules were investigated with light microscopy as well as scanning electron, and transmission electron microscopy in 19 species of Loganiaceae s.l. and 32 related species. The species investigated cover all the recently recognised families segregated from Loganiaceae s.l.: Loganiaceae s.str., Gelsemiaceae, Geniostomaceae, Strychnaceae and Loganiaceae tribe Potalieae which is now included in the Gentianaceae. Orbicules were found in all the species of Loganiaceae s.l. that were investigated with the exception of Buddleja and Retzia. From the related species that were studied, 20 species (18 genera, seven families) possess orbicules. On the basis of observed morphological and ultrastructural variations, five orbicule types are described. The types are based on the orbicule typology previously described for the rubiaceous subfamilies Cinchonoideae and Ixoroideae. The systematic value of this typology is tested by comparison with the most recent classifications within Loganiaceae s.l. The heterogeneity of Loganiaceae s.l. is reflected in the variation of orbicule morphology. The two tribes Antonieae and Strychneae (Strychnaceae) are characterised by the presence of two very distinct types of orbicules (I or IIIa). In the most recent classifications of the Loganiaceae s.l. a relationship between Strychneae and the Loganiaceae s.str. (including Geniostomaceae) is suggested; this is confirmed by the presence of the same type of orbicules in all of these taxa. Gelsemiaceae species have one or other of two orbicule types (III or IV). The inclusion of the Loganiaceous tribe Potalieae in Gentianaceae is supported by the presence of the same orbicule type (V) as found in the genus Exacum and the former Loganiaceous genera Anthocleista and Fagraea now also included in Gentianaceae. In Apocynaceae s.l. orbicules do not occur in all tribes. Orbicules are present in the tribes Plumerioideae, Apocynoideae, and Periplocoideae (Asclepiadaceae). In the more derived genera of Asclepiadaceae (except Periplocoideae) orbicules do not occur. Results show that the systematic application of orbicule characteristics can be of value at the tribal level in the Loganiaceae s.l.

Transverse Young's Moduli and Cell Shapes in Coniferous Early Wood

Summary The relationship between transverse Young's moduli and cell shapes in coniferous early wood was investigated using cell models constructed by two dimensional power spectrum analysis. The calculated values of tangential Young's modulus qualitatively explained the relationship between experimental values and density as well as the difference in experimental values among species. The calculated values of radial Young's modulus for the species having hexagonal cells agreed well with the experimental values, whereas, for the species having square cells, the calculated values were much larger than the experimental values. This result was ascribed to the fact that the bending moment on the radial cell wall of square cell models was calculated to be small. It is suggested that the asymmetrical shape of real wood cells or the behavior of nodes during ell deformation is an important factor in the mechanism of linear elastic deformation of wood cells.

ABNORMAL LIGNIN DISTRIBUTION IN WOOD FROM SEVERELY DROUGHT STRESSED PINUS RADIATA TREES

Radiata pine logs exhibiting concentric shelling were examined for abnormal wood anatomy and cell wall characteristics. The trees from which the logs originated were growing on coastal sand dunes with a shallow impermeable iron pan subsoil, and the abnormal wood properties are assumed to be the result of frequent water stress and possible associated nutritional stress. The wood showed numerous false growth rings alternating with bands of poorly lignified tracheids. Examination of lignin distribution by confocal fluorescence microscopy and transmission electron microscopy revealed abnormal cell wall development associated with a poorly lignified middle lamella and outer secondary cell wall. Affected tracheids showed poor adhesion with development of intercellular checking, particularly on radial cell walls. Some tracheids showed concentric lamellation associated with areas of high and low lignification within the secondary wall. In many cases, the S3 layer was thicker and more heavily lignified than normal. Tracheids with the greatest reduction in lignification of the secondary wall showed evidence of collapse. The shelling behaviour of the wood was thus explained by poor or negligible adhesion between tracheids due to reduced lignification of middle lamellae. This investigation provides some insight into the effect of growth environment on lignification.

