Abstract
Hybridization between diploid cultivars of poinsettia results in abundant seed production. However, with the diploid used as the maternal parent in crosses with tetraploids, less than 1% viable seedlings are realized. Cytological observations indicated a 2:3:2 chromosome number ratio of the embryo, endosperm and maternal tissue in diploid crosses with endosperm development completed in about 45 days after pollination. Embryo differentiation in diploid crosses occurred at 26 days and matured in approximately 54 days after pollination. The chromosome number ratio of the embryo, endosperm, and maternal tissue was not firmly established for the diploid × tetraploid cross due to the small number of viable seeds obtained, but the expected 3:4:2 ratio was frequently observed. Endosperm development in most of the ovules examined did not progress beyond the large fragile cell stage. This study suggests that the endosperm may be the determining factor in setting of seed in the diploid × tetraploid cross.
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