IAWA Journal, Vol. 19 (3),1998: 265-278

A STUDY OF THE VASCULAR ORGANIZATION OF (-) USING A MICROCASTING METHOD by Jean-Pierre Andre

INRA, P. O. Box 2078, 06600 Antibes, France

SUMMARY

A reliable and simple microcasting method is applied to the study of the vascular structure in nodes; it provides new insights into their complexity, revealing the exact arrangement of branched vessels and clustered tracheary elements. Axial differentiation gradients in the metaxylem cell files, probable relics of the intercalary meristem, can also be found using this method. This anatomical finding can be linked to arecent hypothesis on the continuum in the tracheary element differ­ entiation. Key words: Metaxylem, vascular bundle, vessel, bamboo, node-inter­ calary meristem.

INTRODUCTION

Numerous anatomical details on the bamboo vascular organization have been de­ scribed in the last 20 years (Zee 1974; Yulong & Liese 1997) but it is obvious that the classical histological methods - serial sections, macerated sampies, cleared prepara­ tions - hardly give an adequate three-dimensional understanding of such extensive and complex anatomical structures. A new casting method reaches this goal much more easily; it allows one to visual­ ize the vessels of a culm segment and to accurately examine cell file casts in their original arrangement (Andre 1993). This has been recently applied to 25 bamboo species in order to complete earlier observations (Andre 1996). The method reveals the exact features of the bundle ramifications in the form of tracheary element clus­ ters and their mode of mutual connections, allowing hypotheses on the bundle organi­ zation in the wh oie mature . Moreover, by these means it was found that the internodes of the aerial axes show a probable trace of their intercalary meristem in the form of a short differentiation gradient of the secondary wall thickening in the metaxylem vessels. Assumptions conceming these particular 'transition' structures, which have not as yet been describ­ ed for Poaceae, can be made on the basis of the concept of procambium - metacambium continuum according to Larson (1982) and of the differentiation continuum hypoth­ esis by Savidge (1996). Other casting methods have recently been published (Fujii 1993; Fujii & Hatano 1996) but, to our knowledge, have not been applied to this area.

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Table 1, Classification of the studied species.

Subtribe Species

Arundinariinae Arundinaria distichus (Mitf.) Muroi & Okamura* linearis (Hack.) Nakai* DrepanostachyumJalcatum (Nees) Keng f. nitida (Mitf.) Nakai** Pseudosasa japonica Sieb. & Zucc. ex Steud. palmata var. nebulosa (Mak.) s. Susuki Shibateinae Chimonobambusa marmorea (Mitf.) Mak. quadrangularis (Munro Nakai) Fenzi Mak. Hibanobambusa tranquillans var. shirochima (Koidz.)*** aurea Carr./ A. & C. Riv. aureosulcata MacClure bambusoi'des var. castilloni Madiar & Carr. Mak. var. violascens A. & C. Riv. flexuosa Carr./ A. & C. Riv. nigra (Lodd. ex Lindl.) Munro viridiglaucescens Carr./A. & C. Riv. viridis Robert Young kagamiana Mak. fastuosa (Marliac ex. Mitf.) Mak. ex Nakai Bambusinae multiplex (Lour.) Reuschel ex. Schult. f. multiplex cv. eIe gans multiplex cv. golden goddess tuldoi'des (Munro) ventricosa MacClure Guaduinae aztecorum (Mac Cl ure & Smith) Calderon

*: synonym Pleioblastus; **: synonym Sinarundinaria; ***: probably a hybrid of Sasa x Phyllostachys (Demoly 1996).

