A Brassica napus line that produces flowers with virtually no petals (Apetalous) was compared for its physiological significance with Marnoo, a related cultivar bearing conventional yellow petals. Experiments between 1984 and 1987 used different growth regulators, plant population densities and irrigation treatments. Apetalous allowed 30 % more short-wave radiation to reach the base of the inflorescence, and reflected only half as much radiation as Marnoo at peak flowering. This allowed leaves to persist and remain active for longer. Apetalous then consistently retained more seeds/pod, each of greater mean weight, at all heights within the crop canopy, resulting in substantially greater yields than Marnoo in most instances. At the highest population density, it retained productive secondary branches, whereas Marnoo had none. Irrigation after flowering increased the yields of both lines by allowing more seeds/pod to be retained, particularly at lower heights in the canopy. In Apetalous, however, even without irrigation, yields remained high. Apetalous does have larger flower parts, other than petals, which could contribute to the larger pods and seeds. This, combined with lower plant populations than Marnoo in some experiments, could have explained some of the effects seen, but the characteristic is worthy of further study in a range of genetic backgrounds and environments.