Embryo development of woody and perennial Hibiscus species

Embryo development is the precursor of seeds, which are important to produce interspecific hybrid plants. The objective of this study was to investigate the embryo development of interspecific crosses between Hibiscus species and provide useful information, for the timely embryo rescue of potential F₁ progenies. Hibiscus species were hand-pollinated and tagged on the day of anthesis. Starting two days after pollination (DAP), the pods were collected for observation under a stereomicroscope. Hibiscus species showed three stages of embryo development. Endosperm formation in the cross-pollinated ovules occurred from 2 to 7 DAP for H. sinosyriacus and 2–4 DAP for H. moscheutos in stage I. The ovules slowly increased in size. Embryo development was observed from 8 to 13 DAP in H. sinosyriacus and 5–8 DAP in H. moscheutos at stage II. The endosperm triggered the development of the embryo. Then, the endosperm supported the embryo histodifferentiation through globular, heart, and torpedo-shaped stages inside the growing ovule in stage III. After reaching the maximum ovule size, embryo growth until maturation occurred from 14 to 30 DAP in H. sinosyriacus, and 9–15 DAP in H. moscheutos at stage III. The embryo completely absorbed the endosperm, and the mature embryo fully occupied the ovule in stage III. As a result, ovules from cross-pollinated deciduous shrub-type H. sinosyriacus exhibited slower embryo development compared with the perennial H. moscheutos with fast embryo maturation. Embryo rescue may be possible starting at Stage II because the globular embryo can be observed under the microscope.