Increased seed set in down slope-facing flowers ofLilium duchartrei

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Abstract

Flower orientation has been considered one aspect of floral attraction. Plants growing on slopes should orientate their flowers facing down slope towards greater open space to enhance reproduction by attracting more pollinators. Flower angle and floral symmetry may affect this pattern; for example, this trend would be overshadowed in vertical/pendent flowers with radial symmetry because the flowers can attract pollinators and provide landing platforms from many directions. We investigated this hypothesis in Lilium duchartrei, a herb with pendent and actinomorphic flowers, in the Hengduan Mountains region of China by measuring flower direction for individuals growing on flat ground and on slopes. We also changed flower direction from facing down to up slope to test the effects on pollinator visitation frequency and subsequent plant reproduction. Plants growing on flat ground orientate their flowers equally towards the four geomagnetic directions, whereas the flowers on individuals growing on slopes preferentially face down slope. This pattern was more pronounced for individuals growing on steeper slopes. There was a positive correlation between slope angle and the seed set of flowers facing down slope (control), but a negative correlation between seed set and flowers facing up slope. The visitation frequency also tended to be higher for control flowers on steeper slopes and lower for those flowers changed to face up slope. Unexpectedly, floral direction was not affected by flower angle or floral symmetry. The results suggest that a down slope orientation of flowers could function to improve pollination in heterogeneous pollination environments.

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