Pollination ecology of Aconitum gymnandrum (Ranunculaceae) at two sites with different altitudes

In this paper, we compared pollination characteristics of Aconitum gymnandrum at two sites respectively with elevations of 2460 and 3200 in in the northeast Qinghai-Tibetan Plateau. This species is an alpine biennial with unique systematic position and floral morphology in the tribe Delphinieae, Ranunculaceae. We found that the floral longevity and male and female phase durations in the high altitude population were significantly longer than those in the low altitude population. Seeds cannot be set through apomixis in this species. Aconitum gymnandrum is highly self-compatible, but autonomous self-pollination within the individual flower is ruled out through a combination of protandry and herkogamy during floral development, suggesting that pollen vectors were indispensable for successful seed sets of A. gymnandrum. Bumblebees are main pollinators of A. gymnandrum in both populations. Within each raceme, pollinators visited flowers at the bottom first, and then spirally moved upwards; however, 3.9% and 2.7% of the visits were downward respectively in the low altitude population and the high altitude population. In addition, 37.7% and 29.3% of the movements occurred between different racemes within the same plant; therefore geitonogamous self-pollination could not be completely avoided. The higher seed sets of artificially pollinated flowers than the intact flowers suggested the existence of pollination limitation in both populations. Visiting frequency of bumblebees at the low altitude was higher than that at the high altitude, but seed sets of the intact flowers in both populations did not differ remarkably, probably due to that the longer duration of the stigmatic receptivity in the high altitude population compensates the decrease of seed sets because of the low visiting frequency of pollinators at this site.