Variation in fruit set within and among natural populations of the self-incompatible herb Centaurea scabiosa (Asteraceae)

Various factors known to affect fruit set in self-incompatible plants were studied in 11 populations of Centaurea scabiosa. The importance of genetic variation was estimated using isozymes. A low overall genetic differentiation among populations suggested high levels of either current or historical gene flow. Large variation in fruit set was found among populations, among plants within populations, and among flower heads within plants. Most of this variation was explained by differences among flower heads within plants. Within the range in population size studied, there was no indication that plants growing in the smaller populations had lower fruit set than plants in larger ones. A significant positive correlation between genetic variation and population reproductive success was found. Pollinator visitation rate was very high and dominated by bumblebees. It was rendered unlikely that reproduction was limited by pollinator availability. It was even suggested that a very high visitation rate might have negative effects on fruit set in some populations due to an increased proportion of geitonogamous pollination resulting in stigma clogging with non-compatible pollen. The number of flower heads produced by individual plants did not affect fruit set in individual flower heads.