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Anatomy and Morphology |
2Forestry and Forest Products Research Institute, Tsukuba Norin, P.O. 16, Ibaraki 305–8687, Japan; 3Department of Forest Science, Graduate School of Agriculture, Hokkaido University, Sapporo 060–8589, Japan; 4Faculty of Agriculture, Tokyo University of Agriculture and Technology, Fuchu-Tokyo 183–8509, Japan
Received for publication September 19, 2003. Accepted for publication January 30, 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: bordered pit • Fraxinus lanuginosa • hydraulic architecture • resin casting • ring-porous wood • vessels
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Carlquist, 1975
, 1988
; Baas, 1976
, 1982
; Tyree and Zimmerman, 2002
; Sperry, 2003
). Studies of the structure of vessels and their three-dimensional (3-D) networks could lead to a better understanding of the pathways of the ascent and distribution of water in the plant body, as well as of plant strategies for adaptation of xylem structure to various environments, in particular, the way in which water transport is assured in cases of injury or embolism (Utsumi et al., 1999
; Hacke and Sperry, 2001
) and the way in which xylem structure is optimized with respect to its hydraulic and biomechanical properties (Gartner, 1991a
, b
; Niklas, 1992
; Fujii et al., 2001
; Hacke et al., 2001
; Woodrum et al., 2003
).
There has been much interest in the structure and arrangement of vessels, but the 3-D architecture of vessel networks in secondary xylem has been investigated at the microscopic level in only a few studies and in very small segments of wood (for reviews, see Fujii et al., 2001
; Tyree and Zimmermann, 2002
). The 3-D arrangements of vessels in series of transverse sections have been reconstructed for a 0.3 x 1 x 2.5 mm3 segment of wood in Populus sp. (Braun, 1970
) and a 1.5 x 2.0 x 10 mm3 segment of wood in Fraxinus excelsior (Burggraaf, 1972
). Such reconstructions, based on microscopic observations of long series of sections, are tedious, and require considerable effort (for discussion, see Tyree and Zimmermann, 2002
). A cinematographic method has facilitated 3-D reconstructions and proved very useful in studies of vessel networks (Zimmermann and Tomlinson, 1966
; Tomlinson et al., 2001
). Other techniques for the rapid tracing of vessel networks include injection of dye and insertion of wire (Kanai et al., 1996
). The cinematographic and the wire-insertion methods allow studies of larger specimens but fail to provide detailed anatomical information about features such as the structures of pit contacts and of perforation plates, which play important roles in the patterns of water movement. The architecture of vessel networks in most dicotyledonous species and the pathways via which water is conducted, in particular, in the radial direction in xylem, remain poorly understood. The frequency of contacts between vessels on the two sides of a growth-ring boundary remains poorly characterized, and the type of xylem element (vessel or tracheid) that is predominantly involved in the apoplastic transfer of water in the radial direction remains to be identified.
Direct visualization of the vessel-to-vessel connections along the course of individual vessels requires the microscopic analysis of large specimens. Epifluorescence microscopy allows observations of the details of cell structure on the surface of a specimen (O'Brien and McCully, 1981
; Donaldson et al., 1999
; McManus et al., 2002
), and confocal laser scanning microscopy (CLSM), allows, in addition to the dimensions (x, y) for conventional optical resolution, higher resolution in the third (z) dimension, that is useful for studies of the internal structure of thick histological sections (Donaldson and Lausberg, 1998
; Kitin et al., 2000
, 2002b
, 2003
; Funada, 2002
). In addition, a microcasting method was combined with scanning electron microscopy (SEM) to study the secondary cell wall sculptures of vessels in great detail (Fujii, 1993
; Mauseth and Fujii, 1994
; André, 1998
, 2002
; Fujii and Hatano, 2000
; Kitin et al., 2001
) and to examine the arrangement of vessels in the hardwood trees Pterocarya rhoifolia, Ilex macropoda, Acer pictum (Fujii and Hatano, 2000
), and Machilus thunbergii (Fujii et al., 2001
). The use of such recently developed anatomical methods, supported by powerful computer software, should facilitate the 3-D reconstruction of vessel networks in longer segments of stem and, at the same time, should allow detailed analysis of anatomical structures, such as intervessel pits.
