Seedless Mechanism And ISSR Molecular Marker Analyses Of Wuzihong 1-7 Sweet Orange (Citrus Sinensis Osbeck) Originated From Radiation Mutagenesis

Wuzihong 1-7 sweet orange（Citrus sinensis Osbeck）is a new variety of originated from seedy Hong 1-7 sweet orange through ⁶⁰Co-γray induction.It has the advantages of seedless and low acid,which is of great significance for fresh food and processing.However,the reason of its seedlessness is not clear.In this study,seedy Hong 1-7 sweet orange and its mutant Wuzihong 1-7 sweet orange were used as materials to study seedless mechanism of Wuzihong 1-7 sweet orange.Male sterility of Wuzihong 1-7 sweet orange was studied,chromosome behavior of pollen mother cell meiosis observed by carbol fuchsin staining,the development of microspores observed by paraffin-cut section,the morphology and size of pollen grains observed by scanning electron microscopy.The pollen-tube was compared between self-and cross-pollinated of Wuzihong 1-7 sweet orange were observed under fluorescence microscope by tissue pressing.Embryo sac structure and development of self-and cross-pollinated of Wuzihong 1-7 sweet orange was observed by paraffin-cut section.Genetic differences between Wuzihong 1-7 sweet orange and seedy Hong 1-7 sweet orange were analyzed by ISSR molecular markers.The main results were as follows:1.PMC of Wuzihong 1-7 sweet orange was abnormal.The pollen viability and germination ability were weak and the fertility was low.The morphological size of the pollen grain was different from that of Wuzihong 1-7,showing a high degree of deformity.It was speculated that male sterility may be the main cause of seedlessness of Wuzihong 1-7 orange.2.Pollen tube of Wuzihong 1-7 sweet orange growed normally in stigma and style after self-pollination and cross-pollination,but pollen tube was not observed in ovaries of self-pollination and cross-pollination 10 days after pollination suggesting that Wuzihong 1-7sweet orange might take longer to complete the process of pollination and fertilization.3.The embryo sac structure of Wuzihong 1-7 sweet orange was observed by paraffin sectioning.Wuzihong 1-7 sweet orange still has most of the embryo sac in the mononuclear,binuclear and trinuclear stages on the day of blooming,the embryo sac abortion rate is 89.9%.The abortion rate of embryo sac of Wuzihong 1-7 sweet orange was significantly different from that of seedy‘Hong 1-7’sweet orange.4.The embryo development of Wuzihong 1-7 sweet orange was delayed after self-pollination and cross-pollination.Most embryos stopped developing at the stage of globular embryo and heart-shaped embryo,and occasionally one or two embryos could develop to cotyledon embryo.The lag of embryo sac development directly or indirectly led to embryo abortion,which was one of reasons for seedlessness of Wuzihong 1-7 sweet orange.5.ISSR analysis of Wuzihong 1-7 sweet orange and seedy Hong 1-7 sweet orange was carried out.47 ISSR primers were selected from 100 primers to amplify polymorphic bands,of which 3 primers could amplify specific bands in Wuzihong 1-7 sweet orange,indicating the differences in genomic DNA level between Wuzihong 1-7 sweet orange and seedy Hong1-7 sweet orange.