Agriculture Reference
In-Depth Information
VLD types are typical northern European long-storing cultivars. Within these
groupings smaller differences in day-length response give early, maincrop and
late cultivars. Very approximately, short-day onions can initiate bulbs when day-
lengths exceed 11-12 h, intermediate-day types when day-lengths exceed 13-14
h and long-day types require > 16 h. Careful experimentation shows these
criteria to be subject to much qualification (see Chapter 4); however, the division
of cultivars into these broad response groups gives a general indication of their
suitability for bulb production in different regions.
Short-day onions can produce bulbs at low latitudes where photoperiods
remain close to 12 h throughout the year, although many varieties widely used
in the tropics were bred for autumn sowing as early spring-maturing crops at
higher latitudes - for example, the Texas Early Grano types discussed above.
Intermediate-day varieties are typically grown at middle latitudes as over-
wintered crops, and they begin to bulb in the spring ready for harvest in the late
spring and early summer. Long-day varieties are typically sown in the spring at
high latitudes and bulb in the mid- to late summer. If a cultivar has an
inappropriate day-length response for a locality it is useless for bulb production.
For example, a long-day variety growing at near equatorial latitudes will not
bulb at all, since photoperiods do not reach sufficient length to stimulate the
process. On the other hand, a short-day variety sown in spring at high latitudes
will be stimulated by long photoperiods to bulb almost as soon as leaves have
emerged. Consequently, bulbs will develop on tiny plants and will be very small.
Molecular techniques are now being applied to investigate the relatedness
of onion cultivars. Analysis of simple sequence repeats, SSRs, in the DNA of 35
different cvs of diverse origin yielded a 'family tree', with cultivars grouped into
well-defined categories (Jakse
et al.
, 2005). LD storage cvs from the USA were
closely related to those from Japan, consistent with the introduction of these
types of onion to Japan from the USA. European LD storage types formed
another cluster more closely related to a cluster of SD and ID short-storage cvs
than to US and Japanese LD storage types.
Bark and Havey (1995) investigated genetic diversity in 17 open-pollinated
populations of onions that bulb in short days (SD) and long days (LD) using
RFLP polymorphisms probed by random complementary DNA. One hundred
and forty-six polymorphic DNA fragments were scored for presence or absence
and the data were used to calculate how closely related each population was.
The populations did not clearly separate according to the day-length response.
The SD populations were genetically more diverse and it appeared that the more
uniform LD populations were derived from them. SD onions may show greater
genetic diversity because they have been maintained as landraces and open-
pollinated cultivars over a wide geographic area, whereas intermediate and
long-day cultivars are more localized (Currah, 2002).
There is now a danger of diversity in the SD gene pool being lost because
new hybrids and highly bred, open-pollinated cultivars from international seed
companies are replacing traditional cultivars in tropical areas.