We attempted to examine series of thick histological sections by epifluorescence microscopy and CLSM and to examine resin casts by SEM in an effort to characterize the 3-D network of vessels in Fraxinus lanuginosa Koidz., a ring-porous hardwood from the temperate forests in East Asia. We selected this species because large-diameter earlywood vessels and distinctly smaller-diameter latewood vessels are arranged singly or in small groups (Fig. 1). This arrangement allowed the convenient tracing of individual vessels. Our preliminary examinations revealed that solitary vessels and vessel multiples (for definitions of wood features, see IAWA Committee, 1989
) appeared to be isolated from each other and that contiguous vessels on the two sides of a growth-ring border were rare in single transverse sections. This organization led us to ask whether and how the vessels might communicate to provide a continuous network.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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. Two series of transverse sections were cut in downward axial direction. The first one consisted of 213 sequential 100 µm thick sections and the second of 30 sequential 200 µm thick sections. The sections were mounted in glycerol without staining and observed with an epifluorescence microscope (Optiphot XF-Ph-21; Nikon, Tokyo, Japan) with excitation with incident blue light (450–490 nm) from a mercury lamp and a long-pass filter (LP; 520 nm). Images of corresponding areas of 2 mm (radial) x 1.4 mm (tangential) of serial sections were captured with a digital CCD camera (Polaroid PDMC Ie; Polaroid, Cambridge, Massachusetts, USA). Individual vessels were numbered on the image of the first section of each series and each number was stored as a separate layer in a Photoshop multilayered image file (Adobe Photoshop 5.5; Adobe Systems, San Jose, California, USA). Then the image of the first section of series was replaced with the image of the second section, and numbers contained as separate layers in the Photoshop document were adjusted over the corresponding vessels (Fig. 3). This procedure helped us easily to identify corresponding vessels and was repeated for each sequential section in the series. The course of each individual vessel was traced on the sequential images, and vessel networks were reconstructed for two blocks of xylem tissue of 2 x 1.4 x 21.2 mm3 and 2 x 1.4 x 5.8 mm3 (radial x tangential x axial). The deviation in the radial direction of each vessel was determined with respect to the position of the growth-ring boundary in the images. For the evaluation of tangential deviation in the long courses of vessels from a certain radius of the stem, dye-flow experiments are more appropriate than the anatomical analysis of small samples. Therefore, in this analysis, the relative tangential deviation in the course of each vessel was determined with respect to the position of one individual vessel, namely, vessel no. 7 in Fig. 3, which was selected randomly in the first section and designated the marker vessel on the sequential microscopic images.
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Resin casting and SEM
Resin casts were prepared by vacuum suction of low-density polyethylene (LDPE, M281, melt index = 80; Showa Denko, Tokyo, Japan) into wood blocks of the same specimen of F. lanuginosa, as described by Fujii and Hatano (2000)
. Cell walls were dissolved with acids (sulfuric acid and a mixture of hydrogen peroxide and acetic acid) or enzymes (pectinase and cellulase) and sodium hypochlorite as described by Kitin et al. (2001)
. Long casts of vessels remained intact in samples after the digestion of cell walls. These casts were observed with SEM (JSM-840; JEOL, Tokyo, Japan) at an accelerating voltage of 5 kV after Pt-Pd ion-sputter coating.
