Changes in cyclobutyl pyrimidine dimer levels in rice (Oryza sativa L.) growing indoors and outdoors with or without supplemental UV-B radiation

We have examined the changes in the steady-state levels of cyclobutyl pyrimidine dimer (CPD) in rice plants (Oryza sativa L.) growing indoors and outdoors with or without supplemental UV-B radiation. The CPD levels remain between 3 and 6 CPD Mb^-1 throughout the day in plants grown under sunlight although the levels fluctuate. The CPD levels are slightly elevated with UV-B (1 W m^-2) supplementation, but remain constant (between 5 and 8 CPD Mb^-1) throughout the day. In plants grown under visible radiation in a growth chamber, the CPD levels remain constant but are lower compared with those in outdoor-mown plants; they are slightly elevated by supplementation with UV-B radiation. Under exposure to UV-B radiation without visible radiation, the CPD levels gradually increase with lengthening of the irradiation time and reach a level of approximately 35 CPD Mb^-1 at dusk. However, this level decreases during the night and then increases again the next day. It is thus evident that the CPD levels are strictly maintained within constant limits throughout the day in plants growing indoors and outdoors with or without supplemental UV-B radiation. It is also found that excision repair occurs during the night in plants having higher CPD levels.