Some plant species have flowers that open widely and then close slowly over time, resulting in changes in the width of the corolla opening. We examined the adaptive significance of retaining flowers with relatively closed corollas using Gentiana triflora var. japonica, a perennial species with petal movement, in Hakkoda, northern Japan. Under open pollination, floral openness changed with floral age. It increased from days 1-3 after the flower initially opened and decreased thereafter until floral senescence. Hand-pollinated flowers showed a significant reduction in floral openness compared with unpollinated flowers, suggesting that a floral openness reflects its demand for pollination. Pollinators visited inflorescences with many flowers irrespective of their openness, suggesting that the reduction in floral openness does not affect the attractiveness of the inflorescences to pollinators. Within inflorescences, pollinators visited less-pollinated flowers with wider floral openness more frequently and more-pollinated flowers with lower floral openness less frequently, possibly because of the time cost of entering flowers with low openness. This strategy appears adaptive because if all flowers maintain wide floral openness irrespective of their pollination demand, pollinators may visit more-pollinated flowers in an inflorescence and may leave the inflorescence without visiting less-pollinated flowers. Reduction of openness of more-pollinated flowers fits a strategy of enhancing effective pollination within inflorescences.
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