Reproductive Cycle

Most of the information on the reproductive cycle presented in this chapter is based on the following sources: Miyake (1903); HAkAnsson (1956); Sarvas (1968); Mikkola (1969); ANDERSSON et al. (1969); CHRISTIANSEN (1972); JONSSON (1973); ANDERSSON (1980); MOSHKOVICH (1992) and OWENS et al. (2001). Contrary to the other Picea species, the development of reproductive organs of Norway spruce is only sparsely illustrated. However, it seems that there are no fundamental differences in the reproductive cycle between Norway spruce and other spruce species and a number of other species of the Pinaceae family.

6.5.1. Microsporogenesis and microgametogenesis

The determination of the male bud takes place in late spring or early summer. In mid July, the first microsporophyll primordia become visible in the embryonic shoot (Fig. 6.1D). During the following months, two microsporangia with well-defined sporogenic tissue develop inside a microsporophyll. The micro-sporangial initials are hypodermal in origin. The inner layer of microspo-rangium develops into the tapetum. At this stage the male buds enter into the winter dormancy period. During the next spring, the sporogenous cells divide and microsporocytes develop (i.e. microspore mother cells). The common wall of the microsporocytes is lysed. The nuclei of the tapetum cells divide but cytokinesis is absent and mature tapetum cells are predominately binucleate.

In central Europe, meiotic divisions of the microsporocytes take place in late April and last only for a few days. The air temperature has a significant influence on meiosis. At -2°C some irregularities have been observed, whereas at -10°C all the dividing microspore mothers cells die (ANDERSSON 1980). Each functional microsporocyte gives rise to four haploid microspores, which are enclosed within the wall of the microsporocyte for some time. When the microspores enlarge, the wall disintegrates and connections between the microspores disappear. Two bladders or wings are formed on each spore due to the separation of the outer and inner layers of the microspore wall.

Before the pollen is shed, a mitotic division of the microspore gives rise to a two-celled gametophyte comprised of the central and the prothallial cell. The division of the central cell in turn, gives rise to a second prothallial and to an anteridial cell. The latter divides forming a generative cell and tube cell. Both prothallial cells soon become flattened and die. This is usually the stage in which pollen grains are released into the air.

The diameter of the mature pollen grain in spruce is 70-90 am. Pollen grains are two-fold larger than in Pinus species and have smaller air-filled wings. Consequently, they are not dispersed as far as pine pollen grains. There are numerous amyloplasts in the cytoplasm of the mature pollen grains. The generative cell of the germinating pollen grain divides, giving rise to the spermatogenic and stalk cells. CHRISTIANSEN (1972) did not observe the stalk and tube cells in microgametophytes of Picea. On the contrary, HAkAnsson (1956) has noticed both these cells in Picea pollen tubes.

6.5.2. Megasporogenesis and megagametogenesis

Female buds like the male buds are initiated the growing season prior to the year in which flowering occurs. In Poland, the differentiation of female buds usually takes place in June. In July the first scale primordia arise on the shoot apical meristem. A month later the first primordia of ovuliferous scales (seed scales) are initiated in the axils of some scale primordia. A bract subtends each ovuliferous scale. The bracts are much shorter than the associated ovuliferous scales. Up to 10% of the cone scales are sterile. The sterile scales are mainly formed near the end of the growing season and at the beginning of the next one.

Each ovuliferous scale bears a pair of ovules, attached at the base of its adaxial surface. The ovule consists of the nucellus, i.e. the central body ofvege-tative cells, which later encloses the sporogenous cells, and a single integument (ovules are unitegmic). The ovule is inverted and a conspicuous micropyle points inward toward the cone axis. The sporogenic tissue differentiates in early spring (in March) from the subepidermal initial cell at the apex of the nucellus. The megaspore mother cells (megasporocytes) and the nucellar cap, located above them, are formed in sporogenic tissue. The nucellus enlarges and the megasporocyte yields a tetrad of haploid megaspores. Three of them soon degenerate and the fourth, the chalazal megaspore, functions as a mother cell of the megagametophyte. During the free nuclear period of growth, the mitotic divisions of the megagametophyte nuclei are not followed by cytokinesis. When cell walls are finally formed, the coenocytic megagametophyte is converted into cells. The free nuclear period lasts for several weeks. The female gametophyte is completely cellular 10-14 days after pollination. Earlier, the free end of nucellus becomes slightly concave and the pollen grains are deposited into the shallow cavity.

On the micropylar pole of the megagametophyte, one or several superficial cells divide periclinally and differentiate into two cells: a small outer primary neck cell; and a larger inner central cell. By means of two successive anticlinal divisions and two periclinal divisions of these four cells, an eight-celled neck is formed. The neck cells are arranged in two or sometimes four tiers. The central cell is the archegonial mother cell.

The number of archegonia produced by a single gametophyte varies considerably. There are from one to seven (mean 2.8-3.4) archegonia present in a single ovule (Sarvas 1968). Half of the 300 ovules studied by Miyake (1903) had four archegonia, 25% of them had three; 20% had five; and in individual ovules there were two, three or seven archegonia. In mid May each of the archegonium mother cells enlarges and their nuclei divide, giving rise to a small ventral canal cell. The nucleus of the egg cell is about 0.1 mm in diameter. The nucleus moves to a central position in the egg. When mature, the egg is jacketed by a distinct layer of cells with numerous proteinaceous bodies in the cytoplasm.

6.5.3. Pollination and fertilization

In the climatic conditions in Poland, pollen shedding takes place in late April or early May. At this time the axis of the young megasporangiate cone elongates and the ovuliferous scales separate. Pollen grains transported by the wind enter the space between the scales and adhere to the pollination drops, which are exudates of the open ends of the inverted ovules (RUNIONS et al. 1995). Pollen grains in the liquid fill the micropylar canal of the ovule, which is enclosed by the edges of the integument. After pollination the ovuliferous scales are drawn together and remain tightly pressed. A few days after the pollen grains entered the pollen chamber, the growth of pollen tube begins. When the pollen tube approaches half the distance to the egg cell, the spermatogenous cell of the male gametophyte divides to form two male gametes of unequal size. The tube cell, stalk cell, and male gametes move toward the tip of the pollen tube.

Pollen tube growth is dependent upon air temperature, usually quantified by a degree-day index, the sum of the mean daily temperature above a base temperature of 5°C. In Finland, pollen tube growth starts at 220 degree-days (Sarvas 1968). Four to five weeks after pollination, the pollen tube penetrates the nucellus. Since several pollen grains may have reached the apex of

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