Abstract: Evolution of herkogamy has been ascribed to preventing autonomous selfing within flowers, but this argument has received challenges due to the negative effects of sexual interference on reproductive success of plant species. We examined the effects of the two contrasting modes of herkogamy, detailed by the different patterns of style movement, on the foraging time of pollinators on flowers, pollen removal, and deposition in two Chamerion species. Our results suggested that the duration of foraging time of bumblebees was generally higher in C. angustifolium (L.) Holub than in C. conspersum (Hausskn.) Holub during the male-phase of flowers, but the reverse during the female-phase of flowers. As a result, stigma interfered with pollen removal more in C. conspersum than in C. angustifolium, based on examinations of pollen removal after one visit by a pollinator. However, this negative effect on total pollen removal was covered by the high pollinator diversity and the possible high visitation rate in the study sites. In contrast, we found that almost no difference existed in pollen deposition rate after one visit of a pollinator between the two Chamerion species, and emasculation did not increase the total pollen deposition in either species, indicating that anthers did not interfere with pollen deposition in the two Chamerion species. Our results suggested that, compared with C. conspersum, the derived mode of herkogamy in C. angustifolium reduced interference of stigmas in pollen removal. This might contribute to the wide distribution of C. angustifolium, to some degree, in environments of pollinator scarcity and low activity in newly colonized habitats.

Key words: alpine plant, bumblebee, Chamerion angustifolium, Chamerion conspersum, foraging time, pollen export/deposition, southeastern Tibet.