Agriculture Reference
In-Depth Information
intercrossing and seed production; (ii) the use of replicated trials and statistically
based measurements of the average performance of different lines or families in
a breeding programme to choose those to be retained and combined in an
improved cultivar; and (iii) the development of F
1
and triple-cross hybrid
cultivars. The most appropriate technique depends on how well developed the
existing cultivars are, and on the expertise and resources available for breeding.
The following examples of actual onion breeding programmes illustrate these
various approaches. These breeding methods are summarized in Fig. 3.5.
Mass selection
Currah (1985) described a breeding programme to develop onion cultivars
suitable for Ethiopia from rather variable Sudanese cultivars. Approximately
2000 bulbs were produced and selected to eliminate off-colour, double, thick-
necked and small bulbs from within red-, brown- and white-bulbed populations.
About half of the bulbs were saved from each sowing, although in most such
programmes only 1-5% of plants would be selected. These were replanted and
allowed to flower and cross-pollinate within the population of each colour. The
seed was collected and resown as soon as ripe and the whole process was
repeated annually. In most climates a biennial cycle with bulb production in
year 1 and seed production in year 2 is necessary. Five such cycles were sufficient
to develop an improved cultivar of each colour that was capable of producing
export-quality bulbs. This breeding technique is known as 'mass selection'.
Continuing selection of bulbs during routine seed production is essential in
maintaining the quality of such cultivars. Mass selection is effective where, as
in this case, a fairly large shift is needed to adapt a breeding population to a new
locality and, in consequence, off-types are numerous and obvious. Mass
selection was probably widely used for developing open-pollinated European
onion varieties until the 1980s (Dowker
et al.
, 1984). Mass selection is simple,
requires little specialized knowledge of plant breeding and so is suitable where
there is little expertise or technology available. Where populations are already
highly bred and improved, however, it becomes difficult to advance by such
simple techniques.
Line selection
The breeding of improved open-pollinated onion populations in the UK was
described by Dowker
et al.
(1984). A fairly wide range of northern European
cultivars were selfed and inbred for one to three generations. The resulting
inbred lines were grown in replicated trials and the bulbs were assessed for: (i)
appearance (skin retention, skin colour and shape); (ii) yield (including an
allowance for the proportion of thick-necked plants and maturity date); and