MATERIALS AND METHODS

Plant material The 25 bamboo speeies whieh were studied are from 4 of the 9 subtribes and from 11 of the 69 genera reeently reeognized for woody bamboos (Clark 1995) (Table 1). The speeies were eolleeted from three plaees in the South ofFranee ('Jardin Thuret' in Antibes, 'Bambouseraie du Mandarin' in Montauroux, and 'Bambouseraie de Prafranee' in Anduze). Two mature eulms ofless than three years old, plus a growing eulm of Bambusa tuldoides, were cut at soillevel and sampled for eaeh speeies. For one speeies, Phyllostachys viridis, the eulm was sampled with its rhizome. The spe­ eies originated from regions of similar climate, but differed in numerous morphologi-

Downloaded from Brill.com09/25/2021 08:39:40PM via free access Andre - Microcasting of vascular bundles in bamboos 267 cal characters, namely the culm size and axis architecture, which are not analyzed here. The nodes were numbered from the soil level, positively for the aerial part, negatively for the underground part. The diameter and length of the internodes were measured. The nodes to be cast were sawn 2-3 cm above and below the leaf sheath scar. The sawn rims were carefully recut with a razor blade. Slices of adjacent internodes were kept for bundle counting.

Methods For the casting, the air-dried segments were immersed in a liquid mixture of two silicon elastomer components (RTV 141, Rhöne Poulenc, France) and evacuated in a vacuum chamber at room temperature. As soon as the vacuum was broken, the elastomer penetrated each empty space, namely the intracellular lumina of the cut metaxylem vessels and the intercellular cavities which formed in the bundles of age­ ing or dried bamboos, between the partially collapsed protoxylem and protophloem cell files. The injection speed depended on the pore size. In order to limit the catalyzed setting of the mixture, the culm segments were kept at a low temperature (-18°) for 10 hours. The intracellular injection was stopped by intact pits, but easily passed through the perforations. Thus, it was possible to cast files containing several hun­ dred cells. After this step, the elastomer was cured at 50 oe for 4 hours and became a transpar­ ent, elastic and chemically inert material. The cell walls were then destroyed by suc­ cessive bathing in solutions of sulfuric acid (H 2S04, 2H20), sodium bicarbonate (l mole/l) and sodium hypochlorite (2 moles/l). The casts ofthe metaxylem vessels and the cavities where the protoxylem and part of the phloem cell files were embed­ ded, were shaped like tiny filaments soldered at each end to a small elastomer block. They were covered by a water film for optimal observation under a light microscope and photographed in a scanning electron microscope (SEM) after standard coating with gold (Andre 1993).

RESULTS Macroscopical description The bamboos form vegetative axes from the buds, which are in distichous alter­ nate positions at the rhizome, culm and branch nodes. Each axis consists of similar metamers where diameters are relatively constant in the rhizome. They increase from one metamer to the next in the underground part of the culm, then decrease in the aerial part at the rate of approximately 3-5 mm/m of culm, and also in the branches, from the first to the apical metamer. Each internode is cylindrical and hollow with an externall internal diameter ratio depending on the bamboo species. Numerous axial, collateral, parallel bundles run in the intern ode wall, scattered in a ground tissue of parenchyma. From the outside to the inside of the wall, the bundle diameters increase whereas their density decreases, e. g., from approximately 50 to 5 bundles/mm2 in the basal internodes ofthe Phyllostachys species studied. In the ma­ ture culm, the main water conducting vessels are probably the metaxylem vessel pairs,

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------..... vessel . 2 C I 1 nb I, I [] [J 1 I I I ph .a. 1000 I I I I ~------*- - * "- "- ~ " 1 "- o 1500 o 0 0 , * - - - 0 - 15 ------, "- o '", C I ' *"­ , I o ' , , I , 0 , l:> *, 1000 , "- U.p. A.P. ~ , ''

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Downloaded from Brill.com09/25/2021 08:39:40PM via free access Andre - Microcasting of vascular bundles in bamboos 269 whose diameters vary from 80-130 /illl in the internal bundles to 5 -15 /illl in the peripheral ones (Fig. 1). The nodal zones where each adjacent organ is connected are anatomically more complex than the intern odal ones. Moreover a parenchymatous nodal diaphragm is present with numerous transverse connecting bundles. Dur observations focus essen­ tially on the nodal vascularization and the connections between mature axes. The following description is based mainly on the metaxylem casts.

