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, trist ylous Pontederia sagittata. Prior application of self pollen, followed after 2, 4, or 6 h by compatible pollen, was conducted on five genoty pes of each of the three style morphs under uniform glasshouse conditi ons. The greatest reductions in seed set occurred in pollinations of t he 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 sig nificant reductions in seed set were observed in the short-styled (S) morph or in the other morphs at shorter time intervals. Observations o f pollen germination and pollen tube growth indicated that the lack of inhibitory effects in the S morph may occur because relatively few po llen grains adhered to stigmas in self-pollinations. In the L and M mo rphs, early germination of self pollen may cause physical clogging of the stigma and style, resulting in a reduced number of compatible poll en tubes in styles. Observations of the structural integrity of styles indicated that prior germination of self pollen resulted in more rapi d onset of pistil senescence, particularly in the L morph. These influ ences may contribute to the morph-specific differences in seed set obs erved following prior self-pollination of outcrossed flowers. The nega tive effects of incompatible pollen are likely to be most evident wher e ecological factors cause delays in the delivery of outcross pollen t o stigmas.