Ultrastructural Localization of a Bean Glycine-Rich Protein in Unlignified Primary Walls of Protoxylem Cells

A polyclonal antibody was used to localize a glycine-rich cell wall protein (GRP 1.8) in French bean hypocotyls with the indirect immunogold method. GRP 1.8 could be localized mainly in the unlignified primary cell walls of the oldest protoxylem elements and also in cell corners of both proto- and metaxylem elements. In addition, GRP 1.8 was detected in phloem using tissue printing. The labeled primary walls of dead protoxylem cells showed a characteristically dispersed ultrastructure, resulting from the action of hydrolases during the final steps of cell maturation and from mechanical stress due to hypocotyl growth. Primary walls of living protoxylem and adjacent parenchyma cells were only weakly labeled. This was true also for the secondary walls of proto- and metaxylem cells, which in addition showed high background labeling. Inhibition of lignification with a specific and potent inhibitor of phenylalanine ammonia-lyase did not lead to enhanced labeling of secondary walls, showing that lignin does not mask the presence of GRP 1.8 in these walls. Dictyosomes of living proto- and metaxylem cells were not labeled, but dictyosomes of xylem parenchyma cells without secondary walls, adjacent to strongly labeled protoxylem elements, were clearly labeled. These observations suggest that GRP 1.8 is not produced by xylem vessels but by xylem parenchyma cells that export the protein to the wall of protoxylem vessels.     

Cell Wall Thickening of the Xylem Tracheary Elements in Different Zones of the Adventitious Root of Allium cepa L.

Cell wall thickness of the xylem tracheary elements was measuredin the proto- and metaxylem of the Allium cepa L. adventitiousroot. Measurements were taken in root fragments of known age(1, 3, 5, 7 and 9 d) located in either the basal or medio-apicalzone. Tracheary elements in the protoxylem matured within ashorter period of time than those in the metaxylem. Final cellwall thickness was greater in metaxylem than in protoxylem components.The cell wall thickening in the tracheary elements in both proto-and metaxylem was more rapid in the basal zone of the root thanin the medio-apical zone. Additionally, cell walls of the maturetracheary elements were thicker in the basal zone than in areasfurther from the bulb. Allium cepa, onion, root, cell wall, xylem maturation     

Structural cell-wall proteins in protoxylem development: evidence for a repair process mediated by a glycine-rich protein

Polyclonal antibodies were used to localize structural cell-wall proteins in differentiating protoxylem elements in etiolated bean and soybean hypocotyls at the light- and electron-microscopic level. A proline-rich protein was localized in the lignified secondary walls, but not in the primary walls of protoxylem elements, which remain unlignified, as shown with lignin-specific antibodies. Secretion of the proline-rich protein was observed during lignification in different cell types. A glycine-rich protein (GRP1.8) was specifically localized in the modified primary walls of mature protoxylem elements and in cell corners between xylem elements and xylem parenchyma cells. The protein was secreted by Golgi bodies both in protoxylem cells after the lignification of their secondary walls and in the surrounding xylem parenchyma cells. The modified primary walls of protoxylem elements were visualized under the light microscope as filaments or sheets staining distinctly with the protein stain Coomassie blue. Electron micrographs of these walls show that they are composed of an amorphous material of moderate electron-density and of polysaccharide microfibrils. These materials form a three-dimensional network, interconnecting the ring- or spiral-shaped secondary wall thickenings of protoxylem elements and xylem parenchyma cells. The results demonstrate that the modified primary walls of protoxylem cells are not simply breakdown products due to partial hydrolysis and passive elongation, as believed until now. Extensive repair processes produce cell walls with unique staining properties. It is concluded that these walls are unusually rich in protein and therefore have special chemical and physical properties.     

