Primary Growth

The description of bud development presented here is based primarily on the findings of HEJNOWICZ and OBARSKA (1995) and related studies. The embryonic shoots of vegetative and reproductive buds are visibly easy to differentiate in winter. With the aid of a microscope, it is possible to differentiate between bud types some months earlier. For example, in Poland the differentiation of the bud type takes place sometime in June (unpublished data; Fig. 6.1).

The embryonic shoot in the vegetative winter bud, encased by bud scales, possesses all of the next year's needle primordia, but not the lateral bud primordia. At the basal portion of the embryonic shoot in the cortex and pith there is a nodal diaphragm, termed a crown (Fig. 6.2). It is comprised of thick-walled parenchyma cells with irregularly thickened, but not lignified walls. A ring of vascular bundles interrupts the crown. In late autumn a gap arises below the crown due to the autolysis of pith cells (Fig. 6.2). Young needles or needle primordia in vegetative embryonic shoots are spirally arranged on the shoot axis in a specific pattern (phyllotaxis). Occasionally the phyllotaxis may change during shoot ontogeny, whereas the position of the primary vascular system is more stable. This suggests that the vascular system has a role the positioning of successively initiated leaf primordia on the apex (ZAGORSKA-MAREK 1995). In the winter bud, below the base of the embryonic shoot, the bud scales are joined together at the base forming the receptacle.

The organization of the shoot apical meristem in spruce is typical for many Coniferopsida. At the apex there are a fixed number of apical initials, below which the central mother zone is located. Further below there is a pith-rib meristem zone, which produces pith cells. The peripheral meristem zone occurs on the flanks of the apical meristem and produces the scale and needle primordia (Fig. 6.1). The zonation of the apical meristem and the number of

Figure 6.1. Shoot apex in longitudinal section. Feulgen reaction with fast green.

Tannin cells are orange (photo A. Hejnowicz)

Figure 6.1. Shoot apex in longitudinal section. Feulgen reaction with fast green.

Tannin cells are orange (photo A. Hejnowicz)

A -shoot apical meristem of the vegetative bud during the initiation of the bud scales for the following year's bud; early June; B - apical meristem of the male bud during the initiation of the first microsporophylls; early June. Beneath the apical initials (ai), central mother cells are visible (cm). Tannin cells are less numerous than in photo A; pm - peripheral meristem zone, rm -pith-rib meristem; C - apical meristem of the vegetative bud during the initiation of the needle primordia; mid-July; many tannin cells are visible in the apex; D - apical meristem of the male bud during microsporophyll initiation; mid-July; there are a few tannin cells in the apex.

cell layers in the different zones changes during shoot development. The number of vertical files of cells in the pith meristem also depends on stem vigor. For example, the width of the pith meristem decreases during shoot growth and with increasing branch order. The organization of the meristem tissues in the spruce shoot apex typical for a mature plant is achieved in seedlings as young as 30-days old (GREGORY and ROMBERGER 1972).

Lateral bud primordia arise in the axils of the elongating needles in late April, just when the apical meristem of the mother shoot has initiated bud scales for the following year's bud (HEJNOWICZ and OBARSKA 1995). Bud scale initiation lasts for about 2 months during the spring. Afterwards in late

Fig. 6.2. Embryonic shoot of the winter bud in median longitudinal section;

Feulgen reaction with fast green. Tannin cells in the pith are stained yellow or green.

Arrows indicate the nodal diaphragm (crown). (photo A. Hejnowicz)

Fig. 6.2. Embryonic shoot of the winter bud in median longitudinal section;

Feulgen reaction with fast green. Tannin cells in the pith are stained yellow or green.

Arrows indicate the nodal diaphragm (crown). (photo A. Hejnowicz)

A - male embryonic shoot; mi - microsporophyll; B - vegetative embryonic shoot; n - needle

June or early July, needle primordia are formed in the vegetative bud. This process terminates in early September.

As early as June, the two types of embryonic shoot can be distinguished. The main difference between them is in the abundance of tannin cells in the young pith (Figure 6.1A, B). This difference is magnified during the next period of bud growth (Fig. 6.1C, D). In male reproductive buds, the number of tannin cells is much lower than in the vegetative buds (Figs 6.2, 6.3 and 6.4). The abundance of tannin cells in an embryonic shoot makes it possible to determine the bud type before the leaf primordia (needles or microsporophylls) become discernible.

