Cell frequencies of green somatic-variations in the tl chlorophyll mutant of Nicotiana tabacum var 'Samsun'.

The chlorophyll deficient tl mutant of Nicotiana tabacum var 'Samsun' expresses green, clear and twin, green and clear somatic variations spontaneously on leaves at a low frequency. This character is maintained after both vegetative multiplication and sexual reproduction. However a very important phenotypic variability in the capacity for somatic variation appears in in vitro bud neoformations from leaf fragments of tl/tl homozygous plants. This variability is observed in the type of variations and the variation pattern, defined as the frequency and size of the variant areas.The present work was aimed at determining both the cell frequencies of the events which lead to the somatic variation and the preferential sequence of leaf initial development during which these frequencies are at a maximum. It was limited to plant populations with very different patterns for green variations, some having a high frequency of large variation, others having a high frequency of small variations. They were compared with a population of control plants having a low frequency.In the case of plants having a high frequency of large green variations, the events leading to somatic variation occurred between the twenty-first and the twelfth cell cycles preceding the end of the initial division phase, the maximum cell frequencies being in the seventeenth and sixteenth cycles. The maximum frequencies appeared extremely high, being on average about 10(-2). In plants with a high frequency of small green variations the event occurred between cell cycles nine and one, with mean frequencies of 10(-3) but without any clearly marked maximum. In the low frequency control plants the event also took place during the last ten cell cycles but with decreasing frequencies from 10(-4) to 10(-7).The frequency and the starting period of the cell events leading to somatic variation are closely dependent on the state of the cell. This is, on the one hand, strictly linked to the physiology of the plant and, on the other, closely correlated with the stage of differentiation, which may vary according to the genetic back ground of the leaf initial cells.The results are discussed in relation to comparable observations and the relevant interpretations made on other instability mutants.