Measurement of the extent of somaclonal variation in begonia plants regenerated under various conditions. Comparison of three assays

Begonia plants were regenerated from leaf explants treated with increasing concentrations of the chemical mutagen nitrosomethylurea (NMU). In these pl ants, we evaluated three methods to assess the extent of variation: a quali tative, phenotypic assay (the percentage of aberrant plants), a molecular a ssay (changes in RAPD patterns) and a quantitative, phenotypic assay (varia tion in a quantitative trait). The qualitative, phenotypic assay required a large number of plants per treatment (approx. 100) and careful, skilled ju dgement. It was sensitive to physiological variation. The RAPD assay was no t sufficiently sensitive: even at the highest NMU concentration there were no changes in RAPD patterns. The quantitative, phenotypic assay gave the be st results: it was simple, objective and sensitive, and required few plants per treatment (approx. 30). Plants were also regenerated from different ty pes of intermediate callus, and their variation was assessed. The performan ce of the three assays was essentially the same as with plants obtained aft er mutagenesis with NMU. An intermediate nodular- or non-nodular-callus pha se resulted in slightly or strongly increased variation, respectively. In c ontrast to NMU-induced variation, callus-related variation, as determined i n the quantitative, phenotypic assay, appeared to be to a large extent tran sient since it decreased strongly after a second direct-regeneration step. An intermediate callus phase resulted in 2.5% juvenile plants. This aberrat ion, which might be related with changes in the methylation status of DNA, was not observed in NMU-treated plants.