Some effects of ultraviolet radiation on the infection of Nicotiana glutinosa leaves by tobacco mosaic virus

Ultraviolet radiation affects the capacity of Nicotiana glutinosa leaves to support the multiplication of tobacco mosaic virus (TMV) more than was assumed by previous workers, and conclusions drawn from experiments in which leaves were irradiated at different intervals after inoculation are of doubtful validity. The effects vary greatly with changes in the physiological state of leaves, and, although exposing irradiated leaves to daylight can repair irradiation damage, photoreactivation does not produce a constant response; depending on the dose of radiation energy and the state of the leaves, photoreactivation may enhance the original capacity, restore it completely, or restore it only partially. Rubbing leaves twice can prevent daylight from repairing damage done by radiation to the leaf's capacity.

The response to photoreactivation also depends on the type of inoculum. When leaves are inoculated with intact TMV immediately after irradiation, exposure to daylight increases the number of lesions to the same extent as when irradiated leaves are kept in daylight for some hours before they are inoculated. By contrast, leaves inoculated with infective nucleic acid immediately after irradiation produce no more lesions when kept in the light than when kept in the dark, irradiated leaves kept in the light for some hours before inoculation, however, produce more lesions than comparable leaves kept in darkness. Photoreactivation of the leaf's capacity to support infection takes some time, and it seems that the intact virus particles can survive this time unharmed in vivo whereas the unstable nucleic acid is inactivated. Further evidence for the instability of the nucleic acid came from experiments with irradiated inocula; the nucleic acid was photoreactivated when leaves were exposed to daylight immediately after inoculation, but not when they were kept in darkness for 0.5–1 hour before being exposed to light. Nucleic acid that does not establish infection within an hour of inoculation seems to be destroyed.