Observation of tangential vessel walls at growth-ring boundaries by SEM
Small blocks containing cambium and the adjacent phloem and xylem were obtained from 4-year-old branches of Fraxinus lanuginosa in late October, after the cessation of cambial activity. The fresh samples were cut in the longitudinal and radial direction into slabs that were 2–3 mm thick and 1 cm long, and these slabs were treated with pectinase to macerate the cambium, as described by Kitin et al. (1999)
. Then the bark and cambium were removed from the samples, and the exposed tangential surfaces of the xylem boundary at the interface between cambium and xylem were observed by SEM as described earlier.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Figure 5 shows a schematic diagram of the vessel network that was reconstructed from a series of 213 thick transverse sections of a wood block of 2 x 1.4 x 21.2 mm3 (tangential x radial x axial). Some of the sections in this series were discussed earlier and are shown in Fig. 3. A group of vessels with the frequent interchange of intervessel contacts could be defined in the earlywood of the growth ring (group I in Fig. 5). Vessel no. 18, which drifted tangentially in serial sections (Fig. 3A–C) was in contact sequentially with vessels nos. 7, 15, 5, and 1 (not shown in Fig. 3), therefore, it linked the rest of the earlywood vessels, with the exception of no. 14, in a common network (Fig. 5). Vessel 14 moved beyond the field of view and was contiguous with other vessels, but these could not be identified in sequential sections.
Two groups of latewood vessels could be defined in the latewood (labeled II and III in Fig. 5). Group II was connected with the vessel network of the earlywood of the growth ring through a contact between vessels 4, 24, and 40 (not shown in Fig. 3). The vessels in group III remained isolated from the rest of the vessel network in the wood block that we studied, which, in fact, contained only a portion of the routes of the various vessels (Fig. 5). We performed this anatomical analysis using much larger specimens of wood than those used in previous reports, but these larger specimens still did not contain the entire vessels. Individual large-diameter vessels and, in particular, the vessels of ring-porous hardwoods, can be up to several meters long (Zimmermann and Jeje, 1981
; Ewers et al., 1990
). In view of the fact that the visualized vessels were longer than the lengths of our samples and that their courses deviated from the straight longitudinal direction, vessels in group III and vessel no. 14 were probably also part of a common vessel network. Experimental proof of this hypothesis would require analysis of even larger blocks of wood.
Tangential and radial continuity of the vessel network
At the macroscopic level, the wood of F. lanuginosa did not appear to have a spiral or wavy grain. However, in serial sections, substantial tangential drift of the relative positions of vessels was evident (Fig. 3). Earlywood vessels nos. 2, 3, 4, and 18 and latewood vessels nos. 24, 25, 26, and 40 gradually shifted course "to the left" and disappeared from the field of view (Fig. 3A–D). Vessel 14 moved to the right of the field of view. New vessels, for example, latewood vessels 42 and 46, came into the field of view from the right.
In addition to the tangential drift, a radial drift of latewood vessels was also evident. Vessel 27 (Fig. 3A) moved outwards in a radial direction and joined vessel 37, which appeared from the outward direction (Fig. 3C). The radial position of vessels 21 and 22 (Fig. 3A) also shifted along the stem axis (Fig. 3F). Radial shifts in the positions of vessels 23 and 46 were also apparent (Fig. 3D–H). Some vessels, which were not seen in the first section of the series (Fig. 3A), for example, vessels 37 and 39 (Fig. 3B), 41 (Fig. 3C), and 44 (Fig. 3F), came into the field of view in sequential sections. All these vessels appeared from the outward side of the growth ring (the cambium side is the upper side of the images in Fig. 3), which indicated that their courses are inclined slightly or fluctuated in the radial direction. Even a small drift in the radial direction of the courses of individual vessels provided tangential connections between neighboring solitary vessels or vessel multiples, such as vessels 27 with 37 (Fig. 3B and C) and 23 with 46 (Fig. 3D–F).
