CELLULAR-CHANGES DURING THE ACQUISITION OF EMBRYOGENIC POTENTIAL IN ISOLATED POLLEN GRAINS OF NICOTIANA-TABACUM

We used electron microscopical techniques to study ultrastructural cha nges during the acquisition of embryogenic competence in immature poll en grains of Nicotiana tabacum, isolated at the early- or mid-bicellul ar stage and cultured in vitro under starvation conditions. Cytoplasmi c and nuclear changes during the starvation treatment are reported. De differentiation of plastids, dilation of the wall of the generative ce ll, the appearance of a large vacuole, loss of nuclear pores in the ve getative nucleus, changes in chromatin and nucleolar structure, and a decrease in the size of the nucleolus were observed. We suggest that t hese events are the first step in the switch from generative to vegeta tive generation during pollen embryogenesis.