ON THE APOMICTIC SEED DEVELOPMENT IN THE WALNUT (JUGLANS-REGIA)

Some of the Walnut varieties grown in Germany show the tendency of apomictic fruit development. It is not clear from which structures within the ovule the apomictic embryos develop. The investigations aimed to elucidate by histological methods the development of apomictic embryos, especially by investigating sporo- and gametogenesis and the development of the embryo sac until the beginning of the differentiation of the embryos. The apomictic varieties 'Esterhazy II' and 'Nr.26' and the non apomictic 'Weinsberg I' were used for the investigations. The main results are: About 20 days before anthesis differentiation of the nucellus occured, and the embryo sac mother cell was evident from the 13th day before anthesis. After division of the embryo sac mother cell the diade stadium could be observed from 7 to 14 days before anthesis and the tetrade was evident from the 4th day before anthesis. The chalazal cell formed the embryo sac. A aposporic development from a cell of the nucellus was never observed. The embryo sac was fully differentiated only 6 to 8 days after anthesis and it always revealed the 8 nuclei normal type, also in the isolated flowers. After isolation of flowers with cloth bags embryo development originated from divisions of the egg cell. Embryo development from somatic cells of the nucellus was, with one exception, never observed. Thus, the development of the apomictic embryos in the walnut is diplosporic and not by nucellar embryogenesis, as reported by SCHANDERL (1964). Embryo development started according to variety and year between the 4th and 5th week after anthesis. No differences were found between isolated and open pollinated flowers, also in the histological development of the embryos.