Summary: Of seven plant viruses tested, all except tobacco mosaic showed the phenomenon of photoreactivation, i.e. plants exposed to visible light after inoculation with preparations partially inactivated by ultraviolet radiation produced more local lesions than plants kept in darkness. Five strains of tobacco mosaic virus, which differed widely in their pathogenicity and other properties, were tested, but none showed the phenomenon. Of the six viruses that did, potato × showed it much the most strongly, tomato bushy stunt and a tobacco necrosis virus the least; cabbage black ringspot, cucumber mosaic and tobacco ringspot were intermediate.

Photoreactivation does not occur immediately after plants are inoculated. With irradiated preparations of potato virus X, most particles need about 30 min. to reach the state in which their ability to infect is affected by light; once in this state, exposure to daylight for 15 min. gives almost complete photoreactivation. Some irradiated particles respond to light of 80 f.c., but others need brighter light; no additional response occurs when light intensity is increased above 600 f.c. The sensitive state of potato virus X persists for about 1 hr. in plants kept in darkness, after which the particles seem to be permanently inactivated.

With antibodies and enzymes, a constant amount of absorbed energy decreases the activity of a unit weight by a given fraction. This rule does not apply to plant viruses, which lose infectivity with less absorbed energy than the rule predicts. If viruses are more sensitive because they contain nucleic acid, their sensitivity is not a direct function of their content of nucleic acid. Potato virus X has the same nucleic acid content as strains of tobacco mosaic virus, but is inactivated by less absorbed energy and individual strains of tobacco mosaic virus also differ by factors of two in the amount of radiation needed to decrease their infectivity by a given fraction.


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