Pit membrane

The plasmodesmata may be aggregated in primary pit fields or in the pit membranes between pit pahs. The plasmodesmata appear as narrow canals (2 pm) lined by a plasma membrane and are traversed by a des-motubule, a tubule of endoplasmic reticulum. The plasmodemata are dynamic altering their dimensions and are functionally diverse. For example, whereas some transport endogenous plant transcription factors, others transport numerous proteins from companion cells to enucleated sieve elements. 

Uptake of smaller particles including protein molecules occurs by micropinocytosis, a process that can be seen only by electron microscopy. This often takes place via coated pits, indentations of 0.3 pm diameter underlain by a thickened membrane. 5 8 The pit membrane is also coated with protein molecules and appears to have many short bristles or spikes protruding into the cytoplasm (Fig. 8-27A). After endocytosis the coated pits become coated vesicles of 0.15-0.25 ran (Fig. 8-27B). Within a few seconds, however, these vesicles lose their coat and become endosomes. 

Figure 10. View of a bordered pit membrane with the dome-shaped pit border removed. The dark central portion is the torus. The stringlike microfibrils radiating from the torus constitute the margo portion of the pit membrane. Water flows freely from cell to cell through the openings between the margo microfibrils. 3,000X...

Figures 8 and 15 reveal ray crossing pits as seen from the inside of longitudinal tracheids. The considerably higher magnification in Figure 15 shows a solid pit membrane. Openings in the pit membrane would expose the cytoplasm to the hostile environment of the longitudinal tracheid lumen and result in the death of the parenchyma cell. Thus, the membranes are solid and do not provide a passageway for free liquid flow.

Figure 13. Cross-sectional view of an aspirated bordered pit-pair. The pit membrane has moved to the border and sealed a pit aperture with the torus. In this condition, liquid flow no longer occurs between contiguous cells. 5,000X...

Figure 14. Surface view of an aspirated bordered pit membrane. The imprint of the pit aperture through the torus is the result of an extremely tight seal. 6,200X...

When vessels end, they rarely do so in isolation but rather among a group of vessels. Translocation continues into the adjacent vessels via the intervessel pits. These pits differ from softwood bordered pits in that they lack a torus and openings large enough to be readily detected with an electron microscope. Figure 17 depicts a typical intervessel pit membrane. Different arrangements of intervessel pits can be detected and are useful in the identification of hardwood species. 

Figure 15. View from the inside of a longitudinal tracheid showing pits connecting a longitudinal tracheid to a ray cell. Note the lack of openings within the pit membrane. 2,500X...

Figure 17. Pit membrane from an intervessel bordered pit, Note the absence of a torus and detectable openings in the membrane. 2,400X...

Structural Factors. Wood is essentially a closed cellular system, and the cell walls are characterized by the presence of numerous pit pairs which serve as flow paths for liquids in living cells. After the cells die and they are transformed into heartwood, the pits undergo aspiration and become occluded with wood extractives. This results in a reduction in the effective pore size which in turn restricts flow of materials. Nevertheless, evidence suggests that the major flow path from cell to cell (in either rays or trach-eids) is through the pit membrane since this is the path of least resistance. Consequently, an understand-... 

Pit Structure. Because the pit membrane appears to be the controlling factor in flow through wood, an examination of its structure would be pertinent. 

Chemical Factors. Based on the structure and flow paths in wood, the pit membrane appears to be the component which should be chemically altered in order to increase the permeability. In order to accomplish this, knowledge of the chemical composition of the pit membrane is desirable. 

A number of investigators have studied the chemical composition of the pit membrane (10, 11, 12, 13,... 

In sapwood, the pit membranes appear to be principally composed of cellulose and pectin (13). However, in a number of genera, the sapwood pit membranes also contain polyphenols in some cases, but the distribution is not uniform even within species. 

