Liquid Culture Systems for in vitro Plant Propagation

Mass propagation of conifer trees 399
In this system (Timmis et al., 1992), the Miniplug trays are filled by
machine with a soil mix such as vermiculite and sand, and a precise volume
of sucrose-containing nutrient medium is then added. An adhesive is applied
to the upper surface of the tray, which is then autoclaved. Embryos are sown
~5mm beneath the soil surface, and then a sterile transparent polyethylene
film is placed in contact with the adhesive on the upper surface of the cells
to provide protection from airborne contamination. A similar film can be
placed on the undersurface. In this way the cells are isolated
microbiologically from one another, and the trays can be transferred to a
shaded greenhouse for germination.
The somatic embryos germinate into the headspace in about 2 weeks.
The polypropylene film, which is more permeable to CO2 than to water
vapor, allows photosynthesis while slowing water loss, so that the plants
gradually acclimate to the ex vitro environment. The type of adhesive
selected allows the top and bottom films to be easily peeled off when the
epicotyls reach the top of the headspace. The plants are then watered
thoroughly to wash out any remaining sucrose, and thereafter tended much
as ordinary containerized seedlings. An alginate gel treatment was found to
improve the soil plug’s integrity for earlier nursery transplanting (Pullman
and Yancey, 1991).
In general, we found that direct-to-soil sowing of somatic embryos was
superior to gelled medium because it produced an upright plant with wellbranched
root system and root hairs. Tests of variations in this method
showed its potential for use with other container systems (e.g., Styroblocks,
using dry sterile surface powders or particulates for isolation), or even with
uncovered containers in sufficiently clean environments.
10. Manufactured seed delivery system
Agricultural processes are well developed for the use of natural seed.
Artificial seed has the potential of delivering somatic embryos to the
bareroot nursery or farm field using methods that require minimum
alteration of current sowing equipment. Ideally, once artificial seeds are
sown, the performance of the crop and the nursery or field cultural practices
required should not differ from those for current nursery seedlings. No
additional care should be necessary to culture the crop. Redenbaugh (1986)
defined a synthetic seed as a somatic embryo inside a coating, and as being
directly analogous to natural seed. Several names have been used for such
“seed”, including artificial seed, seed analog and somatic embryo seed. At
Weyerhaeuser Company, we use the term “manufactured” seed, to reflect the
nature of the construct more accurately. Several patents and papers have
been published on synthetic seed design exhibiting low percentage