On the evidence of a diffusion barrier in the outer cortex apoplast of cress-roots ( Lepidium sativum), demonstrated by analytical electron microscopy

Summary To mark the apoplastic pathway of ions in the root of the dicotyledonous plant Lepidium sativum we used the heavy element lanthanum, which can be identified by analytical electron microscopy (EELS and ESI). In the front root tip, the primary walls of all meristematic cells contained lanthanum. 10-15 mm behind the root apex, lanthanum was found in the cortex cell walls up to the endodermis, but not in the stele. 20-25 mm from the tip, lanthanum was accumulated in the radial cell walls of the hypodermis, which, however, is not a complete diffusion barrier for ions, so that traces of lanthanum also were found in the cortex cell walls up to the endodermis. This study provides evidence for the presence of two apolastic diffusion barriers in the region of highest water uptake in cress roots.

Apoplastic accumulation of iron in the epidermis of maize (Zea mays) roots grown in calcareous soil

High concentrations of Fe in the roots of plants grown in calcareous soil have been found in a variety of plants, which, nevertheless, show Fe deficiency symptoms. In the present work, energy dispersive X‐ray (EDX) analysis at the cellular level has been used to characterize high root Fe concentrations in maize (Zea mays L.) grown in a calcareous soil in comparison with low root Fe concentrations under acidic soil conditions. Roots were thoroughly washed to remove adhering soil particles from the root surface as far as possible. To avoid any interference with possibly still present soil particles, the excitation beam was focused on radial walls of neighboring cells as well as on the symplast. Under alkaline conditions, high Fe concentrations in the mM range and higher accumulated in the epidermal root apoplast. Symplastic Fe was not detectable. Only traces of Fe were detectable in the apoplast of the cortex parenchyma. Under acidic conditions, apoplastic root Fe concentrations were clearly lower than under alkaline conditions, and no Fe was detectable in the root apoplast by use of EDX analysis. We conclude that, under alkaline conditions, high amounts of Fe are trapped in the epidermal root apoplast (apoplastic Fe inactivation), probably because of a high apoplastic pH and thus restricted translocation towards the root stele and to the upper plant parts. In contrast, on acidic soils Fe translocation towards the root stele and thus Fe supply to the upper plant parts was not impaired. Our findings imply that Fe deficiency on calcareous soils is not caused by restricted acquisition of Fe from the soil.

Hypothesis on the role of cell wall structure in differential transverse shrinkage of wood

Cell wall thickness under dry and wet conditions and the cellulose fibril arrangement on transverse-fracture surfaces of the S2 layer of the tangential and radial walls of Scots pine tracheids were examined using SEM images and ESEM images, respectively. A hypothesis for explaining the anisotropic transverse shrinkage of wood was proposed based on the test results and statistical analysis. A simplified model of a tracheid's cross-section was drawn for calculating the differential transverse shrinkage. Results of this study indicate that the radial cell wall of Scots pine latewood is about 25 percent thicker than the tangential wall. Earlywood does not show such a difference. Furthermore, there is a significant tendency of a preferential orientation (i.e. packing density) of the fibrils on the tangential and radial walls in the overall tangential direction. Despite some phenomena of the cell wall structure not yet clearly understood, this might explain the anisotropic transverse shrinkage of wood to a major extent.

The Importance of Phellogen Cells and their Structural Characteristics in Susceptibility and Resistance to Excoriation in Immature and Mature Potato Tuber ( Solanum tuberosum L.) Periderm

Potato tuber ( Solanum tuberosum L.) periderm maturation is an important physiological process that directly affects the susceptibility and development of resistance to costly excoriation (skinning-type wounds) at harvest. The objectives of this research were to identify the specific types of cells and the cellular changes associated with susceptibility and resistance to tuber excoriation in immature and mature tubers respectively. Epifluorescent microscopic examination of immature tuber periderm (phellem, phellogen and phelloderm cells) from several genetically diverse cultivars has shown that the cellular damage resulting from excoriation occurs within the phellogen (cork cambium), a meristematic layer of cells that gives rise to neighbouring phellem and phelloderm cells. Tuber excoriation is the result of the fracture of radial phellogen cell walls linking the skin (phellem) to the phelloderm. As the tuber periderm matures, phellogen cells become inactive and the radial walls of these cells become more resistant to fracture; resistance to excoriation develops. Ultrastructural studies of immature tuber periderm show that radial walls of active phellogen cells are thin and fragile. During periderm maturation, both radial and tangential phellogen cell walls thicken as they strengthen and become resistant to fracture, thereby providing resistance to excoriation. These results refute previous theories of the physiological changes responsible for the onset of resistance to tuber skinning injury. The combined results establish a paradigm whereby the thickening and strengthening of tuber phellogen cell walls upon periderm maturation are the determinant for resistance to tuber excoriation.

