Arthur and Newell (1929) may have been the first to note that exposure to ultraviolet radiation causes plant leaves to turn yellow. Since this effect of ultraviolet radiation is counteracted by visible light, Tanada and Hendricks (1953) attributed this to dimer formation in DNA, a lesion which can be repaired by photoreactivation (Chessin and Cohen, 1962). Indeed, two different types of photolyase have, as already mentioned, been demonstrated in plants. However, the phenomenon may also have another explanation. Several investigators, and most clearly Wu (1971) and Wu et al. (1973) have produced evidence that a diffusable factor is produced in the epidermis as a result of irradiation, and that this factor upon entering the mesophyll speeds up breakdown of chlorophyll in excised leaves. If the epidermis is removed immediately after irradiation of the leaves, and also if the factor is leached out by allowing the leaves to float on water, irradiated epidermis down, the UV effect is canceled. If a sufficiently high exposure to UV is administered, cells die immediately (necrosis), and the yellowing process does not take place even if the epidermis is left intact.
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