II.3. BRASSICA CROPS (BRASSICA SPP.) – 161
organ (Figure 3.8). Most cultivars are white fleshed except for the exposed above-ground
portion which, when exposed to sunlight, may turn purple, red or green. Yellow, orange
and red fleshed cultivars are also grown. Leaves grow directly from the above-ground
shoulder of the expanded hypocotyl and not from a visible crown or neck as occurs in
rutabagas (Brassica napus var. napobrassica). The leaves may be harvested and eaten as
“turnip greens”. Turnip roots for edible purposes will each weigh about 1 kg but weight
will vary with the variety and growing conditions. Cultivars grown for cattle and sheep
feed produce much larger roots. The flowering stalk bolts from the overwintered root the
following spring, producing a terminal raceme with siliques about 6 cm long.
Figure 3.8. B. rapa subsp. rapa the common turnip
Source: Courtesy Wikipedia, the Free Encyclopedia.
Brassica rapa oilseed and weedy forms
The oilseed form of B. rapa subsp. oleifera includes both annual and biennial
varieties. Both the spring and winter forms of B. rapa mature earlier and withstand cold
temperatures better than their B. napus counterparts. However, the seed and oil yield
is normally lower than B. napus so production of the winter form is limited to the more
rigorous climates of central Sweden, Finland, north-west China and the foothills of the
Himalayan mountains. The plant and growth stages of spring B. rapa are illustrated in
Figure 3.9. Following the emergence of the cotyledons, the plant quickly produces a tap
root and a rosette of leaves that shades the surrounding area reducing weed competition.
The lower leaves are stalked, lyrate-pinnatifid with a large end lobe exhibiting sparse
hairs on the under side. The upper leaves are much smaller and slightly stalked.
In the winter form, the plant remains in the rosette stage until exposed to a long
vernalization period (40 days) at near freezing temperatures. Day length, and where
required vernalization, determine when bolting of the flower stem will occur. Figure 3.9
shows only a single raceme but under field conditions the plant produces many flowering
branches and with B. rapa, as opposed to B. napus, it can be difficult to identify the
primary raceme. The plant grows to a height of a meter or less. The position of the flower
buds on a raceme, relative to the just opened, self-incompatible flowers, can be used to
distinguish plants of B. rapa from B. napus. In B. rapa the flowers over top the buds
while the reverse is true for B. napus. Siliques, some 6 cm long, contain up to 30 brown
to yellow seeds in 2 locules (Figure 3.10).
B. rapa subsp. campestris (formerly subsp. sylvestris), the weedy form of
subsp. oleifera, is morphologically indistinguishable from the cultivated spring oilseed
B. rapa, except that the seed of subsp. campestris exhibits primary dormancy,
a recessively inherited characteristic.