Significant losses of flowers and pods throughout flowering and subsequent pod development have been found (Mendham et al, 1981; Bilsborrow & Norton, 1984), with pod and seed losses as high as 60%. These losses have been mainly attributed to assimilate shortage at critical developmental stages (eg. shading or high flower light interception in canopy) (Mendham et al, 1981). Seed and pod abortion is hierachical, mainly confined to lower axillary branches and distal sites on earlier racemes (Tayo & Morgan, 1975).
The terminal raceme and uppermost branches carry a large number of pods to maturity, subsequently influencing seed yield potential. Alternatively, late flowering branches produce progressively fewer flowers, most of which abort (Tayo & Morgan, 1975). So reproductive losses from lower racemes don't affect seed yield, whereas losses from the terminal raceme directly correlates with a seed yield reduction (Tommey & Evans, 1992). It is therefore of economic interest to restrict the production of lower branches in order to concentrate fruit production to the terminal raceme and upper branches.
The number of seeds per pod is an important factor to be investigated as it too is normally below its potential (Mendham et al, 1981). This is mainly due to an insufficient assimilate supply during early pod development which leads to seed abortion, and more, seriously pod abortion (Bouttier & Morgan, 1992).
If any part of the developmental process involving pollination, fertilisation and seed development is hindered, the affects on yield can be enormous. We have looked at the early reproductive losses of flowers and pods that strongly influence seed yield, however there are a number of important agronomic traits targetted for improvement (Thompson & Hughes, 1986), such as resistance to, pod shatter (can cause significant seed loss prior to harvesting) (Thompson & Hughes, 1986) disease, the cold (improve winter survival) and lodging.
Pod shatter in B. napus is known to have economic significance in terms of seed loss, although the loss of seed dispersal mechanisms is normally considered one of the first steps towards the domestication of a crop plant. For example, Bilsborrow (1985) reported field losses of 5-10% of seed yield due to shatter and a potential loss of 50% of yield during adverse conditions prior to harvest (Macleod, 1981). Subsequently, both timing and method of harvesting are factors in limiting field losses.
For pod shatter to be limited, harvesting tends to occur prior to complete seed maturity
(asynchronous ripening) which also has the adverse affect of collecting immature seeds. These
seeds don't directly affect seed yield, however they do affect the oil quality because they
contain more moisture and chlorophyll which are undesired traits.
