Mechanism of seedlessness in a new lemon cultivar ‘Xiangshui’ (Citrus limon)

Seedlessness is an important economic trait of lemon. Pollen and embryo sac fertility, embryo development,compatibility of self-pollinated ‘Xiangshui’ lemon and cross-pollinated ‘Xiangshui’ lemon were studied forinvestigating mechanism of seedlessness in ‘Xiangshui’ lemon. The results showed that the fertility of pollenand mature embryo sac was normal. The development of embryo sacs belonged to polygonum type. Themature embryo sac had one egg, two synergids, three antipodal cells and one big central cell containingtwo polar nuclei. Pollen tube in self- or cross-pollination grew well in stigma. Pollen tube of cross-pollinated‘Xiangshui’ lemon could grow normally in style and ovary, and then entered into the embryo sac, doublefertilization was accompanied. However, the growth of pollen tube in self-pollination was finally stopped in thebottom of stigma. Embryonic development in cross-pollination was normal. Zygote began to divide 2 weeksafter cross-pollination (WACP). Early globular embryos were observed 3 WACP. Globular embryos and heartshapedembryos were observed 4 WACP. Torpedo-shaped embryos were observed 5 WACP. Cotyledonaryembryos were observed 6 WACP and developed into complete seeds. While in self-pollination, ovules beganto abort at 2 weeks after self-pollination (WASP), disappeared 5 WASP later and produced seedless fruits. Theemasculated flowers without any pollination finally developed into seedless fruits also. Results indicated thatparthenocarpy is contributed to seedlessness. However, gametophytic self-incompatibility has a main role inseedlessness of ‘Xiangshui’ lemon by blocking fertilization in the bottom of stigma.