Agriculture Reference
In-Depth Information
onion seed production is dominated by Dutch seed companies, who operate in
southern Europe - particularly in Italy, southern France and Spain - because
the warm, dry summers are well suited to the production of disease-free seed
crops of onions (and leeks). Small-scale seed production for breeding purposes
still occurs in cooler regions, and can be very satisfactory in polyethylene
tunnels (Dowker
et al.
, 1985).
Methods of production
There are two basic systems of production: (i) bulb-to-seed production, in
which a bulb crop is raised first and seeds are produced from the planted bulbs;
and (ii) seed-to-seed production, in which growing plants are vernalized and
induced to flower and produce seeds without going through a bulb stage. The
former system has the advantage that it is possible to select the bulbs for
maintaining the quality of the seed stock and to discard off-types - for example,
double bulbs, misshapen bulbs or premature bolters (Peters, 1990). On the
other hand, this method normally takes 2 years to produce a seed crop.
Seed-to-seed production is possible where the crop can overwinter as
growing plants. It is important that vernalization is sufficient to induce 100%
flowering, otherwise this method of production will result in the selection of easy-
bolting genotypes. To ensure 100% bolting, seed is sown several weeks earlier
than normal for a bulb crop, and irrigation is applied to ensure rapid emergence
and quick growth to a post-juvenile size so that vernalization starts as the
autumn begins (Jones and Mann, 1963). Heavy soils, with a high water content,
create a slightly cooler microclimate and this is important for vernalization in
warm regions like the Imperial Valley of California.
BULB
-
TO
-
SEED PRODUCTION
Raising mother bulbs
The first stage in the bulb-to-seed method is to raise mother bulbs. This is
essentially the same as producing a normal bulb crop. Bulbs of 4-6 cm diameter
are generally required. During growth of the crop, roguing for abnormal foliage,
thick necks and uneven maturity can be carried out at the appropriate times.
Storage of mother bulbs
Autumn-sown cultivars are normally stored at ambient (often warm) tempera-
tures after an early summer bulb harvest and planted in the autumn, after
which they are vernalized by low temperatures in the field. In hot regions it may
be necessary to vernalize bulbs by cold storage before planting: 90 days at
8-10°C was the optimal treatment for the Sao Francisco valley in Brazil, where
the mean temperature of the coolest months is 27°C (Currah and Proctor,
1990). If cold storage before planting can be used, it is possible to ensure reliable
flowering of seed crops in such regions, while avoiding using genotypes that bolt
too easily when grown for bulbs. Mother bulbs of spring-sown cultivars are
