MORPHOLOGICAL AND PHYSIOLOGICAL-RESPONSES OF BRASSICA-NAPUS TO ULTRAVIOLET-B RADIATION - PHOTOMODIFICATION OF RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE AND POTENTIAL ACCLIMATION PROCESSES/

As the stratospheric ozone layer is depleted, the biosphere will be ex posed to higher levels of ultraviolet-B (UV-B) radiation (290-320 nm). Using laboratory light sources that simulate the spectral quality of sunlight, we are examining some of the mechanisms involved in plant re sponses to UV-B. It was found that exposure of ribulose-1,5-bisphospha te carboxylase/oxygenase (Rubisco) from Brassica napus to UV-B in vivo or in vitro resulted in production of a high molecular weight (HMW) v ariant of the large subunit. Coincident with formation of the HMW prod uct in vitro was a loss in tryptophan fluorescence. To protect against damage, plants can acclimate to UV-B. To this end, we have studied co tyledon curling in B. napus; a photomorphogenic response specific to U V-B. To characterize the photoreceptor for curling, inhibitors of phot ochemical signaling were employed. A quencher of flavin excitation, an d inhibitors of Ca++ and cyclic nucleotide signaling diminished curlin g. Biosynthesis of flavonoids and other UV-absorbing pigments also occ urred in B. napus exposed to the levels of UV-B that caused curling. T o determine which flavonoids and other UV-absorbing compounds were UV- B specific, HPLC analysis was carried out. Approximately 20 distinct U V-absorbing pigments were produced in response to UV-B radiation. Thus , using B. napus we were able to follow UV-B induced damage and acclim ation.