The role of nuclear fusion in pollen embryogenesis of Datura innoxia Mill.

If anthers of Datura innoxia containing microspores are cultured at 28° C for 24 h, two forms of haploid pollen grains are produced; one with typical unequal generative and vegetative nuclei (A), and the other with equal vegetativetype nuclei (B). The A form predominates. After 24 to 72 h in culture, three embryogenic events ensue; 1) division of the vegetative cell in A grains accompanied by rapid degeneration of the generative cell, 2) simultaneous or independent mitosis of the nuclei in B grains, and 3) simultaneous mitosis of both the generative and vegetative nuclei in A grains following two successive cycles of DNA synthesis in the generative nucleus, but only one in the vegetative nucleus (C). The endoreduplicated generative nucleus gives rise to diplochromosomes which it is postulated divide on a common spindle together with the vegetative chromosomes to yield a 3n proembryoid. In other C grains, simultaneous mitosis of an endoreduplicated generative nucleus and two vegetative daughter nuclei can occur. Nuclear fusion in these grains is envisaged as leading to a 4n proembryoid. Diploid proembryoids probably arise by nuclear fusion in A or B grains. At plantlet emergence, mainly n and 2n plantlets are obtained. Higher ploidy proembryoids fail to develop probably as a result of competitive influences.Similar embryogenic events occur in cultures initiated at the stages of the first pollen mitosis and young pollen grains, but C embryogenesis predominates. Competition from lower ploidy embryoids is minimised and 3n, 4n and even 6n plantlets are obtained.