Attributes of Individual Flowers of Brassica napus L. are Affected by Defoliation but not by Intraspecific Competition

To investigate the effect of intraspecific competition on flowering in Brassica napus ‘Westar’, a series of 30 pots was established that spanned a range of one–96 plants per pot. In each pot, the following attributes of individual flowers were quantified: petal length; petal width; stamen length; pistil length; pollen production; diameter of pollen grains; and nectar production. Certain plants contained a transgenic marker that enabled the vigour of their pollen to be quantified by screening the progeny from post-pollination trials against conventional male competitors. Plant size was strongly affected by plant density; plants in the lowest density pots comprised ten-times more dry biomass than individuals in the highest density pots. However, none of the attributes of individual flowers varied with density. In contrast, the number of flowers per plant declined rapidly with density. In the face of resource scarcity, the plants apparently conserved flower size rather than flower number. There was no difference in the response to density between transgenic and conventional plants, but transgenic plants obtained more fertilizations than expected in post-pollination competition against conventional competitors. A separate experiment demonstrated that B. napus is, nevertheless, capable of plasticity in certain flower attributes (petal size, stamen length) in response to defoliation. Explanations for the stability of flower size relative to flower number are discussed.