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Structure and Development |
Laboratory of Wood Biology, Graduate School of Agriculture, Hokkaido University, Sapporo 060-8589 Japan
Received for publication October 11, 2004. Accepted for publication March 24, 2005.
ABSTRACT
The structure of the intervascular pit membranes of four dicotyledonous species (Salix sachalinensis, Betula platyphylla var. japonica, Acer mono, and Fraxinus mandshurica var. japonica) was examined by field-emission scanning electron microscopy. The intervascular pit membranes of F. mandshurica var. japonica had thin surface layers and a dense middle layer, while no similar middle layer was detectable in the other three species. In F. mandshurica var. japonica, the entire area of each pit membrane was densely covered with microfibrils. In the other three species, by contrast, openings were found in the pit membranes. In some of the intervascular pit membranes of S. sachalinensis, B. platyphylla var. japonica, and A. mono, microfibrils were sparsely interwoven in small areas of the pit membranes and openings of up to several hundred nanometers in diameter were present in such regions. These porous regions tended to be located in peripheral areas of pit membranes. In S. sachalinensis and B. platyphylla var. japonica, ethanol-soluble extracts, whose chemical nature and function remain unknown, were heavily distributed over the intervascular pit membranes. Our observations suggest that the structure of intervascular pit membranes is more complicated than has previously been acknowledged.
Key Words: Acer mono • Betula platyphylla var. japonica • field-emission scanning electron microscopy • Fraxinus mandshurica var. japonica • intervascular pit • pit membrane • Salix sachalinensis
The xylem of most dicotyledons includes specialized water-conducting tissues called vessels. Each vessel is composed of a number of vessel elements that are connected, via perforation, with each other. The end of one vessel generally makes contact with another vessel but the junction is not perforated. Individual vessels are partitioned by the pit membranes of intervascular pit pairs. The vessels do not consistently run in parallel, and they frequently exchange positions tangentially. When two vessels cross and touch during the exchange of positions, intervascular pit pairs are also generally found in the common walls of the adjacent vessels (Panshin and de Zeeuw, 1980
; Dickison, 2000
; Tyree and Zimmermann, 2002
).
The basic structure of intervascular pit membranes in dicotyledons is not yet fully understood. Early studies by transmission electron microscopy suggested consistently that intervascular pit membranes are uniform structures with a primary wall texture, without any distinction between torus and margo, consisting of several layers or lamellae and lacking visible openings that can be detected by transmission electron microscopy (Côté, 1958
; Harada et al., 1958
; Schmid, 1965
; Schmid and Machado, 1968
). By contrast, later studies indicated that a torus is present in the intervascular pit membranes of some taxonomic groups (e.g., Ohtani and Ishida, 1978
; Wheeler, 1983
; Dute and Rushing, 1988
; Dute et al., 1992
; Jansen et al., 2004b
), and openings were visualized by electron microscopy in the intervascular pit membranes of several species (e.g., Bonner and Thomas, 1972
; Wheeler, 1982
; Sperry and Tyree, 1988
; Dute et al., 1992
; Sano, 2004
). However, to date, information about structural variations remains limited and fragmentary for a large number of species and types of pit, and descriptions of the structures of intervascular pit membranes in the literature are very brief (e.g., Panshin and de Zeeuw, 1980
; Tsoumis, 1990; Dickison, 2000
; Butterfield, 2003
).
The functions of intervascular pit membranes also remain to be clarified (Zwieniecki et al., 2001
; Choat et al., 2003
; Jansen et al., 2004a
, b
). Since the "air-seeding" model was proposed by Zimmermann (1983)
, particular attention has been paid to the porosity of intervascular pit membranes. According to Zimmermann's model, the susceptibility to the progression of water stress-induced cavitation, from one cavitated conduit to the adjacent water-filled conduit, depends upon the difference in pressure and the maximum size of the micropores in the pit membranes that are located between the two conduits. Attempts have been made to confirm the relationship between the "air-seeding" pressure that is required to advance the cavitation from a cavitated vessel to the adjacent water-filled vessel and the size of the micropores that are present in the intervascular pit membranes. However, the results of such attempts are inconsistent (e.g., Sperry and Tyree, 1988
; Jarbeau et al., 1995
; Choat et al., 2003
). It is now necessary to examine the fine structure of intervascular pit membranes in greater detail if we are to understand the mechanisms of water transport in living trees.
