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Nucellar embryogenesis in undeveloped ovules in some navel orange cultivars. - Nucellar embryogenesis has been studied in some cultivars of navel orange, Citrus sinensis (L.) Osbeck. Histological observations have been carried out in undeveloped ovules excised 2, 4, 6, 8 months after anthesis. Ovules under study derived a) from flowers naturally pollinated or hand-pollinated with pollen of Poncirus trifoliata (L.) Raf.; b) from emasculated flowers, and c) from flowers deprived of stigma. In absence of pollination, as well as in absence of fertilization, the nucellar embryos reached the globular stage.

Characterization by SEM of the pollen of some species of Citrus and related genera at two ploidy levels (2x and 4x).—The pollen grain morphology of some Citrus species, of some related genera, and of some intergeneric hybrids is described. The study made with the SEM has shown: a) the more typical shape of Citrus pollen is spheroidal with four furrows; the sculptures of ektexine in all Citrus species can be an useful character for taxonomy; the ektexine can be pitted (especially in edible citrus fruits) or reticulated as (in some species valuable as rootstock); in two types of ektexine there are differences that can allow the individualization of some species; these preliminary observations do not allow to indicate useful elements to individualize the cultivars of the same species; b) pollen morphology is a valid diagnostic criterion for the ploidy levels 2x and 4x of the same species in the genera Citrus and Poncirus; pollen of tetraploid plants is larger and has a higher number of furrows than ...