Prickly pear stamen movement and the bee's route to pollen

Two taxa, historical names

Schlindwein and Wittmann studied Opuntia megapotamica var. megapotamica (in the paper, O. brunneogemmia) and O. retrorsa (in the paper, O. viridirubra) in southern Brazil. Observations from 1991–1994 covered flower opening, bee visits, stigma contact, pollen loads and seed set. The current name assignments follow the POWO treatment. [Schlindwein and Wittmann 1997][2-WWW] [2-WWW]

Nastic movements inside the flower

Stamens responded to mechanical stimulation by bending towards the style, regardless of the side from which they were touched. This movement is termed thigmonasty. Return towards the initial position took around 10–20 minutes, and the strongest response was observed in the first hours of flower activity. [Schlindwein and Wittmann 1997]

Stamens differed in length. The longer outer stamens formed a layer over the shorter inner ones after bending. Much of the pollen lay deeper, beneath this covering. How an insect entered the flower therefore determined which part of the pollen resource it could reach. [Schlindwein and Wittmann 1997]

Behaviour of specialised bees

Female bees collecting prickly pear pollen entered between the style and the surrounding stamens. They reached the deep anther layer and nectar secreted in a recess at the pistil base. In moving this way, they often touched the stigma and later transferred pollen during flights between plants. [Schlindwein and Wittmann 1997]

The authors identified female Ptilothrix fructifera, Lithurgus rufiventris and Cephalocolletes rugata among the particularly effective visitors. Pollen loads were analysed microscopically, and behaviour was recorded. Here, oligolecty means restricting pollen collection to a particular plant group. Its significance is apparent in the combination of feeding fidelity and the manner of flower contact. [Schlindwein and Wittmann 1997]

From a visit to pollination

A total of 48 bee species was recorded. Their abundance at flowers and importance in pollination did not follow the same pattern. Some insects collected pollen from the readily accessible layer, some rarely touched the stigma, and others spent a long time visiting successive flowers on the same plant. The entire feeding sequence therefore determined transfer of pollen from another plant. [Schlindwein and Wittmann 1997]

Honeybees were observed making repeated visits to one plant, among other behaviours. This can provide pollen transport between flowers of the same individual while contributing less to movement between plants. In the system analysed, specialised females more often combined access to internal anthers with a favourable flight route. [Schlindwein and Wittmann 1997]

The result seen in seeds

Hand self-pollination produced seeds in both taxa. However, their number was lower than after bee pollination, while hand cross-pollination achieved a result similar to open flowers. Permanently isolated flowers without manipulation produced very few seeds. Stamen movement, pollen access and insect visits thus form successive, measurable stages of reproduction. [Schlindwein and Wittmann 1997]