Shapes of Seed Including Well-developed Zygotic Embryo in Polyembryonic Citrus Species/Cultivars

Abstract

Seed shape in relation to the presence of well-developed zygotic embryos with germination capacity was examined in the polyembryonic seeds of ‘Hayashi unshu’, a satsuma (Citrus unshiu) pollinated with 5 citrus species cultivars, ‘Sekito-Yu’ (C. grandis), ‘Valencia’ orange (C. sinensis), ‘Duncan’ grapefruit (C. paradisi), Ponkan (C. reticulata) and ‘Seminole’ tangelo (C. reticulata×C. paradisi).
Such seeds were plump, large in size and flat and even in appearance. One or a few embryos were included in these seeds, seedcoat suture being fit to the line where two cotyledons were united, and generally the color of the zygotic embryo was somewhat different from that of nucellar ones.
In all crosses between ‘Hayashi unshu’ and 5 species/cultivars, there were found one or more seeds carrying such characteristics. The number of embryos per seed was influenced by the male parents, especially, ‘Sekito-Yu’, which gave the least number of embryos.
The seedling resulting from such characteristic seeds had leaves clearly different from those of nucellar seedlings. Therefore, they were confirmed as hybrid seedlings.
When ‘Valencia’ orange was used as a seed parent, no such characteristic seeds were produced in any cross with five other citrus species/cultivars.
Similar observations were carried out on open pollinated seeds of 32 citrus species/cultivars to determine frequency of occurrence of such characteristic seeds with single embryos. As a result, the data were divided into the following three groups based on the frequency of occurrence of monoembryonic seeds; more than 10% (A), 10-0% (B) and 0% (C).
The species/cultivars in A and B group can be utilized as seed parents in breeding, because hybrid seedlings are successfully obtained from the seeds containing well-developed zygotic embryos, which are distinguished by their shape.
When satsuma mandarin was used as a seed parent, 119 hybrid seedlings were obtained from 662 seeds crossed with 5 citrus species/cultivars.