Examination of morphological changes in the first formed protoxylem in Arabidopsis seedlings

We examined morphological changes in the first-formed protoxylem vessels in Arabidopsis seedlings. Between 2.5 and 8 days after imbibition, mean hypocotyl and root length increased 1.52 and 23.3 times, respectively. In the 2.5-day-old seedlings, two continuous protoxylem vessels were present in the hypocotyl-root axis. In the 8-day-old upper hypocotyls, six protoxylem vessels were observed, and in the lower hypocotyls, four protoxylem vessels and one or two metaxylem vessels were observed. In the 8-day-old roots, there were two protoxylem vessels and two or three metaxylem vessels. Two protoxylem vessels in the hypocotyls connected to two metaxylem vessels in the roots of 8-day-old seedlings. At the 0.3-mm part below the hypocotyl-root boundary, the mean intervals of neighboring annular secondary wall thickenings in protoxylem vessels in 8-day-old roots were 12.9% larger than those in 2.5-day-old roots. In more apical parts of 8-day-old roots, the mean intervals fluctuated between 1.71 and 2.29 microm. In 8-day-old seedlings, metaxylem vessels were formed between 0.4 mm above the hypocotyl-root boundary and 17 mm below the boundary. The intervals in these regions were not extended so much as protoxylem vessels were collapsed. The first-formed protoxylem vessels presumably retain their water-conductive function after metaxylem formation.     

Examination of morphological changes in the first formed protoxylem in Arabidopsis seedlings

We examined morphological changes in the first-formed protoxylem vessels in Arabidopsis seedlings. Between 2.5 and 8 days after imbibition, mean hypocotyl and root length increased 1.52 and 23.3 times, respectively. In the 2.5-day-old seedlings, two continuous protoxylem vessels were present in the hypocotyl-root axis. In the 8-day-old upper hypocotyls, six protoxylem vessels were observed, and in the lower hypocotyls, four protoxylem vessels and one or two metaxylem vessels were observed. In the 8-day-old roots, there were two protoxylem vessels and two or three metaxylem vessels. Two protoxylem vessels in the hypocotyls connected to two metaxylem vessels in the roots of 8-day-old seedlings. At the 0.3-mm part below the hypocotyl-root boundary, the mean intervals of neighboring annular secondary wall thickenings in protoxylem vessels in 8-day-old roots were 12.9% larger than those in 2.5-day-old roots. In more apical parts of 8-day-old roots, the mean intervals fluctuated between 1.71 and 2.29 μm. In 8-day-old seedlings, metaxylem vessels were formed between 0.4 mm above the hypocotyl-root boundary and 17 mm below the boundary. The intervals in these regions were not extended so much as protoxylem vessels were collapsed. The first-formed protoxylem vessels presumably retain their water-conductive function after metaxylem formation. Received: April 10, 2001 / Accepted: November 7, 2001     

Light-controlled growth of the maize seedling mesocotyl: Mechanical cell-wall changes in the elongation zone and related changes in lignification

Cell extension in the mesocotyl elongation zone (MEZ) of maize ( Zea mays L.) seedlings is inhibited by light. The growth inhibition by blue light in the MEZ was reversible upon transfer to darkness. This experimental system was used for investigating the modification of mechanical cell-wall properties and the role of cell-wall lignification in cell elongation. The occurrence of lignin in the cortex and vascular bundle tissues of the MEZ was demonstrated by the isolation of diagnostic monomers released after thioacidolysis of the cell walls. Concomitantly with the inhibition of growth, blue light induces an increase in cell-wall stiffness (tensile modulus) as well as an increase in extractable lignin in the outer MEZ tissues (cortex+epidermis). Both effects are reversed when growth is resumed in the MEZ in darkness after a period of growth inhibition induced by 3 h light. In the vascular bundle light produces no comparable change in lignin content. Appearance and disappearance of phenylpropanoid material in MEZ cell walls in the light, or in darkness following a brief light treatment, respectively, can be visualized under the fluorescence microscope by characteristic changes in autofluorescence of tissue sections upon excitation with UV radiation. It is concluded from these results that light-induced lignification of primary walls is involved in cell-wall stiffening and thus inhibition of elongation growth in the MEZ of maize seedlings. Resumption of growth upon redarkening may be initiated by wall loosening in the uppermost MEZ region which displaces the lignified cell walls towards the lower mesocotyl region.     

