AOBPreview originally published online on December 19, 2002
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Annals of Botany 91: 311-317, 2003
© 2003 Annals of Botany Company
Floral Biology and Pollination Mechanisms in Two Viola Species—From Nectar to Pollen Flowers?
1 Departamento de Botânica, Universidade Estadual de Campinas, Caixa Postal 6109, Campinas, São Paulo, 13083-970, Brazil
* For correspondence at: Jardim Botânico do Rio de Janeiro, Rua Pacheco Leão 915, Rio de Janeiro – RJ, 22460-030, Brazil. Fax +55 21 25122077, e-mail leandro{at}jbrj.gov.br
Received: 15 August 2002; Returned for revision: 5 October 2002; Accepted: 25 October 2002 Published electronically: 19 December 2002
The genus Viola is represented by four related species in Brazil belonging to section Leptidium, one of the most primitive sections in the genus. Floral biology and pollination by bees were studied in Viola cerasifolia and V. subdimidiata in high-altitude areas in south-eastern Brazil. Flowers are zygomorphic and spurred. The five stamens are arranged in a cuff around the ovary, and pollen is released by means of apical connective projections, which form a cone surrounding the base of the style. The connective projections of the inferior stamens are elongated and curved to form a hook-shaped structure. Nectar-secreting tissue can occur in the basal connective appendages of the inferior stamens, which project into the spur. Flowers of V. subdimidiata secreted a mean volume of 0·14 µl nectar over a 24-h period; approx. 40 % of flowers did not secrete any nectar. The main pollinators of these Viola species are female bees belonging to the genus Anthrenoides (Andrenidae), which search mainly for pollen. These bees seem to be oligolectic and obtain large amounts of pollen from Viola by vibrating the flowers or by moving the hook repeatedly back and forth. Males of Anthrenoides patrol Viola clusters and also feed on nectar, acting as secondary pollinators. The basic floral structure in the genus Viola fits that of ‘nectar flowers’. The uncommon hook-shaped projections, scanty nectar production, and behaviour of pollinators suggest that V. cerasifolia and V. subdimidiata are shifting their reward for pollinators from nectar to pollen. Based on floral morphology, this shift may be widespread in Viola sect. Leptidium.
Key words: Andrenidae, Anthrenoides, buzz pollination, floral biology, melittophily, nectary, oligolectic bees, pollen flowers, Viola, Violaceae.