Radial widening of the Casparian strip follows induced radial expansion of endodermal cells.

The Casparian strip, the barrier to apoplastic transport that is located at the endodermis in roots and stems, is formed by individual endodermal cells and is constructed as a highly organized mesh within the primary wall. Since little is known about the mechanism of formation of the strip, we tried to obtain morphological evidence for the existence, prior to suberization and lignification, of some regulatory system at the expected site of the strip. Endodermal cells in etiolated pea stems were induced to expand in the radial direction by piercing the stems through the cortex before formation of the strip. The radial width of the strip increased significantly with the expansion of the radial walls of these endodermal cells. The expansion of the cells occurred before the formation of the strip. However, strips that had already been formed when the stems were pierced did not increase in width despite an induced expansion of the radial walls. These observations suggest that some positional information exists in the radial wall of endodermal cells that defines the future site of formation of the strip and its width.

Observation of the Swelling Behavior of Coniferous Cells Using a Confocal Scanning Laser Microscope and a Digital Image Correlation Method

The transverse swelling behavior of coniferous wood cells was observed using a confocal scanning laser microscope and a digital image correlation method (DIC). Since the laser microscope has great depth of focus and high resolution, it was possible to observe the deformation of cells precisely. In order to observe the swelling behavior of specimens (Douglas-fir, Pseudotsuga menziessi) on the stage of the microscope, moisture was supplied using a humidifier. Digital image data of the cells taken by the laser microscope were used to measure the swelling behavior using DIC.The outside diameter of latewood tracheids swelled or changed almost isotropically with increasing moisture content, but the inside diameter (lumen) shrank greatly and anisotropically. Earlywood tracheids expanded a little but underwent great changes in shape. Thickness expansion of cell walls was less than circumferential expansion (perpendicular to the thickness direction), so that we concluded that the anisotropy of expansion was mainly based on thickness swelling. In latewood, circumferential expansion of radial cell walls (changes in length of the middle lamella) was great, but tangential cell walls expanded less than radial walls (in some cases the expansion was nearly zero). The angles between a radial wall and the adjacent tangential wall increased both in latewood and in earlywood by swelling.

Functional structure of the indeterminate Vicia faba L. root nodule: implications for metabolite transport

Summary The structure of N 2 -fixing nodules of faba bean ( Vicia faba L.) was investigated by means of light and electron microscopy to determine possible pathways for metabolite movement. The central infected zone is enclosed by a cortex, which is composed of a loosely arranged outer part and a densely packed inner part. Both are separated by the nodule endodermis, which borders on the root endodermis and continuously envelopes the entire nodule interior, with the exception of the apical meristematic region. Vascular bundles, located peripherally within the inner cortex, are surrounded by another endodermal layer. Radial cell walls of both nodule and vascular endodermis are incrusted and form Casparian bands. Additionally, all endodermal cells are coated by suberin lamellae. These modifications may prevent an apoplastic loss of solutes from the enclosed tissues, and require a symplastic metabolite passage. This is supported by high plasmodesmatal frequencies found between vascular endodermis and inner cortex. Within vascular bundles, transfer cells adjacent to xylem elements may play a role in xylem loading. The central infected tissue contains uninfected cells, possibly representing a symplastic continuity. These cells may form a preferred metabolite pathway since plasmodesmata are extremely sparse between infected cells. The low number of uninfected cells in the central tissue of V. faba nodules favors an additional apoplastic pathway.

Cell wall thickness and tangential Young's modulus in coniferous early wood

To investigate the effect of wall thickening around cell corners on the tangential Young's modulus of coniferous early wood, tapered beam cell models in which the variation of the cell wall thickness in the axial direction was taken into account were constructed for seven species. Their tangential Young's moduli were compared with the experimental results. The calculated Young's moduli of tapered beam cell models were larger than those of the models composed of the cell walls with uniform thickness, although both models showed almost the same density. For some species the calculated Young's moduli of the models in which the cell wall thickness increased curvilinearly in the axial direction were much closer to the experimental values. The reduction of the radial cell wall deflection due to the increase of the stiffness around cell corners was considered to increase the tangential Young's modulus of a wood cell.

