Principles of Plant Genetics and Breeding

PLANT GENETIC RESOURCES FOR PLANT BREEDING 91
genes for resistance to parasites or for characteristics indicated by advances in science or technology or by changing demands of
the consumer”. Alongside the rising importance of the PGRs for providing food there also exist traditional uses like medicine,
feed, fiber, clothing, shelter, and energy.
Over one 5-year period, 6.5% of all genetic research within the plant breeding and seed industry, resulting in marketed innovation,
was concerned with germplasm from wild species and landraces compared with only 2.2% of new germplasm originating
from induced mutation (Swanson 1996; Callow et al. 1997).
Since the beginning of agriculture, natural diversity declined due to agricultural domestication, breeding, and distribution of
crops (Becker 2000). But in recent years crop species and varieties have also become threatened or even extinct. In agriculture,
the widespread adoption of a few varieties leads to a drastic decrease of landraces with their potential valuable genetic resources.
Among the cultivated plants, which represent less than 3% of the vascular plants, only about 30 species feed the world (Hammer
2004).
Conservation and monitoring of PGRs
The monitoring and evaluation of plant material is necessary for the conservation of PGRs. There may be a big difference between
the phenotype and the genotype in a population. With improved biotechnology methods, like the assignment of molecular markers,
the gene level is of increasing interest.
Ex situ conservation
Ex situ conservation stands for all conservation methods in which the species or varieties are taken out of their natural habitat and
are kept in surroundings made by humans. Large collections started with the activities of the Russian scientist N. I. Vavilov at
the beginning of the last century. Even at that time the employment of ex situ measures was necessary because of the rapidly
increasing gene erosion of landraces and other plants (Coats 1969). Alongside these collections, plant breeders contributed to
maintenance by collecting breeding material. This material was often kept in specific institutions, the first-called “gene banks” in
the 1970s. They were established for the collection (Guarino et al. 1995), maintenance, study, and supply of genetic resources of
cultivated plants and related wild species. Gene banks maintain the plant material as seeds, in vivo (when the storage of seeds is
difficult) or in vitro (mostly through cryoconservation). In contrast to the cultivation of plants in botanical gardens, the work in
gene banks is more engaged in intraspecific variability. Unfortunately, a lot of the material stored in gene banks is not in good
condition and urgently needs to be rejuvenated (Hammer 2004).
In situ conservation
As well as ex situ conservation, there is also the attempt to save biodiversity and therefore PGRs in ecosystems (in situ). This can
occur in the natural habitat (especially wild relatives and forestry species) or in locations where the plants (landraces and weeds)
have evolved (on farms, in agroecosystems). As opposed to ex situ conservation in gene banks where only a section of the whole
diversity is covered, the in situ approach is able to save larger parts of biological diversity.
Table 1 summarizes the methods of conservation for the different categories of diversity and evaluates their relative importance. It
is divided into cultivated plants, wild growing resources, and weeds.
Characterization and evaluation of plant genetic resources
The yield levels of many crops have reached a plateau due to the narrow genetic base of these crops. Although the results of some
surveys (Brown 1983; Chang 1994) indicate that the genetic base of several important crops is beginning to increase in recent
years, breeding programs of many important crops continue to include only a small part of the genetic diversity available, and the
introduction of new and improved cultivars continues to replace indigenous varieties containing potentially useful germplasm.
To widen the genetic base for further improvement, it is necessary to collect, characterize, evaluate, and conserve plant biodiversity,
particularly in local, underutilized, and neglected crop plants.
Morphological and agronomic characteristics are often used for basic characterization, because this information is of high
interest to users of the genetic diversity. Such characterization requires considerable amounts of human labor, organizational
skills, and elaborate systems for data documentation although it can be done by using simple techniques and can reach a high
sample throughput. Quantitative agronomic traits can be used to measure the differences between individuals and populations
with regard to genetically complex issues such as yield potential and stress tolerance. The diversity of a population, considering
such complex issues, can be described by using its mean value and genetic variance in statistical terms. The traits detected are
of great interest, but are frequently subject to strong environmental influences, which makes their use as defining units for the
measurement of genetic diversity problematic.
Molecular methods can be employed to characterize genetic resources and for the measurement of genetic variation. The
major advantage of molecular methods for characterization is their direct investigation of the genotypic situation, which allows
them to detect variation at the DNA level, thereby excluding all environmental influences. They can also be employed at very