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Nicotiana crosses were pollinated in situ and in vitro with heavily irradiated pollen (500, 700, 1000 Gy) to induce parthenogenesis and to study the development of embryo and endosperm. Haploids were obtained after in situ pollination only; however, parthenogenetic proembryos occurred also after in vitro pollination. It was demonstrated that ovule culture following pollination offers a better chance to produce haploids than undisturbed pollination does. Pollination with irradiated pollen (PwIP) strongly decreased the number of endosperm cells and the size of embryo sacs, and it affected the development of embryos; no significant differences between applied irradiation doses were found. Ovules with endosperm only, embryo only, and with both embryo and endosperm were observed. The most frequent were the ovules with endosperm only, the rarest with embryo only. A small amount of storage products occurred in the endosperm cells. The diploid chromosome number counted in the endosperm produced after PwIP points to their origin without fertilization. An interesting phenomenon observed after PwIP was vigorous growth of endothelium.
In vitro placental pollination can be employed to overcome barriers of self-incompatibility and cross-incompatibility. Pollen grains of self-incompatible plants are capable of germinating directly on ovules, and the whole process of sexual reproduction can be accomplished. In some combinations of crosses, direct pollination of ovules enables prezygotic incompatibility barriers to be overcome. As a result, hybrid embryos or even hybrid plants can develop, depending on the partners crossed. Pollen grains germinate abundantly on immature ovules irrespective of the stage of megasporogenesis or megagametogenesis, and their tubes can penetrate the integuments and the nucellus cells. Preliminary investigations revealed that pollen grains of seven selected species of gymnosperms germinated on ovules of various species of Angiospermae. Embryological analysis of ovules of Melandrium album fixed 3 days after pollination with pollen of Pinus wallihiana revealed the presence of remnants of pollen tubes and 2-celled embryos in the embryo sacs. Those preliminary observations have not been described heretofore.
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