Agriculture Reference
In-Depth Information
cultivars showed that the double-hybrids were more uniform but smaller and
lower yielding than the existing cultivars (Kim et al. , 2007). The double-
haploids mean bulb weight was 60% of the existing cultivars' mean at one trial
site and 85% at another. The mean size and yield from the F 1 hybrids
approached or exceeded that of existing cultivars depending on site. The use of
double-haploid lines as parents for F 1 hybrids should make it possible to develop
highly uniform onion cultivars expressing maximal hybrid vigour (Bohanec,
2002).
POLLINATION CONTROL IN BREEDING
It is evident from the procedures described above that the control of pollination
is fundamental to breeding. In simple mass selection, the population
undergoing selection must be isolated from extraneous pollen, particularly
that from strains with a different bulb colour. Isolation can be achieved by
producing seed in fields spaced more than 1.5 km apart. The same isolation is
needed for the large-scale production of seed from open-pollinated lines
produced by line selection, or in hybrid seed production. For producing smaller
quantities of seed, contamination with unwanted pollen is achieved by
isolating lines for crossing in insect-proof cages of nylon mesh (see Fig. 3.6a).
For controlled crosses between pairs of lines, flower heads are paired in
small, mesh cages or in cellophane bags and flies are introduced to spread the
pollen. Self-pollination is simply achieved by enclosing a single flower head in
such a cage or bag and introducing flies (see Fig. 3.6b). On a small scale a brush
can be used to hand-pollinate flowers. Controlled crossing while avoiding self-
pollination can be achieved on a small scale by removing anthers from mother
plants before they ripen and shed pollen, a process termed 'emasculation'.
BREEDING OBJECTIVES IN EDIBLE ALLIUMS
The knowledge of genetics and the breeding techniques discussed above are
applied to move crop improvement in the direction of certain explicit goals or
objectives, which the breeder must set. Some objectives, like high yield, a
nutritious end-product and pest and disease resistance, are universally
desirable. Others - for example, the colour and shape of onion bulbs - are
determined by the preferences of the buying public. Objectives of the latter sort
will often vary with locality and with the market niche identified for the
improved variety. Usually, positive selection is confined to a few major traits at
a time, otherwise the breeding programme becomes unmanageably large.
As described in Chapter 1, onion cultivars fall into the major categories: (i)
storage types; (ii) sweet types for consumption as raw bulbs; (iii) dehydrators;
and (iv) green salad types. Universally desirable traits in cultivars grown for
 
 
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