Agriculture Reference
In-Depth Information
based on molecular markers indicate that rhizomatous species are not all
immediately related and rhizomes must have evolved independently several
times, an example of 'convergent evolution'. In the new classification subgenus
Rhizirideum
is a much more restricted group than formerly.
Subgenus
Cepa
This subgenus of the alliums includes: (i) the section
Cepa
, to which belong the
major crops onion,
A. cepa
and Japanese bunching onion,
A. fistulosum
; (ii) the
section
Schoenoprasum
, to which the chive,
A. schoenoprasum
belongs; and (iii)
the section
Sacculiferum
, to which the oriental crop rakkyo,
A. chinense
belongs.
Plants in this subgenus have slender, hollow leaves and short, vertical
rhizomes, an extreme example of which is the disc-like baseplate of onion.
SECTION
CEPA
This small group, which includes onion and Japanese bunch-
ing onion, has cylindrical, opposite leaves and bulbs formed from several leaf
bases covered by thin, dry skins. The leaf sheaths form a pseudostem that
surrounds the hollow flower scape. Axillary daughter bulbs form tufts or
short rhizomes; a progressive reduction of rhizomes within the section
culminates in the baseplate disc of onion.
These species occur mainly in the Tien Shan and Pamir-Altai mountains of
central Asia. They grow in open rocky sites with shallow soil and have a fairly
long annual growth period from spring to autumn, although in arid areas they
may show weak summer dormancy induced by drought. They have a long
juvenile phase of 3 to 10 years before flowers are produced. Some of the
localized species are in danger of extinction, partly because of over-collection
for food by local people. It is important that these species do not disappear, as
they might contribute valuable genes to the crops (Kik, 2002). The section has
been divided into four alliances of closely related species (Fritsch and Friesen,
2002). The
Cepa
alliance includes
A. cepa
, the common onion,
A. vavilovii
and
A. asarense
;
A.vavilovii
is completely interfertile with onion (Kik, 2002) and is
an endangered species. The
Altaicum
alliance includes
A. altaicum
and
A.
fistulosum
, the Japanese bunching onion.
The alliums are distributed widely through the temperate, warm temperate
and boreal zones of the northern hemisphere. In tropical areas they are
confined to mountain areas. In Eurasia the region of greatest species diversity
ranges from the Mediterranean eastwards into the mountains of central Asia,
through northern Iran, Afghanistan and Pakistan, Tajikstan, the Tien Shan
mountains of Kyrgyzstan and north-east China, into the mountains of
Mongolia and southern Siberia. The most important crops originated in this
centre of diversity (see below). A lesser centre of diversity occurs in western