Seed production of a species may vary among fruits, individuals and populations, but few studies have investigated seed production of all individuals in various populations to assess factors limiting plant fecundity. The hair-like styles of Pulsatilla cernua, an insect-pollinated, spring flowering endangered herb in Japan, will largely elongate after pollination. We found that style length is an indicator of seed setting. This finding permits us to investigate seed production per flower of all individuals in four populations and a large number of individuals in two large populations. Seed set (seeds to ovules ratio) varied widely among fruits (0-100%), individuals (0-97%) and populations (8-46%). Seed set was pollen limited in each population in that a proportion of aggregate fruits did not produce a seed. We tested the effects of plant size and distance from the nearest neighbor within population on seed set. Both were found a weak correlation in only one of six populations, indicating large plants with multi-flowered display and closed plants rarely increased seed set in P. cernua. In these populations major pollinators were bees and their activities only occurred in sunshine time. Insect irregular visitation caused by unfavorable weather may confuse these correlations. Pollination experiments demonstrated that stigma receptivity lasted six days but decreased with age in this protogynous and self-compatible species, however, seed production depended on insect visits even through selfing. We suggested weather condition limited insect activity and ultimately limited seed production in this spring flower.

Key words: Plant size, pollen limitation, pollinator activities, Pulsatilla cernua, seed set, stigma receptivity, weather