8. 1. REPRODUCTIVE DEVELOPMENT 8.1.1. Juvenile phase and first flowering
Initiation of the first reproductive organs (also termed strobili or flowers) is commonly considered the end of the juvenile phase and the onset of reproductive maturity in forest trees (WAREING 1959; GIERTYCH 1976c; POETHIG 1990). The evidence to date suggests that this phase change is correlated with some minimum number of growth cycles and/or minimum tree height or size (see CHALUPKA and Cecich 1997). Consequently, DORMLING et al. (1968) proposed two ways of shortening the juvenile phase in Picea abies under controlled conditions: (1) if the deciding factor is the number of completed growth cycles, it would be possible to accelerate a phase change by shortening the individual growth cycles; (2) if the deciding factor is determined by tree size, then an accelerated change in phase could be achieved by artificial stimulation of growth in phytotron conditions.
Under natural conditions, the average length of the juvenile phase in Picea abies is 20-25 years (WAREING 1959); however, first flowering in this species may be observed as early as 9-10 years (WINIARSKI1886; STARCHENKO 1964; CHALUPKA 1972; SZYDLARSKI1980). More abundant flowering coupled with a substantial production of mature seeds typically occurs much later. PANOV (1950) reported seed setting by P. abies trees in Bosnia at 25 years in open-grown trees, 25-30 years in trees along stand edges and greater than 35 years in trees of the stand interior. In Poland, individual open-grown trees mature at the age of 30-40 years, whereas individuals tend to mature at 60-70 years when grown in forest stands (TOMANEK 1966). Cone and seed production increases with tree age, attaining its maximum at 110-120 years in the Bialowieza Primeval Forest (CHALUPKA 1972). After a period of maximum production, cone and seed production diminishes with tree age (HAGNER 1955; USKOV 1962).
8.1.2. Distribution of strobili
On open-grown trees of Picea abies, the female strobili cover the outer portions of the crown, frequently extending to the lowest branches, especially in a year of abundant flowering (KOZUBOV 1974). However, most of the female strobili cover the upper and southern part of the crown (HAGNER1955) and it is assumed that this is related to a higher insolation and temperature of the buds (Pukacki 1980). Male strobili are typically found mostly in the lower half of the crown (LONGMAN 1989). The crowns of Norway spruce trees growing in a dense stand differ in terms of strobili distribution. One can recognize three zones: an apical zone, in which about 60% of the female strobili and 43% of the male strobili are distributed, a middle zone with 31% of the female and 39% of the male strobili, and the lower zone with 9% of the female and 18% of the male strobili (ELIASON and CARLSON 1968).
According to Tiren (1935), male strobili can develop from apical buds, lateral buds, buds between whorls, and from buds subtending the branches, whereas female flowers as a rule develop from apical buds only. It appears that there is a sex gradient with an increasing proportion of female strobili in the lower shoot orders and an increasing proportion of males in the higher shoot orders (Minina 1960). Debazac (1965) associates the sex differentiation of shoots with the vigor of the apical meristem, and classifies Norway spruce shoots from the point of view of their function and degree of sexual specialization, distinguishing juvenile vegetative shoots, shoots with female strobili, and shoots with male strobili.
126.96.36.199. Initiation and differentiation of reproductive buds Norway spruce, as other Pinaceae, is a monoecious species and its strobili are initiated on current shoots a year before flowering (LONGMAN 1989). Flower bud initials are laid down towards the end of the height growth period. In Byelorussia, flower buds of Picea abies are formed in late July (JURKEVICH and GOLOD 1966), whereas in Bulgaria flower buds are formed in late May through the end of June (PLOSHCHAKOVA-BALEVSKA1970).
The factors governing vegetative and reproductive bud differentiation are not well understood and remain an intriguing problem. According to Deba-ZAC (1965) the timing of bud differentiation is strictly related to the rate of mitotic activity of the apical meristem. VARNELL and ROMBERGER (1967) believe that bud function in Norway spruce is determined after their initiation.
At the time of flushing, the apical meristems are indistinguishable. In June, the male strobili primordia are first differentiated taking on a hemispherical shape. At that time both the female strobili and the primordia of vegetative buds take on a parabolic shape and remain similar until July, when the first ovuliferous scale primordia are initiated. The differentiation of reproductive buds in Picea abies seems to be closely related to the metabolism of endogenous gibberellins, and the ratio between GA9 and GAi concentrations in the shoots may be used as a specific indicator of reproductive potential of the buds (MORITZ and ODEN 1990; ODEN et al. 1994).
