POLLEN QUANTITY AND QUALITY AS EXPLANATIONS FOR LOW SEED SET IN SMALL POPULATIONS EXEMPLIFIED BY EUPATORIUM (ASTERACEAE)

As plant populations decrease in size, reduced seed set may contribute to their ultimate extirpation. In this study, effects of pollen quantity and compatibility relationships (quality) on seed set were investigated in a rare species (Eupatorium resinosum) and a closely related common species (E. perfoliatum). The impact of pollen quantity was studied through pollen supplementation experiments in two populations of each species. Addition of pollen increased seed set only in the smaller population off. resinosum Compatibility relationships (pollen quality) were investigated in a diallel crossing experiment using ten genotypes from the same populations. Plants from the smaller population of E. resinosum were found to be 40% cross-incompatible, which was higher than the larger population off. resinosum and the two populations off. perfoliatum, the latter showing signs of self-compatibility in some individuals. In addition the variance in number of compatible matings per individual was higher in the smaller population off. resinosum. These results are consistent with a computer simulation model that investigated the effect of small population size on S-allele diversity. Sufficient pollination accompanied by a partial breakdown of the incompatibility system may account, in part, for the relative success of E. perfoliatum.