ALLELOPATHIC POLLEN EXTRACT FROM PHLEUM-PRATENSE L (POACEAE) REDUCES SEED SET IN SYMPATRIC SPECIES

We studied the effect of pollen extracts from Phleum pratense L. (Poac eae) on seed set in sympatric species to test whether pollen-allelopat hic effects exist in situ. Stigmas of six species of Poaceae (Agropyro n repens, Agrostis stolonifera, Bromus inermis, Danthonia compressa, P oa compressa, and Poa pratensis) were examined in a field study. The s tigmas of A. repens, D. compressa, and B. inermis received means of 9. 35, 7.20, and 7.10 pollen grains of P. pratense, respectively. Based o n data from previous in vitro experiments, the quantities of pollen of P. pratense found on these stigmas were sufficient to cause allelopat hic reduction in sexual reproductive success. We applied acidic extrac ts of the equivalents of 0.1, 1, 2, 3, 4, 5, 7.5, 10, 15, 20, and 25 p ollen grains of P. pratense/mu L to stigmas of each of A. repens and D . compressa. In A. repens, extract concentrations as low as five grain s/mu L caused approximately a 65% reduction in mean seed set. The decr eases in mean seed set were nonlinear and similar to previous studies of the effect in vitro. In D, compr essa the results were similar; how ever, the decreases in mean seed set were limited by the seed set by c leistogamous flowers. We suggest that this indicates that allelopathic pollen of P. pratense limits seed set in A. repens and D. compressa i n oldfield communities.