Distyly has been interpreted as a mechanism that favors cross-fertilization. In this research we describe floral attributes and ancillary floral polymorphisms typically associated to heterostylous plants in Palicourea demissa (Rubiaceae), a distylous shrub of the Venezuelan Andes cloud forests. A hand-pollination experiment was done to evaluate self- and intramorph incompatibility and female reproductive output in both floral morphs. The studied population was morphologically distylous but morph differences in most ancillary floral polymorphisms and reciprocity of the sexual organ heights were found. The floral morphs were self-incompatible and did not differ in fruit set under controlled cross-pollination conditions, but at the population level they exhibited imperfect reciprocal herkogamy. Fruits and seeds of short-styled plants were larger than those of long-styled plants and fruit set was higher in short-styled plants under natural conditions, suggesting a higher reproductive potential among short-styled plants. Given the 1:1 morph ratio within the studied population, further evidence is needed to determine the influence of floral visitors and seed dispersers in the expression of heterostyly in P. demissa under natural conditions.

Abstract:We investigated morph differences in attributes that contribute to rewarding floral visitors of the distylous shrub Palicourea demissa at La Mucuy cloud forest in Venezuela. In both morphs, we measured nectar production from flowers subjected to repeated removals at 2-h intervals (10 plants per morph) and flowers that accumulated nectar for 24 h (10 plants per morph). In both cases, floral visitors were excluded. In addition, we quantified nectar availability (30 plants per morph), floral visitation (10–12 plants per morph) and legitimate pollination (30 plants per morph) throughout the day. We explored morph differences in the variables mentioned above using analyses of variance, and the effects of nectar variation on floral visitation and legitimate pollination using regression models. We observed 1205 floral visits, grouped into six hummingbird (94.7%) and three insect species (5.3%), across observations (264 h). Coeligena torquata was the most frequent floral visitor (34%) in both morphs (1.4–1.7 visits per plant h−1). Nectar production and availability, and visitation rate were similar between morphs. Visitation rate and legitimate pollen deposition increased with the nectar production in both morphs, but levels of legitimate pollination were higher on short-styled flowers than long-styled flowers. These results show that short-styled and long-styled flowers reward floral visitors equally, but frequency and foraging behaviour of long-billed pollinators can promote asymmetrical legitimate pollination.

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