UV-A Mediated Modulation of Photosynthetic Efficiency,Xanthophyll Cycle and Fatty Acid Production of <Emphasis Type="Italic">Nannochloropsis</Emphasis>

Nannochloropsis, a green microalga, is a source for commercially valuable compounds as extensively described and, in particular, is recognised as a good potential source of eicosapentaenoic acid (20:5ϖ3), an important polyunsaturated fatty acid for human consumption for prevention of several diseases. Climate change might include variation in the ultraviolet (UV) levels as one of the consequences derived from the anthropogenic activity. This paper shows the response of Nannochloropsis cultures exposed for 7 days to UV-A (320–400 nm) added to photosynthetically active radiation (PAR; 400–700 nm). Growth rates and photosynthetic activity were assessed to determine the impact of UV-A increased levels on the cell growth and basic metabolism activity. Xanthophyll pigments (zeaxanthin and violaxanthin), carotenoids (canthaxanthin and β-carotene) and polyunsaturated fatty acids (myristic, palmitic, palmitoleic, arachidonic and eicosapentaenoic acids) were measured for assessing the antioxidant response of the microalgae to added UV-A radiation to PAR. The results show that the modulated use of UV-A radiations can lead to increased growth rates, which are sustained in time by an increased light transduction activity. The expected antioxidant response to the incident UV-A radiation consisted of increases in zeaxanthin and β-carotene contents—synthesis of antioxidant carotenoids—and increases in the saturated fatty acids to polyunsaturated fatty acids ratio. The results suggest that modulated UV-A radiation can be used as a tool to stimulate value molecules accumulation in microalgae through an enhanced both light transduction process and antioxidant response, while sustaining cell growth.     

Elevated UV-B radiation incident on Quercus robur leaf canopies enhances decomposition of resulting leaf litter in soil

We examined whether the exposure of Quercus robur L. to elevated UV-B radiation (280–315 nm) during growth would influence leaf decomposition rate through effects on litter quality. Saplings were exposed for eight months at an outdoor facility in the UK to a 30% elevation above the ambient level of erythemally weighted UV-B radiation under UV-B treatment arrays of fluorescent lamps filtered with cellulose diacetate, which transmitted both UV-B and UV-A (315–400 nm) radiation. Saplings were exposed to elevated UV-A alone under control arrays of lamps filtered with polyester and to ambient radiation under unenergised arrays of lamps. Abscised leaves from saplings were enclosed in 1 mm² mesh nylon bags, placed in a Quercus–Fraxinus woodland and were sampled at 0.11, 0.53, 1.10 and 1.33 years for dry weight loss, chemical composition and saprotrophic fungal colonization. At abscission, litters from UV-A control arrays had ≈ 7.5% higher lignin/nitrogen ratios than those from UV-B treatment and ambient arrays (P < 0.06). Dry weight loss of leaves treated with elevated UV-B radiation during growth was 2.5% and 5% greater than that of leaves from UV-A control arrays at 0.53 and 1.33 years, respectively. Litter samples from UV-B treatment arrays lost more nitrogen and phosphorus than samples from ambient arrays and more carbon than samples from UV-A control arrays. The annual fractional weight loss of litter from UV-B treatment arrays was 8% and 6% greater than that of litter from UV-A control and ambient arrays, respectively. Regression analyses indicated that the increased decomposition rate of UV-B treated litters was associated with enhanced colonization of leaves by basidiomycete fungi, the most active members of the soil fungal community, and that the frequency of these fungi was negatively associated with the initial lignin/nitrogen ratio of leaves.     

