Abstract
Vertical raceme or spike inflorescences that are bee-pollinated tend to present their flowers horizontally. Horizontal presentation of flowers is hypothesized to enhance pollinator recognition and pollination precision, and it may also ensure greater consistency of pollinator movement on inflorescences. We tested the hypotheses using bee-pollinated Corydalis sheareri which has erect inflorescences consisting of flowers with horizontal orientation. We altered the orientation of individual flowers and prepared three types of inflorescences: (i) unmanipulated inflorescences with horizontal-facing flowers, (ii) inflorescences with flowers turned upward, and (iii) inflorescences with flowers turned downward. We compared number of inflorescences approached and visited, number of successive probes within an inflorescence, the direction percentage of vertical movement on inflorescences, efficiency of pollen removal and seed production per inflorescence. Deviation from horizontal orientation decreased both approaches and visits by leafcutter bees and bumble bees to inflorescences. Changes in floral orientation increased the proportion of downward movements by leafcutter bees and decreased the consistency of pollinator movement on inflorescences. In addition, pollen removal per visit and seed production per inflorescence also declined with changes of floral orientation. In conclusion, floral orientation seems more or less optimal as regards bee behavior and pollen transfer for Corydalis sheareri. A horizontal orientation may be under selection of pollinators and co-adapt with other aspects of the inflorescence and floral traits.
Highlights
Flower and inflorescence characteristics are thought to influence pollinator behavior [1,2,3,4,5]
Pollinator recognition: The bumble bees were reported to have a preference for horizontal flowers [16]
Compared with Unmanipulated inflorescences, the occurrence of visits following approaches decreased in manipulated inflorescences for both pollinator types (GLMM, leafcutter bee, Up: b = 2 1.43360.309, z = 24.638, P,0.001; Down: b = 20.69860.275, z = 22.538, P = 0.011; bumble bee, Up: b = 22.45560.669, z = 2 3.669, P,0.001; Down: b = 21.76960.354, z = 24.997, P, 0.001; Unmanipulated is used as the baseline and a negative value of b implies a negative effect of the treatment)
Summary
Flower and inflorescence characteristics are thought to influence pollinator behavior [1,2,3,4,5]. The angle between a flower’s main axis and the horizontal, is thought to affect pollinator attraction [6,7,8,9,10], foraging behavior [10,11,12], and pollen transfer [11,13,14]. Deviation from horizontal may result in decreased pollen transfer These hypotheses predicted the significance of floral orientation on plant reproductive success from different aspects. The effects of horizontal orientation on pollinator recognition and pollination precision have been investigated for relatively few plant taxa pollinated by certain pollinator groups, e.g. syrphid fly [11], hummingbird [13]. Experimental investigations are needed to directly test the effect of floral orientation on the consistency of pollinator movement on inflorescences
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