In single sections of F. lanuginosa, contact between vessels across annual ring borders was seen infrequently (Fig. 2). In the first section of the series (Fig. 3A), only vessels 2 and 10 were contiguous with latewood vessels of the previous annual ring. However, in the subsequent sections in the series, vessels 1, 6–9, and 13 also appeared adjacent to latewood vessels (data not shown). Therefore, in our 21.2 mm long sample of wood of F. lanuginosa, all earlywood vessels at the growth-ring boundary made contact with small-diameter vessels in the latewood of the previous ring. By contrast, we observed only one contact between earlywood and latewood vessels within the growth ring itself (Fig. 5).
We found numerous, densely arranged intervessel pits in the tangential walls of adjacent vessels (Figs. 11–13). Longitudinally aligned, latewood vessel elements were frequently found as terminal cells at growth-ring boundaries and densely arranged bordered pits, facing the cambium, were visible on their tangential walls (Fig. 14). Such intervessel pits illustrate the radial pathway that allows movement of water via the vessel network. In addition, marginal fiber tracheids (Fig. 13), which are imperforated cells that can transport water via bordered pits, were seen at the growth-ring boundaries. To our knowledge, this is the first report of fiber tracheids (fibers with distinctly bordered pits; IAWA Committee, 1989
) in F. lanuginosa.
Twisting of adjacent vessels and changes in vessel-to-vessel contacts
As described earlier, earlywood vessels at the annual ring boundaries made contact with vessels in the latewood of the previous annual ring. In addition, individual latewood vessels or vessel multiples at a growth-ring boundary made contact with more than one earlywood vessel in the next growth ring, as demonstrated in Fig. 4A and B, in which the vessel multiple designated "c" makes contact sequentially with vessels nos. 4 and 5. The radial drift towards the growth-ring boundary of another vessel multiple, designated "a," can also be seen in Fig. 4A–C. The establishment of contacts between an individual latewood vessel and more than one of the earlywood vessels of the next annual ring appeared to be facilitated by the exchange of relative positions between adjacent vessels, which twisted around one another. Such twisting of earlywood vessels is demonstrated in Fig. 4A–C. Note that the sections shown in Fig. 4 were separated by comparatively large distances from one another (z = 0–5.6 mm) and, therefore, the changes in the relative positions of vessels are somewhat magnified.
A similar exchange of positions among neighboring vessels was easily recognizable in 3-D renderings of a series of optical tangential sections by CLSM (Fig. 6). The resin-casting method revealed earlywood vessels that had twisted around one another, as shown in Fig. 7, and casts of similarly arranged latewood vessels are shown in Fig. 8. A helical arrangement of vessels is suggested by such exchanges of position among individual vessels, but no complete helices were observed in this segment of wood. By contrast, helical arrangement of bordered pits was clearly seen in some earlywood and latewood vessels (Figs. 9 and 10), demonstrating that the contact cells in the vasicentric axial parenchyma were arranged in a helical pattern around the vessels. It remains to be determined whether this helical arrangement of vessels and vessel-associated cells is predetermined by the initial arrangement of cambial cells or whether it results from readjustments in the positions of cambial derivative cells during the expansion of the developing vessel elements.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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that, in a 10-mm length of wood of Fraxinus excelsior, 3–5% of the latewood vessels and 30% of the earlywood vessels are isolated from the vessel network. However, 10-mm length of wood is not enough to reveal the intervessel contacts along the courses of individual vessels. Following vessels from section to section in a 21.2-mm length of wood, we found that all the vessels within a growth ring tended to be integrated into a single network.
There were frequent connections between vessels within the earlywood and within the latewood. By contrast, connections between earlywood and latewood vessels within a growth ring were infrequent. Interestingly, a very similar pattern of vessel network in Fraxinus excelsior can be seen in a drawing by Burggraaf (1972)
in his Fig. 7. Vessel network between earlywood and latewood is better developed across growth rings rather than within a growth ring. Such phenomenon might be related to the seasonality of the water-conduction functions of the earlywood and latewood. Earlywood vessels in ring-porous hardwoods are formed early in the growing season and allow the rapid transport of sap during a period of intensive tree growth and high environmental humidity (Utsumi et al., 1996
; Kitin et al., 2002a
). Then the large earlywood vessels are gradually filled with tyloses or gums (Fujita et al., 1978
; Saitoh et al., 1993
). They are empty of water by the fall of the growing season (Utsumi et al., 1996
, 1999
). By contrast, the latewood vessels allow the slower but, nonetheless, continuous and reliable transport of sap and remain functional in the next growing season (Ellmore and Ewers, 1986
; Cochard and Tyree, 1990
).