Appropriate controls in outdoor UV-B supplementation experiments

Quercus robur L. saplings were exposed in an outdoor experiment to supplemental levels of UV-8 (280–315 nm) radiation using treatment arrays of cellulose diacetate-filtered fluorescent lamps that also produce UV-A (315–400 nm) radiation. Saplings were also exposed to UV-A radiation alone using control arrays of the same lamps filtered with polyester and to ambient levels of radiation, using arrays of unenergized lamps. The UV-B treatment was modulated to maintain a 30% elevation above the ambient level of UV-B radiation, measured by a broad-band sensor weighted with an erythemal action spectrum. Saplings exposed to UV-B radiation beneath treatment arrays developed thicker leaves than those beneath ambient and control arrays. Despite the fact that supplemental levels of UV-A radiation were only a small percentage of ambient levels, apparent UV-A effects were also recorded. Significant increases in sapling height, lammas shoot length and herbivory by chewing insects were observed under treatment and control arrays, relative to ambient, but there were no differences between the responses of saplings under treatment and control. These data imply that supplemental UV-A radiation or other effects associated with energised lamps can significantly affect plant growth parameters and herbivory in outdoor studies. We conclude that the results from current outdoor UV-B supplementation experiments that lack control exposures using polyester-filtered lamps need to be interpreted with caution and that future supplementation experiments should include appropriate controls.     

Responses of Strawberry (Fragaria × ananassa) to Supplemental UV-B Radiation and Selenium Under Field Conditions

In greenhouse experiments, selenium (Se) has been shown to defend plants against detrimental effects of heavy UV-B radiation stress. The aim of this study was to investigate whether this positive effect can be found in open-field conditions with enhancement of UV-B radiation. In the experiment, conducted with strawberry (Fragaria×ananassa, cultivars “Jonsok” and “Polka”) over two growing seasons, plants were exposed to UV-B radiation (including UV-A) and cultivated without Se or supplied with Se added at two levels (0.1 and 1.0 mg kg⁻¹). The plants were monitored for growth, flavonoids, chlorophyll fluorescence, net photosynthesis as well as tissue and cell structure. Photosystem II was observed to be sensitive to UV-B stress under field conditions. In the leaves, a decrease in F_v/F_m was seen at the end of the growing season, implying a cumulative effect of UV-B stress. Several parameters, especially cell and tissue structures, were affected by UV-B and UV-A treatments, which proves the need for UV-A control in outdoor UV-B supplementation studies. Addition of Se did not ameliorate the harmful effects of UV-B but the lower Se-increment level increased leaf growth. The effects of UV-B and Se differed during the two experimental years, indicating the need to repeat experiments during several growing seasons.     

An integrated wavelength-shifting strategy for enhancement of microalgal growth rate in PMMA- and polycarbonate-based photobioreactors

This study investigates the spectral shifting of UV-A radiation, using fluorescent material, as a tool for enhancing Chlorella sp. growth rate in simulated photobioreactors made of UV-stabilized polycarbonate (PC) and poly (methyl methacrylate) (PMMA). The feasibility of using a fluorescent coating as a wavelength shifter layer, to shift UV-A radiation to the PAR range, was explored. For this purpose, a variety of concentrations of fluorescent dye dissolved in thermoplastic acrylic resin were prepared and used to coat PMMA and PC sheets which then were placed between the radiation source and the culture flask. Compared with the uncoated sheets, the panels coated with the wavelength shifter layer exhibited 74% and 45% (for PC and PMMA substrates, respectively) increase in biomass productivity during the same culture period. It was also found that the elimination of UV-A radiation increased chlorophyll a content of the cells.     

The Role of Flavonol Glycosides and Carotenoids in Protecting Soybean from Ultraviolet-B Damage

