Agriculture Reference
In-Depth Information
Hanelt (1990) describes how domestication of onions may have originated
as illustrated by
A. pskemense
, another wild, large-bulb species of the section
Cepa
. This grows in the western Tien Shan mountains of Uzbekistan and
Kazakhstan, and inhabitants of some valleys transplant it to their gardens
where it is cultivated and propagated. During domestication of onions, selection
for faster growth, leading to a biennial rather than a longer life cycle, and for
larger bulbs must have occurred. Also, barriers to crossing with related species
must have developed.
Cultivated types of
A. cepa
fall into two broad horticultural groups, the
Common Onion group and the
Aggregatum
group (Hanelt, 1990).
The Common Onion group constitutes the vast bulk of the economically
important varieties. These form large, single bulbs (see Plate 1) and are mostly
grown from seed. Varieties grown for salad onions and as small bulbs for
pickling are mainly from this group. There is great diversity in adaptation to
photoperiod and temperature, in bulb storage life, dry-matter content, flavour
and skin colour. Many of these aspects are discussed in more detail in later
chapters and in the section on onion cultivars below.
The bulbs of the
Aggregatum
group are smaller than the common onion
because they rapidly divide and form laterals, hence forming clusters of bulbs.
Jones and Mann (1963) distinguished two bulb-forming subgroups: multiplier,
or potato, onions and shallots (see Plate 1). Multiplier onions divide into
between three and 20 bulbs that are wider than they are long, and which are
encased by the dry, outer bulb skins. Shallots form clusters of narrow, separate
bulbs, and the leaves and flowers are usually smaller than those of common
onions. The
Aggregatum
group is usually vegetatively propagated but, recently,
improved seed-reproduced varieties of shallots have been bred and are being
widely grown in Europe, Israel and North America. Hybrid cultivars have been
developed in Israel and Holland using cytoplasmic male sterility (Rabinowitch
and Kamenetsky, 2002). There has, however, been some dispute as to whether
such seed-reproduced cultivars truly constitute shallots from the producers of
traditional, vegetatively propagated varieties in France.
When flowers form, they are interfertile with the Common Onion group,
and therefore they are the same species. In view of this, the specific name
A.
ascalonicum
, used for shallots in the past, is not justified. The
Aggregatum
group
are not so important commercially as the Common Onion group, and many
are grown as home garden crops; however, large-scale cultivation of shallots
takes place in Europe, North America, Argentina and in some tropical regions.
In France they are favoured for their special flavour and cooking quality as
compared with common onions. Vegetatively propagated onions like multiplier
or potato onions are traditionally grown in Finland and northern Russia
(Aura, 1963). Shallots are also suitable for such high-latitude regions with a
short growing season.
Interestingly, at the other extreme of the latitude range, in the humid
tropics - particularly in lowland coastal regions, shallots and multiplier onions
