II.3. BRASSICA CROPS (BRASSICA SPP.) – 205
which resulted in the overloading of the small 42 m^2 recipient plots with donor pollen.
Similar high outcrossing rates were recorded by Ramsey, Thompson and Squire (2003)
where blocks of ten male-sterile plants were placed at increasing distances from a large
commercial field. These results have implications for feral populations situated near
commercial fields. Other observations suggest that field-to-field crossing is likely to be
highest in fields just commencing or finishing flowering when a nearby field is in full
bloom.Table 3.9. Predicted outcrossing rates for spring and winter oilseed rape at three isolation
distances (with 95% confidence limits), based on 2000-03 multilocation UK field trialsOilseed rape type Percent outcrossing
2 m 50 m 150 m
Spring 0.46 (9.97)^1 0.02 (0.39) 0.01 (0.14)
Winter 0.76 (12.25) 0.04 (0.84) 0.02 (0.40)Note: 1. 0.46 is the average percent outcrossing with a 5% chance that outcrossing could be as high as 9.97%.
Source: Weekes et al. (2005).
Downey and Beckie (2002) and Friesen, Nelson and Van Acker (2003) illustrated
how easily pedigree seed can be contaminated in breeding nurseries. Admixture during
seeding, harvesting or cleaning was also identified as a contaminant source (Downey and
Beckie, 2002). These studies alerted seed companies to the problem of contamination in
breeders seed stocks, leading to tighter controls (see the section “Oilseed certified seed
production”).
However, the present rapid development and acceptance of B. napus hybrid varieties
dictates that certified seed-production fields will contain at least 66-75% male sterile
plants. This increases the risk of outcrossing. In Canada, all hybrid producing seed fields
are regulated and inspected to ensure that they are isolated from other rapeseed plants and
fields by at least 800 m and free of certain Brassica weeds within the production field and
the regulated isolation area. The isolation distance used by most seed companies for
hybrid seed production of B. napus in Canada is at least 1.6 km (Wescott and Nelson,
2001). To further reduce the possibility of fertilisation by foreign pollen, the fields are
heavily stocked with honey bees. Such fields are also saturated with leaf cutter bees
(Megachile rotundata [Fabricius]), which have a short foraging range, to ensure the
desired rapid and complete fertilisation of the male sterile female parent.Seed development, production and natural dispersal
After fertilisation the endosperm develops rapidly, while embryo growth does not
start for some days. The embryo is generally still small two weeks after pollination but by
three to five weeks has almost completely absorbed the endosperm and filled most of the
seed coat. Nutrient reserves for germination are stored in the cotyledons which are folded
one over the other so that there is a smaller inner and a larger outer cotyledon
(Figure 3.37).