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Flavonoid induction is regulated by complex signal transduction pathways involving cryptochrome, phytochrome and UVB photoreceptors. Previously, we identified the UVB-inducible flavonoids in Brassica napus cv. Topas leaves and showed that UVA affected accumulation of the quercetin (Q) and kaempferol (K) glycosides (Wilson et al. [2000] Photochem. Photobiol. 73, 678-684). In this study, we examined the effects of far-red light (FR, 700-780 nm) on UVB-mediated flavonoid accumulation in B. napus. Plants were grown under photosynthetically active radiation (PAR, 400-700 nm, 150 micromol m(-2) s(-1)) plus a moderate level of FR (35 micromol m(-2) s(-1)) for 14 days, and then transferred to five different irradiation regimes (PAR +/- [UVA + UVB] + moderate, intermediate or low fluence FR) for 4 days. Kinetics of flavonoid accumulation were assessed via HPLC. Accumulation of flavonoids, in general, was suppressed by increasing the amount of FR in the spectrum. Furthermore, addition of UVB (290-320 nm) to the spectrum altered the flavonoid composition by causing significant changes in the quantities of individual flavonoids. The relative levels of acylated K glycosides were diminished whereas the relative levels of nonacylated Q glycosides increased dramatically. With UVB exposure there was a five-fold increase in the Q:K ratio. In contrast, increasing the level of FR in the presence of UVB decreased the Q:K ratio by half.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants known for their photoinduced toxicity. This toxicity may occur through two mechanisms: Photosensitization, and photomodification. Photosensitization generally leads to the production of singlet oxygen, a reactive oxygen species that is highly damaging to biological molecules. Photomodification of PAHs, usually via oxidation, results in the formation of new compounds and can occur under environmentally relevant levels of actinic radiation. The toxicities of 16 intact PAHs to Daphnia magna were assessed under two ultraviolet radiation conditions. The toxicity of intact PAHs generally increased in the presence of full-spectrum simulated solar radiation relative to that in the presence of visible light plus ultraviolet A only. Despite the knowledge of a bipartite mechanism of phototoxicity that includes photosensitization and photomodification, few studies have examined the effects of PAH photoproducts on animals. To expand the existing data, 14 PAH photoproducts (oxy-PAHs) also were assayed, most of which were highly toxic without further photomodification. Two photoproducts of benzo[a]pyrene, 1,6- and 3,6-benzo[a]pyrenequinone, were the most toxic compounds tested, followed closely by benz[a]anthraquinone. Each of these three compounds had a median effective concentration in the low nanomolar range. The data presented highlight the effects of ultraviolet radiation on mediating PAH toxicity and the need to analyze absorption spectra of contaminants in the prediction of photoinduced toxicity. The importance of the role of photomodification also is stressed, because several oxy-PAHs, an unregulated group of contaminants, were highly toxic to D. magna, a key bioindicator species in aquatic ecosystems.

Among the photomorphological responses in plants induced by ultraviolet-B radiation (UVB; 290 nm-320 nm) are leaf asymmetry, leaf thickening and cotyledon curling. We constructed an action spectrum of cotyledon curling in light-grown Brassica napus to characterize the UVB photoreceptor that initiates this response. Cotyledon curling was also characterized in Arabidopsis thaliana. Peak efficiency for this response occurred between 285 and 290 nm. Additionally, UVB-induced changes in epidermal cells from A. thaliana cotyledons were assessed because they are the likely site of UVB photoreception that leads to curling. Investigation of cellular structure, chlorophyll a fluorescence and chlorophyll concentration indicated that cotyledon curling is not concomitant with gross cellular damage or inhibition of photosynthesis, which only occurred in response to wavelengths <280 nm. Many UVB effects are apparently an indirect consequence of UVB radiation, dependent on UVB-mediated increases in reactive oxygen species (ROS) that either act as a signal in the UVB transduction pathway or cause oxidative damage. The cotyledon curling response was impeded by ascorbate and cystine, ROS scavengers and was promoted by H(2)O(2), a ROS. We suggest that following absorption by a UVB chromophore, ROS are generated via photosensitization, ultimately leading to cotyledon curling.