Bundle and metaxylem vessel counting - Casting efficiency As observed in the transverse sections, the number of bundles remains constant along an internode, but varies from one internode to the next. It increases or decreases with the diameter of the culm, from its junction with the rhizome to its apex. Accurate counting of the bundles along successive metamers was limited to the aerial internodes of a culm of Phyllostachys aurea and to some internodes of the lower and under­ ground part of a culm of Phyllostachys viridis. These data were multiplied by 2 in order to give the total number ofthe metaxylem vessels (t). In addition, the number of the effectively cast metaxylem vessels (c) was counted in each segment of the Phyllostachys aurea culm. A relative constancy of t was observed in the first aerial internodes, where nodal buds and branches were absent. In the branched part ofthe Phyllostachys aurea culm, t decreased at the rate of about 100 vessels/node and in the underground part of the Phyllostachys viridis cu Im, it increased at the rate of about 250 vessels/node (Fig. 2). The casting efficiency was 75-80% for the Phyllostachys aurea culm. The deficit of 20-25% between c and t resulted from tyloses which partially or totally blocked several vessels and from the minute diameters of the most peripheral vessels. These c values indicate that a nodal cast thus gives reliable, albeit not entirely complete mate­ rials for observations. The protoxylem vessels were always embedded together in a thick sheath of elastomer through which the annular and helical thickenings were visible under a light microscope. They were not examined in this study.

Fig. 1. Transverse seetion of a Bambusa tuldoides internode (b: bund1e with fibre sheaths; c: central cavity; mx: metaxylem; p: ground parenchyma; ph: phloem; px: protoxylem; square = I mm2). - Fig. 2. Total number (t) of vessels along the underground and aerial parts of a culm. Examples of Phyllostachys aurea aerial part (ph.a) and of Phyllotachys viridis under­ ground and basal parts (ph.v). Total number (c) of the cast vessels in the same Phyllostachys aurea culm. The broken lines are intuitively fitted. - Fig. 3-10. Vascular structures in the node. - 3: Vessel pair crossing the node without any con-nection (ip np: internodal and nodal pitting; arrowhead: oblique plate). - 4: Vessel pair with lateral ramification (arrowhead: first 3-perforated element). - 5: Vessel with a lateral branch ending in an adjacent axis (a: arrow­ head, first 3-perforated element; b: arrowhead, second vessel pairing the branch; c: ramified end of the vessel pair). - 6: Upper ramified end. - 7: Lower ramified end. - 8: Connection of two opposite bundles. - 9: Ring of connections around the lower rim (1.r.) ofthe diaphragm. - 10: radial seetion of connection between two axes showing the partly I-vessel transversal and partly 2-vessellongitudinal bundles.

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Fig. 11-18. SEM photographs of terminal clusters casts in Phyllostachys aurea (11-14) and Fargesia nitida (15-18) nodes, observed at different scales. The arrowheads refer to the suc­ cessive enlargements. The compact casts were opened with a tiny needle in order to show the inner vascular ramifications. - 11: An entire terminal ramification (bar 500 flm). - 12: Detail of Fig. 11 (bar 100 flm). - 13: Cluster element showing dense pitting (bar 20 flm). - 14: Pit casts (bar 5 flm). - 15: Connection between lower and upper vessel ends (bar 500 flill). - 16: Detail of Fig. 15 (bar 100 flm). - 17: Partial view of pitted elements (bar 20 flm). - 18: Pit casts (bar 5 flm).

