Pre-zygotic factors influencing the reproductive success of the federally endangered plant Florida ziziphus (Ziziphus celata)

Florida ziziphus (Ziziphus celata) is a clonal shrub endemic to central Florida’s Lake Wales Ridge .  Due to habitat loss and fragmentation, most populations are uniclonal.  In addition, Z. celata has a gametophytic self-incompatibility system (GSI) which limits the number and availability of mates. Previous research has shown that even after hand-pollination of compatible genotypes, fruit set is low and fruits often abort or contain inviable seeds.  We hypothesized that pollen sterility, ineffective pollen tube growth, and post-zygotic failure contribute to sexual reproductive failure in Z. celata.  In 2005 and 2006 we hand-pollinated approximately 400 flowers with self pollen, pollen from incompatible donors, or pollen from compatible donors and stained them for viewing using fluorescence microscopy.  In 2005 only 41% of hand-pollinated flowers retained pollen grains on the stigma surface after staining; 86% did so in 2006. As expected for a GSI species, on average 23% of the self crosses and crosses between incompatible genotypes resulted in pollen tube inhibition in the style. Between 7% (2005) and 13% (2006) of hand-pollinated flowers had pollen tube growth continue to the ovules, including a few selfs and crosses between putatively incompatible genotypes.  However, these results were isolated to two plants notable for their high rates of fruit set.  Thus it appears that low levels of reproduction in Z. celata are influenced by a number of factors including pollen sterility and ineffective pollen tube growth.  Future research will involve dissections of aborted fruits to learn more about potential sources of post-zygotic failure.