The course of individual vessels in Fraxinus and other genera is known to deviate from the straight longitudinal direction, and neighboring vessels have been shown to touch and communicate with one another (Burggraaf, 1972
; Tyree and Zimmermann, 2002
). Vessels arranged in a zigzaging pattern were shown in the secondary xylem of Fraxinus excelsior by Burggraaf (1972)
, Fagus crenata (Fujii, 1993
), and Machilus thunbergii (Fujii et al., 2001
). Zigzagging or circular vessels at branch junctions, stem-root junctions, and reaction wood formed after wounding were shown in several dicotyledonous species by André (2002)
. Tangential shifts of vessel axes in Machilus thunbergii were demonstrated in serial cross sections and in resin casts by Fujii et al. (2001)
. Dye-flow experiments have shown that the ascent of injected dyes usually follows a helical pathway within a growth ring and along a tree stem, indicating that tangential deviations in the course of vessels occur and that vessel networks are well established in the tangential direction (Chaney and Kozlowski, 1977
; Tyree and Zimmermann, 2002
). The tangential continuity of vessel networks is considered to be of great importance for the reliable transport of water in plants in case of injuries to roots or stem (for a discussion, see Tyree and Zimmermann, 2002
). Our analysis of the relative positions of vessels in our series of transverse sections revealed that vessels do not run parallel to one another but deviate from such a course in the tangential direction, for the most part. Moreover, individual vessels regroup as different clusters of vessels along the stem axis. Most vessels tended to drift tangentially in relation to the position of the marker vessel, and some vessels were twisted around one another. Because of the small size of the specimen that we studied, we were unable to visualize or demonstrate the tangential drift of an entire group of vessels around the stem axis but, as noted, dye-flow experiments have shown that such drift is a general occurrence.
The radial drift of latewood vessels was less marked than the tangential drift. Even very limited drift in the radial direction of individual vessels allowed tangential connections to be made between neighboring solitary vessels or vessel multiples. Small radial drifts and fluctuations of vessels occur in the wood of Fraxinus excelsior (Burggraaf, 1972
), Fagus sylvatica (Bosshard and Ku
era, 1973
), and Machilus thunbergii (Fujii et al., 2001
).
The tangential deviations of vessels mirrored the structure of the cambium and corresponded to similar deviations in the orientation of cambial cells (Kitin et al., 2003
). However, the radial deviations of vessels are not so easily explained. Variable radial position of individual vessels suggests that the initiation and completion of differentiation of the mother cells of such vessels might not have been simultaneous. In many cases, reactivation of cambium after the beginning of growing season does not occur uniformly along tree stems (for reviews, see Aloni, 1988
; Larson, 1994
; Lachaud et al., 1999
). Moreover, as demonstrated in Ulmus, the sequential differentiation of vessel elements from cambial cells has a basipetal pattern, from the top to the bottom of the stem (Pomerleau, 1966
). A time delay in the sequential formation of axially arranged elements of individual vessels might be related to the occurrence of small radial shifts of individual vessels within a growth ring. However, a full understanding of this phenomenon will require more research into the nature of morphogenic events in the cambium.
