Cambial

Figure 3. PME isoform patterns in cell wall extracts fiom active and resting cells, a cell wall extracts from successive segments (A, B, C and D) sectioned along mui bean hypocotyb and exhibiting decreasing elongation rates a, and y m c the main PME isoforms present in the extracts, tteir pi are respectively around 7.5, S.5 and above 9.1. b cell wall extracts obtained from poplar cambium and inner bark tissues during cambial active (may) and rest (december and march) periods 1, 2 and 3 represent the activity of 3 groups of PME isoforms with pi around 5-6, 7.5 and above 9.1. Activities expressed as percent of total PME activity present in each extract.

Preteatment with pectinases improves meehanieal debarking of wood significantly. In addition to polygalacturonase, enzymes hydrolysing the neutral polymers in cambial tissue (arabinanase, galactanase, and glucanase) are needed for effective solubilization of spruce cambium. 

Several groups have identified non-radioactive metabolic intermediates of lignification in cambial sap or sapwood extracts 54, 58 b, 68). The compounds detected agree well with the schemes for the biosynthesis of lignin precursors set forth in Figs. 1 and 2 and with the data described later for the third stretch of lignification. 

The three glucosides (IV-VI) are present in the cambial sap of spruce, V being by far the most abundant (49). Furthermore, spruce cambial sap contains very small amounts of coniferyl alcohol (I), coniferaldehyde (VII), the dilignols (XVII), (XIX), (XXII), and the trilignol (XXXVI) (19). Other lignols are present but in amounts too small for convenient identification. Extensive work has been done to examine the pathway of lignin formation in vivo and in vitro. 

Although in the latter case gibberellin appears to be primarily responsible for the increased mitotic activity and auxin for differentiation of the newly formed cells, auxin may also be involved in the primary cambial activation mechanism (25). After the discovery of the gibberellins several studies showed conclusively that auxin and gibberellin have separate, sometimes additive and sometimes syner-... 

Fig. 1-2. Transverse section of xylem and phloem of red spruce (P/cea rubens). CZ, cambial zone DP, differentiating phloem MP, mature phloem with sieve cells (sc) and tannin cells (tc) DX, differentiating xylem with ray cells and tracheids (tr) MX, mature xylem, earlywood (EW) with resin canals (rc), lined with epithelial cells (ec) LW, latewood. Note that each ray continuous from the xylem, through the cambial zone, and into the phloem. Light micrograph by L. W. Rees. Courtesy of Dr. T. E. Timell.

Radial growth begins in the cambium which is composed of a single layer of thin-walled living cells (initials) filled with protoplasm (cf. Fig. 1-2). The cambial zone consists of several rows of cells, which all possess the ability to divide. On division the initial cell produces a new initial and a xylem mother cell, which in its turn gives rise to two daughter cells each of the latter is capable of further division. More cells are produced toward the xylem on the inside than toward the phloem on the outside phloem cells divide less frequently than xylem cells. For these reasons, trees always contain much more wood than bark. 

Timell, T. E. (1973). Ultrastructure of the dormant and active cambial zones and the dormant phloem associated with formation of normal and compression woods in P/ cea abies (Karst.). Tech. Publ.—State Univ. N.Y., Coll. Environ. Sci. For., Syracuse No. 96, pp. 1 -23. 

Delcosine Reduction of cambial growth, gibberellic acid (GA) — 249,252... 

Schmid, G. and Grisebach, H. (1982) Enzymic synthesis of lignin precursors purification and properties of UDP-glucose coniferyl alcohol glucosyltransferase from cambial sap of spruce (Picea abies L.). Eur.. Biochem., 123, 363-70. 

Fig. 68.—Photomicrograph of cross-section of stem of Aristolochia sipko, where cambial activity is just beginning, a, Epidermis b, coUenchyma c, thin-walled parenchyma of the cortex, the innermost cell layer of which is the starch sheath or endodermis d, sclerenchyma ring of the pericycle e, thin-walled parenchyma of the pericycle /, primary medullary ray g, phloem h, xylem interfascicular cambium medulla or pith. X 20. (From Stevens.)...

Between the bundles certain cells of the primary medullary rays become very active and form interfascicular cambium which joins the cambium of the first-formed bundles (intrafascicular cambium) to form a complete cambium ring. By the rapid multiplication of these cambial cells new (secondary) xylem is cut off internally and new (secondary) phloem externally, pushing inward the first-formed, or protoxylem, and outward the first-formed, or prolophloem, thus increasing the diameter of the stem. The primary medullary rays are deepened. Cambium may also give rise to secondary medullary rays. 

Fimre SB. Light micrograph of the location and manufacture of wood cells in the cross section of a young pine (softwood) stem. The cambial phloem or inner bark cells to the outside and wood... 

Wood Cell Production. The site of wood cell production, the vascular cambium, is illustrated in Figure 3B. Technically, it is a microscopic sheath of meristematic cells. However, the exact circumferential line of cambial cells is very difficult to locate precisely, particularly during the tree s growing season, because of the presence of recent xylem and phloem derivatives. Therefore, it is more common to reference this lateral meristem as the cambial zone (2). 

All cells in the cambial zone are living. However, as xylem derivatives (i.e., developing wood cells) begin a sequence of transformations that will convert them into mature wood elements, they embark on a path of cell specialization or differentiation that will lead eventually (for fibers, vessel elements, and certain other cells) to cell death. 

Recent xylem derivatives may function for a period of time as mother cells, dividing to form still other derivatives. Nevertheless, the ultimate fate of most xylem derivatives is self destruction, auto-lysis, of their living contents, protoplast, and the eventual products are fully differentiated, or specialized, wood cells possessing rather elaborate walls and hollow centers, lumens. Only a relatively small number of cells in wood—called parenchyma—retain a viable protoplast after exiting the cambial and differentiation zones. Parenchyma are small, nonfibrous cells that have special storage or secretory functions. 

The overall process of wood cell production is influenced by numerous factors, including genetics, climate, photoperiod or day length, and forest soil conditions. However, these factors are only indirectly involved in the control of wood cell development, but they directly affect the tree crown—the leaves and buds—and the crown then exerts a directing influence on cambial zone activities, including the form and number of derivatives produced. 

As mentioned earlier, the initial portion of a fiber cell wall is manufactured in the cambial zone and is referred to as the primary wall. Here, cellulose microfibrils form a random, irregular, and interwoven network (Figure 17) to facilitate cell expansion during the enlargement phase of fiber development. In addition to cellulose, the primary wall contains a large proportion of matrix carbohydrates, particularly pectic materials and hemicelluloses see Chapter 2). The combination of two adjacent primary walls and the interdisposed true middle lamella zone is collectively referred to as the compound middle lamella. Microscopically, it is difficult to separate wall substance here from the interfiber substance. 

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