Changes in frequencies of variegated leaves in NMU treated tobacco : Evidence for a differential response to NMU.

Seeds of Nicotiana tabacum were allowed to imbibe water for 1 h and were then treated with 5-20 mM N-nitroso-N-methylurea (NMU) for 1 h. Seedlings were planted out separately and leaves no. 1-6 were scored individually after expansion was complete: frequencies of leaves with mutant sectors and the percentage of leaf area that was mutant were determined for the numbered leaves. Treatment with 5 mM NMU gave few mutant plants but after 10-20 mM NMU 50-98% of plants were mutant. The frequency of mutant leaves increased from leaf no. 1 to leaves no. 3-4; as much as a 5.6-fold increase, from leaf no. 1 to no. 3 was found. There appears to be differential sensitivity to NMU: it is lower in the initial cells for leaf no. 1 than in the initials for leaves no. 3 or 4. Leaves no. 1-4 arise from four different groups of initial cells: mutations appearing in two or more of leaves no. 1-4 must, therefore, arise independently of one another. From mutants found in leaves no. 1-4 it is estimated that the mean number of mutations per seedling was 2.68 after treatment with 20 mM NMU. Mean percentage of leaf surface area occupied by mutant sectors increased from 14% to 29.4% as NMU concentration increased from 10 to 20 mM. It also increased significantly from leaf no. 1 to leaves no. 3-6 after treatment with 15 or 20 mM NMU but not after 10 mM NMU: this suggests that mutagen treatment may affect the formation of mutant homoplasmon cells and their contribution to leaf primordia.