170 – II.3. BRASSICA CROPS (BRASSICA SPP.)
B. juncea, oilseed and condiment mustards
Plants of this species, grown for their seed oil or condiment production, are normally
referred to simply as B. juncea without the attachment of a subspecies name. However,
Spect and Diederichsen (2001) classify this plant group as B. juncea subsp. juncea.
Plant for both oil and condiment are similar in their morphology but differ in seed oil
percentage and the type and amount of glucosinolates present in the seed. These forms are
annuals that grow to about 1.2 metres as spring-sown crops in western Canada and
Europe. On the Indian sub-continent they are grown as a winter crop where, under short
days, plants grow up to 2.1 metres tall. The plants are green and sometimes slightly
glaucous. The lower leaves of the rosette are rather thin, elliptic to obovate and
lyrately-lobed or divided. The upper stem leaves are small, narrow and not clasping
(Figure 3.21). Depending on the day length and temperature the flowering stalk bolts and
produces a raceme with no terminal flower. As with B. napus, the buds are borne above
the open flowers. Apical dominance is present with the secondary racemes initiated about
three days after flowers open on the main raceme. The silique is about 7 cm long
containing seed weighing 2.5-3.0 g/1 000 seeds.
Geographic distribution, ecosystems and habitats, cultivation and management
practices, centres of origin and diversity
Introduction
From an ecological and agronomic point of view, both the spring and winter forms of
oilseed rape exhibit two undesirable characteristics. First, mature pods tend to shatter,
leaving large but variable amounts of seed on the ground at harvest (see below on the
contribution of B. napus harvest losses to persistence). Pod shatter not only results in lost
yield but also sets the stage for large numbers of volunteer plants in subsequent crops.
Fortunately B. napus seeds have no primary dormancy so if moisture and temperature are
adequate, the vast majority of these seeds germinate and are killed by frost, herbicides,
cultivation or predators (see below). The opportunity for B. napus to acquire primary
dormancy is limited due to the vast majority of fields being sown each year with high
germination certified seed.
The second undesirable characteristic is the tendency for a proportion of the shattered
seed to acquire secondary dormancy. Such dormancy is induced by abiotic stresses
(see section on persistence below). Although most of the shattered seed will quickly be
reduced by fatal germination, predation, disease and abiotic stress, a small percentage can
remain dormant and viable for ten years or more (Schlink, 1998; Lutman, Freeman and
Pekrun, 2003). Thus, B. napus is able to establish seed banks within cultivated fields (see
Lutman et al., 2005 and below). As a result, traits or genes that have been genetically
silenced or augmented within improved varieties may be reintroduced. Examples would
be the genetic blocking of the biosynthesis of erucic acid in rapeseed oil, the reduction in
linolenic acid content and the augmentation of oleic fatty acids in the oil, or reduction of
glucosinolates in the oilseed meal.
It should be noted, however, that there is considerable genetic variability within the
species and its close relatives in both the degree of pod shatter and the percentage of
induced dormancy. Until recently these characteristics have not been a priority for oilseed
rape breeders but progress is possible. Wang, Ripley and Rakow (2007) have clearly
demonstrated that selection for reduced pod shatter in B. napus can be achieved. In
addition, Østergaard et al. (2006) have shown that expressing the Arabidopsis
