Pollen: ovule ratio and its relationship with other reproductive traits in some Passiflora species (Passifloraceae)

The pollen/ovule (P/O) ratio has been used as predictor of the reproductive system of angiosperms (lowest P/O values correspond to obligate autogamous species while the highest correlate with obligate xenogamous species) but it does not keep for all the taxa. The relation of P/O index with pollen and stigma size, flower diameter, pollen vitality, stigmatic area/pollen-bearing area of the pollinators ratio (SA/PBA), taxonomy and breeding system was analyzed in four Passiflora species with different degree of compatibility, pollination system and taxonomic placement. P/O of the self-compatible species (with shorter anthesis, scarce pollinator visits or minor longevity) was lower than P/O of the self-incompatible ones. The P/O values of the outcrossers could be related with the highly efficient pollination performed by the frequently visiting Xylocopa species; these bees transport Passiflora pollen in a usually monospecific mass on the thorax, resembling grouped pollen dispersal. All the taxa exhibited pollen with unimodal size and high vitality, except for P. misera, in which pollen size was bimodal, with similar quantities of grains in both classes, the large being more vital than the small ones. Correlations of P/O were positive with flower size, negative with stigma area/pollen-bearing area of the pollinator, null with stigma area and not significant with pollen size. The P/O ratio did not reflect taxonomic affinities; this index and the breeding system were more related with pollinator type, anthesis, flower size and SA/PBA than with pollen grain or stigma size.