Factors affecting the density of Brassica napus seeds

Brassica napus seed is composed of low density oil (0.92 g.cm(-3)) and higher density solids (1.3-1.45 g.cm(-3)). Seed buoyant density may potentially be used to determine seed oil content and to separate seeds with different oil contents, however, we have found that seeds with the lowest buoyant density had lower than expected oil contents. It is proposed that the low oil content observed in the lowest density seed is a function of air gaps or pockets within the seed coat with sufficient volume to disrupt the linear relationship between oil content and density. The air gaps make density-based selection of seeds for oil content more difficult. Seed solid and liquid components are essentially incompressible with increased hydrostatic pressure but air gaps in seeds may be compressed or filled if density sorting is conducted in a pressurised apparatus. Experimental evidence shows that compression of air pockets with hydrostatic pressure or filling of air pockets with solvent simplifies the relationship between seed density and oil Content. In spite of applied pressure it was observed that pressure was not sufficient to eliminate the buoyancy of all seeds with air pockets. Seeds containing air pockets may have been immature or improperly filled during maturation. Interrupted maturation, premature maturation or desiccation may potentially interrupt storage compound accumulation and lead to morphological changes. Chlorophyll fluorescence was used in this study as an indicator of seed maturity. An inverse relationship between chlorophyll content and oil content was observed, but was not observed for chlorophyll content and density. This relationship between oil content, chlorophyll content and density suggests that the lowest and highest density seed groups were enriched in immature seed.