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SUMMARY: Of seven plant viruses tested, all except tobacco mosaic showed the phenomenon of photoreactivation, i.e. plants exposed to visible light after inocula- tion with preparations partially inactivated by ultraviolet radiation produced more local lesions than plants kept in darkness. Five straiIis 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 X 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 sensi- tive 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. Visible light mitigates the damaging effects that ultraviolet radiation has on many kinds of cells (Kelner, 1949,1951 ; Bawden & Kleczkowski, 1952). It can also partially restore the activity of irradiated preparations of some viruses ; exposure of the irradiated viruses to light in vitro has no effect on their ability to multiply, but more of the irradiated particles multiply when recently in- fected cells are exposed to light than when they are kept in darkness (Dulbecco, 1950; Bawden & Kleczkowski, 1953). The alleviating effect of visible light both on irradiated cells (Kelner, 1949) and bacterial viruses (Dulbecco, 1950) has been called ' photoreactivation ', and for simplicity and uniformity we shall also use this term. However, we stress the fact that there is no evidence to show whether the phenomenon occurs because changes caused in virus par- ticles by the radiation are reversed in illuminated cells, or because conditions in such cells favour infection and allow damaged particles to multiply. Of the three viruses we studied earlier, tomato bushy stunt and the Rotham- sted tobacco necrosis virus showed the phenomenon of photoreactivation, and

Studies on the Ability of Light to Counteract the Inactivating Action of Ultraviolet Radiation on Plant Viruses