Seed Development in Citrus With Special Reference to 2x X 4x Crosses

Comparative cytological and histological studies during embryogenesis of seeds from 2x X 2x and 2x x 4x crosses indicate that the ratio of ploidy level between embryo and endosperm is the most important factor affecting the course of seed development. The crosses produced seeds with the expected ploidy relationships between embryo, endosperm, and maternal tissue of 2:3:2 and 3:4:2 as well as the anomalous relationships 3:5:2, 4:6:2, and 6:10:2. All but 3:4:2 resulted in normal, germinable seeds. The ploidy level of the maternal tissue in relation to that of the embryo or endosperm did not appear to have any effect on seed development. About 92–99 % of seeds from 2x x 4x crosses containing triploid embryos with tetraploid endosperm aborted at different stages of embryogenesis. The abortion in all cases was preceded by abnormalities in the tetraploid endosperm. It is postulated that the unbalance of chromosome number between embryo and endosperm disturbs physiological relationships between these two tissues, leading first to the abortion of the endosperm and then of the embryo.