Somaclonal variation in tomato: effect of explant source and a comparison with chemical mutagenesis

Plants were regenerated from leaf, cotyledon, and hypocotyl explants of tomato cv Moneymaker. Various phenotypic alterations were observed among regenerated plants (R1), but were not transmitted to the progenies, except for ploidy variation. Variation in ploidy level, mainly tetraploidy, occurred in R1 plants and their R2 progenies, and the frequency of polyploid plants depended on the explant source. More than 50% of the regenerants derived from hypocotyl explants were found to be polyploid. A correlation was observed between the percentage of polyploid cells present in the explant material in vivo and the frequency of polyploid plants. Several monogenic mutations were recovered in the R2, four of which were shown to be allelic to known, recessive, single-gene mutants. No significant effect of explant source or duration of tissue culture period on mutant frequency or spectrum was found. For several mutant types that could be scored unambiguously, somaclonal variation was compared to variation induced by treatment of seeds with ethyl methane sulphonate (EMS). The results showed that the mutant frequencies were higher after EMS treatment than those generated through tissue culture. With respect to the mutant spectrum, no clear differences were observed between the spectra obtained after EMS treatment and those after tissue culture. However, tissue culture gave rise to polyploid plants, whereas no ploidy variants occurred after EMS treatment.