Fruit characteristics and seed anatomy of 'Majia' pomelo pollinated with cobalt-60 gamma-ray-irradiated pollen

The 'Majia' pomelo (Citrus maxima (Burm.) Merr., Rutaceae) is a distinctive local cultivar of Jiangxi province, China, that is famous for its good flavour and abundant lycopene content; however, each 'Majia' pomelo fruit usually contains approximately 120–200 large seeds. To obtain seedless 'Majia' pomelo fruit, we used pollen from sour pomelo (a local variety generally used for pollination) irradiated with 1000 Gy cobalt-60 gamma rays to pollinate 'Majia' pomelo. This cross produced seedless fruit that displayed similar features to those of fruit with fertile seeds in terms of fruit weight, fruit diameter, peel thickness, total soluble solid (TSS) content, vitamin C (Vc) content, sugar content and titratable acidity. Interestingly, pollen viability was not affected by irradiation in an in-vitro germination test and the irradiated pollen tubes grew normally along the stylar canals into ovules. The pulp of the seedless fruit contained higher concentrations of IAA, GA3, and ZR and a higher (IAA+GA3+ZR)/ABA ratio than that of seeded fruit; however, the hormone concentrations and ratios in the seeds of these two kinds of fruit were opposite to those in the pulp. These results may be important reasons that the fruit fertilized with irradiated pollen could maintain fruit growth and fruit quality. Paraffin sectioning showed that, the endosperm nuclei extended along the wall of the embryo sac under both pollen treatments at two weeks after pollination; at seven weeks after pollination, some of the endosperm had entered the cellularization stage in the embryo sacs fertilized with nonirradiated pollen, but endosperm degeneration and shrinkage were observed in most ovules fertilized with irradiated pollen. The endosperm plays a very important role in embryo growth by providing nutrients stored in its cells. Our study demonstrates that early endosperm degeneration in developing seeds pollinated with irradiated pollen may be a crucial reason for seed abortion.