II.3. BRASSICA CROPS (BRASSICA SPP.) – 171
FRUITFULL gene in B. juncea, using a CaMV 35S promoter, produces shatter-resistant
plants. Although the shatter-resistant pods held their seed too tightly for combine
harvesting, a weakened form of the FRUITFULL gene could result in an economically
and environmentally valuable advance. It is unlikely that conventional breeding will lead
to complete elimination of the shattering characteristic, but there appears to be
considerable room for improvement. Further, Pekrun, Potter and Lutman (1997); Gruber,
Pekrun and Claupein (2004); Gruber, Emrich and Claupein (2009); and Gulden, Thomas
and Shirtliffe (2004) have all shown that among B. napus varieties, of both spring and
winter forms, there is a wide range in the percentage of seed susceptible to induced
dormancy. Thus, the application of conventional breeding techniques to select varieties
producing seed resistant to secondary dormancy should greatly reduce the presence of
volunteers in subsequent crops.
Ecologically, B. napus is described as a cultivated crop where escaped plants become
colonisers of waste places. However, they are not invasive of natural habitats. Colonisers
are defined as species that occupy disturbed sites or habitats but with populations that
keep moving, founding new populations while losing old ones (Williamson, 1996).
Feral populations of B. napus are most frequently found along road and rail verges, field
margins and in disturbed soils. The reports on the abundance and persistence of such feral
populations vary considerably from country to country and between the spring and winter
forms. Williamson (1996) noted that colonising species are not the same as invaders, even
if they have high intrinsic rates of increase, as exhibited by B. napus. He classifies
B. napus in Britain as intermediate between naturalised and casual. On the other hand,
recent intensive surveys of feral sites in mainland Europe have identified feral
populations in higher frequencies than anticipated, with some sites able to sustain
themselves in a semi-permanent state (Pivard et al., 2008). Such reports have given rise to
concerns by some that a proportion of feral populations could become permanent and in
time result in the invasion of natural habitats.
Although the species does have the weedy characteristics noted above, producing
many propagules (seeds), plus the ability to cross with some weedy relatives, it is not
competitive with perennial grasses that dominate the natural habitat. It should also be
noted that oilseed rape has been part of the European landscape for a very long time as
have the truly weedy, related species, Sinapis arvensis and B. rapa. However, none have
become invasive of natural habitats. In recent years, the area of oilseed rape cultivation
and intensity of production has increased worldwide. For example, since 1970 oilseed
rape production in France and Germany has increased 4.5- and 8-fold, respectively.
In the same period, the Canadian oilseed rape acreage has quadrupled, thus a wider and
more frequent occurrence of feral populations is to be expected. The spring form of
oilseed rape is much less likely to form feral populations or to be self-sustaining since fall
germination is normally fatal while frosts will kill many seedlings that germinate in the
spring. Although Knispel et al. (2008) has reported some transient feral population in the
province of Manitoba, Canada, such roadside populations are rare over most of the Prairie
Provinces, except near collection points and to a limited extent along railroad verges.
This is because in western Canada most road verges are mowed in late August before
feral populations set viable seed. Such roadside mowing is essential to prevent snowdrifts
across roads that tall vegetation can cause. In contrast, in Europe, the winter form can
avoid being killed by the fall road maintenance since mowing does not usually affect the
established first year rosettes, leaving some plants to flower and set seed before the next
fall mowing.