Effects of pollen load composition and deposition pattern on pollen performance in Clarkia unguiculata (Onagraceae)

This study explores how variations in pollen load composition and deposition patterns affect pollen performance in growth chamber pollinations of Clarkia unguiculata. Pollen load composition was varied to include either a single pollen donor (self or outcross) or two pollen donors (self + outcross outcross + outcross). Deposition patterns were varied by either mixing pollen from donors together or applying pollen from each donor as separate clumps. The clumps were either placed adjacent to one another on a single stigma lobe or separated on different stigma lobes. The pollen performance factors measured included levels of pollen grain germination on the stigma, pollen tubes in the style, pollen tube attrition levels, and numbers of seed set. Both pollen source and deposition scheme were found to influence pollen performance. The strongest effects were observed during pollen grain germination on the stigma. Pollen loads consisting of pollen from two donors showed significantly lower germination than single-donor pollinations. This reduction occurred even when pollen from different sources was deposited in separate clumps without direct contact between individual grains. Deposition patterns also influenced pollen grain germination. Pollen germination was found to decrease as the degree of contact between the different types of pollen grains increased. Pollen source, but not deposition pattern, also had a significant effect on pollen tubes reaching the upper style, with single-donor pollinations producing the highest proportion of pollen tubes. Pollen tube attrition in the style and numbers of seeds produced were unaffected by pollen source or deposition scheme.