190 – II.3. BRASSICA CROPS (BRASSICA SPP.)
for seed production. Thus, these crops are more winter hardy than cole crops and can be
grown for seed over a much wider environmental range. However, the market for their
seed is relatively small, so seed companies tend to contract their production with growers
in areas already producing seed of other Brassica vegetable crops.
The B. rapa vegetables prefer a soil pH between 6.0 and 7.5 with an N-P-K
fertilisation ratio at planting of 2:1:1. Additional nitrogen fertiliser is normally applied at
anthesis (George, 2009). The seed is produced by either the head-to-seed or the
seed-to-seed method described by Opeña, Kuo and Yoon (1988). As with the cole crops,
the ratio of male to female in hybrid production fields is 1:1 or 1:2 (Takahashi, 1987).
Centres of origin and ancestors
Introduction
There are few areas of the world where members of the family Brassicaceae are
totally absent. The exceptions are parts of the tropics, were the family is thinly
represented, but where some introduced cosmopolitan weeds have become established.
The genera and species of the family occur in greatest number and diversity in the
temperate zone of the northern hemisphere and in particular, the areas surrounding the
Mediterranean basin and throughout the southwest and central regions of Asia
(Figure 3.31; Hedge, 1976). Although the generic and specific endemism in the family is
highest in the Irano-Turanian region, the centre of origin of the current subtribe
Brassicinae, lies in the Mediterranean basin (Hedge, 1976).
Using chloroplast DNA restriction sites together with cpDNA probes, Warwick and
Black (1991) surveyed 33 diploid taxa of the Brassicinae. The phylogenetic results
indicated there were clearly two ancient and distinct evolutionary lineages within the
subtribe. They found the “Nigra” lineage to include B. nigra, B. fruticulosa,
B. tournefortii, Sinapis pubescens, S. alba, S. flexuosa, S. arvensis, Coincya cheiranthos,
Erucastrum canariense and Hirschfeldia incana. The other lineage, termed
“Rapa/Oleracea”, was made up of Brassica rapa, B. oleracea and subsp. alboglabra, the
B. rupestris-villosa complex (B. rupestris, B. drepanensis, B. macrocarpa, B. villosa),
B. barrelieri, B. deflexa, B. oxyrrhina, B. gravinae, Diplotaxis erucoides, D. tenuifolia,
Eruca sativa, Raphanus raphanistrum, R. sativus and Sinapis aucheri. In the “Nigra”
lineage, B. nigra was most closely related to the annual Sinapis species S. arvensis and
S. alba (Figure 3.31). Only a single mutation difference was found between the crop and
weedy accessions of B. rapa and between crop accessions of B. oleracea and wild
accessions of B. oleracea subsp. oleracea and subsp. alboglabra (Warwick and Black,
1991). The weedy species R. raphanistrum and the crop species R. sativus differed by
only four mutations.
Although the economically important Brassica species arose from ancestors in the
Mediterranean region, wars and trade ensured their wide dispersal, resulting in islands of
isolated environmental and selection pressure. The earliest widely distributed species
were those that exhibited seed dormancy combined with useful traits. Seed dormancy
allowed the introduced seed to survive long after its introduction. The fast-growing,
weedy type of B. rapa, providing lamp oil and animal feed, and B. nigra as an oil and
spice source, would be prime candidates. The Mission Trail in Southern California is a
case in point: priests scattered B. nigra seed to mark the trail between the early Missions.
Parts of those trails can still be seen each year as the black mustard blooms on the
California hillsides.
