Could microechinate orbicules be related to the release of pollen in anemophilous and 'buzz pollination' species?

The function of orbicules has been a matter of speculation for a long time and until now no satisfactory answer has been put forward. We propose two hypotheses that could contribute to the elucidation of their function: (1) that anemophilous and 'buzz pollination' species have microechinate orbicules; and (2) that microechinate orbicules are advantageous for pollen release in both pollination modes. To test the first hypothesis, orbicule morphology of species in which the pollen is released by anther shaking (20 anemophilous and three with buzz-pollination) was analysed. We also conducted a literature review, noting the orbicule surface type and the presence of floral traits suggesting that pollen is released by anther shaking. Ninety-two percent of the species with microechinate orbicules are anemophilous or present 'buzz pollination'. Orbicules without spinules are present in species that lack floral traits linked to anther shaking for pollen release. To test the second hypothesis, a computational simulation that reveals the electric field produced by electrostatically charged orbicules was used. Results showed that the field is increased at the tip of each orbicule spinule. Therefore, an anther loculus surface coated by pointed structures increases the repulsion force of the pollen grains. This is an advantage when pollen is released by shaking the anther. Orbicules are small bodies coating the interior of the anther locule that present the same reactions than the exine of pollen grains. Although many speculations were made trying to understand the orbicules role, none of them could be confirmed. We found that anemophilous and 'buzz pollination' species have microechinate orbicules and that the anther loculus surface coated by pointed structures increases the repulsion force of the pollen grains. This is an advantage when pollen is released by shaking the anther. [ABSTRACT FROM AUTHOR]