Relationship between microtubule orientation and patterns of cell expansion in developing cortical cells of Lemna minor roots

In actively growing cortical cells in the elongation zone of Lemna minor L. roots, both longitudinal (radial and tangential) and transverse walls expand in both length and width. The longitudinal walls of the three types of cortical cells in the root (i.e. outer, middle and inner) showed the largest expansion in the longitudinal axis. In contrast, the inner cortical cells exhibited the least expansion in width, whereas the middle cortical cells displayed the largest expansion in width. Thus, the profiles of the expansion of longitudinal walls were characteristic for the three types of cortical cells. In this study, both the orientation of cortical microtubule (MT) arrays and their dynamic reorientation, and the density of cortical MTs, were documented and correlated to the patterns of cell wall expansion. Significantly, transverse arrays of cortical MTs were most prominent in the radial walls of the inner cortical cells, and least so in those of the middle cortical cells. Toward the base of roots, beyond the elongation zone, the orientation of cortical MTs shifted continuously from transverse to oblique and then to longitudinal. In this case, the rate of shift in the orientation of cortical MTs along the root axis was appreciably faster in the middle cortical cells than in the other two types of cortical cells. Interestingly, the continuous change in cortical MT orientation was not confirmed in the transverse walls which showed much smaller two-dimensional expansion than the radial walls. Additionally, the presence of fragmented or shortened cortical MTs rapidly increased concomitantly with the decrease of transversely oriented cortical MTs. This relationship was especially prominent in the transverse walls of the inner cortical cells, which displayed the least expansion among the three types of cortical cells investigated. In the root elongation zone, the density of cortical MTs in the inner cortical cells was about three times higher than that in the other two cortical cell types. These results indicate that in the early stage of cell expansion, the orientation of cortical MTs determines a preferential direction of cell expansion and both the shifting orientation and density of cortical MTs affect the magnitude of expansion in width of the cell wall.

Um Novo Lenho Gimnospérmico da Formação Rio Bonito, SC, Brasil - Aterradoxylon solidum gen. et sp.nov.

The morphoanatomic study of a taphoflora from the Rio Bonito Formation, Pouso Redondo County, Santa Catarina State, Brazil, resulted in the determination of new gymnospermous genera, among them a fossil wood that, by being represented by 9 of the 25 samples studied, constitutes the second form with significant representation in the assemblage (36%). By virtue of the solid and heterocelular pith, the centrifugal differentiation of the primary vascular system and the large pits in the cross-field of the secondary vascular system, the wood in question is similar to the morfogenus Megaporoxylon KRÄUSEL, 1956 from the Permian of South Africa. However, the presence of singular characters, like the nature of the secretory system and the outline of pith as well as the configuration of the bordered pits on radial walls of the secondary xylem cells, evidences the necessity of its segregation in new taxon which is named Aterradoxylon solidum gen. et sp. nov.

Electron-translucent angular areas in developing tapetum cell walls and pollen grains ofTilia platyphyllos

InTilia platyphyllos, the anther tapetal cell walls undergo significant modifications from the tetrad stage onwards. During the tetrad stage the inner tangential and radial parts of the tapetal walls begin to dissolve, while the distal parts swell. After the tetrad stage, the distal and outer radial tapetal cell walls become covered by a thick, irregular, highly electron-dense, polysaccharide layer. Striking features of the maturing tapetal walls (microspore stage and later) are electron-translucent, structureless, unstainable angular areas of variable dimensions. Similar electron-translucent areas occur in the exine arcades and apertures, but also isolated in the locular fluid ofT. platyphyllos. Electron-translucent areas, that are also found in the exine arcades and tapetal cells of other angiosperms, can be interpreted as the products of poorly understood metabolic processes.

