A survey of watermelon germplasm for inhibitory seed exudates

Watermelon [Citrullus lanatus v. lanatus (Thunb.) Matsum & Nakai] seed exudates are inhibitory to germination and seedling growth of other plant species. A miniature bioassay experiment that measured proso millet (Panicum miliaceum L.) radicle growth was used to assess the inhibition caused by seed exudates of 125 genotypes of watermelon and related Citrullus species. Exudates of most genotypes were not inhibitory; however, exudates of 53 accessions reduced radicle growth in comparison with the control. In subsequent proso millet radicle growth experiments, genotypes were found to vary in inhibitory potential, and concentration response curves generated using filtered, pasteurized exudates were different among genotypes. Filter-sterilized seed exudates of Citrullus accessions also varied in the level of inhibition in a bioassay that measured their effect on sporangia formation by the watermelon pathogen, Phytophthora capsici. These observations suggest that consiituents in Citrullus seed exudates affect organisms in the spermosphere and that the inhibitory potential of seed exudates varies among genotypes. Differences in concentration response curves in the millet bioassay and differences in the relative inhibition of genotypes in the millet and fungus bioassays indicate that the inhibitory constituents in seed exudates vary among genotypes.