198 – II.3. BRASSICA CROPS (BRASSICA SPP.)
as S. alba are very day-length sensitive while some cultivars of B. napus and B. juncea
are day neutral.
Both the onset of flowering and duration of the flowering period are variable and
quite dependent on weather, particularly temperature. Low temperatures decrease the rate
of plant development and hence the onset and rate of flowering is delayed. Low plant
density results in secondary branching, thus extending the flowering period. If plants are
pruned back when still green, regrowth and a second flush of flowers can be obtained.
Flowers produced on regrowth are typically smaller and less productive than the first
formed flowers (Downey, Klaasen and Stringham, 1980).
The flowers of the Brassica species are regular, bisexual and hypogenous.
The differentiation of the flower proceeds through the successive development of four
free sepals in two whorls, medium and transverse, six stamens, two carpels and four free
diagonally placed petals (Figures 3.2 and 3.35). The flowers have one pair of lateral
stamens with shorter filaments and four median stamens with longer filaments. When the
anthers are a few millimetres in length, the pollen mother cells, after meiosis, give rise to
the tetrads. The pollen grains are 30-40 μm in diameter and have three germination pores.
The sutures of the anthers are introse in the bud stage, but the four long anthers become
extrose as the flower opens (except in the B. rapa Yellow Sarson form where they remain
introse).
Figure 3.35. Typical flower of B. napus
Note: This photo shows the typical four petals with the stigma in the centre surrounded by four median stamens
and a pair of shorter lateral stamens.
Source: Downey, Klaasen and Stringham (1980).
Two functioning nectaries are located at the base of the short stamens and
two non-functional nectaries at the base of the pairs of the long stamens. The anthers
dehisce when the petals completely unfold. The pollen is shed through two longitudinal
slits on the upper side of the anthers. If the weather is warm and dry, nearly all the pollen
is shed the day the flower opens. In the evening the flowers tend to close, approaching a
funnel shape but open again the following morning. On the third day the flower remains
almost closed and the petals and sepals begin to wilt.
Studies on pollen-tube growth indicate that fertilisation is effected within about
24 hours of pollination (Khanna and Chowdhury, 1974). The two carpels (although
flowers on some plants may produce three or four carpels) form a superior ovary with a
“false” septum and two rows of campylotropous ovules. After fertilisation, the ovary
develops into a bivalve silique with a longitudinal septum (Figure 3.2). When the buds