Highly substituted glucuronoarabinoxylans (hsGAXs) and low-branched xylans show a distinct localization pattern in the tissues of Zea mays L

Polyclonal antibodies which recognized highly substituted glucuronoarabinoxylans (hsGAXs) and low-branched xylans and did not cross-react with each other, were raised in order to examine localization of these epitopes in internodes of maize. Immunofluorescent labeling revealed different pattern between two succeeding developmental stages. The hsGAX epitope was localized evenly in primary walls in all tissue types, and strongly in unlignified secondary walls in phloem. However, lignified secondary walls in protoxylem, parenchyma and a part of fibers were faintly labeled with this epitope. Moreover, the epitope showed limited binding in lignified parenchyma and fiber walls at ultrastructural level. Low-branched xylan epitope was localized evenly throughout lignified walls in all tissue types. This epitope was also localized only in lignified walls of other organs such as leaf, root apex and dark-grown mesocotyl. Low-branched xylans are significantly related to lignification. Localization of hsGAX epitope in their organs was similar to that in internodes. The hsGAX epitope was distributed both in unlignified walls of all tissues and in lignified walls of parenchyma and annular thickening of protoxylem. We propose that hsGAX has separate functions in lignified and unlignified tissues. In conclusion, at tissue level, hsGAX is localized mainly in unlignified walls, and low-branched xylans in lignified walls.     

PROCAMBIUM VS. CAMBIUM AND PROTOXYLEM VS. METAXYLEM IN POPULUS DELTOIDES SEEDLINGS

The concept of a procambium-cambium continuum was examined in Populus deltoides by following its development in serially sectioned bud and stem tissues. As in other species, the term cambium is used to refer to that part of the continuum associated with the formation of secondary vascular tissues; i.e., with secondary growth. However, that part of the continuum associated with the formation of primary vascular tissues is subdivided to facilitate interpretation of the consecutive stages of primary xylem differentiation. Thus, the procambium as envisioned by other authors is subdivided into procambium, initiating layer, and metacambium, all of which develop acropetally and in complete continuity. The procambium is derived from the residual meristem in the form of acropetally developing strands and traces. The initiating layer is represented by the first, tangentially separated, periclinal divisions that delineate the position of the prospective cambium. The metacambium is a later stage during which additional periclinally dividing cells unite the initiating layer into a tangentially continuous meristem within a trace bundle. After establishment of the initiating layer, the procambial trace is completely phloem dominated. Protoxylem differentiation begins in an originating center at the base of the leaf primordium and it progresses basipetally to form the protoxylem pole. Cells of the initiating layer do not contribute to the formation of either protoxylem or protophloem. However, those cells of the initiating layer directly opposite the protoxylem pole divide precociously and later differentiate to metaxylem, thus forming a radial file of protoxylem-metaxylem elements. Protoxylem elements of lateral traces are longitudinally continuous with the protoxylem of their parent traces, whereas those of a central trace are longitudinally continuous with the metaxylem of its parent trace. Metaxylem is formed later than protoxylem and it is derived from the metacambium. Metaxylem does not form a continuous system with protoxylem of the same trace because of the different temporal and spatial origins of the two kinds of xylem. Rather, metaxylem is longitudinally continuous with secondary xylem of older traces below. An attempt was made to determine the functional significance of the pattern of protoxylem and metaxylem differentiation in relation to primary and secondary plant development.     

Perturbation of polyamine catabolism can strongly affect root development and xylem differentiation

