II.3. BRASSICA CROPS (BRASSICA SPP.) – 163
Autotetraploid B. rapa varieties have been developed for use as leafy vegetables,
fodder (turnips) and green manure. Tetraploid plants have larger leaves, thicker stems,
greater height and larger seeds than their corresponding diploids (Abel and Becker,
2007). However, tetraploids are not used as oilseed crops as their seed and oil yields are
significantly lower than their diploid progenitors (Downey and Armstrong, 1962).
The much larger tetraploid pollen also takes significantly longer to germinate than
B. rapa diploid pollen. Thus, pollen from B. rapa diploid plants has a selective advantage
resulting in triploid embryos, which abort (Downey and Armstrong, 1962; Håkansson,
1956), providing strong selection pressure against B. rapa tetraploid plants growing in
B. rapa diploid populations. The slower pollen germination of tetraploid plants could
predispose them to out-crossing with related species. On the other hand, since tetraploid
B. rapa crops are normally consumed or ploughed down before flowering they are
unlikely to be a significant factor in gene flow.Brassica oleracea
The B. oleracea vegetables are often referred to as the “cole crops” and comprise
cabbage, cauliflowers (including broccoli), kales and kohlrabi, but not the B. rapa
vegetables.Wild B. oleracea
Wild B. oleracea var. oleracea or wild cabbage is native to the western and southern
seaboard of Europe where its tolerance of salt and lime, but its intolerance to competing
vegetation, tends to restrict its presence to limestone sea cliffs (Heywood, 1964; Rich,
1991). The plants of this subspecies are biennial or perennial and in the first year produce
a rosette of thick, fleshy leaves (Figure 3.11). Following vernalization a flowering stalk
1-2 metres tall arises from the centre of the rosette bearing a raceme of self-compatible,
yellow flowers.Figure 3.11. Wild B. oleracea plants in their first year of growth
Source: Wikipedia, the Free Encyclopedia.
B. oleracea var. capitata, cabbage
The cabbage is a biennial plant that in the first year of growth produces a dense,
terminal head of tightly wrapped leaves on a short stout stem. The head is surrounded by
a rosette of large fleshy leaves (Figure 3.12A). Three main types of heads – smooth
green, red and Savoy – are commercially produced (Figure 3.12B). In the second year,