One of the signs of the onset of bud activity is the accumulation of starch. In spruce, starch accumulation precedes the resumption of mitotic activity by about a month (GUZICKA 2001). In a winter bud, starch is undetectable with cytochemical methods under a light microscope (HEJNOWICZ and OBARSKA 1995). Nevertheless, under a transmission electron microscope, small starch grains were observed in some cells of the embryonic shoot of buds collected in January (GUZICKA and WOZNY 2003).

Figure 6.3. Embryonic shoot in longitudinal section; Feulgen reaction with fast green

(photo A. Hejnowicz)

Figure 6.3. Embryonic shoot in longitudinal section; Feulgen reaction with fast green

(photo A. Hejnowicz)

A - male embryonic shoot in mid-August; mi - microsporophyll; there are some tannin cells in the pith; B - vegetative embryonic shoot in mid-August; many tannin cells in the pith; C -vegetative embryonic shoot; late September; n - young needle; D - an enlarged part of C; pc - procambial strands; n - young needle

Figure 6.4. Development of the embryonic shoot of the male bud (photo A. Hejnowicz)

A - early June; B - mid-July; C - late September (arrow - crown); D - microsporophylls with sporogenic tissue; mid-November

Before bud burst, the length of the embryonic shoot increases twofold due to internode elongation. About a month earlier, mitotic activity starts in different parts of the embryonic shoot (HEJNOWICZ and OBARSKA1995). The first cells to divide are those of the scale and needle primordia, as well as the peripheral meristem of the apex. Eventually, the cells of the apical zone divide distally. The resumption of bud development depends primarily on air temperature. Temperature influences the time interval between the first mitoses in the embryonic shoot and bud burst. The time interval is the shortest when air temperature rises quickly. Typically, the first mitoses occur 3 to 4 weeks before bud burst (HEJNOWICZ and OBARSKA 1995). Shoot elongation begins already in the closed bud. Shoot elongation lasts for about 6 weeks in the lower portions of the tree crown. It appears that rainfall has no influence on the period of shoot elongation, but influences the elongation rate and the final shoot length (Hejnowicz and Obarska 1995).

6.2. SECONDARY GROWTH 6.2.1. Vascular cambium

In spruce the resting cambial zone is comprised of four to eight layers of radially arranged cells. In the cambial zone there is a single layer of initial cells, several layers of xylem mother cells located internally to the initials, and phloem mother cells located externally. Some of the mother cells remain as meristems, while the initials produce new mother cells. Sometimes in undiffer-entiated xylem and phloem, files of cells, two or four cells surrounded by a common primary cell wall can be seen (SANIO'S fourth).

The cambial initials, whose derivatives produce the axial system, are called fusiform initials. In winter they have a folded cell membrane (plasmalemma), large elongated nuclei, numerous small vacuoles, plastids, amyloplasts, and small amounts of endoplasmic reticulum (TIMELL 1980). The ray initials are rich in lipid bodies. When the spring growth starts, the small vacuoles join into one large vacuole. Simultaneously, golgi bodies (which were invisible in the resting cambium) appear together with multi-vesiculate structures that participate in cell wall formation (TIMELL 1973).

The resumption of cambial activity in the spring depends on air temperature. Initially it also influences mitotic activity and the size of the cambial derivatives. Water is the main factor affecting cambial activity during the next phase of shoot growth. Water deficit can be compensated by enhanced mineral nutrition (DUNISH and BAUCH 1994). Cambial activation in central-European climatic conditions takes place in April. It may be delayed due to low temperatures in March and April. In years with high March and April temperatures, 10% of the xylem growth ring may arise in April, whereas growth may be delayed by a month during a cold spring (WENK and FIEDLER cit. Schmidt-Vogt 1986).

Three periods of seasonal cambium activity may be distinguished in spruce (Gregory 1971): the starting phase when the meristematic mother cells initiated in the previous year divide; the main phase during which 80% of the growth ring arises; and the last phase when the rate of cell division decreases. The cessation of cell division takes place in September. As with cambium activation in spring, the growth cessation depends on climatic conditions, mainly temperature.