The purpose of the present study was to examine the fine structure of the intervascular pit membranes of four dicotyledonous species, namely, Salix sachalinensis Fr. Schmidt, Betula platyphylla var. japonica Hara, Acer mono Maximowicz, and Fraxinus mandshurica Rupr. var. japonica Maximowicz. My colleagues and I used S. sachalinensis, B. platyphylla var. japonica, and F. mandshurica var. japonica in previously published studies of the seasonal progression of cavitation of vessels (Utsumi et al., 1996
, 1998
, 1999
). Acer is a commercially important genus and is physiologically interesting because of the bleeding phenomenon that occurs in early spring. Therefore, for the present study, I selected A. mono and the other three species to examine structural variations in intervascular pit membranes by field-emission scanning electron microscopy.
MATERIALS AND METHODS
Materials
Samples of outer sapwood (longitudinal, radial, and tangential dimensions were 10, 2, and 5 cm, respectively) were excised at breast height with a chisel from sample trees (Table 1). Two to four samples were collected from each tree. Half of the samples were put into 30% ethanol and stored at room temperature. The remaining samples were immersed in liquid nitrogen, brought to the laboratory, and stored at –80°C.
|
. In brief, samples were cut into small cubes (5 x 5 x 5 mm3) that included the outermost annual ring. The blocks cut from frozen samples were freeze-dried in a freeze-drying apparatus at approximately 0°C and 10 Pa. The blocks cut from samples that had been stored in 30% ethanol were air-dried after dehydration in absolute ethanol. All the dried blocks were split along a tangential plane in the inner, middle, or outer layer of the outermost annual ring, and then the specimens were affixed to aluminum stubs with electron-conductive carbon paste. They were coated with platinum by vacuum evaporation and examined with a field-emission scanning electron microscope (FE-SEM; JSM-6301F, JEOL, Tokyo, Japan) at an accelerating voltage of 2.5 kV. More than 500 undamaged pit membranes in more than 20 intervessel faces were examined for each species. For measurements of diameters of intervascular pit membranes, 50 pit membranes were selected at random on scanning electron micrographs of samples that had been split tangentially at the middle layer of the outermost annual ring, at a magnification of 4000–8000x. Longitudinal and tangential diameters of pit membranes were measured on these micrographs. In the case of Fraxinus mandshurica var. japonica (ring-porous wood), diameters were also measured at the earlywood.
RESULTS
Diameters of intervascular pit membranes
The diameters of intervascular pit membranes were largest in Salix sachalinensis (Table 2; Fig. 1) and smallest in Betula platyphylla var. japonica (Table 2; Fig. 2). The diameters of intervascular pit membranes in Acer mono were slightly smaller than those in S. sachalinensis (Table 2). In Fraxinus mandshurica var. japonica, diameters differed between inter-earlywood vessel pits and inter-latewood vessel pits, being larger in the earlywood (Table 2).
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There were fine curly fibrils in the intervascular pit membranes of large earlywood vessels of F. mandshurica var. japonica (Fig. 3, arrow). The curly fibrils were not only present on the surface of the pit membranes but also penetrated the middle layer of the pit membranes (Fig. 3, arrow). There were no similar curly fibrils in the intervascular pit membranes between the small latewood vessels of F. mandshurica var. japonica or in the other three species.
Porosity of intervascular pit membranes
The porosity of intervascular pit membranes differed among species. In F. mandshurica var. japonica, the surface layer of each pit membrane was densely and evenly covered with randomly oriented microfibrils. Although narrow inter-microfibrilar spaces were noted in the thin surface layers, no such spaces were apparent in the middle layer (Fig. 3).
