The sensitivity of withe (W) and green (G) areas of variegated tobacco leaves (NIcotiana tabacum cv. Samsun Nc) to infection and multiplication of tobacco mosaic virus (TMV) was compared. 1. The number of local lesions produced on W was double of that on G. 2. The enlargement of local lesions was relatively faster on W, however, TMV concentration at 3-4 days after inoculation was 2-6 times higher in the lesions produced on G as compared to that in the lesions on W. 3. The central and marginal tissues of local lesions developed on W and G were observed by electron microscopy to clarify the processes of collapse and necrotization of cells in relation to TMV amultiplication. The results are as follows. 1) In the central tissues of lesions on both areas of W and G, all cellsd were completely collapsed, shrinked and star-shaped. In these cells, TMV particles were aggregated in crystalline form in altered electron-dense cytoplasm. The size and number of the virus-aggregates in G cells were greater than those in W cells. 2) In marginal tissues of the lesions, various stages of cell necrotization were observed. The first events of cell necrotization were plasmolysis and rupturing of tonoplast. Following these events, the plastids or chloroplasts burst and the continuity in the cytoplasmic membrane occurred earlier than bursting of plastids. TMV particles were scattered in the cytoplasm with bursting of the tonoplast in W and G cells. 3) No remarkable difference was observed in the ultrastructure of margins of the lesions from W and G. TMV particles were observed in the cytoplasm of several cells beyond the necrotic region. 4. From these results, the fact that TMV concentration in local lesions on W is lower than that in the lesions on G, contrasting to highersensitivity (in terms of lesion formation) of the former than the latter, may be attributed to the rapid cell necrotization in W cell as compared to that in G cells resulting in early inhibition of virus multiplication.
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