Pollen limitation, reproductive success and flowering frequency in single-flowered plants

Flowering plants exhibit striking interspecific and intraspecific variation in flower number, which strongly influences the reproductive success of animal-pollinated plants. However, the reproductive consequences of producing a single flower are poorly understood. Here, we test if plants producing a single flower have a reproductive disadvantage compared with plants producing multiple flowers by combining field investigation of five deceptive orchids and a survey of published literature. Pollen limitation was estimated by comparing fruit production between hand pollination and open pollination. Flowering frequency was monitored over years to assess the potential advantage of producing a single flower. Both single- and multiple-flowered species suffered strong pollen limitation. However, single-flowered species had significantly lower fruit set and produced fewer seeds per individual, although they had a significantly higher pollen removal than species with multiple flowers. A phylogenetically independent contrast of 28 species representing four of the five subfamilies of Orchidaceae revealed that fruit set was significantly positively associated with flower number. Both pollen removal and fruit set had a positive relationship with flower number within the multiple-flowered species. Synthesis. Current data and phylogenetically independent contrast support the hypothesis that producing a single flower has reproductive disadvantages. Single-flowered species may compensate for low female success through high flowering frequency over years. This study provides insight into costs and benefits of producing a single flower in deceptive orchids.