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Bundle ramifications Whatever the species, the following features were observed in each vascular nodal cast: Most of the bundles crossed the node without any lateral connection. Their metaxylem vessel pairs ran from the lower to the upper internode with only a shorten­ ing of the vessel elements and a much denser pitting of their walls in the nodal cross­ ing (Fig. 3 & 24). The other vessel pairs ramified. They formed either lateral or terminal cell clusters, comprised of several dozen to several hundred densely pitted vessel elements con­ nected together by numerous 3-perforated elements. A cluster started from a first 3-perforated element, either included in the cell file of a metaxylem vessel, for the lateral cluster, or ending the vessel, for the upper or lower terminal ones (Fig. 4-8). These ramifications were more or less regularly dichotomous. They were randomly distributed in the culm wall at the level of the lower rim of the diaphragm. Each ramification was linked to the neighbouring ones, the whole forming a ring of con­ nections around the diaphragm (Fig. 9). In particular, the upper and lower bundles ending in the same culm node were closely linked in the ring by their opposite termi­ nal cluster pairs. But if the injection of the elastomer was restricted to just one side of the nodal culm segment, the other having been previously sealed, only the upper or lower terminal clusters were cast. This was proof that there were only pits, imper­ vious to the elastomer, and no perforations between the ramifications of different bundles. The casts were accurate and allowed the size and shape of the ramifications to be observed and compared at the scale of the cluster, the element and its pitting (Fig. 11-18). As observed in an immature node of Bambusa tuldoides, it seemed that the clusters progressively developed as the culm was growing. Only the first branched elements of the ramification were cast, as if the distal ones were still alive and differentiating (Fig. 19). Numerous trans verse vessel casts were observed in the rhizome and culm diaphragms of the nodes bearing adjacent axes. It was found that each transverse vessel was con­ nected by a 3-perforated element to a longitudinal vessel crossing the node. But, un­ like the 'regular' lateral cluster, which developed close to the vessel, it ramified far from its origin, at the end of a long element file, in one of the nodes of the adjacent axis (Fig. 5). As seen in diaphragm sections, the transverse bundles contained only one metaxylem vessel. Effectively, the vessel casts were not paired in their course through the diaphragm, but if they were followed outside the diaphragm, a second metaxylem vessel was discovered alongside each one in the adjacent axis. The diam­ eter of the second vessel increased acropetally and reached the size of the first one at about 10 mm from its end (Fig. 10 & 20). In addition, preliminary observations conceming the junction of the roots with the underground nodes led us to think that their metaxylem was also connected to the nodal vasculature by pitted ramifications.

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Bundle length The proportion of the vessels ending in or crossing a given node along an axis allowed a qualitative first estimation of the mean bundle length. This ratio depended on the node position, the nature of the axis and species or genus. In the Phyllostachys

21

20

Fig. 19. Immature ramification in a growing node of Bambusa tuldoides (SEM) (bar 500 iJl11). - Fig. 20. Above: connections through 3-perforated element between vessel and lateral 'branch' in Phyllostachys viridis rhizome (arrowhead: air bubble); below: end of the second metaxylem vessel alongside the 'branch' in the adjacent culm (arrowhead). Light microscope (bar 1 mm). - Fig. 21. Three oblique plates in Phyllostachys aurea metaxylem vessels with different angles. Pigment particles are visible in the transparent elastomer on one side and on the plate (arrowhead: plate ends; bar as in Fig. 20).

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22 23

24 25

Fig. 22-25. Diagrams and SEM photographs of the symmetrie al gradients of traeheary ele­ ment differentiation. - 22: Loeation of the symmetrical sequences (s. s.) at the ridge level (r). - 23: Conventional degrees of complexity of the symmetrical sequences from B to F (ip, np: intern odal and nodal pitting; sc, re, he, an: scalariform, reticulate, helical, annular pattern, re­ spectively). - 24: Above: internodal pitting in Phyllostachys flexuosa; below: nodal pitting in Ph. viridiglaucescens. - 25: From left to right: Phyllostachys viridis (type B); ibid. (type D); Ph. aureosulcata (type D); Bambusa ventricosa (type E with double spiral); B. tuldoides (type F). Vessel segments: lower at left, upper at right, for every species (bar 100 11m).

Downloaded from Brill.com09/25/2021 08:39:40PM via free access 274 IAWA Journal, Vol. 19 (3),1998 aurea culm, the sum ofthe ending vessels being relatively constant (100-150/node) for a decreasing total number (t), the ratio acropetally increased from about 0,005 to 0.5. This led to the hypothesis that the bundles traversed a large but acropetally de­ creasing number of internodes, According to the ratio, the mean bundle length seemed larger in the rhizome than in the culm of the Phyllostachys viridis species, and, for the same reason, it seemed to be larger in the Phyllostachys sp. than in the Bambusa sp. culms, for similar nodal positions. No individual bundle lengths were measured. The identification of the same given bundle in successive cut culm segments could be possible, but would require very careful manipulation.

