Phosphorus pratense (Poaceae) has been shown to reduce pollen germination and seed set of heterospecific Poaceae. Danthonia compressa has a mixed breeding system, ie , both cleistogamous and chasmogamous florets. Previous studies revealed that extracts from 1-5 pollen grains of P. pratense reduce pollen germination and seed set Therefore, we tested whether the presence or absence of pollen from P. pratenseaffected seed set in D. compressa . In 1991, inflorescences of P. pratense were clipped to reduce pollen dispersal in an abandoned hayfield. In an adjacent abandoned hayfield,P. pratense flowered unimpeded. Compared the unclipped versus unclipped fields, the mean number of pollen grains of P. pratense / stigma of D. compresseddecreased from 7.6 to 0.19 while mean seed set / increased from 50% to 76.9%. All of the increased seed set is related to the seed set in chasmogamous florets. In 1992, when P. pratense was allowed to flower in both fields, the average seed set was less than 50%. In a third population of D. compressa , P. pratense was not present, mean seed set was over 76% in both years. Cleistogamy in D. compressaminimum 41.9% in all fields. Hence, there is an effective barrier against allelopathic pollen of P. pratense . The implications of the loss of chasmogamously produced seed are unclear. There were no differences in mean seed size between chasmogamous and cleistogamous florets. Hence fitness can be unaffected. May result in increased susceptibility to seedling, but this hypothesis has been tested. The relative numbers of chasmogamous and cleistogamous florets did not change in 1991-1992, although there were more chasmogamous florets produced by P. compressa at the site where P. pratensedid not exist. Tests of whether pollen allelopathy acts as a selection on the relative numbers of chasmogamous and cleistogamous florets in D. compressa will require long-term field studies.
Tagged inflorescences, collected inflorescences