POLLEN VIABILITY AND THE POTENTIAL FOR SELF-POLLEN INTERFERENCE IN PHLOX HIRSUTA, AN ENDANGERED SPECIES

摘要

Premise of research. Hermaphroditic plants with self-incompatible mating systems often produce flowers that fail to mature seed-containing fruits. A variety of factors can affect reproductive success in these species, including pollen load size, pollen viability, pollen-pistil compatibility, and self-pollen interference. We combined field, growth chamber, and laboratory studies to identify factors that are and are not responsible for high ovule-to-seed ratios in Phlox hirsuta, a self-sterile, hermaphroditic, endangered species. Methodology. We (1) determined whether pollen load size affected seed set by counting the number of grains on stigmas following natural pollinations and following outcross-only pollinations, (2) assessed pollen viability by staining grains collected from flowers of different ages, (3) quantified the degree of autonomous self-pollination by counting the number of self-pollen grains that accumulated on stigmas over time, and (4) determined whether self pollen on stigmas interfered with the ability of outcross pollen to sire seeds by performing experimental hand-pollinations and staining pollen tubes in vivo. Pivotal results. Both pollen inviability and autonomous self-pollination increased as flowers aged. Self-pollen grains, which were able to grow tubes through styles, interfered with the ability of outcross pollen to sire seeds when deposited on stigmas 24 h before outcross pollen. Conclusions. Pollen inviability and self-pollen interference have the potential to contribute to the high ovule-to-seed ratios in P. hirsuta. Individual flowers remain open 8 d, but the ability of outcross pollen to sire seed progressively decreases as flowers age. Outcross pollen has the best chance of siring seeds if grains are not only collected from newly opened flowers (before pollen viability decreases) but also transferred to the stigmas of freshly opened flowers (before autonomous and geitonogamous pollinations occur).