The phenology of gender in homogamous flowers: temporal change in the residual sex function of flowers of Oil‐seed Rape ( Brassica napus )

1. We investigated the phenology of the male and female sexual functions in flowers of Oil‐seed Rape ( Brassica napus ) that were exposed to pollinators in an experimental garden. The female ‘residual sex function’ (RSF) of flowers was measured by hand‐pollinating with genetically marked pollen in order to determine the proportion of ovules that remained available for fertilization by incoming pollen. Male RSF was measured by estimating the proportion of pollen grains that remained in dehiscing anthers. 2. Following flower opening, an average flower’s male and female sexual functions each required for completion c. 13 h of exposure to pollinators. One hour after opening, c. 50% of a flower’s ovules were unavailable to incoming pollen whereas only c. 10% of pollen was removed. Therefore, a flower’s sexual function was predominantly female for the first hour and predominantly male thereafter. 3. We found a fairly close correspondence between the proportion of the stigma covered with pollen and the depletion of female RSF. 4. On average, floral senescence occurred after c. 14·5 h of exposure to pollinators. Our observations are fairly consistent with a simple, economic model of optimal senescence time because the flowers remained open for approximately the same length of time as was necessary for the completion of their sexual functions. 5. A flower’s senescence was hastened when pollen was removed from the anthers, but not when pollen was deposited on the stigma. 6. When flowers were either left undisturbed or hand‐pollinated, senescence occurred after c. 24 h. Pollen removal caused senescence after c. 13 h, which also approximated the longevity of flowers in the experimental garden. Apparently, pollen removal governed floral longevity under field conditions.