Some effects of ultra-violet radiation on leaves of french bean (Phaseolus vulgaris L.)

Exposing the primary leaves of French bean to ultra-violet light produces a variety of effects, their magnitude depending on the extent of the exposure and the subsequent treatment of the leaves. Effects include some that are externally obvious, such as glazing and bronzing; others are detectable by changes in susceptibility to virus infection, by increased sensitivity to damage by ultra-violet, and by an increased tendency for cells to collapse when the leaves are kept in darkness. Some of the effects can be counteracted by exposing leaves to visible light. Effects of radiation are not confined to epidermal cells, and measurements with detached epidermis indicate that from a quarter to a half of incident radiation of wave-length 2536 A. may be transmitted by the epidermis. The susceptibility of irradiated leaves to infection by a tobacco necrosis virus, as measured by the number of lesions produced by a given inoculum, depends on the irradiation dose, the time elapsing between irradiation and inoculation, and on whether the plants are in the light or dark after the treatments. When inoculated immediately after irradiation, leaves produce fewer lesions than unirradiated ones even if they are kept in the light, but the difference is much enhanced if they are in the dark. When leaves are inoculated a day after irradiation, provided the dose was not excessive, those kept in daylight after irradiation produce more lesions than unirradiated ones, whereas those kept in the dark produce many fewer. Thus, the temporary resistance produced by ultra-violet irradiation is not only counteracted by exposure to visible light, but is succeeded by a period of enhanced susceptibility, a period that extends over several days.