ABSTRACT
Many studies have shown the negative effects of pollen limitation on fruit or seed production in angiosperms (reviewed in Knight et al. 2005). For animal-pollinated plants, the effects of pollen limitation on fruit or seed production may result from lack of suitable pollinators (Bierzychudek 1981), a lack of co-flowering conspecific plants (Ågren 1996) or an abundance of heterospecific pollen (i.e. pollen from a different plant species). Heterospecific pollen deposition can reduce fruit or seed production of the flowering plant being pollinated, especially if the heterospecific pollen arrives before conspecific pollen (i.e. pollen from a different flower or a different plant of the same species) arrives (Galen and Gregory 1989; Caruso and Alfaro 2000). Though heterospecific pollen may not limit the amount of conspecific pollen deposited, it competes for limited stigmatic space
capable of hydrating conspecific grains. This has the same effect of decreasing the conspecific pollen load, in that space becomes less available to conspecific pollen that could potentially fill the stigma, germinate, and fertilize ovules. High conspecific pollen loads, on the other hand, are more likely to initiate fruit production (Bertin 1990) and increase seed production and weight (Winsor et al. 1987; Bertin 1990) in flowering plants.