Weathering of Radial and Tangential Wood Surfaces of Pine and Spruce

Summary The development of cracks and changes in appearance have been investigated on radial and tangential surfaces of pine (Pinus silvestris L.) and spruce (Picea abies Karst.) which have been exposed outdoors for 33 months. The degradation of the surfaces has also been studied at the micro-level. Untreated samples, samples impregnated with a CCA-agent and samples surface treated with linseed oil have been tested. The annual ring orientation is the most important factor for crack development on weathering. The type of wood, impregnation treatment and surface treatment with linseed oil have only a marginal effect on the crack development. No relation has been found between the density of the samples and the crack development. After 33 months of outdoor exposure, tangential surfaces of pine have 13 times more total crack length per unit area than the corresponding radial surfaces. In spruce, the total crack length on the tangential surfaces is 6 times greater than on the radial surfaces. Tangential surfaces of both pine and spruce have a greater number of cracks per unit area and wider cracks than the corresponding radial surfaces. Tangential and radial surfaces show the same colour change in the surface as a result of weathering. On the micro-level, tangential surfaces have more and deeper cracks than radial surfaces. The cracks on the tangential surfaces occur frequently in both earlywood and latewood. On radial surfaces, cracks occur primarily at the annual ring borders, but to a certain extent also in the earlywood. The radial cell wall of the earlywood has a large number of pits which are degraded at an early stage. Decomposition of the cell wall takes place on both radial and tangential surfaces. Cracks arise which follow the S2 fibril orientation in the cell-wall. Delamination in the middle lamella is especially noticeable in the latewood on tangential surfaces. No differences have been observed regarding linseed oil treatment, impregnation or type of wood.

Shell disintegration and taphonomic loss in rudist biostromes

Radiolitid biostromes in the Upper Cretaceous of Austria and Italy record a marked taphonomic loss controlled mainly by the composition of the biocoenosis, by the density of rudist colonization, by the style of radiolitid shell disintegration and by early diagenetic processes. Radiolitid shells consisted of a calcitic ostracum and an originally aragonitic hypostracum. The attached valve of most radiolitids was built of (1) an outermost ostracal layer of delicate calcite lamellae, (2) a thick layer of ‘boxwork ostracum’ built of radial funnel plates and cell walls, (3) a thin, inner ‘ostracal layer 3’ of thick-walled boxwork, and (4) the hypostracum that formed the innermost shell layer. The attached valve disintegrated by spalling of radial funnel plates of layer 2, and by selective removal of the boxwork ostracum. In the free valve, the ostracum consisted of two layers: (a) an inner, lid-shaped layer of dense calcite, and (b) an outer layer composed of calcite lamellae. The free valve disintegrated by spalling into ostracal and hypostracal portions, by spalling of the ostracum into layers a and b, and by disintegration of layer b into packages of calcite lamellae and individual lamellae. The specific style of disintegration of the radiolitids was aided or induced by discontinuities in shell structure. Lamellar fragments from the ostracum of the upper valve and from the radial funnel plates of the lower valve locally are abundant in free-valve-funnel-plate floatstones that comprise the matrix of or occur in lenses within radiolitid biostromes. In biostromes with an open parautochthonous fabric, selective removal of the boxwork ostracum of the attached valve occurred by mechanical spalling and, most probably, by early diagenetic dissolution. Complete removal of the boxwork ostracum yielded thin, relict shells composed of the ‘ostracal layer 3’ and the hypostracum. During early diagenesis, the hypostracum was replaced by blocky calcite spar, or was dissolved and became filled by internal sediments. The combination of both selective removal of boxwork ostracum and early diagenetic dissolution of aragonite locally resulted in the formation of ghost biostromes that entirely or largely consist of faint relics of radiolitids. The syndepositional formation of radiolitid shell relics and the presence of radiolitid ghost biostromes produced by bios-tratinomic and early diagenetic processes show that rudist biostromes can undergo marked taphonomic loss during fossilization. The presence of ghost biostromes with a burrowed, open parautochthonous rudist fabric indicates that the final preservation of a rudist biostrome was directly influenced by the characteristics of the biocoenosis, including unpreserved burrowing taxa. Rudist biostromes may be of markedly different taphonomy as a result of the taxonomic composition of the entire assemblage and the density of colonization by the rudists.

