Agriculture Reference
In-Depth Information
increased above normal. Onion storage can be assessed as the percentage of
bulbs sprouted or rotten after a fixed time in typical storage conditions. The
soluble solids content, important in breeding cultivars for dehydration, can be
assessed by a refractometer reading of juice extracted from a small plug of the
bulb (see Fig. 8.7). Selected bulbs can still be used for planting. Various
techniques exist for flavour assessment (see Chapter 8), but most breeders still
use tasting panels. Many special tests have been devised for pest and disease
resistance to supplement gross field observations (see the following section).
In the absence of continuous selection to maintain the quality of open-
pollinated cultivars, some desirable features may degenerate. Continuous
selection is necessary to maintain resistance to bolting, single-centred bulbs, a
round bulb shape, high dry-matter content and long dormancy as, in the
absence of selection, cultivars tend to 'drift' away from these ideals (Pike,
1986). Any feature that tends to increase seed yield will increase in promi-
nence with repeated seed multiplication, in the absence of selection against it.
For example, a flatter bulb shape is associated with internal branching, as are
multiple centres, and both are likely to result in multiple flower heads per plant,
hence an increased seed yield per plant.
All important traits are affected by environment as well as genotype. By
comparing the scores from breeding lines against well-characterized control
cultivars, usually grown with more replication than the lines under test, some
environmental influences can be discounted. Many traits, including bulb size,
shape, maturity date, the percentage of thick-necked and of double bulbs, are
influenced by the population density at which plants are grown (Dowker and
Fennell, 1974). The breeder should bear these trends in mind when making
their selections. Bulb yield is also strongly density dependent. When onion
trials are directly sown, it is impossible to achieve constant plant population
densities. To overcome this difficulty, a control cultivar can be grown at a range
of population densities and a mathematical relationship between yield and
population density derived. Then, the yield of a breeding line can be compared
with that of the control cultivar at the same population density, as predicted by
this relationship (Dowker, 1990). Some breeders use transplanted seedlings
grown at a fixed density to overcome the problems associated with differences
of plant density. It is important to avoid the unwitting selection of the most
vigorous seedlings in transplanting such a trial.
Plant density also affects important selection traits in leeks. At higher
densities the ratio of shaft length to width increases, and consequently shafts
appear longer and thinner. In transplanted crops, deeper planting results in
longer shaft lengths.
Selection for yield in vegetatively propagated allium crops is difficult
because the size of the planting material, e.g. garlic clove or shallot bulb, and
also the conditions in which it was produced, e.g. plant density, affect its yield
potential (Chapter 6). Simple mass selection of the largest, best-looking bulbs
gives little improvement (Messiaen
et al
., 1993). Instead, by planting cloves