Spermidine (Spd) treatment inhibited root cell elongation, promoted deposition of phenolics in cell walls of rhizodermis, xylem elements, and vascular parenchyma, and resulted in a higher number of cells resting in G(1) and G(2) phases in the maize (Zea mays) primary root apex. Furthermore, Spd treatment induced nuclear condensation and DNA fragmentation as well as precocious differentiation and cell death in both early metaxylem and late metaxylem precursors. Treatment with either N-prenylagmatine, a selective inhibitor of polyamine oxidase (PAO) enzyme activity, or N,N(1)-dimethylthiourea, a hydrogen peroxide (H(2)O(2)) scavenger, reverted Spd-induced autofluorescence intensification, DNA fragmentation, inhibition of root cell elongation, as well as reduction of percentage of nuclei in S phase. Transmission electron microscopy showed that N-prenylagmatine inhibited the differentiation of the secondary wall of early and late metaxylem elements, and xylem parenchymal cells. Moreover, although root growth and xylem differentiation in antisense PAO tobacco (Nicotiana tabacum) plants were unaltered, overexpression of maize PAO (S-ZmPAO) as well as down-regulation of the gene encoding S-adenosyl-l-methionine decarboxylase via RNAi in tobacco plants promoted vascular cell differentiation and induced programmed cell death in root cap cells. Furthermore, following Spd treatment in maize and ZmPAO overexpression in tobacco, the in vivo H(2)O(2) production was enhanced in xylem tissues. Overall, our results suggest that, after Spd supply or PAO overexpression, H(2)O(2) derived from polyamine catabolism behaves as a signal for secondary wall deposition and for induction of developmental programmed cell death.     

DEVELOPMENT AND STRUCTURE OF THE VASCULAR CONNECTION BETWEEN THE PRIMARY AND SECONDARY ROOT OF GLYCINE MAX (L.) MERR.

The primary vascular connection between primary and secondary root of Glycine max (L.) Merr. was derived from stelar parenchyma and pericycle. Inner stelar parenchyma, associated with the parent metaxylem and outer stelar parenchyma adjacent to the pericycle, were resonsible for the histogenesis of the primary xylem connection. Acropetal maturation of the diarch xylem connection occurred after the lateral root emerged from the parent root. Development of tetrarchy occurred distal to the diarch xylem connection. The concentric primary phloem connection was derived from the pericycle and outer stelar parenchyma. Acropetal maturation of the primary phloem connection occurred prior to lateral root emergence from the parent root. Secondary growth quickly augmented the primary vascular connection. A substantial amount of mature secondary xylem formed prior to maturation of the secondary phloem. The structure of the primary and secondary vascular connections is described.     

Effect of moderate salinity on patterns of potassium, sodium and chloride accumulation in cells near the root tip of barley: Role of differentiating metaxylem vessels

X-Ray microanalysis of fully hydrated, bulk-frozen samples was used to measure concentrations of potassium, sodium and chloride in various cell types along seminal roots of barley ( Hordeum vulgare L. cv. California Mariout) seedlings (1 to 150 mm from the tip). In the cytoplasm of all meristematic cells 1 mm from the root tip, the average concentrations of potassium and chloride were ca 200 and 15 m M , respectively. The potassium level was also high in the vacuoles of incipient xylem elements and did not drop to significantly lower values until 10 mm from the tip in protoxylem, 50 mm in early metaxylem and 150 mm in late metaxylem (LMX). Light microscopy observations (Nomarski optics) of hand-cut sections showed the presence of cytoplasmic strands and also the presence of intact cross walls in LMX up to a distance of 100 mm. Both quantitative analysis of ion contents and structural observations suggested that LMX elements act as a large transitional sink of accumulated ions and therefore may not function as a main pathway of transport until perforation of the end wall takes place 100–150 mm from the root tip. Treatment with 50 m M NaCl resulted in higher concentrations of sodium and chloride in LMX elements than in the surrounding cells, suggesting that living xylem elements, which develop a large central vacuole at an early stage of root differentiation, may assist in alleviating salinity stress in the meristematic region of barley root tips. Further, it is proposed that reabsorption of sodium and chloride from the LMX, especially before the disappearance of the cross walls, may provide a means of salinity tolerance.     

