Healthy and tobacco mosaic virus-infected tobacco (Nicotiana tabacum var. Havana 38) cells from suspension cultures, transferred to aseptic microcultures, exposed to gamma irradiation, and observed with phase microscopy showed damages to nuclear and cytoplasmic structures and modifications of normal dynamic activities. Deformation and displacement of cell nucleus, thinning of cytoplasmic strands, and reduction in starch granule size and number followed irradiation. The type and amount of damage and cellular modification were influenced by the radiation dose. Healthy and virus-infected cells exposed to 250,000 R or more showed decreased cyclosis immediately after irradiation. This effect was temporary except for cells exposed to <tex-math>$1\times 10^{6}\ {\rm R}$</tex-math>. A high cell death rate occurred during the first 2 weeks after irradiation. Cells surviving irradiation treatments showed high starch contents in certain stages. Cells surviving for 4 weeks after irradiation lived for several months. Some irradiated cells survived severe cellular damages. Survival of TMV-infected cells was lower than that of healthy cells after exposure to the same radiation dose. Low radiation doses did not alter division of healthy cells, but 100,000 R or more decreased divisions to 10 to 20%. TMV-infected cell divisions were reduced progressively with increasing radiation doses from 100 to 50,000 R. Some cells resumed normal growth activities and either formed large cells or divided to form cell colonies.

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