Cytophotometric study and statistical analysis of ploidy evolution in cultured tissues of Nicotiana tabacum

Evolution of the ploidy in four strains of Nicotiana tabacum ev. P 19 was studied by cytophotometry after transformation of the cultured tissues into protoplast suspensions.
The limits of these variations were analysed numerically and statistically. The method used is capable of wide application. It can be used to predict the evolution of the average ploidy level of a strain maintained on a given medium if the average ploidy level of the explant is known. Comparison is made relative to a standard of known ploidy (diploid protoplasts). Increase of the average ploidy level is faster when the explant is mainly diploid (leaf parenchyma) than when it is composed of more heterogeneous tissues as regards ploidy (shoot tip). The appearance of a random drift depends on the composition of the culture medium: it is immediate in one medium (MS₂), but is preceded by a phase of relative stability in another medium (MS₁) which is richer in kinetin.
While the results presented specifically concern the strains cultured on media MS₁ and MS₂, the statistical method employed is applicable to cultures of any species for which one can experimentally justify a linear regression analysis.