Changes in frequencies of variegated leaves in NMU treated tobacco

Summary 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.