It should be pointed out that most of the research on chemical composition of the pit membrane has been limited to the bordered pits. The chemical composition of the simple pit membranes in the ray parenchyma cells may or may not be the same. However, since the parenchyma cells produce the precursors for the formation of polyphenolic compounds, it is anticipated that the membrane occlusions would be similar. 

Chemical Treatments. Over the years, a considerable amount of research has been conducted on the possibility of using various chemical pretreatments to improve the permeability of wood. The basic principle behind such treatments is either to extract extraneous material from the pit membrane or degrade the pit membrane in order to enlarge the openings. 

Pre-extraction of Wood. Since the major factor causing a reduction in the permeability of wood during heartwood formation is occlusion of the pit membranes with extraneous material, one would anticipate that pre-extraction of the wood with a suitable solvent would be a method of increasing the permeability of wood. This contention has been verified by a number of studies (15, 16, 17, 18, 19, 20, 3, 21). However, it appears doubtful that such treatments would be commercially feasible since the solvents are expensive and excessive time is required for the additional step in the treating process. 

For example, the work by Emery and Schroeder (24) indicates that wood can be chemically oxidized with an iron catalyzed reaction under acidic conditions. It is conceivable that this type of treatment could degrade the pit membrane and increase the permeability. Furthermore, Tschernitz (25) has shown that treatment of Rocky Mountain Douglas fir sapwood with hot ammonium oxalate improved the treatability of this material. In this latter case, the ammonium oxalate probably solubilized the pectins in the pit membrane. 

There are two major limitations in the processes described above. First of all, the fact that these fungi are effective only in the sapwood, which in most instances is readily treatable, reduces its usefulness. Secondly, it has been shown by Johnson and Gjovik (41) that extraneous bacteria, rather than fungi, may actually be responsible for the pit membrane degradation... 

Since it has been shown that the degradation of the pit membranes in wood is the most logical method of increasing permeability, fungi must be capable of attacking this structure in order to be successful. 

If the right fungi could be found, then it may be possible to pre-treat wood with a spore suspension which would be allowed to react sufficiently to open up the structure before treatment. This, of course, assumes that the fungi will sufficiently degrade the pit membranes before significantly damaging the cell wall structure and cause excessive strength loss. This may be possible because of the accessibility of the pit membranes. Furthermore, the incubation period must be relatively short in order for such a process to be feasible. Kirk (42) indicates that these reactions proceed rapidly under the proper conditions so this may not be a serious problem. 

In pine, it was found that Bacillus polymyxa was the major species involved (45, 55), whereas, in spruce the major species were Bacillus subtilis and Flavobacterium pectinovorum (49). In another study (53), Clostridium omelianskii was identified as the species attacking softwoods. In all studies, it was found that the bacterial attack on the pit membranes was the reason for increased permeability of the wood. Furthermore, it was shown by Fogarty and Ward (49) that bacteria degraded the pit membranes by excreting the enzymes amylase xylanase, and pectinase. A typical sapwood pit membrane that has been attacked by bacteria is shown in Figure 2. As can be seen, the torus is severely degraded and has well defined openings. 

The important thing to remember in most of these experiments is that the bacteria have been used to improve the permeability of sapwood. For species like Sitka spruce which has sapwood that is impermeable to creosote, this type of treatment may prove to be commercially feasible. However, in order to fully exploit this method for increasing permeability, it will be necessary to find bacteria which have the ability to degrade heartwood pit membranes. Indeed, this may be possible since Greaves (50) has shown that some bacteria can increase the permeability of heartwood. In addition to this, it would be desirable to accelerate the reaction as much as possible in order to make it compatible with a commercial operation. 

Figure 1. A fungal hyphae growing inside the lumen of a tracheid. Note how the hyphae is branched to penetrate the pit membrane. (Courtesy of I. B. Sachs)...

Enzymes. Since both bacteria and fungi utilize enzymes to degrade the pit membrane in wood, it is not surprising that treatment with isolated enzymes produces similar effects. This was shown to be the case by Nicholas and Thomas (26) using cellulase, hemi-cellulase and pectinase. Similar results were subsequently obtained by other researchers (62, 63, 64, 65). ... 