The increase in ultraviolet-B (UV-B; 0.290-0.320 [mu]m) radiation received by plants due to stratospheric ozone depletion heightens the importance of understanding UV-B tolerance. Photosynthetic tissue is believed to be protected from UV-B radiation by UV-B-absorbing compounds (e.g. flavonoids). Although synthesis of flavonoids is induced by UV-B radiation, its protective role on photosynthetic pigments has not been clearly demonstrated. This results in part from the design of UV-B experiments in which experimental UV-A irradiance has not been carefully controlled, since blue/UV-A radiation is involved in the biosynthesis of the photosynthetic pigments. The relationship of flavonoids to photosynthetic performance, photosynthetic pigments, and growth measures was examined in an experiment where UV-A control groups were included at two biologically effective daily UV-B irradiances, 14.1 and 10.7 kJ m-2. Normal, chlorophyll-deficient, and flavonoid-deficient pigment isolines of two soybean (Glycine max) cultivars that produced different flavonol glycosides (Harosoy produced kaempferol, Clark produced quercetin and kaempferol) were examined. Plants with higher levels of total flavonoids, not specific flavonol glycosides, were more UV-B tolerant as determined by growth, pigment, and gas-exchange variables. Regression analyses indicated no direct relationship between photosynthesis and leaf levels of UV-B-absorbing compounds. UV-B radiation increased photosynthetic pigment content, along with UV-B-absorbing compounds, but only the former (especially carotenoids) was related to total biomass (r2 = 0.61, linear) and to photosynthetic efficiency (negative, exponential relationship, r2 = 0.82). A reduction in photosynthesis was associated primarily with a stomatal limitation rather than photosystem II damage. This study suggests that both carotenoids and flavonoids may be involved in plant UV-B photoprotection, but only carotenoids are directly linked to photoprotection of photosynthetic function. These results additionally show the importance of UV-A control in UV-B experiments conducted using artificial lamps and filters.     

UV-A mediated induction of carotenoid accumulation in Dunaliella bardawil with retention of cell viability

The effect of adding UV-A radiation (320–400 nm) to photosynthetically active radiation (PAR, 400–700 nm) during growth of the photosynthetic marine microalga Dunaliella bardawil was investigated in this work in terms of cell growth and carotenoid production. Although signs of slow cell growth (slight reduction of chlorophyll and protein content) were observed after 24 h of cell exposure to UV-A (40 mol photons m^–2 s^–1 and 70 mol photons m^–2 s^–1) plus 140 mol photons m^–2 s^–1 PAR , 84 h exposure to these UV-A conditions slightly stimulated cell growth and increased the photosynthetic efficiency of the exposed cultures. The enhanced cell growth was coupled with an increase in total carotenoid content. Besides -carotene as the major pigment, increases in the well-known antioxidants lutein and zeaxanthin of about 3-fold and 5-fold, respectively, were determined in cultures exposed to UV-A radiation of 70 mol photons m^–2 s^–1for 84 h. As a consequence, far from being negative to cell growth, low and medium UV-A radiation are stress factors that could be successfully applied to long-term processes for large scale carotenoid production using D. bardawil cultures with retention of cell viability. UV-A exposure has the advantage of being a factor either easily applied or removed as required, in contrast to other nutrient stresses, which require medium replacement for their application.     

Effects of elevated ultraviolet radiation on Quercus robur and its insect and ectomycorrhizal associates

Saplings of pedunculate oak (Quercus robur L.) were exposed at an outdoor facility to modulated levels of elevated UV-B radiation (280–315 nm) under treatment arrays of cellulose diacetate-filtered fluorescent lamps which also produced UV-A radiation (315–400 nm). Saplings were also exposed to UV-A radiation alone under control arrays of polyester-filtered lamps and to ambient levels of solar radiation under arrays of unenergized lamps. The UV-B treatment corresponded to a 30% elevation above the ambient level of erythemally weighted UV-B radiation. Sapling growth and the occurrence of associated organisms were examined over two years. In both years, leaves of saplings exposed to UV-B treatment were thicker and smaller in area relative to leaves exposed to ambient and control levels of radiation. UV-B treatment also retarded bud burst at one sampling in the first year of the study. Some responses were recorded which were common to both treatment and control arrays, implying that UV-A radiation, or some other factor associated with energized lamps, was responsible for the observed effects. Saplings under treatment and control arrays were taller in the first year of the study, suffered greater herbivory from chewing insects, and had lower root dry weights and greater insertion heights of secondary branches than saplings exposed to ambient levels of radiation. Exposure of saplings to elevated UV-A radiation alone under control arrays increased estimated leaf volumes in the second year of the study and reduced the number of secondary branches and the total number of branches per sapling after two years, relative to both treatment and ambient arrays. There were no effects of elevated ultraviolet radiation on shoot or total plant weight, root/shoot ratios, stem diameter, the numbers or insertion heights of primary or tertiary branches, total leaf number, timing of leaf fall or frequency of ectomycorrhizas. Our study suggests that any increases in UV-B radiation as a result of stratospheric ozone depletion will influence the growth of Q. robur primarily through effects on leaf morphology.     

