Allelopathic Pollen Extract from Phleum pratense L. (Poaceae) Reduces Germination, In vitro, of Pollen of Sympatric Species

In vitro studies were performed to quantify the putative allelopathic effect of pollen from Phleum pratense (Aveneae: Poaceae). Extract from ground pollen was partitioned, using ion-exchange chromatography. Acidic, basic, and neutral fractions were isolated. These fractions, along with extract from intact pollen, were applied in increasing concentrations of the equivalent of 0 (control), 0.1, 1, 3, 5, 10, and 25 pollen grains of P. pratense/μL. Pollen from five sympatric, wind-pollinated species of Poaceae and P. pratense was germinated on agar medium that contained different extract concentrations. In response to increasing extract concentrations, from intact pollen or from acidic fractions, mean percentage of germination of pollen of Agropyron repens, Bromus inermis, Danthonia compressa, and Poa compressa decreased in a nonlinear manner. Basic, neutral, and solvent fractions did not show any allelopathic activity; hence, the allelochemicals may be acidic in nature. An extract concentration equivalent to 0.1 grains/μL was sufficient to inhibit pollen germination in these four species. Hence pollen allelopathy requires fewer pollen grains than may exist on foreign stigma in situ. Pollen from P. pratense was not autotoxic. This, together with the lack of a pollen-allelopathic effect on the contribal Agrostis stolonifera, indicates that members of the Aveneae may be able to detoxify the pollen allelochemicals. Similar experiments, using extracts from intact pollen from 11 populations of Phleum across eastern Canada revealed that the pollen-allelopathic effect on sympatric populations of Agropyron repens was consistent, save for the lone population of Phleum arenarium. Allelopathic pollen may occur in most populations of P. pratense in eastern Canada but not in P. arenarium.