A plant of the brassicaceae family, rapeseed (Brassica napus) originated from a natural crossing between a cabbage (Brassica oleracea) and a turnip (Brassica rapa). Molecular dating indicates that this species likely appeared ~7,500 years ago at the most (Chalhoub et al. 2014). It was selected by humans and there are no wild forms. The precise origin of this cross is not elucidated, but it most likely occurred somewhere around the Mediterranean, in the wild or in vegetable gardens where cabbages were grown for food and turnips for oil-based lighting. Since then, rapeseed has adapted to numerous territories and it is now found in China, Europe, Canada and Australia.
Semi-autogamous reproduction
The ovules of rapeseed flowers are generally fertilised by self-pollination. An outcrossing rate of 10 to 30% is estimated and varies between varieties. The pollen, which is heavy and sticky, cannot be carried by the wind over long distances. It can be transported by insects, and mainly by bees, as the rapeseed flower is very attractive. It secretes 0.2 to 2 mg per day of nectar, containing 40 to 60% sugars; this, thanks to the extent of its cultivation and the duration of its flowering, makes it one of the most important nectariferous and polleniferous plants along with the sunflower due to its abundance of nectar and pollen.
Varietal types
The mode of reproduction has allowed the selection of pure lines, i.e. having homogeneous genetic information across the entire genome. These lines therefore produce homogeneous progeny identical to their parent plants. Crosses between lines have shown that it is possible to produce higher yielding F1 hybrids, combining the characteristics of the parental lines. These F1 hybrid varieties are now predominant on the market.
There are two types of rapeseed. Winter rapeseed, mainly grown in Europe, requires a period of cold to flower, which is why it is sown in September. Spring rapeseed, which is widely grown in Canada, Asia and Australia, can develop without cold.