Immunohistochemical localization of hemicelluloses and pectins varies during tissue development in the bamboo culm

The distribution of hemicelluloses and pectins in bamboo internodes was studied immunocytochemistrically at various stages of development. The ultra-structures of bamboo cell walls have been reported previously at various stages. The internodes were identically classified into three developmental phases: primary wall stage (phase I), unlignified secondary wall stage (phase II) and lignified wall stage (phase III), using the same bamboo culm. (1-->3, 1-->4)-Beta-glucans were distributed in nearly all tissues in an actively elongating stage. Limited amounts of beta-glucans were deposited in primary walls and the middle lamellae, but were limited to the phloem in secondary walls. This suggests that the function of beta-glucans might be different in phloem vis-à-vis other tissues. Highly-substituted xylans were located in nearly all tissues of early phase I, but had disappeared in all tissues immediately prior to lignification. In contrast, low-branched xylan epitopes were present only in the protoxylem in phase I, but were present in all tissues immediately prior to lignification in phase II. In phase III, the epitopes were densely localized in lignified walls, suggesting that the substitution of xylans is closely related to maturation. Methyl-esterified (but not unesterified) pectins were present in all tissues of early phase I. Just before and after lignification, both types of pectins were concentrated in the phloem and protoxylem. Xyloglucans were largely distributed in the phloem and in lignified tissues, suggesting that they might be closely correlated with maturation. This represents the first account of the distribution of hemicelluloses and pectins at the tissue and ultrastructural level in bamboo internodes at various stages of development.     

Immunohistochemical Localization of Hemicelluloses and Pectins Varies During Tissue Development in the Bamboo Culm

The distribution of hemicelluloses and pectins in bamboo internodes was studied immunocytochemistrically at various stages of development. The ultra-structures of bamboo cell walls have been reported previously at various stages. The internodes were identically classified into three developmental phases: primary wall stage (phase I), unlignified secondary wall stage (phase II) and lignified wall stage (phase III), using the same bamboo culm. (1→,1→4)-β-Glucans were distributed in nearly all tissues in an actively elongating stage. Limited amounts of β-glucans were deposited in primary walls and the middle lamellae, but were limited to the phloem in secondary walls. This suggests that the function of β-glucans might be different in phloem vis-à-vis other tissues. Highly-substituted xylans were located in nearly all tissues of early phase I, but had disappeared in all tissues immediately prior to lignification. In contrast, low-branched xylan epitopes were present only in the protoxylem in phase I, but were present in all tissues immediately prior to lignification in phase II. In phase III, the epitopes were densely localized in lignified walls, suggesting that the substitution of xylans is closely related to maturation. Methyl-esterified (but not unesterified) pectins were present in all tissues of early phase I. Just before and after lignification, both types of pectins were concentrated in the phloem and protoxylem. Xyloglucans were largely distributed in the phloem and in lignified tissues, suggesting that they might be closely correlated with maturation. This represents the first account of the distribution of hemicelluloses and pectins at the tissue and ultrastructural level in bamboo internodes at various stages of development.     

Late Maturation of Large Metaxylem Vessels in Soybean Roots: Significance for Water and Nutrient Supply to the Shoot

The large, late metaxylem (LMX) in the roots of soybean beginsdevelopment in the centre of the stele after lignification ofthe early metaxylem poles. Subsequent maturation of the firstappearing LMX elements is gradual. They were never mature inthe 8-d-old seedlings examined. In 10 to 15-d-old plants thefirst LMX matured to open vessels at a mean of 17 cm proximalto the root tip. Additional LMX vessels developed in more proximalregions of the roots and these also matured gradually. Based on calculations from relative vessel diameters, the potentialflow of xylem sap in a single central LMX vessel is 50 timesthat in the total of all the early metaxylem (EMX) vessels ofa typical primary root of soybean. There was a marked dependence of relative leaf area on the lengthof primary root with open LMX vessels. This may result fromthe predicted increased water and nutrient flow to the shoot,facilitated by the opening of the large vessels. It is suggestedthat, as in maize, the living LMX elements may function in ionaccumulation. Dicotyledonous roots, soybean, Glycine max, xylem vessels, xylem maturation, water conduction     

