Cleistogamy limits reduction in seed set inDanthonia compressa(Poaceae) caused by allelopathic pollen fromPhleum pratense(Poaceae)

Abstract

:Allelopathic pollen of Phleum pratense (Poaceae) has been shown to reduce pollen germination and seed set of heterospecific Poaceae. Danthonia compressa has a mixed breeding system, i.e.,both cleistogamous and chasmogamous florets. Previous studies revealed that extracts from 1-5 pollen grains of P. pratense reduce pollen germination and seed set although direct field evidence was lacking. Therefore, we tested whether the presence or absence of pollen from P. pratense affected 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. Comparing the clipped versus unclipped fields, the mean number of pollen grains of P. pratense/stigma of D. compressa decreased from 7.6 to 0.19 while mean seed set/culm increased from 50% to 76.9%. All of the increased seed set was related to increased seed set in chasmogamous florets. In 1992, when P. pratense was allowed to flower in both fields, mean seed set/culm was less than 50%. In a third population of D. compressa, at a site where P. pratense was not present, mean seed set/culm was over 76% in both years. Cleistogamy in D. compressa maintained a minimum seed set of 41.9% in all fields. Hence, there is an effective barrier against allelopathic pollen of P. pratense. The implications of loss of chasmogamously produced seed are unclear. There were no differences in mean seed mass or size between chasmogamous and cleistogamous florets. Hence fitness may be unaffected. Although loss of chasmogamous seeds may result in increased sibling competition in cleistogamously produced seedlings, this hypothesis has yet to be tested. The relative numbers of chasmogamous and cleistogamous florets did not change in 1991–1992, although there were more chasmogamous florets produced by D. compressa at the site where P. pratense did not exist. Tests of whether pollen allelopathy acts as a selection pressure on the relative numbers of chasmogamous and cleistogamous florets in D. compressa will require long-term field studies.