Two kinds of assay, particle counts by electron microscopy and infectivity for leaves of were used to estimate the amounts of virus in extracts made from tobacco leaves kept at 14 to 34° for 1 to 6 days after inoculation, and in extracts from uninoculated tip leaves sampled 14 days after the plants were inoculated. One, 2 and 6 days after inoculation, virus isolate was obtained in largest amount from leaves at 22°, 18° and 14°, respectively. Isolate showed the same trend, but equivalent temperatures were 4 to 6° higher. At 14° infectivity and particle numbers continued to increase between 1 and 6 days after inoculation but at 30° infectivity decreased after 1 day. At some temperatures the number of particles first increased and then decreased. For instance, at 26° the number of particles of isolate tripled between 1 and 2 days, and then decreased at least 100-fold between 2 and 6 days after inoculation. Temperature had only small effects on the ratio of long ( 1900 Å) to short (mainly 450 to 900 Å) virus particles. Specific infectivity increased slightly when particle number was increasing and decreased greatly when particle number was decreasing. Specific infectivity sometimes decreased slightly before particle number. As with some small isometric plant viruses, the amount of tobacco rattle virus extracted from leaves apparently represents the resultant between synthesis and degradation, and the two processes are differently affected by temperature. At 14 to 22° but not at 26°, isolate produced necrotic lesions in inoculated leaves, whereas isolate did not cause visible lesions. This difference seemed to have little effect on the changes in virus titre but although isolate multiplied optimally at a lower temperature than , it was more stable at 26°. Isolate invaded uninoculated leaves only sporadically and accumulated in only small amounts. Isolate readily invaded uninoculated leaves, in which it reached its greatest concentration at 22°.


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