Pod Development

Most of the oilseed rape grown in the UK is winter sown and belongs to the genus Brassica, species napus. Oilseed rape has a tap root system supporting a well-branched stem (0.5-2m tall). Axillary branches and the main stem terminate in racemes which bear yellow flowers. The dark green lyrate leaves and the stems are photosynthetic prior to flowering. In oilseed rape, flowering initiates at the main terminal raceme, and progresses to the primary branches. Flowering also depends on their branch position as those at the base of branches open first, then progressively towards the tips. After the development of the flowering racemes, they senesce rapidly (Tayo and Morgan, 1975; Bilsborrow, 1985) to leave a virtually naked stem, concentrating resources into seed production.

Nutrient competition from the earlier set, lower pods results in the abortion of the youngest pods, flowers and buds. This mechanism therefore enables the plant to limit its pod production to a level it can support. The pod canopy is another factor that shows plasticity during the early months and during this period the plant is indeterminate, retaining its ability to stimulate new reproductive racemes. For example, following severe lodging new racemes can be initiated to produce fertile pods, however this increases the distribution of pod maturity and is therefore agronomically undesirable (Daniels et al, 1986).

Tayo & Morgan (1979) believed that a seed must be present for pod expansion to proceed. This was supported by Pechan et al (1980) who demonstrated that pod wall growth only occurred adjacent to developing seeds and following seed abortion the wall contracts. Pod expansion is facilitated by assimilates from other parts of the parent plant, however this dependency is short lived, after which the pod wall produces the necessary assimilates for further seed growth (Morgan, 1974). Carbon fixation is essential for seed growth and it has been shown that pods become the primary sites of photosynthesis as the post-anthesis leaf area index declines (Tayo & Morgan, 1975). In situations of high assimilate competition between the developing structures and the newly formed pods, losses can occur of up to 25% of the 15,000/m² potential pods (Norton et al, 1991).

Bilsborrow & Norton (1984) described four phases of growth and development (seed-to-seed cycle) of oilseed rape:

Vegetative.
Development of reproductive framework & flowering.
Development of the hulls of the pod (maximum pod area and hull weight).
Seed development.

The pods develop to form a dense canopy in the upper layers of the crop, but only when the pods as a whole have reached maturity are assimilates available purely for seed development (Meakin, 1988). This process occurs 20-30 days after anthesis (DAA) and is coupled with a rapid decline in chlorophyll levels, followed by a decrease in pod moisture content. At 60 DAA the pods senesce and yellow and by 80 DAA they are completely dry (Meakin, 1988).

The graphs below illustrate the rapid growth in length and mass of pods and the increase in seed mass after anthesis:

Pod development in B. napus can be separated into three stages:

0-20 DAA. Newly formed silique reach their full length of up to 10cm, having differentiated into 2 seed containing carpels separated longitudinally by an internal false septum and a vascular replar region between the valve edges. Seeds begin to grow only as the pods near full size (Hocking & Mason, 1993).
10-20 DAA the replar region differentiates into replar cells, large valve edge cells (VEC) and a region 1-3 cells wide comprising the dehiscence zone (DZ) (Meakin & Roberts, 1990).
20-50 DAA. Pod elongation completed and secondary cell wall material deposited in VEC, and further lignification of replar cells (NB. DZ cells don't undergo cell wall thickening).
From 40 DAA DZ cells show progressive loss of organelles, later becoming separated due to enzymic hydrolysis of the middle lamella (Meakin & Roberts, 1990).
50-70 DAA. Lignified replar cells senesce to create tensile and torsional forces within the desiccated pod which eventually causes it to shatter and release the mature seeds. Dehiscence zones develop at carpel margins, adjacent to septum running the length of the silique.