Agriculture Reference
In-Depth Information
are again found to be the most satisfactory way of producing
A. cepa
bulbs
(Currah, 2002). In these conditions, pressure from pests and diseases causing
leaf damage is intense, and shallots and multiplier onions can complete a cycle
of leaf growth and bulbing in 60-75 days, whereas onions grown from seed
need a longer season of leaf growth, during which they are vulnerable to pest
and disease attack (Currah and Proctor, 1990). Furthermore small seedlings,
unlike larger, vegetatively propagated plants, lack the reserves to recover from
leaf damage.
Onion cultivars
Because onions have been cultivated for so long, and because their bulb and
inflorescence development must be closely adapted to the temperatures and
photoperiods that prevail where they are grown, there exists a huge range of
cultivars and landraces, developed over the centuries to fit the diverse climates
and food preferences of the world. Onions show particular diversity in the
eastern Mediterranean countries, through Turkmenia and Tajikistan to
Pakistan and northern India and these regions are, therefore, important
sources of genetic diversity (Astley
et al.
, 1982). Modern varieties sold by
international seed companies - in particular F
1
hybrids, which have a narrow
genetic base - are supplanting these old varieties, with a danger that these
latter, and therefore the many potentially valuable and adaptive genes they
contain, will be lost. This problem, common to many crop species, is termed
'genetic erosion'. Hence, the characterization and collection, preservation and
regeneration of seeds and vegetative clones of these old varieties and landraces
is very important, and a network of 'gene-banks' concerned with this task has
been established (Astley, 1990).
Broadly speaking, several phases of cultivar development have occurred.
First, there must have been the original domestication from a wild bulbing
species, similar to the utilization of
A. pskemense
described above. Then onion
seeds or bulbs must have spread through travel and trade and slowly became
adapted to each region to which they were carried. Onion was a common
cultivated garden plant in the Greek and Roman Empires. The Romans are
thought to have taken the onion into northern Europe and onion was widely
cultivated throughout Europe during the Middle Ages. Onion was probably
introduced into Russia during the 12th or 13th century. Europeans first took
onions to the Americas starting with Columbus. Settlers carried onions from
Europe into North America in the early 1600s. Europeans introduced bulb
onions to East Asia during the 19th century.
The development of a commercial seed trade with the deliberate selection,
testing and introduction of improved varieties was the next phase. Magruder
et al.
(1941) give examples of 18th- and 19th-century seedsmen who introduced
onion cultivars to the USA. Development of new cultivars by seed companies
continues today. Beginning in the 19th and early 20th centuries, onion develop-
ment and breeding began in state-funded agricultural experiment stations in