DEVELOPMENTAL ANATOMY IN ROOTS OF IPOMOEA PURPUREA. II. INITIATION AND DEVELOPMENT OF SECONDARY ROOTS

In seedlings of Ipomoea purpurea secondary roots are initiated in the primary root pericycle opposite immature protoxylem. Cells derived from immature endodermis, pericycle, and incipient protoxylem and stelar parenchyma contribute to the primordium. The derivatives of the endodermis become a uniseriate covering over the tip and flanks of the primordium and emerged secondary root; the endodermal covering is sloughed off when the lateral root reaches 1–5 mm in length. A series of periclinal and anticlinal divisions in the pericycle and its derivatives gives rise to the main body of the secondary root. The initials for the vascular cylinder, cortex, and rootcap-epidermis complex are established very early during primordium enlargement. After emergence from the primary root, the cortical initials undergo significant structural modifications related to enlargement of the ground meristem and cortex, and the rootcapepidermal initials are partitioned into columellar initials and lateral rootcapepidermal initials. Procambium diameter increases by periclinal divisions in peripheral sectors. The mature vascular cylinder is comprised of several vascular patterns, ranging from diarch to pentarch, that are probably related ontogenetically. Cells derived from incipient protoxylem and stelar parenchyma cells of the primary root form the vascuar connection between primary and secondary roots.     

GROWTH STUDIES OF THE ROOT OF INCENSE CEDAR,LIBOCEDRUS DECURRENS. I. THE ORIGIN AND DEVELOPMENT OF PRIMARY TISSUES

Wilcox , Hugh . (State U. Coll. of Forestry, Syracuse, New York.) Growth studies of the root of incense cedar, Libocedrus decurrens. I. The origin and development of primary tissues. Amer. Jour. Bot. 49(3): 221–236. Illus. 1962.—The anatomical features of active and dormant roots of incense-cedar seedlings are described and discussed in relation to various problems of differentiation and morphogenesis. Autoradiographs confirm the presence of a group of relatively inactive cells at the site of the apical initials. During periods of maximum growth activity, the presence of a quiescent center is accentuated by a peak in number of divisions in adjacent tissues. With diminution in growth activity, the peak occurs closer to the quiescent center and the size of the meristem appears to diminish. During dormancy, the configuration of the initial region seems to indicate the existence of apical initial cells which coincide with a minimal constructional center, as determined by studies of cell lineage. Roots whose apical cells retain their meristematic appearance are able to resume growth after a period of dormancy, whereas roots whose apical cells undergo vacuolation are likely to perish. Graphs are presented to show the functional relationships between growth rate and the varying distances from the apical meristem at which the tissues of the root differentiate and mature. Although early differentiation of precursory phloem could be discerned almost as soon as early vacuolation of metaxylem, its recognition was more dependent upon subjective judgment. The functional relationship between differentiation and growth rate was most pronounced in the maturation of protoxylem elements, the development of Casparian strips in the endodermis, the development of suberin lamellae in the endodermis, and by the development of phi layers in the inner cortex.     

Preliminary Studies of Root-Stem Transition in Brassica napus L.

Seedlings of Brassica napus L. 2–11 days after germination were used. However, the most investigation was concentrated on the 6-day old seedlings. The primary root has a diarch protostele, the two groups of primary phloem alternate with the primary xylem. At higher level, the metaxylem is gradually differentiated in a lateral direction. Being coincident with this changes of the metaxylem, the groups of phloem cell are extended. The stele of the lower hypocotyl is root-like and has no pith. In the middle hypocotyl, there is a further lateral differentiation of the metaxylem. At the higher level, four metaxylem arms appear and the groups of phloem are extended circumferentially to form two crescent shaped sectors. In the upper hypocotyl below 0.2 cm of the cotyledonary node, a central pith has been formed which separates the differentiating primary xylem into two distinct units. At a slightly higher level, each primary phloem divides into two small groups, at this time, each xylem unit and the two adjacent groups of phloem constitute a cotyledonary trace. The foliar traces of the first two foliage leaves appear in the inter-cotyledonary plane between the vascular elements of the cotyledonary traces. At this level, the vascular tissue of the hypocotyl forms a siphonostele made up of two cotyledonary traces and the two foliage leaves, where the root-stem transition has nearly been completed, while the endarch condition is not attained in the hypocotyl. At incresing distances from the cotyledonary node upwards, in the cotyledonary petiole, the protoxylem occupies a more and more adaxial position and the metaxylem a more and more abaxial direction and, thus, the endarch condition is attained. The primary system of the root, hypocotyl, and cotyledons forms a complete circular system, the plumular vascular elements are directly connected by secondary elements formed by the cambium in the region of the hypocotyl. As for the results mentioned above, the authers have not detected that the primary xylem has a rotation of 180˚, as described by Van Tieghem.     

