Does interference competition among pollen grains occur in wild radish?

Interest in the possibility of sexual selection in plants has focused primarily on competition among pollen donors based on the speed of pollen-tube growth. However, when pollen arrives on stigmas, there is the opportunity for both races for access to ovules (exploitation competition) and interference with the germination and growth of pollen from other donors (interference competition). We considered whether this second form of competition might occur among pollen grains of wild radish in two experiments. In the first, interference likely occurred because the amount of pollen germination was less in mixed-donor than in single-donor pollinations. This result was duplicated in a second experiment, which also showed that interference occurred only when pollen grains from different donors were in direct contact with each other. In addition, in the second experiment, the opportunity for interference affected the frequency of seeds sired by different pollen donors. Because pollen loads are often mixed in nature, interference competition among pollen grains may be important in the ecology and evolution of plant reproduction.