Liquid Culture Systems for in vitro Plant Propagation

284 Sara von Arnold et al.
1. Introduction
Forest trees play a vital role in the lives of humans and functioning
ecosystems. They provide renewable sources of wood, fibres and chemicals
for human societies. They provide habitats for numerous organisms and
essential ecological functions such as water purification and carbon storage.
Forests are managed in a diversity of ways ranging from intensively
managed short rotation tree-farms to old-growths reserves. Whatever the
goals for forest management and conservation, the methods of clonal
propagation and especially somatic embryogenesis provide powerful options
for breeding and management. The pressure to increase productivity of the
forest will increase dramatically in the future. At the same time, pressure
will be brought to bear to increase forest conservation and sustainability. The
possibility to meet the increased need for wood products by intensive
forestry in highly productive plantations (fibre farming) and thereby making
it possible to release large areas as natural forests, are an attractive
alternative. Before this alternative can be accepted, it is important to develop
methods and to establish field trials showing that this alternative is safe and
sustainable.
The possibility to propagate trees vegetatively creates significant
advantages both for the deployment of selected genotypes through masspropagation
and for capturing and enhancing the genetic gain in the breeding
programme. To ensure that maximum genetic gain is achieved, the influence
of environmental factors on field performance of the genotypes has to be
determined. Today the common way to propagate plants vegetatively is via
cuttings. However, large scale cutting propagation may be limited in some
species owing to problems with rooting, aging of mother trees and survival
of cuttings, as well as high costs. Some of the problems can be overcome by
using tissue culture techniques and especially somatic embryogenesis
combined with cryopreservation. An embryogenic cell line established from
one seed can generate a high number of somatic embryos, therefore, it is
possible to produce a large number of genetically identical plants within a
short period.
It has been shown that somatic embryogenesis combined with
cryopreservation is an attractive method to propagate Norway spruce
vegetatively both as a tool in the breeding programme and for large-scale
clonal propagation of elite material (Högberg et al., 2001). Somatic embryos
of Norway spruce are also valuable as a tool for studying regulation of
embryo development. In addition, the somatic embryos can be used for
producing transgenic plants of Norway spruce (Clapham et al., 2000;
Brukhin et al., 2000).