Abstract
Successive stamen movement directly controls pollen presentation schedules through sequential stamen maturation and changes the extent of herkogamy by altering the positions of sexual organs. However, the implications of such movements in terms of pollination are not well understood. Pollen presentation theory predicts that staggered pollen presentation should be favoured when plants are subject to diminishing returns on pollen transfer. Herkogamy on the other hand, has been interpreted as an adaptive trait that reduces sexual interference in hermaphrodite flowers. In this study, we conducted floral manipulations to determine the function of successive stamen movement in pollen transfer. By artificially manipulating the flowers to present two anthers simultaneously in the floral centre, we attempted to investigate whether changes in the anther presentation strategy affect pollen removal, deposition and the efficiency of pollinators. Compared with the natural treatment, the pollen transfer efficiency of halictid bees decreased significantly when the flowers were manipulated to present two anthers simultaneously. Although the presentation of two anthers simultaneously led to a similar pollen removal rate, there was a significant reduction in pollen deposition on neighbouring stigmas. To evaluate the effect of movement herkogamy on pollen export and deposition and seed set, the flowers were manipulated with or without the movement of stamen bending out from the floral centre. Pollen export decreased significantly when the central anther was moved away from the pistil, and pollen deposition and seed set declined significantly when the five spent anthers were retained on the pistil. Our study provides good support for the pollen presentation theory and provides direct experimental evidence that successive stamen movement could increase pollen transfer efficiency by sequential stamen maturation. Moreover, movement herkogamy promotes pollen export, deposition and seed set, and could therefore be regarded as an effective mechanism to reduce interference between male and female functions.
Highlights
The majority of flowering plants rely on animals for pollination
To assess the successive stamen movement in a protandrous herb Parnassia wightiana (Celastraceae) (Bremer et al 2009), we investigated whether and how staggered pollen presentation affects the pollen transfer efficiency of the pollinators, i.e. the relationship between the amount of pollen removed in a visit and the amount deposited onto the stigma in a subsequent visit
When the flower was at the budding phase, all five immature anthers were aggregated around the pistil with an anther-stigma distance of 1.67 6 0.30 mm
Summary
The majority of flowering plants rely on animals for pollination. The attraction of more floral visitors to participate in the process of pollen transfer and to improve pollination efficiency is one of the most pervasive driving forces in the evolution of floral traits (Faegri and Van der Pijl 1979; Bawa and Beach 1981; Real 1983). Due to the activities of pollinators, pollen loss during the process of pollen transfer is always very high, and normally
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