While the rarity of an endangered plant species can seldom be ascribed to its breeding system, knowledge of its breeding system may be critical to its recovery. The federally-listed endangered Ziziphus celata, a woody clonal shrub endemic to xeric upland habitats of the Lake Wales Ridge in central Florida, is known from only five populations, four of which are sterile and perhaps uniclonal. Altogether only eleven genotypes of Z. celata have been identified, based on allozyme electrophoresis. We conducted field experiments, including hand-pollinations of bagged flowers, over a four year period to determine the breeding system of Z. celata. We found that it is an obligate outcrosser and that some genotypes appear to be cross-incompatible. Determination of cross-compatibility is complicated by the high percentage of seed abortion and by the presence of parthenocarpic fruit. Germination trials and seed dissections are therefore required to confirm cross-compatibility. While eleven of 44 test crosses performed to date have yielded fruit, we have obtained germinants from only four crosses. Our objective is the identification of cross-compatible genotypes to be used for the genetic enhancement of sterile populations and the creation of new fertile populations. The translocation of compatible mating types to create reproductively viable populations is essential for the recovery of Z. celata.

Key words: breeding system, endangered species restoration, Lake Wales Ridge, rarity, Ziziphus celata