Cambial activity in Picea abies begins beneath the emerging new shoots and proceeds basipetally, toward the trunk and root. Cell divisions begin about 2 weeks before bud break, followed by cell divisions in the trunk a few days later (at a height of 1.3 m) and about a month later in roots (LADEFOGED 1958). The stimulus inducing cambial activity is hormonal. The direction in which the cambial cell divisions proceed coincides with the direction of the displacement of the hormonal stimulus. Factors other than temperature, such as photo-period, light, and environment influence the rate of mitotic activity of cambium cells. Consequently, these environmental factors determine the width of the annual growth ring (among others see: ZELAWSKI and WODZICKI 1960, WODZICKI and Witkowska 1961, ESCHRICH and BLECHSCHMIDT-SCHNEIDER 1992, DUNISCH and BAUCH 1994).

The circumference of cambial cylinder increases as the core of the secondary xylem becomes thicker. Both multiplicative (BANNAN 1963) and anticli-nal-pseudotransversal divisions of the cambial initials contribute to circumference growth. Each of the new cells elongates by apical intrusive growth. As a result, the new sister cells come to lie side by side in a tangential plane. The sister cells are shorter than the mother cell. As the anticli-nal-pseudotransversal divisions commonly occur at the end of the growth period, their derivatives (xylem and phloem) near the boundary of the annual growth ring are the shortest (see "Secondary xylem")

The thickness of Norway spruce bark is small in comparison with other Coniferopsida, attaining 10-12 % of the trunk diameter on average. Bark is comprised of tissues external to the cambium: the secondary and primary phloem, cortex, periderm, and rhytidome. The latter is comprised of tissue layers isolated by the periderm. Rhytidome constitutes the outer bark in older stems and roots.

The secondary phloem occupies 50-85% of the total bark width. Growth rings of the secondary phloem are typically 0.2-0.3 mm wide. In trees from nutrient poor sites, the phloem growth rings are two-fold narrower than in trees from optimal sites (KARTUSCH et al. 1991). Within the youngest phloem, the borders between growth rings are distinct (Fig. 6.5A). Within the oldest phloem, the ring boundaries are less distinct. Nevertheless, the borders can be estimated based on the shape of the sieve cells. The sieve cells differentiating at the beginning of the growing season are square in cross section, whereas at the end of the growth period they appear flattened in a tangential plane.

A growth ring of secondary phloem is comprised of 10-12 layers of sieve cells and a single layer of parenchyma cells (Fig. 6.5). In narrow rings the number of layers is reduced. From 60-90% of the secondary phloem is comprised of sieve cells, parenchyma cells, and rays. Fibers are absent. In older phloem some parenchyma cells become sclereids. There are also numerous crystals of calcium oxalate. Sieve cells are about 2.8 mm long and 0.02 mm wide. The radial walls of sieve cells possess sieve-fields. Their pores remain open for about two years (HOLDHEIDE1951). As a rule, sieve cells in spruce function only for one season and a short part of the next one (HUBERcit. ESAU 1969). The sieve cells are crushed in old phloem.

Phloem parenchyma forms discontinuous strands in the middle of the growth ring. The length of parenchyma cells is about 75 ¿am and their width is initially smaller than that of sieve cells. Later the parenchyma cells increase in

Figure 6.5. Living bark (photo A. Hejnowicz)

A - secondary phloem in cross section; B - cambium and secondary phloem in cross section; arrows - eliminated files of cambium cells; sc - sieve cells, pr - ray, po - phloem parenchyma cells; C - secondary phloem in radial section; al - albuminous cells of the ray, pr - ray parenchyma cells, po - phloem parenchyma cell width. Some of the parenchyma cells become sclereids. Their walls become thick and lignified, but owing to the presence of pits, these walls maintain contacts between adjacent cells. Sclereids are numerous in the phloem of trees from poorer sites (KARTUSCH et al. 1991).

Phloem rays are arranged in a single series and contain parenchyma and albuminous cells at their margins (Fig. 6.5C). Albuminous cells lack starch, whereas parenchyma cells are very rich in starch. Albuminous cells are functionally associated with the sieve cells. Thus, they resemble the companion cells of angiosperms, but do not originate from the same precursor cells as the sieve tube members (SRIVASTAVA 1963, TIMELL 1973). In spruce, in addition to the uniseriate rays, there are also multiseriate ones with horizontal resin ducts. In phloem ray cells there are numerous calcium oxalate crystals, depending upon the content of available calcium in the soil (KARTUSCH et al. 1991).

