Agriculture Reference
In-Depth Information
usually vernalized by cool temperatures during storage, and they are planted
out in spring. Storage temperatures between 4.5 and 14°C are suitable, with an
optimum of about 11°C.
Planting mother bulbs
Bulbs are normally planted 10-15 cm deep in rows 50-100 cm apart. Closer
inter-row spacing can give higher yields, but this increases the risk of attack by
fungal disease. To minimize this risk, ventilation can be improved by orienting
rows in the direction of the prevailing wind.
Seed yield increases with plant population. For example, in October-
planted trials near Salerno, Italy, grown in rows 80 cm wide using bulbs with a
mean weight of 100 g, seed yields increased linearly from about 700 to 1200
kg/ha when density increased from four to 12 bulbs/m
2
. The number of
umbels increased from 11 to 35/m
2
, but seed weight per umbel declined from
about 6.0 to 3.6 g (Cuocolo and Barbieri, 1988).
As mother bulb size increases, given a constant planting density, the yield of
seeds increases, chiefly because the number of umbels per plant increases, and this
more than outweighs a slight decrease in the weight of seeds per inflorescence. For
example, Cuocolo (1989), using a population of 12.5 bulbs/m
2
, found that seed
yield increased linearly from about 1200 to 1800 g/m
2
as the median weight of
the mother bulbs increased from 30 to 190 g, corresponding to an increase in
planting rate from 3750 to 23,750 kg/ha. The mean number of umbels per plant
increased linearly from about 2.7 to 5.0 over the same range of bulb size, whereas
the mean weight of seed produced per umbel decreased from about 4.7 to 3.9 g.
The proportion and arrangement of pollinator and male-sterile mother
bulbs planted is important in F
1
hybrid seed production (see Chapter 3). Ratios
of fertile:sterile of 1:3 or 1:4 are used, frequently with two pollinator rows
alternating with eight male-sterile rows (Currah, 1981).
Isolation of seed production fields
Because onions are easily cross-pollinated by insects from neighbouring fields, it is
important to maintain varietal purity by separating the seed production of
different cultivars. Similar cultivars should be kept at least 400 m apart, but seed
fields of different-coloured cultivars should be 5 km apart (Jones and Mann,
1963). Male-sterile lines and their maintainers should be isolated by 5 km, or
grown in insect-proof cages to prevent contamination by fertility-restoring pollen.
Irrigation and fertilizer
To maximize and to help stabilize yields, irrigation is important in most onion
seed-producing regions (see Table 6.6). Irrigation can increase the risk of foliar
disease if humidity is high, and furrow or drip irrigation is preferable to
sprinklers to reduce this risk.
Field studies of flowering onion plants show that the lowest water potential
(greatest water deficit or stress) is found in the individual flowers and pedicels
