Principles of Plant Genetics and Breeding

98 CHAPTER 6
Approaches to germplasm conservation
There are two basic approaches to germplasm conservation
– in situ and ex situ. These are best considered as
complementary rather than independent systems.
In situ conservation
This is the preservation of variability in its natural habitat
in its natural state (i.e., on site). It is most applicable
to conserving wild plants and entails the use of legal
measures to protect the ecosystem from encroachment
by humans. These protected areas are called by various
names (e.g., nature reserves, wildlife refuges, natural
parks). Needless to say, there are various socioeconomic
and political ramifications in such legal actions by governments.
Environmentalists and commercial developers
often clash on such restricted use or prohibited use
of natural resources. This approach to germplasm conservation
is indiscriminatory with respect to species
conserved (i.e., all species in the affected area are
conserved).
Ex situ conservation
In contrast to in situ conservation, ex situ conservation
entails planned conservation of targeted species (not all
species). Germplasm is conserved not in the natural
places of origin but under supervision of professionals
off site in locations called germplasm or gene banks.
Plant materials may be in the form of seed or vegetative
materials. The advantage of this approach is that small
samples of the selected species are stored in a small
space indoors or in a field outdoors, and under intensive
management that facilitates their access to breeders.
However, the approach is prone to some genetic erosion
(as previously indicated) while the evolutionary process
is halted. The special care needed is expensive to provide.
Other aspects of this approach are discussed later in
this chapter.
Germplasm collection
Planned collections (germplasm explorations or expeditions)
are conducted by experts to regions of plant origin.
These trips are often multidisciplinary, comprising
members with expertise in botany, ecology, pathology,
population genetics, and plant breeding. Familiarity
with the species of interest and the culture of the regions
to be explored are advantageous. Most of the materials
collected are seeds, even though whole plants and vegetative
parts (e.g., bulbs, tubers, cuttings, etc.) and even
pollen may be collected. Because only a small amount of
material is collected, sampling for representativeness
of the population’s natural variability is critical in the
collection process, in order to obtain the maximum
possible amount of genetic diversity. For some species
whose seed is prone to rapid deterioration, or are bulky
to transport, in vitro techniques may be available to
extract small samples from the parent source. Collectors
should bear in mind that the value of the germplasm
may not be immediately discernible. Materials should
not be avoided for lack of obvious agronomically desirable
properties. It takes time to discover the full potential
of germplasm.
Seed materials vary in viability characteristics. These
have to be taken into account during germplasm collection,
transportation, and maintenance in repositories.
Based on viability, seed may be classified into two main
groups – orthodox and recalcitrant seed:
1 Orthodox seeds. These are seeds that can prolong
their viability under reduced moisture content and
low temperature in storage. Examples include cereals,
pulses, and oil seed. Of these, some have superior
(e.g., okra) while others have poor (e.g., soybean)
viability under reduced moisture cold storage.
2 Recalcitrant seeds. Low temperature and decreased
moisture content are intolerable to these seeds (e.g.,
coconut, coffee, cocoa). In vitro techniques might be
beneficial to these species for long-term maintenance.
The conditions of storage differ depending on the
mode of reproduction of the species:
1 Seed propagated species. These seeds are first dried
to about 5% moisture content and then usually placed
in hermetically sealed moisture-proof containers before
storage.
2 Vegetatively propagated species. These materials
may be maintained as full plants for long periods of
time in field gene banks, nature reserves, or botanical
gardens. Alternatively, cuttings and other vegetative
parts may be conserved for a short period of time
under moderately low temperature and humidity.
For long-term storage, in vitro technology is used.
Types of plant germplasm collections
There are four types of plant genetic resources maintained
by germplasm repositories – base collections,