Metaxylem vessellength The examination of hundreds of long vessel casts showed that the element files connected by simple perforations were interrupted here and there, at intervals of 5-1 0 cm, by oblique pitted plates which extended between two elements or more, accord-

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Fig. 26. Location of the symmetrical sequences found in the culms of the 25 species studied (names abbreviated, see Table I): the degrees of complexity are here indicated by smallIet­ ters, b to f (see Fig. 23), on the culms (below) and axillary branches (above) of first (1) and second (2) order. In the culms, the trans verse bars are spaced proportionally to the intemode lengths. Vertical bars at left = 0.50 m. Double oblique bars denote incomplete culms.

Downloaded from Brill.com09/25/2021 08:39:40PM via free access Andre - Microcasting of vascular bundles in bamboos 275 ing to their angle. The ornamentation of the plates was visible under a light micro­ scope and looked like a dense field of small round to elliptic pits. However, the plates were pervious to the elastomer, but pigment particles in suspension, of about 1 11m in diameter, could not easily pass and their deposits cluttered the plates (Fig. 21).1t was concluded that at least a certain number of the pits were perforated as a possible result of a selective adaptation to easier axial water transport. So, it was therefore difficult to decide if the plates were really vessel ends, i. e., if a longitudinal bundle contained avessei pair or two vessel files.

Trace 0/ the intercalary meristem The metaxylem vessel elements presented a different pitting in the nodes and the internodes, as described above (Fig. 24). However, different patterns of ornamenta­ tion were observed at the base of the internodes on a short segment of the vessels, comprising 3 to 15 elements. This particular feature was discovered in every aerial internode for certain species and only in the distal culm internodes and in the branches for others (Fig. 22). It consisted of roughly symmetrical and more or less complete sequences of the thickening patterns commonly found in protoxylem and first metaxylem: i.e., scalatiform, reticulate, helical and annular thickening patterns. Five conventional types of sequences indicated by the letters B to F (Fig. 23) were defined in order to approximately characterize the different sequences observed for each spe­ cies in the successive internodes of their culm and branches (Fig. 25): B: absence of any pattern. C: scalariform pattern only. D: scalariform - reticulate - scalariform sequence. E: scalariform - reticulate - helical - reticulate - scalariform sequence. F: scalariform - reticulate - helical- annular - helical- reticulate - scalariform sequence.

Frequently a sequence overlapped successive vessel elements, the same elements some­ times having two different thickening patterns. Such sequences generally became more complex, from B to F, acropetally in the culms and branches (Fig. 26). In a given node, each metaxylem vessel, whether large or small, had the same sequence. These symmetrical differentiation gradients were located at the bottom of the internodes at the level of the nodal ridge. They probably reflected the final activity of the intercalary meristem. In fact, the fibrous bundle sheaths were frequently less developed or partially absent at the same level. These preliminary observations are summarized in Figure 26. Without a thorough knowledge of the organogenesis in this large plant family, it was impossible to inter­ pret the specific differences relating to this particular feature, unknown until now.

DISCUSSION

The two main histological structures revealed by vessel casting - the three-dimen­ sional structure of the ramifications and the thickening sequences in metaxylem - focus the discussion on two distinct concepts: vascular connection and differentia­ tion.