Thus, depending on latitude, flower buds are microscopically distinguishable at the end of July and in August (Tiren 1935; KOZUBOV et al. 1981). Presumably at that time, the normal course of flower bud development can be disturbed, resulting in a number of abnormalities in strobili formation, e.g. female cones proliferated with shoots (JONEBORG 1945), or split cones (Leanderson 1970; Kozioe and KRUPSKI1994). Flower buds differentiated in the summer continue their development in the fall until the end of October, and undergo only quantitative changes during the winter by increasing in size (Ploshchakova-Balevska 1970).
Meiosis in the micro and megaspore mother cells precedes spring bud burst and elongation, representing the first stage of flowering (JURKEVICH and Golod 1966). The timing and rate of these events in different years are related to the air temperature, often quantified by a relatively consistent annual heat sums or degree-days (SARVAS 1972).
A few days after the buds open, female strobili enter a phase of receptivity. In Norway spruce, metandry is observed and receptivity is generally attained prior to pollen shedding on the same tree (ANDERSSON 1965; SARVAS 1968; ERIKSSON et al. 1973). Pollination takes place in Poland between late April and early May (TYSZKIEWICZ 1949), and in Byelorussia during the first half of May (JURKEVICH and Golod 1966). In Finland, flowering begins at the end of May, and pollen shedding lasts about two weeks (SARVAS 1957; 1968; NIKKANEN 2001). Latitude and elevation have a significant effect on the phenology of Norway spruce flowering. On the basis of several studies conducted by Russian authors, JURKEVICH and Golod (1966) claim that with a shift from south to north, the delay in the time of anthesis is on average two days per degree of latitude. In Romania at an elevation of 500-600 m Norway spruce begins to flower in early May, and above 1000 m in late May and early June (TOMESCU et al. 1967), whereas in Bulgaria flowering begins five weeks later at an elevation of 2100 m than at an elevation of 1000 m (VELKOV et al. 1967).
The diurnal and seasonal course of pollen shed is closely associated with temperature and relative humidity. During the day, pollen flight is maximal
Figure 8.1. Diurnal course of pollination of Norway spruce in Finland (according to Sarvas 1955)
between 8 AM and noon, followed by a decline during the afternoon and a complete cessation at night (Fig. 8.1). Strong winds may hasten pollen shed and rains can stop it for some time (SARVAS 1955). The total production of pollen in mature Norway spruce stands varies among years. During a year of abundant flowering, pollen deposition may reach 120-160 kilograms of pollen per hectare (SARVAS 1968).
pollen grains are transported by the wind to the female strobili and deposited at the bottom of ovuliferous scales. The receptive period of female strobili lasts 9-16 days as was observed by Prof. RISTO SARVAS (Nikkanen 2001). The number of grains received by a single ovule can exceed 100, but only a few of them are transferred through the micropylar canal to the pollen chamber, which is able to hold 5.1 pollen grains on average. Pollen transfer with the aid of the so-called pollination drop takes place during the night and can be disturbed by night frosts. Yet, pollen grain germination in a pollen chamber frequently exceeds 90% in southern Finland (SARVAS 1968). Immediately after reaching the chamber, the pollen grains germinate, the pollen tubes penetrate the nucellus, and finally fertilization occurs 3-4 weeks later (MIYAKE 1903; HAKANSSON 1956; CHRISTIANSEN 1972; KOZUBOV 1974).
Figure 8.1. Diurnal course of pollination of Norway spruce in Finland (according to Sarvas 1955)
After fertilization, embryo development continues during the summer months. At the end of August, the embryo is fully differentiated with cotyledons, plumule, primary roots, and provascular bundles in the hypocotyl. Seeds attain full maturation at the end of September in Sweden, and in October in Poland and Byelorussia (HAKANSSON 1956; TYSZKIEWICZ 1949; JURKEVICH and GOLOD 1966). Although some seeds are released from the cones already in September (SOKOLOWSKI1921; OPSAHL 1951), most seeds fall in late winter, when the moisture content of the cones is reduced to 18% (MESSER1956).
The quality of Norway spruce seeds is dependent on many factors. The proportion of full seeds in the total crop and their weight vary with crown position and are higher in the lower than upper crown. Higher percentages of full seeds
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