In the other three species, the porosity varied among individual pit membranes. In B. platyphylla var. japonica, many pit membranes were densely and evenly packed with randomly oriented microfibrils (Fig. 5). However, there were easily visible openings of up to 300 nm in diameter in some of the pit membranes (Figs. 2 and 5). Most of such openings were less than 150 nm in diameter. Openings more than 150 nm in diameter were found in a few pit membranes of a few intervessel faces. The openings tended to be located near the periphery of the respective pit membranes (Figs. 2 and 5). The frequency of occurrence of such porous pit membranes was approximately 5%.
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The porosity of intervascular pit membranes in A. mono was intermediate between that in S. sachalinensis and that in B. platyphylla var. japonica. In approximately 20% of the pit membranes in A. mono, microfibrils were sparsely interwoven in small areas and there were openings of up to 500 nm in diameter in such regions (Figs. 7 and 8). Most of such openings were less than 200 nm in diameter. Openings more than 200 nm in diameter were found in a few pit membranes of a few intervessel faces. The microfibrils were more densely packed in the remaining regions of intervascular pit membranes that had such localized porous areas (Fig. 8) than those in S. sachalinensis.
Presence of ethanol-extractable material
In freeze-dried specimens of S. sachalinensis and B. platyphylla var. japonica, intervascular pit membranes were covered by a conspicuous coating (Figs. 9 and 10). The extent of the coating varied among individual pits. The texture of microfibrils was detectable in cases of limited deposition (Fig. 10) but not in cases of heavy deposition (Fig. 9). This type of coating was not apparent in air-dried specimens that had been dehydrated in ethanol prior to drying (Figs. 1, 2, 4–6). Indeed, the above-described observations of the appearance of the pit membranes of these two species (Figs. 1, 2, 4–6) were based on examinations of air-dried specimens after dehydration in ethanol.
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No similar differences were detected between freeze-dried and air-dried specimens of A. mono and F. mandshurica var. japonica, indicating that a similar ethanol-soluble coating was absent in both of these species.
Aspirated pit membranes
Aspirated pit membranes were often found in S. sachalinensis and A. mono (Fig. 13), but not in the other two species. These aspirated pit membranes were often torn (Fig. 13, arrow).
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The present study revealed not only interspecific differences in the porosity of intervascular pit membranes, but also intraspecific differences in the porosity of the pit membranes. In some of the pit membranes of Salix sachalinensis, Betula platyphylla var. japonica, and Acer mono, microfibrils formed a sparse network in a small area of the entire pit membrane, and larger openings of up to several hundred nanometers in diameter were present in such regions. There are no previous reports, to our knowledge, of the existence and frequency of such pit membranes with small areas of sparse, interwoven microfibrils. However, some electron-microscopic studies have revealed porous intervascular pit membranes in Liriodendron (Bonner and Thomas, 1972
), Acer (Wheeler, 1982
; Sperry and Tyree, 1988
), and Daphne (Dute et al., 1992
).
Scanning electron microscopy is a useful method for examining and characterizing such structural variations. The porosity of intervascular pit membranes has been estimated by perfusion tests with minute particles of known size (e.g., Jarbeau et al., 1995
; Shane et al., 2000
; Choat et al., 2003
). This indirect method allows the quantitative demonstration of the sizes of micropores. However, it is difficult to demonstrate variations in the pore sizes and in their uneven distribution within an individual intervascular pit membrane using this technique. Thus, visualization by electron microscopy seems to be the method of choice for studies of the porosity of intervascular pit membranes and for efforts to better understand their true nature.
