Studies on Nucellar Embryogenesis in Citrus

II. Formation of the Primordium Cell of the Nucellar Embryo in the Ovule of the Flower Bud, and its Meristematic Activity

Abstract

The mechanism of nucellar embryogenesis was studied in histological experiments and ovule cultures. Seven citrus cultivars were used as materials.
A primordium cell of the nucellar embryo, designated by the authors in a previous paper (6), was observed in the ‘Trovita’ orange ovule from 4 days before anthesis. The primordium cell was observed in the other polyembryonic cultivars from the time of flowering. However, the primordium cell was not observed throughout the developmental stages of the ovule in any of the monoembryonic cultivars studied. In the culture of ovules excised from flower buds and young fruits, nucellar embryoids were formed only in the polyembryonic cultivars. The embryoids formed in the monoembryonic cultivars originated in the fertilized egg. The nucellar embryoids formed in the polyembryonic ones were derived either from the primordium cell of the nucellar embryo or other cells which might have such a potential as to develop into the primordium cell. Moreover, the nucellar embryoids were formed in an agar medium containing sucrose only. The mechanism of nucellar embryogenesis is discussed on the basis of these results.