ADVENTIVE EMBRYOGENESIS IN CITRUS (RUTACEAE) II. POSTFERTILIZATION DEVELOPMENT

Cytological and histological studies on postfertilization development of ovules were carried out in six facultatively apomictic Citrus cultivars. At the time of anthesis, adventive embryo initial cells (AEICs) were detected mainly in the cell layers of the nucellus around the chalazal half of the embryo sac. During the approximately 40 days rest period of the AEICs after fertilization, rapid cell division and enlargement in the endosperm and the chalazal half of the nucellus resulted in the split of AEICs into several separated areas forming the micropylar, lateral and chalazal islands surrounding the enlarging embryo sac. Both in diploid seeds with triploid endosperm and triploid seeds with pentaploid endosperm, the AEICs located in the micropylar half successfully developed into adventive embryos. In diploid seeds, almost all AEICs located in the chalazal half did not develop beyond the initial-celled stage, while in the triploid seeds, those located in the chalazal half occasionally developed into cotyledonary embryos. In seeds with aborted endosperm, the AEICs located in the chalazal half often developed into cotyledonary embryos. The chalazal expiants from normal seeds produced a large number of embryos in vitro. Four results can be summarized from these studies on adventive embryogenesis as follows: 1) All AEICs are initiated prior to anthesis. 2) Whether or not the AEICs successfully developed into adventive embryos is dependent upon their position in the seed. 3) The farther the AEICs are located from the micropylar end, the more adventive embryogenesis is suppressed by endosperm. 4) The degree of adventive embryogenesis in the chalazal half is affected by time and extent of malfunction of the endosperm. Under natural conditions, these regulatory systems of adventive embryogenesis contribute to high production of zygotic seedlings in apomictic Citrus species and cultivars.