Influences of Floral Variation on Pollen Removal and Seed Production in Wild Radish

Abstract

The effects of corolla size, pollen production, and pollinator visitation frequency on pollen removal (an estimate of male reproductive success) and pollen deposition and seed production (female success) were studied in wild radish (Raphanus sativus, Brassicaceae) in 1987. Wild radish exhibits variation in corolla size, petal coloration, and anther size. A previous study (Stanton and Preston 1988) found no correlation between corolla size and estimates of female success, suggesting that flower size may be selected upon through variation in male reproductive success. For flowers receiving a single honey bee visit, corolla size was positively correlated with pollen removal, but not with pollen deposition on stigmas or seed production. Flowers receiving an average of 3.7 honey bee visits (in the 1987 study) also showed a correlation between corolla size and pollen removal (seed production was not monitored in this study). In contrast, when flowers were visited an average of 5.7 times by honey bees (the 1988 study), there was no significant relationship between corolla size and either pollen removal or estimates of female reproductive success (pollen deposition on stigmas and seed production). Honey bees demonstrated a preference for large flowers, either by significantly overvisiting large flowers or by visiting the large flowers, before visiting smaller flowers. Under natural conditions, most radish flowers receive numerous visits, but even with frequent visitation, at least 20% of the pollen a flower produced was not removed. At the same time, flowers with greater pollen production had a larger proportion of pollen removed. Pollen removal was a positive decelerating function of honey bee visitation frequency. The number of pollen grains removed per visit declined with increasing honey bee visits. The male fitness function (pollen removal plotted against pollen production) was positive and decelerating in two experiments; positive and linear in one study. Large flowers may have greater male success than small flowers by virtue of their greater pollen production, their greater pollinator visitation frequency, and their earlier pollinator visitation. If selection is acting on floral traits in wild radish through variation in male success, increases in both corolla size and pollen production are likely to be selected for.