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 pyrim idine dimer (CPD) in rice plants (Oryza sativa L.) growing indoors and outd oors with or without supplemental W-B radiation. The CPD levels remain betw een 3 and 6 CPD Mb(-1) throughout the day in plants grown under sunlight al though 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-grown 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 tim e and reach a level of approximately 35 CPD Mb(-1) at dusk. However, this l evel decreases during the night and then increases again the next day. It i s thus evident that the CPD levels are strictly maintained within constant limits throughout the day in plants growing indoors and outdoors with or wi thout supplemental W-B radiation. It is also found that excision repair occ urs during the night in plants having higher CPD levels. (C) 1999 Elsevier Science S.A. All rights reserved.