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Vascular connection: In order to describe the mode of vascular connection, namely in grass and bamboo nodes, the term 'anastomosis' is frequently used, without any precise definition, and the diagrams which represent it in numerous publications suggest a striking similarity with true vascular anastomosis in animal organisms (Metcalfe 1960; Kumazawa 1961; Yulong & Liese 1997). Casting clarifies this point, distinguishing the perforated con­ nections between the tracheary elements of the same vessel and its ramifications, and the pitted connections between the tracheary elements of two distinct vessels. How­ ever, the specific question of the oblique 'pitted-perforated' plates need to be further clarified through ultrastructural examination of the walls. The whole structure of the 'dichotomous' ramifications forming tracheary clusters, and the connection, through perforated elements, between transverse and longitudinal vessels were clearly vis­ ible, for the first time. The transverse bundles containing only one vessel had previ­ ously been observed in the diaphragm (Yulong & Liese 1997). Their exact nature, partly transverse with one vessel, partly longitudinal with two vessels, can now be more precisely described. In serial sections and cleared slices of bamboo nodes, Zee (1974) noted the pres­ ence of numerous aggregated xylem (and phloem) cells, which he called 'transfer cells' using a term earlier proposed by Gunning et al. (1970), and claimed that the xylem transfer cells (shown on his own figures 10 & 11) belonged to the bundles entering the leaf sheaths. Later, Busby and O'Brien (1979) hypothesized on the water conducting role of the transfer cells in the wheat node. Recently Yulong and Liese (1997) assumed that "the intensively pitted sm all cells lying between vessels ... are derived from the transfer cells as described by Zee" and presented, as an example of their own figure 10, a small densely pitted metaxylem element, taken from a macer­ ated preparation which had numerous perforations. It was easy to identify such an element, as weIl as the xylem transfer ceIls, with the tracheary elements belonging to the ramifications. It remains to be examined how the leaf sheaths are linked to the node. Numerous other anatomical details such as the size and shape of the cluster ele­ ments which seem to be specific or generic characters of taxonomic interest require further study.

Vascular differentiation Larson (1982) clearly highlighted the spatio-temporal continuity between the procambium and the cambium, and defined the metacambium from whose divisions the metaxylem elements could derive. The possibility for a given cell file deriving partly from proto and metacambium to differentiate partly as a proto- and metaxylem vessel has not been precisely stated but can be assumed. Savidge (1996) hypothesized the existence of a continuum in primary tracheary element differentiation, and that this continuum results from the increased duration of gene expression from the annu­ lar to the bordered-pitted cell type, during the differentiation process.

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For bamboos, it could be supposed that the metaxylem vessel elements which bear the differentiation gradient observed were the last cells in each vessel acropetally and basipetally derived from the intercalary meristem with an increasingly shorter differ­ entiation time. Such a simple explanation is to be carefully investigated, by examin­ ing the cell file cast. Indeed, the limits between the successive cell types of the same sequence do not always coincide with the strict celllimits. How could the genes of a given cell induce a heterogeneous differentiation in that same cell, or the same differ­ entiation over two adjacent cells? The hypothetical shortening of the duration of the gene expression does not seem to occur in the basal nodes where there is no differen­ tiation gradient. Is it possible that these nodes and intemodes without gradients were preformed in the buds? Similar axial gradients are frequent in many cereal species, as revealed by this method. Apparently, it is a particular property of the metaxylem vessels, which are, above all, 'transition vessels'. Numerous dicotyledons show similar features in the abscission zones, which will be described elsewhere (Andre et al. 1998).

ACKNOWLEDGEMENTS

I would like to thank Mr. B. Beraud and Mr. Y. Crouzet for providing the bamboo material, Prof. Walter Liese for his advice, A. Sechon and C. Latouche-Halle for the collaboration during their training courses, Mrs. Repoux for the SEM service, Mr. C. Slagmulder for the photo graphie illustration, and Mrs. Boutard-Lerede for the presentation of the text.

REFERENCES

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Metcalfe, C.R. 1960. Anatomy of the . 1. Gramineae. Clarendon Press, Ox­ ford. Savidge, R.A. 1996. Xylogenesis, genetic and environmental regulation. A review. IAWA J. 17: 269-310. Yulong, D. & W. Liese. 1997. Anatomical investigations on the nodes of bamboos. In: G.P. Chapman (ed.), The Bamboos: 269-283. Academic Press, London. Zee, S.Y. 1974. Distribution of vascular transfer cells in the culm nodes of bamboo. Can. J. Bot. 52: 345-347.

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