Relationship between Seedlessness of Keraji (Citrus keraji hort. ex Tanaka) and Female Sterility and Self-incompatibility

Abstract

We characterized the mechanism of seedlessness of Keraji (Citrus keraji hort. ex Tanaka), cultivar grown in Kikaijima island, Kagoshima. This cultivar is a diploid (2n=18) that produces a medium amount of fertile pollen which readily fertilizes other citrus cultivars. It is highly parthenocarpic, setting 30-50% of unpollinated flowers. When Keraji flowers are hand-pollinated with 'Hassaku' pollen, these fruits produce only 2 to 3 seeds. However, 19-20% of these hand-pollinated flowers set seedless fruits, whereas 30-38% of open-pollinated flowers set parthenocarpically. Self-pollinated flowers resulted in 91-93% parthenocarpic fruits ; the remaining 7-9% of the fruits had only one seed. Six days after pollination, the self-pollinated flowers had 1.4 pollen tubes at the base of the style, a figure that is much lower than that in cross-pollinated flowers with 'Hassaku' pollen (61±7.4). These observation demonstrated that seedlessness of Keraji results from strong female sterility, self-incompatibility and parthenocarpy.