Adaptive nature of floral nectar production rate for the hummingbird-pollinated plant Ipomopsis aggregata

Nectar production is thought to be an adaptation that increases pollination success by encouraging pollinator visitation. I investigated this hypothesis for the hermaphroditic plant Ipomopsis aggregata.; Plants with naturally high nectar production rates did not receive more pollen, but rather acted as males, contributing more pollen to other plants (as estimated by the dispersal of fluorescent dyes). Experimental nectar addition also produced a significant increase in male function and no effect on female function.; Path analysis indicated that plants producing more nectar not only were visited more often by hummingbirds, but (in contrast to the pollination success results above) also turned a larger proportion of their flowers into fruits. This probably indicates the importance of post-pollination events in determining overall plant reproductive success by affecting fruit production independent of pollination events.; The pollination success results are consistent with measurements of the effects of pollinator behavior (probe duration and probe number per flower) on pollen receipt, pollen removal, and dye dispersal for single flowers probed by hummingbirds in an aviary. Increasing floral nectar volume from 1 to 5 microlitres significantly increased probe duration, but had no effect on pollen deposition on stigmas, on pollen removal from anthers, or on number of fluorescent dye particles exported to other flowers. In contrast, these same estimates increased significantly with the number of times a hummingbird probed a flower. The opportunity for increased female function success for plants with high nectar production rates is probably not realized in the field because of low visitation rates, and because stigmas frequently close after one floral probe.; The concordance of results across these observational and experimental studies indicates that nectar production does affect pollination success, but that the effects are not equal for each sexual function. For Ipomopsis aggregata, nectar production appears to have its primary effect on male function pollination success.