Growth, structure, stomatal responses and secondary metabolites of birch seedlings (Betula pendula) under elevated UV-B radiation in the field

Three-year-old birch (Betula pendula Roth.) seedlings were exposed, in the field, to supplemental levels of UV-B radiation. Control seedlings were exposed to ambient levels of UV radiation, using arrays of unenergized lamps. A control for UV-A radiation was also included in the experiment. Enhanced UV-B radiation had no significant effects on height growth, and shoot and root biomass of birch seedlings. Leaf expansion rate increased transiently in the middle of the growing period in enhanced UV-B- and UV-A-exposed plants; however, final leaf size and relative growth rate remained unaffected. Leaf thickness and spongy intercellular spaces were increased in UV-B-exposed seedlings along with increased density of glandular trichomes. At the ultrastructural level, enhanced UV-B increased the number of cytoplasmic lipid bodies, and abnormal membrane whorls were found. Both enhanced UV-B and UV-A radiation induced swelling of chloroplast thylakoids. Stomatal density and conductance were significantly increased by elevated UV-B radiation. UV-A radiation increased the length and width of stomata, whereas UV-B radiation had only a marginal effect on stomatal size. UV-A and enhanced UV-B radiation attenuated the appearance of necrotic spots in autumn, probably caused by the fungus Pyrenopeziza betulicola, suggesting a direct harmful effect of UV on pathogens or reduced susceptibility to pathogens in UV-exposed seedlings. Secondary metabolite analysis showed increases in (+)-catechin, quercetin, cinnamic acid derivative, apigenin and pentagalloylglucose in birch leaves under enhanced UV-B radiation. Negative correlations between apigenin, and particularly quercetin concentrations and lipid peroxidation levels indicated an antioxidant role of secondary metabolites in birch leaves exposed to UV-B radiation.     

Phenolic compounds and antioxidant properties in the snow alga Chlamydomonas nivalis after exposure to UV light

The snow alga Chlamydomonas nivalis was collected from the Sierra Nevada, California, USA, and examined for its ability to produce phenolic compounds, free proline, and provide antioxidant protection factor in response to UV-A and UV-C light. Exposure of C. nivalis cells to UV-A light (365nm) for 5 days resulted in a 5–12% increase in total phenolics, where as exposure to UV-C light (254 nm) resulted in a 12–24% increase in phenolics after 7 days of exposure. Free proline was not affected by UV-A, but increased markedly after UV-C exposure. A three-fold increase in free proline occurred within two days after exposure to UV-C, but then dropped as cells became bleached. Antioxidant protection factor (PF) increased after treatment of cells with UV-A and remained constant throughout UV-C exposure. Spectral analysis of algal extracts revealed a decrease in absorption in the 215–225 nm region, short-term (2day) stimulation of pigment at 280 nm, and an increase in carotenoids (473 nm), after exposure to UV-A. Snow alga exposed to UV-C light had a different spectrum from that of UV-A exposed cells, i.e. an enhancement of three major peaks at 220, 260, and 280 nm, and loss of absorption in the carotenoid region.We report that UV light exposure, especially in the UV-C range, can stimulate phenolic-antioxidant production in aplanospores of C. nivalis effecting biochemical pathways related to proline metabolism. This revised version was published online in August 2006 with corrections to the Cover Date.     