The periderm includes the cork cambium (i.e. phellogen) that produces the periderm, composed of phellem formed on the outer side by the phellogen, and phelloderm (living parenchyma cells) formed on the inside of the phellogen layer. In spruce the first phellogen arises in the cortex and functions only for one season. Subsequent phellogen cells arise in the deeper layers of secondary phloem. The periderm is layered and comprised of sclerified, spongy, and phlobaphene cork layers. Four or more layers of sclerified cork cells alternate with those of the other cork layers and indicate seasonal growth increments. Phelloderm is comprised of two to three layers of parenchyma cells. As a tree grows, the number of cork cells decreases. In roots the spongy and sclerified phellem of the periderm are each comprised of one layer of cells. Old phellogen layers are eventually cut off to form the outer bark scales or plates. Each scale is comprised of periderm and secondary phloem cut off by phellogen. Scales on the stem surface are typically small and are sloughed off. In roots they are much larger and remain intact.

In the periderm there are isolated regions distinguished from the phellem by the presence of intercellular spaces, called lenticels. These structures permit the entry of air through the periderm. The phellogen of a lenticel is continuous with that of the periderm. The outer loose tissue formed by the lenticular phellogen defines the border of the lenticel. Like the phellem, it is comprised of layers of stained, spongy, and phlobaphene cells (WUTZ 1955).

In Picea abies, bark thickness and microscopic structure depend to a high degree on light environment. Trees from shaded sites have bark two- to three-fold thinner than individuals growing in high-light sites. Differences in microscopic structure are also quantitative. The secondary phloem in shade-grown trees is up to two-fold narrower than in high-light trees. The length and width of sieve cells are greater in high-light than shaded trees. High-light grown trees have six times more sclerenchyma cells in the bark of stems than in shaded trees (EREMIN 1982).

6.2.3. Secondary xylem

Secondary xylem of Picea abies is macroscopically homogenous. The border between sapwood and heartwood is not sharp, both types of wood are histologically identical. Annual growth rings are distinct (Fig. 6.6). The width of the growth rings varies along a radius from the pith outward. Ring widths are greatest for a relatively short period (until about the 20th growth ring, depending on the distance from the tree base), after which annual growth increments decline slowly. Environmental factors have a profound effect on the growth rate and ring widths in mature wood. Each growth ring can be divided into early and late wood (Fig. 6.6A, B, D) Late wood occupies from 2 to 30% of the total growth ring thickness (HEJNOWICZ 1969). In trees from poor habitats, the late wood constitutes a greater proportion than in trees growing in favorable conditions.

Up to 90% of the wood structure is comprised of tracheids. These are narrow, elongated cells 2-5 mm long on average and 0.01-0.06 mm wide. The mean radial diameter of an early wood tracheid (0.026-0.058 mm) is twice greater than in late wood (0.01-0.025 mm) (HEJNOWICZ 1969). Growth rate and tracheid length are related. The longest tracheids are found in growth rings 1-2 mm thick. The usual inverse relationship between ring thickness and tracheid length tends to be reversed in very narrow rings. Finally, the dependence of cell length on growth rate (in terms of ring thickness) is also related to the frequency of pseudo-transverse divisions of cambial cells. Within a growth ring, the mean tracheid length is the greatest in the transition zone from early to late wood, and the smallest near the growth ring border (BANNAN 1963).

The tracheid wall is built of a compound middle lamella and the secondary walls frequently comprised of three major layers: S1, S2, S3. The thickest is the S2 layer, which occupies 80% of the entire width of the secondary wall. The separation of the secondary wall into three layers is the result of the different orientations of microfibrils (constituents of the cell wall consisting of cellulose molecules). There exists a positive correlation between cell wall thickness and several environmental factors, such as day length, light intensity, and temperature. For example, in tracheids formed at 7°C, cell walls are twice thicker than those formed at 23°C (RICHARDSON 1964).

In the walls of the tracheids there are the bordered pits typical for Coniferopsida (Fig. 6.7). The pit diameter is 0.011-0.022 mm. Two opposing pits are called a pit-pair. Each pit has a pit cavity. The secondary wall may overarch the pit cavity forming a border. The pit cavity is enclosed by the border and opens into the cell lumen. A thickened membrane is present in the middle of the pit. This thickening forms the torus. Pits are located mainly on the radial walls of early wood and are abundant on the overlapping ends of tracheids (Fig. 6.7B). Tangential tracheid walls may bear pits in the late wood. In spruce some wide early-formed tracheids may have two rows of pits. The mean number of pits per tracheid ranges from 70-210.

