Low legitimate pollen flow in distylic Turnera hermannioides (Passifloraceae) and its consequences on fruit and seed set

Turnera hermannioides is a ruderal distylic subshrub, native to NE-Brazil. In the Catimbau National Park, situated within the semi-arid Caatinga region, we studied the pollination ecology of this species, emphasizing (1) effective pollinators; (2) characteristics of short- and long-styled flowers; (3) intra- and intermorph pollen flow; and (4) fruit and seed set. Short and long-styled morphs differ in pollen size and ornamentation, stigmatic surface, style and stamen length and nectar production. The flowers are obligate intermorphic outcrossers and depend on animals for pollination. The flowers of T. hermannioides attracted insect visitors of 25 species, among them butterflies, beetles, but mainly bees. Polylectic bees, such as Apis mellifera, stingless bees, and solitary Callonychium brasiliense were the most frequent visitors and the principal pollinators. The frequency of visits, however, was very low (on average 1.9 visits/flower/day), resulting in a low and unbalanced legitimate pollen flow, i.e. a much lower number of pollen grains from short-styled flowers reaching stigmas of long-styles than vice versa, which in turn compromised fruit and seed set. Surprisingly, Protomeliturga catimbaui and P. turnerae, both oligolectic on flowers of the Turneraceae-clade of the Passifloraceae s.l., were very rare flower visitors and did not contribute significantly to the pollination of Turnera hermannioides. We discuss how the low visitation rate and the inefficiency of pollinators (i.e. inefficiency in promoting legitimate pollination) compromise pollen flow and, consequently, fruit and seed set in this species.