Effects of ultraviolet-A exposure on ultraviolet-B-induced accumulation ofspecific flavonoids in Brassica napus

Many plant species are able to acclimate to changes in ultraviolet-B radiat ion (UVB) (290-320 nm) exposure. Due to the wide range of targets of UVB, p lants have evolved diverse repair and protection mechanisms. These include increased biosynthesis of UVB screening compounds, elevated antioxidant act ivity and increased rates of DNA repair. We have shown previously that Bras sica napus L. cv Topas plants can acclimate quite effectively to environmen tally relevant increases in UVB through the accumulation of specific flavon oids in the leaf epidermis. However, B. napus was found to lose other flavo noids when plants are exposed to ultraviolet-A radiation (UVA) (320-400 nm) and/or UVB (Wilson et al. [1998] Photochem, Photobiol, 67, 547-553). In th is study we demonstrate that the levels of all the extractable flavonoids i n the leaves of B, napus plants are decreased in a dose-dependent manner in response to UVA exposure. Additionally, the accumulation of the extractabl e flavonoids was examined following a shift from photosynthetically active radiation (PAR) + UVA to PAR + UVB to assess if preexposure to UVA affected UVB-induced flavonoid accumulation. UVA preexposures were found to impede UVB-induced accumulation of some flavonoids, This down regulation was parti cularly evident for quercetin-3-O-sophoroside and quercetin-3-O-sophoroside -7-O-glucoside, which is interesting because quercetins have been demonstra ted to be induced by UVB and correlated with UVB tolerance in some plant sp ecies. The photobiological nature of these UVA-mediated effects on flavonoi d accumulation implies complex interactions between UVA and UVB responses.