Pollen deposition, pollen tube growth, seed production, and seedling performance in natural populations of Clarkia unguiculata (Onagraceae)

In natural plant populations, pollen grain load sizes may vary greatly and have the potential to affect the intensity of pollen competition for ovules. Pollen competition can in turn, affect seed production, fruit set, and progeny vigor, with important ecological and evolutionary consequences. In this investigation, we determined the potential for pollen competition in four adjacent natural populations of Clarkia unguiculata during three growing seasons. We collected stigmas and styles after 6, 24, and 48 h of exposure to pollinators and determined pollen load sizes, the number of pollen tubes reaching the upper and lower portions of the style, and pollen tube attrition rates. We also collected mature seeds to determine the effect of pollen load size on seed set, seed germination, and seedling vigor. Pollen load sizes increased with increasing length of exposure to pollinators and varied from year to year and from population to population. In most plants, pollen loads were large enough to result in pollen competition on the stigma. The number of pollen tubes reaching the upper and lower portions of the style was affected by pollen load sizes. Pollen tube attrition rates in the style were moderate and were not affected by pollen load sizes. However, as pollen tubes completed growth in the style, the opportunity for pollen competition decreased as the number of pollen tubes became smaller than the number of ovules in the ovary. Seed production in this experiment was dependent on pollen load sizes, even though the number of seeds produced was well below the number typically recorded for this species. Our measures of seedling vigor (percentage seed germination, cotyledon length, and first foliar leaf length) were not affected by pollen load size.