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SUMMARY Residual infectivities of u.v.-irradiated preparations of potato virus X were assayed on three different kinds of plants. When the plants were kept in darkness to prevent photoreactivation of the inactivated virus, the residual infectivity appeared the same whether Chenopodium amaranticolor or either of two varieties of tobacco was used for assay. When the plants were exposed to daylight, photoreactivation occurred in all three kinds of plants, possibly to a somewhat greater extent in Chenopodium than in tobacco plants. Thus, in contrast to the results previously obtained with tobacco necrosis virus, no evi- dence was obtained for dark reactivation of u.v.-inactivated potato virus X in Chenopodium. In the absence of photoreactivation, the quantum yields for inactivation were about 0.8 x IO -~ and I. 3 x io -3 for the whole virus and for the RNA in- side the virus, respectively. INTRODUCTION Kleczkowski (I968a) obtained evidence that some damage to tobacco necrosis virus, caused by exposure to ultraviolet radiation (u.v.), was repaired in darkness in Chenopodium amaranticolor but not in tobacco (Nicotiana tabacum) or French bean (Phaseolus vulgaris) plants. The residual infectivity was increased by photoreactivation in tobacco and in French bean, but not in Chenopodium, suggesting that the kind of damage repaired in darkness in Chenopodium includes all or most of the damage that can be repaired by photoreactivation. The results obtained with tobacco necrosis virus are not generally applicable to all plant viruses, because Werbin et al. (I966), who used the same range of host plants to test the residual infectivity of u.v.-irradiated RNA from tobacco mosiac virus, obtained no indication of dark reactivation and observed photoreactivation in all the test plants. Therefore, it seemed desirable that some other plant viruses should be investigated for the phenomenon of dark reactiva- tion and we describe experiments with potato virus X, chosen because it shows more photoreactivation than any other plant virus yet tested (Bawden & Kleczkowski, 1955). The results give no evidence of dark reactivation in Chenopodium of irradiated potato virus X and there was at least as much photoreactivation in Chenopodium as in tobacco. METHODS Potato virus X. The virus (strain cs 35) from systemically infected leaves of tobacco plants (var. White Burley) was purified by clarifying sap with chloroform, followed by two cycles of differential centrifugation and centrifugation through a sucrose gradient. The preparation was finally dialysed against o.oi M-borate buffer at pH 7"5, sedimented

Residual Infectivity and the Extent of Photoreactivation in Three Different Host Plants of u.v.-irradiated Potato Virus X