Field testing for pollen allelopathy. A review.

Abstract

Field testing for pollen allelopathy comprises several approaches. In vitro tests must be done first to establish the existence of a pollen allelopathic effect. The number of pollen grains required to elicit an in vitro pollen allelopathic effect then is compared to observations of the number of pollen grains of pollen allelopathic species adhering to stigmas of heterospecifics. In animal-pollinated species, the amount of potential heterospecifics may first be assessed by observing pollinator behavior, but actual enumeration of grains is necessary. In all species, enumeration of the amount of heterospecific pollen transfer requires that stigmas be clipped before they senescence, carefully preserved in small test tubes, and assessed immediately thereafter with aid of a compound microscope. Stigmas may be examined for relative amounts of allelopathic and conspecific pollen to determine if the number of conspecific pollen tubes declines hyperbolically with increasing amounts of allelopathic pollen. Enumerating the number of heterospecific pollen grains on stigmas is used to support testing of the ecological impact of pollen allelopathy by direct manipulation. In habitats (e.g., old fields surrounded by forest tracts) where atmospheric conditions prevent export or import of other pollen from surrounding areas, the influorescences of a pollen allelopathic species can be clipped within a large area and the subsequent effect on seed set of target species can be assessed. The hypothesis that pollen allelopathy causes an ecological impact (reduced sexual reproductive success) is not rejected under two conditions. An absence of pollen from the allelopathic species must result in an increase in seed set in target species and the amount of pollen transferred from allelopathic species to heterospecifics is similar to that causing in vitro reductions in sexual reproduction in the targets. This experimental approach must be repeated over several sites and years in order to provide an unequivocal test for ecological effects of pollen allelopathy. This methodological approach has allowed me to demonstrate that pollen allelopathy is an important ecological interaction between Hieracium pratense and five sympatric Asteraceae species and between Phleum pratense and three sympatric Poaceae species.