FLOWER HELIOTROPISM IN AN ALPINE POPULATION OF RANUNCULUS-ACRIS (RANUNCULACEAE) - EFFECTS ON FLOWER TEMPERATURE, INSECT VISITATION, AND SEED PRODUCTION

Some plants in arctic and alpine habitats have heliotropic flowers tha t track the sun. This results in a heating of the flower's interior, w hich may improve the possibilities for insect pollination and seed pro duction. Here, I examine whether flower heliotropism in an alpine popu lation of the self-incompatible Ranunculus acris L. (Ranunculaceae) en hances pollinator visitation and seed production. Flowers of Ranunculu s acris tracked the sun during the day. Tracking accuracy was greatest during the middle of the day. The temperature elevation in flowers wa s negatively correlated with the flower's angle of deviation from the sun. Despite the increased temperature, insects did not discriminate a mong flowers on the basis of their angle of deviation from the sun, or tend to stay longer in the flowers aligned closest towards the sun. A tethering experiment was conducted on three groups of plants flowerin g at different times in the 1993 season and on one group the following season. Manipulation plants were constrained not to track the sun, wh ereas control plants tracked the sun naturally. Solar tracking had no effect on seed:ovule ratio, seed mass, or abortion rate in any of the groups. There is probably a very narrow range of weather conditions (c old, sunny, and calm) where flower heliotropism may enhance visitation rate to flowers and their seed production.