Protocols for Micropropagation of Woody Trees and Fruits

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3. To improve and succeed in the regeneration of genetically transformed leaf
and/or nodal stem pieces of silver birch, the preculture period for 4 to 8
days (Keinonen, 1999; Valjakka et al., 2000) has proved to be appropriate.
Although TDZ was necessary for shoot induction, the induced shoots deteriorate
if cultivation on TDZ medium is continued. Therefore, the induced
shoots have to be transferred into medium containing BA as a cytokinin.
2.5.3. Acclimatisation and Hardening
1. Wash the rooted shoots to remove all agar using tap water. It is also possible
to transfer non-rooted in vitro shoots to ex vitro conditions. However, in vitro
rooted shoots are usually of more uniform quality at least during the early
phases of ex vitro development than the shoots rooted ex vitro. However, if
shoots are submerged in IBA before transfer to soil ex vitro, rooting is more
uniform and faster.
2. Transfer the rooted shoots or shoots for instance into multipots including
a wet peat-perlite (1:1 v/v) mixture with (Jones et al., 1996) or without
(Ryynänen, 1999) slow-releasing fertilizer or to perlite, unfertilized peat
and birch forest soil (2:2:1 v/v) (e.g. Laitinen et al., 2004). Cultivate under
decreasing humidity e.g. in propagators for the first 2 weeks before transferring
them to the greenhouse conditions.
3. After 4 weeks fertilize the rooted shoots with commercial fertilizers (e.g.
0.2% and followed by 0.3% Superex from Kekkilä, Finland) especially if
unfertilized peat has been used before and transplant the plants in individual
pots.
3. CONCLUSIONS
Silver birch is an important forest tree species due to wide distribution of the species,
economic importance, long conventional breeding practices as well as its potential in
molecular breeding and basic research. For many of these approaches the in vitro
propagation technology available is of utmost importance. For silver birch the
micropropagation method is appropriate and applicable for several genotypes although
in specific genotypes further optimisation might be needed. Micropropagation of
selected genotypes has been used to establish controlled seed orchards in greenhouses
and thereby improved the plant material for afforestation. The in vitro protocols
have enabled the preservation of genetic resources of the species by cryopreservation
and it has also been optimised for genetically transformed material. At present,
however, silver birch is not directly micropropagated for afforestation purposes in
Europe which is more due to high labour costs and legislation regulating the use
of clonal material than a technical question. In this paper we are summarising the
detailed micropropagation protocols applied for silver birch micropropagation in
several tissue culture laboratories.
Acknowledgements. We acknowledge the research funding from the Academy of Finland
(grant 105214 to HH).