Intraspecific variations in the porosity of intervascular pit membranes are important when we discuss the validity of the "air-seeding" model (Zimmermann, 1983
; Tyree and Zimmerman, 2002
). As stated in the introduction of the present paper, attempts have been made to examine the relationship between the air-seeding pressure and the sizes of micropores in intervascular pit membranes. In some studies, the diameter of pores in intervascular pit membranes that was calculated from the air-seeding pressure, which was estimated from a determination of the loss of hydraulic conductivity, corresponded to the measured diameter (e.g., Sperry and Tyree, 1988
; Jarbeau et al., 1995
). By contrast, Choat et al. (2003)
reported that, in two drought-deciduous and two evergreen species in a seasonally dry rainforest, the diameters of pores in intervascular pit membranes, as measured by scanning electron microscopy, were much smaller than the diameters estimated from the air-seeding pressure. Choat et al. (2003)
mentioned, as one possible explanation for this discrepancy, that larger pores were indeed present in the intervascular pit membranes that they examined but that these pores were too rare to be detected by electron microscopy. After a subsequent analysis, Choat et al. (2004)
confirmed that this explanation was more plausible than other possible explanations. In the present study, the large openings found in some of the intervascular pit membranes were more frequent than predicted by Choat et al. (2003
, 2004
). It is likely that the frequency of pit membranes with small loose zones differs among species. Although Choat et al. (2003)
did not find any such pit membranes among 30–50 pit membranes, such porous pit membranes might be found in the four species that they studied if more pit membranes had been examined. The studies by Choat et al. (2003
, 2004)
and the present study suggest that much higher numbers of pit membranes need to be examined in the future for an accurate characterization of the porosity of intervascular pit membranes.
The present study also highlighted differences in the layered structure of intervascular pit membranes among species. Variations in the layered structure of intervascular pit membranes have not previously been fully appreciated. Early reports of the fine structure of the intervascular pit membranes of several species stated consistently that pit membranes are uniform structures with a primary wall texture, without a distinctive torus and margo and that they consisted of several layers or lamellae (Côté, 1958
; Harada et al., 1958
; Schmid, 1965
; Schmid and Machado, 1968
). Interspecific differences in the layered structure have not been fully considered, and more attention was focused on the occurrence and structure of torus-bearing pit membranes (e.g., Ohtani and Ishida, 1978
; Wheeler, 1983
; Dute and Rushing, 1988
; Dute et al., 1992
; Jansen et al., 2004b
). Information on the layered structure of intervascular pit membranes remains limited. The presence of a middle layer probably influences both the porosity and the rigidity of pit membranes. It is very likely that the properties of pit membranes, such as the tendency toward pit aspiration, differ between pit membranes with a dense middle layer and those without a middle layer. Thus, further investigations are necessary to characterize the layered structure if we are to develop a better understanding of the structure and function of intervascular pit membranes.
We also need to examine the nature and function of the ethanol-soluble material on some of the intervascular pit membranes. Such material is also present on the intervascular pit membranes of B. platyphylla var. japonica (Sano, 2004
). In the present study, a similar material was also noted on those of S. sachalinensis. The material might also be present in many more species. It has been reported that intervascular pit membranes are heavily covered by deposits in heartwood (Bonner and Thomas, 1972
; Kininmouth, 1972
; Thomas, 1976
; Wheeler, 1981
, 1982
, 1983
; Wheeler and Thomas, 1982
; Sano and Fukazawa, 1994
). However, the chemical components of the ethanol-soluble material found in the outer sapwood probably differ from those of the materials that are secondarily deposited during heartwood formation cannot be removed by organic solvents.
Interesting hypotheses have recently been proposed with respect to functions of the lipids in vessel lumina and the cell wall matrix in intervascular pit membranes. Schneider et al. (1999)
found a lipid layer that covered the vessel side of vessel-to-parenchyma pit pairs in the resurrection plant Myrothamnus flabellifolia. They suggested that such a lipid layer might play a role in preventing loss of water from the protoplast of the parenchyma under severe drought. By contrast, Zwieniecki et al. (2001)
demonstrated that the flow rate through stem segments was reversibly enhanced by switching the perfusion medium from deionized water to dissociating solutes, such as KCl, NaCl, and KNO3. They suggested that such reversible enhancement of water flow might have been caused by the reversible shrinkage of the pectin that covered the cellulose microfibrils in the intervascular pit membranes. The ethanol-soluble material might also serve some function that is associated with water conduction, although the solubilities of lipid and pectin in organic solvents are quite different from that of the ethanol-soluble material found in the present study (Wagner et al., 2000
). The ethanol-soluble material might also play an important role in the flow of water in B. platyphylla var. japonica and S. sachalinensis. Further studies are required to clarify the nature and function of such extraneous or matrix material in the flow of water in living trees.
FOOTNOTES
1 This study was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan (nos. 12760112 and 16580127). 
2 E-mail: pirika{at}for.agr.hokudai.ac.jp
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