Effects of prior self-pollination on outcrossed seed set in tristylous Pontederia sagittata (Pontederiaceae)

The potential inhibitory effects of incompatible pollen on outcrossed seed set were investigated in mass-flowering, self-incompatible, tristylous Pontederia sagittata. Prior application of self pollen, followed after 2, 4, or 6 h by compatible pollen, was conducted on five genotypes of each of the three style morphs under uniform glasshouse conditions. The greatest reductions in seed set occurred in pollinations of the long-styled (L) morph at the 6 h time interval. Smaller reductions were also found for this treatment in the mid-styled (M) morph. No significant reductions in seed set were observed in the short-styled (S) morph or in the other morphs at shorter time intervals. Observations of pollen germination and pollen tube growth indicated that the lack of inhibitory effects in the S morph may occur because relatively few pollen grains adhered to stigmas in selfpollinations. In the L and M morphs, early germination of self pollen may cause physical clogging of the stigma and style, resulting in a reduced number of compatible pollen tubes in styles. Observations of the structural integrity of styles indicated that prior germination of self pollen resulted in more rapid onset of pistil senescence, particularly in the L morph. These influences may contribute to the morph-specific differences in seed set observed following prior self-pollination of outcrossed flowers. The negative effects of incompatible pollen are likely to be most evident where ecological factors cause delays in the delivery of outcross pollen to stigmas.