CHARACTERIZATION AND BIOLOGICAL IMPLICATIONS OF SCYTONEMIN,A CYANOBACTERIAL SHEATH PIGMENT1

Scytonemin, the yellow-brown pigment of cyanobacterial (blue-green algal) extracellular sheaths, was found in species thriving in habitats exposed to intense solar radiation. Scytonemin occurred predominantly in sheaths of the outermost parts or top layers of cyanobacterial mats, crusts, or colonies. Scytonemin appears to be a single compound identified in more than 30 species of cyanobacteria from cultures and natural populations. It is lipid soluble and has a prominent absorption maximum in the near-ultraviolet region of the spectrum (384 nm in acetone; ca. 370 nm in vivo) with a long tail extending to the infrared region. Microspectrophotometric measurements of the transmittance of pigmented sheaths and the quenching of ultraviolet excitation of phycocyanin fluorescence demonstrate that the pigment was effective in shielding the cells from incoming near-ultraviolet-blue radiation, but not from green or red light. High light intensity (between 99 and 250 μmol photon · m^?2· S^?1, depending on species) promoted the synthesis of scytonemin in cultures of cyanobacteria. In cultures, high light intensity caused reduction in the specific content of Chl a and phycobilins, increase in the ratio of total carotenoids to Chl a, and scytonemin increase. UV-A (320–400 nm) radiation was very effective in eliciting scytonemin synthesis. Scytonemin production was physiological and not due to a mere photochemical conversion. These results strongly suggest that scytonemin production constitutes an adaptive strategy of photoprotection against short-wavelength solar irradiance.     

Preillumination of excised spinach leaves with red light increases resistance of photosynthetic apparatus to UV radiation

Leaves of spinach (Spinacia oleracea, cv. Ispolinskii) were preilluminated by low-intensity light (1.0 and 1.5 W/m², 0.5?C3.0 h) with wavelengths ranging from 530 to 730 nm to study the effect of this pretreatment on the activity of photosystem II (PS II), content of photosynthetic pigments, and peroxidase activity in excised leaves exposed to UV-A irradiation. Irradiation of leaves with UV-A suppressed the activity of PS II, reduced the content of chlorophylls (a + b) and carotenoids, and increased the peroxidase activity. Preillumination of leaves with red light (RL, 620?C660 nm) alleviated the inhibitory action of UV-A on PS II activity and reduced the pigment losses but increased the peroxidase activity in leaves and thylakoid membrane preparations, as compared to the respective effects of UV-A light applied without preillumination. The preexposure of leaves to red light alternating with far-red light (FR, 730 nm) removed partly the influence of RL on the parameters under study, which indicates the involvement of phytochrome active form, P_FR into stress-induced defense responses in leaves. It is supposed that elevated resistance of photosynthetic apparatus to UV-A radiation was formed with the involvement of P_FR and the antioxidant system induced by oxidative stress after preillumination of leaves with red light     

UV Effects on Pigments and Assimilation of 15N-Ammonium and 15N-Nitrate by Natural Marine Phytoplankton of the North Sea

Effects of ambient solar UV radiation in the field and of artifical UV irradiation under controlled laboratory conditions were studied with natural phytoplankton populations from Helgoland, German Bight, North Sea. The pattern of pigments varied after UV-A or UV-B plus a low dose of UV-A radiation: UV-A usually induced a stimulation of pigment biosynthesis; whereas UV-B plus UV-A led to a reduction of the contents of chlorophyll a, diadinoxanthin, fucoxanthin, peridinin and an unknown carotenoid; content of diatoxanthin was significantly enhanced. The damaging effect on nitrogen assimilation by UV was more pronounced after artificial UV-B plus UV-A irradiance compared to the influence of ambient solar UV under field conditions. The uptake of inorganic nitrogen was dependent on the dose and exposure time of UV radiation as well as on the species composition. The uptake of ¹⁵N-nitrate by natural phytoplankton collected in spring was more sensitive to UV irradiation than the assimilation of ¹⁵N-ammonium. UV-A radiation with a small part of shorter wavelengths at 315 nm (Philips-lamps in conjunction with the cut-off filter WG 320) caused a reduction of up to 12% whereas a stimulation of the ¹⁵NH₄⁺ uptake was observed after exposure to UV-A without any UV-B (Philips lamps TL 60W/09N). Pattern of ¹⁵N-incorporation into free amino acids and pool sizes varied in dependence on the applied nitrogen compound and on the irradiation conditions. The impact of UV radiation on the pattern of ¹⁵N-Iabelled free amino acids and the pool sizes was different. ¹⁵N enrichment into all the tested amino acids was reduced after 5 h UV-B plus UV-A exposure and after application of ¹⁵NH₄⁺. A depression of the glutamate and glutamine pools was observed after addition of ¹⁵N-nitrate alone. Pools of all main amino acids from phytoplankton in summer 1993/94 were inhibited by UV irradiance. Results are discussed with reference to the UV target (e.g. enzymes, pigments) and the adaptation to the environmental conditions.     