Selenastrum capricornutum was grown in two lake waters of differing dissolved organic carbon content (1.8 vs. 9.1 mg DOCl(-1)) to determine the responses of population dynamics and photosynthesis to Cu, and to assess the modifying effects of varying ultraviolet radiation (UVR) exposure. In the absence of UVR, the mean EC(50) for Cu effect on population growth rate was 2.3-2.6 microg l(-1) in the low DOC water and 17.4-26.2 microg l(-1) in the high DOC water. The variable chlorophyll a fluorescence ratio, F(v)/F(m), decreased approximately in parallel with the diminished growth rates. Exposure of the higher DOC lake water to full spectrum artificial radiation caused an increase of Cu(2+) concentration, compared to samples held in darkness or in photosynthetically active radiation (PAR) only. Full spectrum exposures also resulted in a lower (although not significantly so) EC(50) for Cu effect on growth rate, consistent with response to the moderately elevated Cu(2+) concentration. Cu(2+) concentration was unaffected by radiation exposure in the low DOC water, and EC(50)s for growth were also unaffected except in the most severe UVR treatment, which was >40% inhibited even in the absence of added Cu. Using F(v)/F(m) as an end-point, there was no evidence of interactions between UVR and Cu under the relatively low PAR exposures used here. Algal growth and photosynthesis was extremely sensitive to Cu in these soft lake waters, with EC(50)s close to current water quality standards in the low DOC water.

Plectonema boryanum UTEX 485 cells were grown at 29 degrees C and 150 mumol m-2 s-1 photosynthetically active radiation (PAR) and exposed to PAR combined with ultraviolet-A radiation (UV-A) at 15 degrees C. This induced a time-dependent inhibition of photosystem II (PSII) photochemistry measured as a decrease of the chlorophyll a fluorescence ratio, Fv/Fm, to 50% after 2 h of UV-A treatment compared to nontreated control cells. Exposure of the same cells to PAR combined with UV-A + ultraviolet-B radiation (UV-B) caused only a 30% inhibition of PSII photochemistry relative to nontreated cells. In contrast, UV-A and UV-A + UV-B irradiation of cells cultured at 15 degrees C and 150 mumol m-2 s-1 had minimal effects on the Fv/Fm values. However, cells grown at 15 degrees C and lower PAR irradiance (6 mumol m-2 s-1) exhibited similar inhibition patterns of PSII photochemistry as control cells. The decreased sensitivity of PSII photochemistry of P. boryanum grown at 15 degrees C and 150 mumol m-2 s-1 to subsequent exposure to UV radiation relative to either control cells or cells grown at low temperature but low irradiance was correlated with the following: (1) a reduced efficiency of energy transfer to PSII reaction centers; (2) higher levels of a carotenoid tentatively identified as myxoxanthophyll; (3) the accumulation of scytonemin and mycosporine amino acids; and (4) the accumulation of ATP-dependent caseinolytic proteases. Thus, acclimation of P. boryanum at low temperature and moderate irradiance appears to confer significant resistance to UV-induced photoinhibition of PSII. The role of excitation pressure in the induction of this resistance to UV radiation is discussed.

It has recently been demonstrated that light dramatically enhances the toxicity of polycyclic aromatic hydrocarbons (PAHs) to the duckweed Lemna gibba L. G-3 (L. Ren, X.-D. Huang, B.J. McConkey, D.G. Dixon, and B.M. Greenberg, 1994, Ecotoxicol. Environ. Saf. 28, 160-171). To extend this research to terrestrial plants, Brassica napus L. (oil seed rape) seeds were germinated in the presence of three PAHs; anthracene (ANT), benzo[a]pyrene (BAP), and fluoranthene. The chemicals were applied both in intact form and following photomodification in UV-B radiation; toxicity was assessed in simulated solar radiation (SSR), a light source with a visible light:UV-A:UV-B ratio similar to that of sunlight. Germination efficiency, root and shoot growth, and chlorophyll content, measured after 6 days of exposure, were used as toxicity endpoints. Intact and photomodified PAHs had little impact on shoot fresh weight or chlorophyll content, but markedly inhibited root fresh weight, with the photomodified PAHs having greater impacts than the intact PAHs. The decline in root fresh weight was not attributable to a decline in germination frequency or delayed germination. However, the seedlings produced shorter roots in the presence of either intact or photomodified PAHs. To explore the role of actinic radiation on PAH toxicity, seedlings were incubated in SSR, visible light and darkness with either intact or photomodified PAHs. Inhibition of root growth was only achieved by the intact chemicals if actinic radiation was present. However, with photomodified ANT or photomodified BAP, root fresh weight accumulation was inhibited in SSR, visible light and darkness. Thus, intact PAHs are hazardous to terrestrial plants in the presence of light, but once the compounds are photomodified, actinic radiation is no longer an absolute requirement for phytotoxic activity.

A component of the photosystem II reaction center, the 32-kDa protein, is rapidly turned over in the light. The mechanism of its light-dependent metabolism is largely unknown. We quantified the rate of 32-kDa protein degradation over a broad spectral range (UV, visible, and far red). The quantum yield for degradation was highest in the UVB (280-320 nm) region. Spectral evidence demonstrates two distinctly different photosensitizers for 32-kDa protein degradation. The data implicate the bulk photosynthetic pigments (primarily chlorophyll) in the visible and far red regions, and plastoquinone (in one or more of its redox states) in the UV region. A significant portion of 32-kDa protein degradation in sunlight is attributed to UVB irradiance.