COMPETENCE FOR REGENERATION DURING TOBACCO INTERNODAL DEVELOPMENT - INVOLVEMENT OF PLANT-AGE, CELL ELONGATION STAGE, AND DEGREE OF POLYSOMATY

This study deals with internodal development in vegetative plants of N icotiana tabacum cv Samsun NN and its reflection in changes of the cel lular competence for regeneration. During elongation of the internodes , the cells of the epidermis, subepidermis, and cortex exclusively exp anded and increased their DNA content cell type specifically, generall y from 2C to 4C. Cells with the 8C DNA content were found mainly among the cortex cells of mature internodes. The frequency of shoot regener ation (directly from subepidermal and epidermal cells together) on thi n cell layer explants increased to an optimum along with elongation of the internodes and decreased in mature internodes along with aging. T he frequencies of diploid shoots among the regenerants from elongating and mature internodes were high (88 and 75% on the average, respectiv ely), indicating that most cells that had achieved the 4C DNA content generally retained the G2 phase of the diploid cell cycle. Shoots rege nerated from explants of young plant material mainly had a vitrified a ppearance. The occurrence of this type of malformed growth was already determined by the physiological state of the cells in the internode a nd did not interfere with their acquisition of competence. Vitrificati on was unrelated to the degree of polysomaty of the internodal tissue. Using the occurrence of tetraploid root regenerants (from intermediat e cortex-derived callus), up to a frequency of 50%, we show that the p osition in the plant where a majority of the 4C cortex cells switched to the G1 phase of the tetraploid cell cycle was at the transition fro m the elongation phase to the mature phase.