Changes in the relaxation of electrochromic shifts of photosynthetic pigments and in the levels of mRNA transcripts in leaves of Pisum sativum as a result of exposure to supplementary UV-B radiation. The dependency on the intensity of the photosynthetically active radiation

Supplementary ultraviolet-B radiation (UV-B) caused decreases in mRNA levels for photosynthetic genes and for the chloroplastic defensive enzyme Cu/Zn-superoxide dismutase. The magnitude of these decreases depended on the intensity of the photosynthetically active radiation (PAR): the mRNA levels being considerably higher under high PAR (400-600 mu E m(-2) s(-1)) than under low (150 mu E m(-2) s(-1)). Transcript levels for the non-chloroplastic defence protein chalcone synthase also were PAR-dependent, since an increase of mRNA transcripts occurred under low PAR only. UV-B also caused a PAR-independent increase in the rates of relaxation of the single turnover flash-induced electrochromic shift of thylakoid pigments. The UV-B doses needed were about 10-fold lower than the doses needed to affect photosynthetic components. Chlorophyll content and chlorophyll fluorescence of PSII were unaltered by these doses. We suggest that the lowered sensitivity of mRNA levels under high PAR may be caused by a protective mechanism at the level of DNA or transcription. This mechanism would not be involved in repair of the thylakoid membrane or its lipids. These results also highlight the importance of PAR intensity for the assessing of UV-B effects on plants.