Factors affecting low seed: ovule ratios in a spring woodland herb, Trillium grandiflorum (Melanthiaceae)

Female fertility in flowering plants is commonly observed to be submaximal. The fraction of ovules in a flower that produce seeds is commonly less than 1.00, particularly in outcrossing species. Here we investigate through controlled pollinations factors that may contribute toward low seed : ovule ratios in Trillium grandiflorum, a self-incompatible woodland herb from eastern North America. Reduced fruit and seed set in open-pollinated plants in comparison with hand cross-pollinated plants demonstrated that pollen limitation was a contributing factor to low female fertility. However, seed : ovule ratios of hand cross-pollinated flowers averaged only 0.66. Experimental pollinations involving the manipulation of pollen age, the number of pollen donors, the timing of pollination, and the application of self-pollen before cross-pollen each had no effect on fertility. This indicated that aspects of pollen delivery were not responsible for low seed : ovule ratios and that resources might influence variation in seed : ovule ratios. An analysis of the relation between plant size and seed set in hand cross-pollinated plants provided evidence that resources limited fertility when ovules of T. grandiflorum were not pollen limited. We discuss these results in the context of the stochastic pollination environment that characterizes the early spring flowering period of T. grandiflorum.