Figure 6.6. Stem wood in cross section (photo A. Hejnowicz)

A - parts of the two adjacent growth rings; Ew - early wood, Lw - late wood; arrow - trabecula; B - stem wood from the peripheral part of an old tree; C - reaction (compression) wood; D -axial resin duct with thick-wall epithelial cells; late wood

Figure 6.7. Stem wood in longitudinal radial (A and B) and tangential (C and D) section (photo A. Hejnowicz)

A - axial resin duct (ar) in late wood; B - bordered pits in tracheid walls; C - wood rays; rr -horizontal resin duct; D - bordered pit (bp) in radial tracheid wall

It has long been known that spruce wood exhibits excellent resonating properties, particularly in slowly growing trees. Therefore, spruce is frequently used in the construction of musical instruments. When selecting spruce wood for its resonating properties, it is of the utmost importance that it possesses a regular, even structure. The growth rings should be of the same width, with a smaller proportion of late wood and larger diameter bordered pits in the early wood tracheids and rays in comparison to nonresonant wood (KOCZWANSKA 1970). For this purpose the wood must be floated in water for a long period of time. Presumably, bacteria damage the pit membranes in the tracheids during floating so that the tori do not occlude the pits any longer. However, in examined samples of spruce wood from old Italian violins dating from the 16th to 18th centuries, the pit membranes remained intact (BARLOW and WOOD-HOUSE 1990).

Spruce wood possesses trabecula, rod like parts of the cell wall extending radially across the lumen of axial tracheids (Fig. 6.6A). They are more numerous in tracheids of trees growing under unfavorable conditions and often arise near injured tissues (GROSSER 1986). Spruce wood rays are heterogenic and uniseriate (Fig. 6.7A). Some of them include horizontal resin ducts, which usually occur near the center of the fusiform ray (Fig. 6.7C). There are 30-50 rays per 1mm2 of a tangential section. The rays are numerous near the pith. They are comprised of two kinds of cells: parenchyma cells and ray tracheids, which are located usually on the ray margins. The rays are about 0.1-0.2 mm in height and are comprised of 6-12 cells.

In radial longitudinal section, the characteristic arrangement of pits can be observed in a cross-field over the radial wall. Cross-field is the portion of a radial section bounded by the upper and lower horizontal walls of a ray parenchyma cell, and the wall of an axial tracheid. The pitting at the cross-fields is used for the identification of conifer wood types. In spruce the pitting is of the piceoid type, but some other kinds of pitting (cupressoid, taxodioid) can also occur in the first several growth rings (HEJNOWICZ 1969).

Resin ducts in Picea abies wood run axially between the axial tracheids, and radially in the fusiform rays. The axial ducts occur mainly in the transitional region of the growth ring (Figs 6.6B, D and 6.7A). Resin ducts in Picea abies are schizogenous in origin, meaning they are formed from the splitting of adjacent cells. They are lined with 7-12 resin-producing epithelial cells. There are two kinds of these cells: thin- and thick-walled. Thin-walled cells have no secondary walls. Simple pits are present in the anticlinal walls between the thick-walled cells and absent in tangential walls facing the canal lumen (TAKAHARA et al. 1983). In spruce heartwood, the thin-walled epithelial cells grow into intercellular resin duct lumens, forming the thylosoids. Resin ducts can also develop in response to injury, occurring as very large cysts or pitch pockets formed in the same manner as the resin ducts.

The type of reaction wood in spruce is compression wood. The stimulus of gravity and the distribution of endogenous growth hormones are important factors in initiating the development of reaction wood. Compression wood in gymnosperms is formed in regions of high auxin concentration. Tracheids in compression wood are shorter than those in normal wood. Their cell walls are thicker, much more lignified, and appear rounded in transverse sections. The inner (S3) layer of the secondary wall is absent and the S1 layer is more loosely attached to the primary wall and the S2 secondary wall layer. The microfibril angle in the S2 layer is much greater (30-50°) than in tracheids of normal wood (10-30°).

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