| The production of seedless nonastringent germplasm is of great importance for the genetic improvement of persimmon (Diospyros kaki L. f.). In this thesis, the plant material of 19 so-called seedless cultivars and 11 seeded cultivars was used to study the mechanism by which seedless and rarely seeded fruits were produced, cytology of 2n pollen formation, potential value of 2n pollen for breeding, and thereafter approaches to produce seedless germplasm through 2n pollen in nonastringent persimmon. The main results could be summarized as follows:1 Chromosome numbers of shoot-tips in above cultivars were investigated. It was found that most of the so-called seedless cultivars were hexaploid (2n=6x=90), except for 'Hiratanenashi', which was nonaploid (2n= 9x =135). In addition, chromosome numbers of root-tips in hexaploid cultivars were also detected, which showed that the progenies and their parents shared the same ploidy. Based on the ploidy of parents and their progenies, it was concluded that the seedlessness of studied hexaploid cultivars was not caused by their ploidy variation.2 Possibilities of parthenogenesis and sexual reproduction in a so-called seedless cultivar 'Mopanshi' were investigated. The results strongly showed the occurrence of parthenogenesis. The normal growth of pollen tubes in the pistils after controlled pollination, and an increase in seed production were observed, but some abnormal growths of pollen tubes leading to barriers of fertilization were found as well, suggesting that sexual reproduction could occur occasionally.3 Random amplified polymorphic DNA (RAPD) analysis was conducted to assess genetic variations between parents and both their open-pollinated and artificially pollinated progenies in 'Mopanshi' as well as in 'Luotiantianshi'. The paternal bands in the progenies indicated the presence of hybrids in both cases.4 Distinct differences in seed development between hexaploid and nonaploid cultivars were found: the fruits in hexaploid were either truly seedless or seeded with intact seeds, meanwhile, some abortive seeds which have degenerated to some extent were found in the immature fruits of nonaploid. It suggested that the ploidy level was responsible for the seedlessness of nonaploid.5 RAPD analyses were conducted to assess genetic variations between theopen-pollinated progenies and their maternal parents in 19 Chinese so-called seedless cultivars. Either one or more polymorphic bands could be detected in the progenies of all studied cultivars, showing the presence of sexual hybrids. All these cultivars were found to be able to produce seeded fruits.6 The frequency of 2n pollens estimated by the diameters of pollens in nonastrigent cultivar 'Zenjimaru' was 6.39%. The occurrence of 2n pollens was confirmed by the analysis of pollen nuclear DNA flow cytometry.7 Cytological mechanisms of 2n pollen formation were studied in nonastringent cultivar 'Zenjimaru'. The meiotic abnormalities resulting in the 2n pollen production revealed that: (1) disoriented spindles, including parallel, fused and tripolar spindles, were formed at metaphase II and anaphase II; (2) the nuclei at telophase II were arranged to two poles, each pole contained two nuclei, or to three poles, one of which contained two nuclei, the other two contained one nucleus respectively; (3) dyads and triads were produced at tetrad stage. The dyad would develop into two 2n pollens, and the triad into one 2n and two n pollens. The frequency of 2n pollen production predicted by frequencies of dyads and triads agreed with that estimated by pollen diameters.8 The purification and restoration of viability of 2n pollens in 'Zenjimaru' were studied. The frequency of 2n pollen could increase from 6.39% to 82.2% by screening, and the viability of purified pollens could be fully restored by the treatment with 15% sucrose solution. The 2n and n pollens showed different sensitivities to soft X radiation. Being radiated with a dose ranging from 104.4C/kg to 150.8C/kg, the 2n pollens coul... |
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