II.3. BRASSICA CROPS (BRASSICA SPP.) – 159
leaves are large with upper leaves reduced in size. B. nigra can be easily distinguished
from the commercial Brassica crops in that B. nigra does not produce a rosette of basal
leaves. A typical plant image, including the tap root, is shown in Figure 3.2. The siliques
are short (2-5 cm), hirsute and appressed to the stem of the flowering raceme, with a beak
about 0.6 cm long. The small, brown to black seeds exhibit primary dormancy and tend to
germinate throughout the growing season.Brassica rapa
Plants of B. rapa species are widely cultivated as leaf and root vegetables, fodder and
oilseed crops. In addition, they can be a weed of cultivated land and disturbed sites.
The widest array of vegetable forms evolved in the People’s Republic of China (hereafter
“China”) with many of the selected forms corresponding to or mimicking those found in
the B. oleracea complex. Because the selected forms exhibited significantly different
morphological traits, early botanists classified them as separate species. Today they are
more correctly classified as subspecies or varieties of B. rapa.Brassica rapa vegetables
The plants in the B. rapa subsp. pekinensis group of vegetables are biennials that have
been classified into three variant forms. The var. cylindrica has broad but thin, crinkled
and conspicuously veined green leaves with white petioles (Figure 3.3). The leaves are
usually tightly wrapped in a cylindrical formation to form a head with a length of
30-60 cm and a diameter about 10-17 cm. The var. cephalata forms a flat head similar to
a drum-head cabbage (Figure 3.3) while the var. laxa forms a loose heart. In the second
year of growth bolting occurs and the flowering stem is quickly thrust upwards reaching a
height of 1.5 metres and bearing the characteristic raceme with typical Brassica yellow
flowers. Common names for this group include pe-tsai, celery cabbage or Chinese
cabbage.Figure 3.3. B. rapa subsp. pekinensis
A. var. cylindrica B. var. cephalata(^)
Source: Courtesy Evergreen Seeds.
The B. rapa subsp. chinensis group includes both annual and biennial forms. Bailey
(1930) described the subspecies as “a very smooth biennial with large ladle-shaped
upstanding radial leaves with thick ivory-white but not wing-margined or toothed
petioles.” The clasping, entire leaves have prominent veins and resemble leaves of Swiss
chard (Figure 3.4). The common name for this plant group is pak choi or bok choy. If the
plants are harvested in the early stages of growth they may be called “baby bok choy” or
“Shanghai bok choy” (Figure 3.5). The subsp. parachinensis is usually included within