Vessel networks between growth rings and radial pathways for apoplastic conduction of water
Tangential contacts between vessels and intervessel pits that allow the radial flow of sap in hardwoods were first studied by MacDougal et al. in 1929 (see Tyree and Zimmermann, 2002
) and later by Braun (1970)
and Burggraaf (1972)
. However, to date, the structure of vessel networks that extend in the radial direction across growth-ring boundaries has not been fully clarified, although its occurrence has been clearly demonstrated in Fraxinus excelsior in series of transverse sections (Burggraaf, 1972
) and in Machilus thunbergii by air-permeability measurement through the growth-ring boundary supported by dye-flow experiment and resin casting (Fujii et al., 2001
).
Contacts between vessels across annual ring borders were infrequent in our single sections of F. lanuginosa. However, because of the considerable length of individual vessels, the presence or absence of intervessel contacts across growth-ring borders can be clarified unequivocally by 3-D analysis of xylem tissue. In the present study, 3-D analysis of a 21.2 mm long sample of wood revealed that all earlywood vessels at the growth-ring boundary made contact with small-diameter vessels in the latewood of the previous ring. Shifts of vessels from the longitudinal direction and twisting of adjacent vessels facilitated the establishment of frequent contacts between vessels on both sides of the growth-ring border. Braun in 1959 (see Tyree and Zimmermann, 2002
) estimated that, in Populus wood, about 30% of the earlywood vessels at the annual ring boundary make contact with vessels of the previous annual ring. However, the percentage of vessels that make contact across ring borders obviously depends on the nature and the size of the sample studied. Contacts between vessels have been analyzed quantitatively in several papers (Skene, 1969
; Braun, 1970
), but in no analysis, including our report, have intervessel contacts been studied along the entire length of vessels.
In study of the hydraulic properties of individual xylem vessels of Fraxinus americana, the radial conductance of water per unit area is approximately six orders of magnitude lower than the longitudinal conductance (Zwieniecki et al., 2001a
). Within a vessel network, water must move laterally through the membranes of bordered pits (Fujii et al., 2001
), a requirement that explains the low radial conductance of water in xylem. Bordered pits might provide not only a pathway for the lateral movement of water, but also a mechanism for the control of the movement of water. The axial and lateral flow of water from vessel to vessel is mediated by the concentration of ions in the xylem sap (Van Ieperen et al., 2000
; Zwieniecki et al., 2003
), and the structure of hydrogels in pit membranes and, consequently, the permeability to water of bordered pits, might depend on the ionic strength of the sap (Zwieniecki et al., 2001b
). Numerous, densely arranged intervessel pits in the tangential walls of adjacent vessels in vessel multiples are a characteristic feature of the wood of many hardwood species including F. lanuginosa. In addition, we frequently observed longitudinally aligned, latewood vessel elements as terminal cells at the growth-ring boundary, with densely distributed bordered pits on their tangential walls that faced the cambium. This arrangement suggests that the vessel mother cells in the cambium might be adjacent to terminal vessel elements of the xylem that will provide radial continuity to the vessel network.
The density of vessel network across growth-ring borders in various species might depend on the type of imperforated tracheary elements in the xylem. Braun (1970)
found no contacts between vessels across growth-ring borders in species with tracheids in the xylem, such as Quercus robur and Fagus sylvatica. In such species, the transfer of water from vessel to vessel might be mediated by the imperforated tracheary elements, which are present between vessels. By contrast, direct contacts between vessels across growth rings were shown in Acer pseudoplatanus, Populus sp., Aesculus hippocastanum (Braun, 1970
), Fraxinus excelsior (Braun, 1970
; Burggraaf, 1972
), and Machilus thunbergii (Fujii et al., 2001
). Interestingly, all those species, as well as Fraxinus lanuginosa, lack tracheids in the xylem. Our detailed investigation of the wood of F. lanuginosa found marginal fiber-tracheids that can conduct apoplastic water across tree rings. However, the amount of water that can flow between fiber tracheids is probably negligible compared with the water that can be transferred via intervessel pits.
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FOOTNOTES |
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5 E-mail: kitin{at}iwt-pu.ac.jp
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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