In a recent study by Tshernitz (25), it was verified that enzymes could be used to increase the permeability of Rocky Mountain Douglas fir. By pretreating the wood with pectinase, a completely uniform treatment of the sapwood zone was possible with creosote. This is in contrast to erratic treatment normally obtained when this material is treated. A typical pit membrane which has been degraded by pectinase is shown in Figure 3. 

To date, it has not been possible to degrade heartwood pit membranes with isolated enzymes. Since co-factors are probably required (42), it appears that use of a whole organism rather than isolated enzymes may be required if the permeability of heartwood is to be increased. 

Water conduction in a tree is made possible by pits, which are recesses in the secondary wall between adjacent cells. Two complementary pits normally occur in neighboring cells thus forming a pit pair (Fig. 1-5). Water transport between adjacent cell lumina occurs through a pit membrane which consists of a primary wall and the middle lamella. Bordered pit pairs are typical of softwood tracheids and hardwood fibers and vessels. In softwoods the pit membrane might be pressed against the pit border thus preventing water transport, since the torus is impermeable. The pits connecting tracheids, fibers, and vessels with the ray parenchyma cells are half-bordered. Simple pits without any border connect the parenchyma cells with one another. 

The normal structure of the cell wall is broken by pits. Changes appear already in the growth period of the cell. For instance, early stages of pit formation in softwoods are visible in the primary wall just before the cell reaches its final dimensions (primary pit fields). The microfibril network is loosened and new microfibrils are oriented around these points. The structure in the middle of the circles is tightened and the radially oriented microfibril bundles finally form a netlike membrane, permeable to liquids (margo) (Fig. 1-17). The central, thickened portion of the pit membrane... 

As all pits develop in softwoods and hardwoods, a specialized pit membrane remains within the pit complex (Figure 19, D and E). This membrane is initially constructed from the compound middle lamella in all cases, but in its fully difierentiated state the membrane can differ considerably between various cell types, between softwoods and hardwoods, and to some extent even between different species (3). In hardwoods, pit membranes are observed to be thin and generally nonporous partitions of microfibrils, matrix materials, and lignin (Figure 20). Movement of liquids through the pit complex to an adjacent cell must therefore occur largely by diffusion rather than by free liquid translocation. Fortunately, hardwoods have an effective alternate mechanism for liquid movement, at least in the vertical direction, and that mechanism is the vessel system. 

The interfiber-pit membranes in softwoods are substantially different from the pit membranes in hardwoods. Much of the membrane periphery is quite open, with only the central portion being nonpo-... 

Copyright 1982, American Chemical Society.) (A) SEM of interfiber pits in earluwood as seen on the wood radial face. Note the donut-shaped borders. (B and C) SEM of pit pairs between adjacent fibers cross-sectional suiface. (D) SEM ofbordered-pit membranes (PM) in face view of a split wood radial surface. (E) Light micrograph of pit pairs as seen in cross section with a light microscope. Key PM, pit membranes PB, pit border and PA, pit aperture. 

In the standing, living tree the bordered-pit membranes between softwood fibers act as valves to prevent the spread of air or bubbles into sap-filled cells in the event of tree injury and potential rupture to vertical water columns. Unfortunately, they perform a similar function in the processing of wood into commercial products. For example, during wood drying, substantial capillary and surface tension forces are developed upon water retreat from the fiber lumens through the pits, and the membranes move effectively (particularly in earlywood) to seal the apertures in the direction of water... 

Figure 20. SEM of intervessel-pit membranes in hardwoods. (A) Vessel in western red alder. The cell wall at the lumen is partly tom away to...

The secondary wM (S) at the lumen has been removed to expose the nonporous pit membranes (PM) and a special structure known as vestures. The latter can be found in the pit complex of various hardwoods (2). (Reproduced with permission from Ref. 40. Copyright 1982, Technical Association of the Pulp and Paper Industry Press.)... 

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