Non-invasive measurements of leaf epidermal transmittance of UV radiation using chlorophyll fluorescence: field and laboratory studies

Ratios of chlorophyll fluorescence induced by ultraviolet (UV) and bluegreen (BG) radiation [F(UV)/F(BG)] were determined with a Xe‐PAM fluorometer to test the utility of this technique as a means of non‐intrusively assessing changes in the pigmentation and optical properties of leaves exposed to varying UV exposures under laboratory and field conditions. For plants of Vicia faba and Brassica campestris, grown under controlled‐environmental conditions, F(UV‐B)/F(BG) was negatively correlated with whole‐leaf UV‐B‐absorbing pigment concentrations. Fluorescence ratios of V. faba were similar to, and positively correlated with (r²=0.77 [UV‐B]; 0.85 [UV‐A]), direct measurements of epidermal transmittance made with an integrating sphere. Leaves of 2 of 4 cultivars of field‐grown Glycine max exposed to near‐ambient solar UV‐B at a mid‐latitude site (Buenos Aires, Argentina, 34° S) showed significantly lower abaxial F(UV‐B)/F(BG) values (i.e., lower UV‐B epidermal transmittance) than those exposed to attenuated UV‐B, but solar UV‐B reduction had a minimal effect on F(UV‐B)/F(BG) in plants growing at a high‐latitude site (Tierra del Fuego, Argentina, 55° S). Similarly, the exotic Taraxacum officinale did not show significant changes in F(UV‐B)/F(BG) when exposed to very high supplemental UV‐B (biologically effective UV‐B=14–15 kJ m^?2 day^?1) in the field in Tierra del Fuego, whereas a native species, Gunnera magellanica, showed significant increases in F(UV‐B)/F(BG) relative to those receiving ambient UV‐B. These anomalous fluorescence changes were associated with increases in BG‐absorbing pigments (anthocyanins), but not UV‐B‐absorbing pigments. These results indicate that non‐invasive estimates of epidermal transmittance of UV radiation using chlorophyll fluorescence can detect changes in pigmentation and leaf optical properties induced by UV‐B radiation under both field and laboratory conditions. However, this technique may be of limited utility in cold environments where UV and low temperatures can stimulate the production of BG‐absorbing pigments that interfere with these indirect measurements of UV‐transmittance.     

Photosynthetic responses to solar UV-A and UV-B radiation in low-and high-altitude populations of <Emphasis Type="Italic">Hippophae rhamnoides</Emphasis>

Two contrasting sea buckthorn (Hippophae rhamnoides L.) populations from the low (LA) and high (HA) altitudinal regions were employed to evaluate the plant physiological responses to solar UV-A radiation and near-ambient UV-B radiation (UV-B+A) under the sheltered frames with different solar ultraviolet radiation transmittance. LA-population was more responsive to solar UV-A. Some modification caused by UV-A only existed in LA-population, such as significant reduction of leaf size, relative water content, and chlorophyll (Chl) b content as well as δ¹³C elevation, coupled with larger increase of contents of total carotenoids (Cars). This higher responsiveness might be an effective pre-acclimation strategy adapting for concomitant solar UV-B stress. Near-ambient UV-B+A radiation caused significant reduction of leaf size and Chl content as well as slight down-regulation of photosystem 2 activity that paralleled with higher heat dissipation, while photosynthetic rate was modestly but significantly increased. The higher photosynthesis under near-ambient UV-B+A radiation could be related to pronounced increase of leaf thickness and effective physiological modification, like the increase of leaf protective pigments (Cars and UV-absorbing compound), constant high photochemical capacity, and improved water economy.     

