Pollen load, capsule weight, and seed production in three orchid species

Abstract

Relationships between pollen load, seed production, and capsule weight in orchids are important both theoretically and pragmatically. It has been hypothesized that pollen packets (pollinia) evolved in orchids because of selection to produce packages capable of fertilizing all ovules in an ovary; testing this hypothesis requires information about the relationship between pollen load and seed production. As well, because capsules contain thousands to millions of seeds, an easily measured correlate of seed number, such as capsule weight, would be valuable for studies of reproductive success. We hand-pollinated Cypripedium calceolus, Amerorchis rotundifolia, and Calypso bulbosa with different pollen loads, weighed capsules, and estimated seed number by subsampling from liquid suspension. Pollen load affected seed number in Calypso but had no significant effect in Cypripedium or Amerorchis. Capsule weight was positively associated with seed number in all species, but there was considerable variation in R 2 across species (from 0.40 to 0.85), indicating that it is unwise to assume that capsule weight is a good measure of relative reproductive success. As well, our data suggest that size of the typical unit of pollen deposition rather than that of the entire pollinarium evolved to match ovule number in orchids. Key words: Orchidaceae, pollen load, capsule weight, seed set.