Mechanism for deformation of wood as a honeycomb structure II: First buckling mechanism of cell walls under radial compression using the generalized cell model

Coniferous woods were modeled as honeycomb cellular solids consisting of hexagonal-prism tracheids to examine the mechanism for radial compression. Because of the abrupt breaks of radial cell walls, it was assumed that the flrst break followed Euler's equation of buckling. The nominal stress at the buckling of the radial cell wall was theoretically obtained based on this assumption, and the actual nominal stress was obtained experimentally. The theoretical stress was found to correspond almost to the experimental value. This finding suggests that the abrupt first break that occurs in wood under radial compression can be mainly attributed to the buckling of radial cell walls.

Mechanism for deformation of wood as a honeycomb structure I: Effect of anatomy on the initial deformation process during radial compression

The mechanism for radial compression of coniferous wood was examined from the viewpoint of the porous structure of wood. The compressive test was carried out in a wet-type scanning electron microscopy (WET-SEM) chamber to observe continuously the deformation process of wood. The initial stress-strain relation of the cellular solids or single cell was measured with image analyses of SEM photographs. The first fracture occurred in one tangential row of earlywood tracheids just after the load-displacement curve exceeded the proportional limit. The fracture occurred because of abrupt breaks of the radial cell walls. The first fractured cells had a tendency to have the smallest percentage of cell wall within an annual ring, and the cells suffered shearing deformation in a radial direction until the occurrence of the first fracture. On the basis of the results of image analyses, it was concluded that this shearing deformation of cells was almost linearly related to the compressive load.

Tangential Youngs Modulus of Coniferous Early Wood Investigated Using Cell Models

Summary The relationship between the tangential Young's modulus and the transverse cell shape in coniferous early wood was investigated by using cell models constructed by power spectrum analysis. The calculated Young's moduli of the cell models explained qualitatively the change of the experimental Young's moduli with density as well as the difference in the experimental values among species. The calculated Young's moduli differed significantly among species depending on the cell model shapes when compared at the same density. With increasing element angle in the model, the Young's modulus greatly increased without a significant change in the density, especially at the larger ratios of the axial length of the tangential cell wall to that of the radial cell wall.

Differential deposition of suberin phenolic and aliphatic domains and their roles in resistance to infection during potato tuber (Solanum tuberosumL.) wound-healing

Rapid suberization of wounded potato tubers is critical in avoiding infection byErwinia carotovorasubsp.carotovora(a causal organism of bacterial soft rot) andFusarium sambucinum(a causal organism of fungal dry rot) in cut seed and stored potatoes. However, until now the reason for the differential development of resistance to bacterial and then fungal penetration during suberization has not been shown to be related to the differential deposition of the two major suberin components (phenolic and aliphatic domains) during wound-healing. Tubers of four varieties of diverse genetic background were wounded and inoculated withE. carotovorasubsp.carotovorathroughout a 5-day time course andF. sambucinumthroughout an 11-day time course during wound-healing (18 °C and 98% RH). During wound-healing, the tubers were examined at the cellular level for deposition of suberin phenolic and aliphatic domains. The percentage of inoculated tubers which became infected was determined for each wound-healing time point and was related to the deposition of suberin phenolic and aliphatic domains. Total resistance to infection byE. carotovorasubsp.carotovoraoccurred after the completion of phenolic deposition on the outer tangential wall of the first layer of cells (2–3 days). However, this suberin phenolic matrix offered no resistance to fungal infection byF. sambucinumeven after phenolic deposition was complete on adjoining radial and inner tangential cell walls of the first layer of cells. Resistance to fungal infection did not begin to develop until after deposition of the suberin aliphatic domain was initiated. Total resistance to fungal infection was attained after completion of deposition of the suberin aliphatic domain within the first layer of suberizing cells (5–7 days). These results indicate that the suberin phenolic domain provides resistance to infection byE. carotovorasubsp.carotovorabut not infection byF. sambucinum.They further suggest that deposition of the suberin aliphatic domain is responsible for final resistance to infection byF. sambucinum.This is believed to be the first evidence indicating separate depositional patterns, at the cellular level, for the two major domains of suberin and separate roles for each of these domains in the development of resistance to bacterial and fungal infection during suberization.