Polinização de espécies de Blumenbachia Schrad. (Loasaceae): oferta de recursos florais e comportamento de forrageio de polinizadores

Abstract

The interactions between angiosperms and their pollinators are characterised by morphological, functional, phenological and behavioural attributes of the species involved, varying widely in degree of specialisation/generalisation. Plant species with complex floral morphology, in which floral resources are hidden or gradually offered to flower visitors, require specialised floral handling, which can restrict the guild of pollinators and result in specialised plant-pollinator interactions. Melittophilous species of Loasoideae (Loasaceae) share such exclusive and complex flower morphology. The androecium is organized in staminode complexes alternating with clusters of fertile stamens. Each staminode complex consists of a conspicuous concave nectar scale, opposite to two free staminodes. The nectar is stored at the base of the nectar scales and free staminodes. The flowers are protandrous and release pollen gradually through successive stamen movements to the center of the flower from an initial position, hidden in naviculate petals. The movements occur autonomously or after mechanical stimulation in the nectar scales (thigmonasty) by pollinators. After all the stamens have moved, the pistillate phase starts, the stigma grows and becomes prominent in the flower centre. To increase the knowledge about the interactions between species of Loasoideae and their pollinators, I studied the pollination of two species of Blumenbachia, B. insignis and B. catharinensis, focusing on the dynamics of pollen and nectar presentation, the reproductive systems of the plants and the foraging behaviours of the pollinators. Blumenbachia insignis is pollinated exclusively by Bicolletes indigoticus (Colletidae) a species of oligolectic bee. The partitioned offer of pollen and nectar by the flowers, combined with the floral fidelity of the females of B. indigoticus, assures them an exclusive supply of floral resources. The specialized foraging of the bees, in turn, guarantees cross-pollen flow in the population of Blumenbachia insignis. To understand the foraging strategy of the bees, I have experimentally manipulated the availability of nectar in the flowers and carried out experiments with chemical extracts of conspecific females. The results revealed that the females of B. indigoticus optimize the search for nectar by concentrating the visits on flowers with accumulated nectar. In addition, females rejected recently visited flowers and flowers in which chemical extracts form conspecific females were added. In Blumenbachia catharinensis we found a new species of oligolectic bee as the main floral visitor (Rhophitulus ater, Andrenidae), which was described. The relationship between B. catharinensis and R. ater was revealed as one of the rare cases in which an oligolectic bee species causes a deleterious effect on the reproduction of its host plant species. Females of R. ater constantly inspect the flowers of B. catharinensis whether they present a new moved stamen in the center of the flower and collect, in this way, almost all the released pollen. Moreover, they remove pollen from already deposited on the stigmas in flowers of the pistillate phase. The results achieved contribute to the general understanding of interactions between plants and pollinators and reinforce that the pollination systems of Loasoideae species are predominantly specialised.