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 floweringin Brassica napus‘Westar’, a series of 30 pots wasestablished that spanned a range of one–96 plants perpot. In each pot, the following attributes of individual flowerswere quantified: petal length; petal width; stamen length; pistillength; pollen production; diameter of pollen grains; and nectarproduction. Certain plants contained a transgenic marker thatenabled the vigour of their pollen to be quantified by screeningthe progeny from post-pollination trials against conventionalmale competitors. Plant size was strongly affected by plantdensity; plants in the lowest density pots comprised ten-timesmore dry biomass than individuals in the highest density pots.However, none of the attributes of individual flowers variedwith density. In contrast, the number of flowers per plant declinedrapidly with density. In the face of resource scarcity, theplants apparently conserved flower size rather than flower number.There was no difference in the response to density between transgenicand conventional plants, but transgenic plants obtained morefertilizations than expected in post-pollination competitionagainst conventional competitors. A separate experiment demonstratedthat B. napus is, nevertheless, capable of plasticity in certainflower attributes (petal size, stamen length) in response todefoliation. Explanations for the stability of flower size relativeto flower number are discussed. Copyright 2001 Annals of BotanyCompany Brassica napus, defoliation, density effects, flowers, flower size, intraspecific competition, nectar production, oilseed rape, pollination, transgenic plants