Silicon-induced changes in viscoelastic properties of sorghum root cell walls

Silicon is deposited in the endodermal tissue in sorghum (Sorghum bicolor L. Moench) roots. Its deposition is thought to protect vascular tissues in the stele against invasion by parasites and drying soil via hardening of endodermal cells. We studied the silicon-induced changes in mechanical properties of cell walls to clarify the role of silicon in sorghum root. Sorghum seedlings were grown in nutrient solution with or without silicon. The mechanical properties of cell walls were measured in three separated root zones: basal, apical and subapical. Silicon treatment decreased cell-wall extensibility in the basal zone of isolated stele tissues covered by endodermal inner tangential walls. The silicon-induced hardening of cell walls was also measured with increases in elastic moduli (E) and viscosity coefficients (eta). These results provided new evidence that silicon deposition might protect the stele as a mechanical barrier by hardening the cell walls of stele and endodermal tissues. In contrast to the basal zone, silicon treatment increased cell-wall extensibility in the apical and subapical zones with concomitant decrease in E and eta. Simultaneously, silicon promoted root elongation. When root elongation is promoted by silicon, one of the causal factors maybe the silicon-enhanced extensibility of cell walls in the growing zone.     

Alfalfa Stem Tissues: Cell-wall Development and Lignification

Alfalfa stems contain a variety of tissues with different patternsof cell-wall development. Development of alfalfa cell wallswas investigated after histochemical staining and with polarizedlight using light microscopy and scanning electron microscopy.Samples of the seventh internode, from the base of stems grownon cut stems, were harvested at five defined stages of developmentfrom early internode elongation through to late maturity. Internodeseven was elongating up to the third sample harvest and internodediameter increased throughout the entire sampling period. Chlorenchyma,cambium, secondary phloem, primary xylem parenchyma and pithparenchyma stem tissues all had thin primary cell walls. Pithparenchyma underwent a small amount of cell-wall thickeningand lignification during maturation. Collenchyma and primaryphloem tissues developed partially thickened primary walls.In contrast to a recent report, the formation of a ring shaped,lignified portion of the primary wall in a number of cells inthe exterior part of the primary phloem was found to precedethe deposition of a thick, non-lignified secondary wall whichwas degradable by rumen microbes. In numerous xylem fibres fromthe fourth harvest date onwards, an additional highly degradablesecondary wall layer was deposited against a previously depositedlignified and undegradable secondary wall. The pattern of lignificationobserved in alfalfa stem tissues suggests that polymerizationof monolignols by peroxidases at the luminal border of the primarycell wall creates an impermeable zone which restricts lignificationof the middle lamella region of tissues with thick primary walls.Copyright1998 Annals of Botany Company Alfalfa,Medicago sativaL., stem tissue, cell wall, development, lignification, degradation.     

SEM studies on vessels in ferns. 17. Psilotaceae

Perforation plates are reported in aerial and subaerial axes of Psilotum nudum and in aerial axes of Tmesipteris obliqua. In Psilotum, both perforations lacking pit membranes and perforations with pit membrane remnants were observed. Perforation plates in Psilotum may consist wholly of one type or the other. In Tmespteris, perforations have threadlike pit membranes or consist of porose pit membranes. Wide perforations alternating with narrow pits, a conformation observed in various ferns, were observed in Psilotum (subaerial axes). In Psilotum, perforations are more common in metaxylem than in protoxylem; perforations in protoxylem consist of primary wall areas containing small circular porosities or relatively large circular to oval perforations. There are no modifications in the secondary wall framework of protoxylem or metaxylem in Psilotum or Tmesipteris that would permit one to distinguish presence of perforations or perforation plates with light microscopy, and scanning electron microscopy (SEM) is required for demonstration of porose walls or perforations. The tracheary elements of the Psilotaceae studied have no features not also observed in other ferns with SEM.