Most of the oil in canola seed is contained in the cotyledons and so an increase in cotyledon size may lead to increased oil concentration in the whole seed, provided compensatory changes in non-oil bearing tissues are minimised. In addition, in in vitro studies, light has been shown to affect fatty acid synthesis. In two glasshouse experiments, we manipulated seed size in 3 cultivars of canola by increasing the source–sink ratio through removal of lateral inflorescences, restricting the plants to flowering on the main axis. We manipulated the ability of the growing seed to use light for the synthesis of fatty acids for oil by shading the siliques at different stages of seed development. The growth of ovules and embryos in the first experiment was assessed by evaluating changes in the projected area of the organs during growth and the final seed weight. We examined the pattern of organ development in the embryo by fitting appropriate curves and comparing the effect of the treatments on the coefficients.

Pruning axillary branches increased seed weight by 14–43% but did not change the pattern of development of the cotyledons or radicle in the seed. Embryo growth over time was sigmoid in form with the most rapid growth occurring 12–27 days after flowering (daf). The removal of axillary branches and inflorescences reduced oil concentration in the larger seeds by 2.6–4.5% but only in one experiment.

Light was excluded from the siliques from 2, 10, or 30 daf or not at all. Excluding light from 2 or 10 daf reduced ovule weight at maturity by 63% and excluding light from 30 daf reduced ovule weight by 20%. Excluding light reduced the number of seeds per silique by up to 90%, especially when excluded from 2 daf. Excluding light slightly reduced oil concentration, suggesting that, provided maternal substrates are available to the seed, it can use these to obtain energy for oil synthesis, even in the dark. We conclude that manipulation of the canopy of canola to change the source–sink ratio or the distribution of light within the canopy may have large effects on seed size, but little effect on the accumulation of oil in the seeds. This conclusion assumes that the relationships found in our glasshouse experiments apply in the field.