Vegetative Propagation

8.2.1. Grafting

Grafting of forest tree species began in Sweden by ANDERSON in 1906 (WERNER 1979). Grafting is used for the propagation of ornamental varieties and the establishment of seed orchards. A key advantage of grafting compared to other methods of asexual reproduction is the absence of an age barrier in the ortet, permitting the propagation of very old trees. Spruce can be grafted either in the field or in a greenhouse. Grafting in a greenhouse requires planting rootstocks in pots. The most common rootstocks are 3 or 4-year-old Picea abies. For production of large quantities of vigorous shoots for grafting, SYRACH-LARSEN (1956) recommends grafting permanent rootstocks with a diameter of 5-10 cm at the grafting union. In such situations, the rapid growth of grafted shoots will enable further collection of material for grafting after one 1 or 2 years. Grafting 6- or 7-year-old permanent rootstocks at a height of 0.5 m and cutting the rootstock above the grafting union guarantees rapid growth and a regular form of the graft (NAESS-SCHMIT and SOEGAARD 1960, after WERNER 1979).

The best scions are obtained from the tops of vigorous one-year-old shoots from the upper or outer parts of the crown (GIERTYCH and PRZYBYLSKI1965; Hrynkiewicz-Sudnik et al. 1991). Grafts developed from scions obtained from the upper tree crowns flower more frequently and more abundantly compared to grafts developed from the lower crowns, an important trait for seed orchard grafts (DORMLING 1970, 1976). While it is possible to graft twigs (Slaski and S^KOWSKI 1988), older scions are more difficult to establish. Scions that exhibit discolored needles or necrotic apices are rarely successful grafts, likely a result of poor cell division.

The most appropriate times of the year for grafting Norway spruce is February-March in greenhouses (KRUSSMANN 1964) and May in the field (Syrach-Larsen 1956; Giertych and PRZYBYLSKI 1965) or in August-September in field or greenhouses (KRUSSMANN 1964; GIERTYCH and PRZYBYLSKI 1965). Rootstocks may be grafted near the root collar at a height of 5-10 cm or in the middle of the shoot apex (HRYNKIEWICZ-SUDNIK et al. 1991). Grafting last-year's shoots produces better results in comparison to three-year old shoots (MEHNE1990). Old and thick rootstocks can be grafted at a height of 1.3 m (SYRACH-LARSEN 1956). The success of grafting depends on accurate technique and sterile conditions. In the Pinaceae, resins may prevent proper bonding between the graft and rootstock. This necessitates grafting at low temperatures and frequent washing.

8.2.2. Grafting methods

The majority of grafting methods of Norway spruce place scions on the side of the rootstock. GIERTYCH and PRZYBYLSKI (1965) demonstrated a higher effectiveness of lateral grafting compared to scion grafting into a cleft in the shoot apex. Whenever rootstocks are thick, SYRACH-LARSEN (1956) recommends cutting the rootstock, especially when using the method in which the scion is placed beneath the bark within two parallel incisions.

Lateral grafting is carried out using the side-grafting method (DORMLING 1963). A firm union between the scion and the rootstock prevents tissue isolation by resin flow, and is often strengthened with polyethylene straps, rubber bands (BARTELS 1982), or cotton strings (HRYNKIEWICZ-SUDNIK et al. 1991). In addition, PROKAZIN'S method (1962) also known as the "cambium to cambium" or "pith to cambium" method is also effective. In this procedure, a cut is made to expose the cambium on the rootstock and the scion is cut on one side at an angle or along the pith. The advantage of this method is that the two parts fuse properly even in the case of less accurate work. When grafting weak scions, scions may be cut on two sides forming a wedge (both cuts must have a common longitudinal edge) and inserted into a notch cut into the rootstock.

To date, bud grafting of coniferous species has not been practiced widely. SEVEROVA (1951) described a method of grafting P. abies rootstocks with P pungens scion resting buds. Norway spruce trees can also be grafted using the chip-budding method (KOHNERT 1991). Micro-budding is another form of bud grafting in which buds are collected from ortets or from in vitro meristematic cultures on rootstocks from several-week old seedling cultures in vitro or ex vitro. The objective of this kind of propagation is the rejuvenation of ortet tissue, but this is not always achieved (Ewald et al. 1991).

The fusing of Picea abies grafts typically begins with cell divisions of the rootstock cambium on the fourth day after grafting. The functional merger of phloem and xylem tissues occurs in the 5th-8th week (MEHNE 1990). Physiological indicators of the fusion process include the water potential and water content of the scion tissues (BEASON and PROEBSTING 1988). It is possible to accelerate the growth of grafted plants in controlled-environment chambers (NIENSTAEDT1959).

In contrast to forestry, the nursery practice does not recommend outdoor spruce grafting (TERPINSKI 1984; SLASKI and S^KOWSKI 1988; HRYNKIE-WICZ-SUDNIK et al. 1991), since graft success is often highly weather dependent. In forest nurseries, rootstocks are typically planted at 50 X 25 cm spacing and grafted after attaining a diameter of 6-8 mm. In the case of spring grafting, shoots soon begin to develop from buds of the established scions. Apical portions of the stock may then be removed the following year. In the case of summer grafting, the formation of grafted plants may be terminated in spring of the following year.

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