Agriculture Reference
In-Depth Information
an even skin and flesh colour, which may vary from white to red or purple
according to consumer preference.
Objectives for leek breeding usually emphasize a long shaft length and a dark
green leaf blade in addition to high yield. In contrast to onion, leek development
is not dependent on day-length and temperature, and so leek cultivars are
adapted over a much wider area than onions. Climatic adaptation is a matter of
having fast-growing cultivars for spring and early summer production and hardy
types able to stand outside throughout the winter. Winter hardiness is usually
associated with a short shaft length. Uniformity is important: leeks vary
considerably in size at harvest, even after transplanting uniformly sized seedlings.
With direct-drilled or module-raised crops there is a need for uniformly vigorous
seedlings. Seedlings of existing leek cultivars vary in vigour, possibly because of
the large number of deleterious recessive gene alleles that exist in the populations
(see Genetics, above). As mentioned previously, hybrid cvs are less variable than
open-pollinated cvs. Resistance to leek yellow-stripe virus and leek rust (
Puccinia
allii
) are also important breeding objectives (Pink, 1992).
In recent years commercial cultivars of Japanese bunching onion have
increasingly supplanted farmer-saved seed in Japan. New cultivars, including
F
1
hybrids, have been bred for greater resistance to bolting, more tolerance of
high temperature and improved quality. The quality objectives diverge between
those grown for long pseudostems and those grown for tender green leaves
(Inden and Asahira, 1990).
Plant breeding of chives has concentrated on cultivars for growing as a
'forced' crop in winter. Objectives are erect, long leaves of large diameter (>
3.6 mm), vigorous shoot branching, late flowering and high yield. F
1
hybrid
cultivars have been bred that outyield open-pollinated forcing cultivars
(Poulsen, 1990).
To select for the breeding objectives listed in the previous section, the breeder has
to devise appropriate ways of measuring the characteristics selected. For some
objectives he/she must ensure the trials impose a high 'selection pressure' for
traits of interest. Also, efficient selection methods are dependent on knowledge of
how environment influences the traits under selection. Since bulb onion
development is so closely dependent on climate, it is essential that selection trials
are performed in the region where it is intended to grow any improved cultivars
that result.
Many objectives of onion breeding can be assessed visually and quantified
by a simple scoring system. This applies to shape, colour, skin retention, skin
thickness and uniformity. Maturity dates can be assessed, as when 50 or 80% of
plants show collapsed foliage. The early sowing of breeding lines of overwintered
crops to maximize bolting is an example of how selection pressure can be