Roles of PprA,IrrE, and RecA in the resistance of <Emphasis Type="Italic">Deinococcus radiodurans</Emphasis> to germicidal and environmentally relevant UV radiation

To study the role of different DNA repair genes in the resistance of Deinococcus radiodurans to mono- and polychromatic UV radiation, wild-type strain and knockout mutants in RecA, PprA, and IrrE of D. radiodurans were irradiated with UV-C (254 nm), UV-(A + B) (280–400 nm) and UV-A (315–400 nm) radiation, and survival was monitored. The strain deficient in recA was highly sensitive to UV-C radiation compared to the wild-type, but showed no loss of resistance against irradiation with UV-(A + B) and UV-A, while pprA and irrE-deficient strains exhibited elevated sensitivity to UV-A and UV-(A + B) radiation. These results suggest that the repair of DNA double-strand breaks is essential after treatment with highly energetic UV-C radiation, whereas protection from oxidative stress may play a greater role in resistance to environmentally relevant UV radiation.     

Spectral balance and UV-B sensitivity of soybean: a field experiment

Soybean [Glycine max (L.) Merr.] cultivar Essex was grown and tested for sensitivity to UV-B radiation (280–320 nm) under different combinations of UV-A (320–400 nm) and PFD (400–700 nm) radiation in four simultaneous field experiments. The radiation conditions were effected with combinations of filtered solar radiation and UV-B and UV-A lamps electronically modulated to track ambient radiation. Significant UV-B-caused decreases in total aboveground production and growth were seen only when PFD and UV-A were reduced to less than half their flux in sunlight. When PFD was low, UV-A appeared to be particularly effective in mitigating UV-B damage. However, when PFD was high, substantial UV-A did not appear to be required for UV-B damage mitigation. Leaf chlorophyll fluorescence did not indicate photosynthetic damage under any radiation combination. With UV-B, leaves in all experiments exhibited increased UV-absorbing pigments and decreased whole-leaf UV transmittance. Results of these field experiments indicate difficulties in extrapolating from UV-B experiments conducted in glasshouse or growth cabinet conditions to plant UV-B sensitivity in the field. Implications for UV radiation weighting functions in evaluating atmospheric ozone reduction are also raised.     

BLUE LIGHT AND UV-A RADIATION CONTROL THE SYNTHESIS OF MYCOSPORINE-LIKE AMINO ACIDS IN CHONDRUS CRISPUS (FLORIDEOPHYCEAE)

The induction of UV-absorbing compounds known as mycosporine-like amino acids (MAAs) by red, green, blue, and white light (43% ambient radiation greater than 390 nm) was examined in sublittoral Chondrus crispus Stackh. Fresh collections or long-term cultures of sublittoral thalli, collected from Helgoland, North Sea, Germany, and containing no measurable amounts of MAAs, were exposed to filtered natural radiation for up to 40 days. The MAA palythine (λ_max 320 nm) was synthesized in thalli in blue light to the same extent observed in control samples in white light. In contrast, thalli in green or red light contained only trace amounts of MAAs. After the growth and synthesis period, the photosynthetic performance of thalli in each treatment, measured as pulse amplitude modulated chlorophyll fluorescence, was assessed after a defined UV dose in the laboratory. Thalli with MAAs were more resistant to UV than those without, and exposure to UV-A+B was more damaging than UV-A in that optimal (F_v/F_m) and effective (φ_II) quantum yields were lower and a greater proportion of the primary electron acceptor of PSII, Q, became reduced at saturating irradiance. However, blue light-grown thalli were generally more sensitive than white light control samples to UV-A despite having similar amounts of MAAs. The most sensitive thalli were those grown in red light, which had significantly greater reductions in F_v/F_m and φ_II and greater Q reduction. Growth under UV radiation alone had been shown previously to lead to the synthesis of the MAA shinorine (λ_max 334 nm) rather than palythine. In further experiments, we found that preexposure to blue light followed by growth in natural UV-A led to a 7-fold increase in the synthesis of shinorine, compared with growth in UV-A or UV-A+B without blue light pretreatment. We hypothesize that there are two photoreceptors for MAA synthesis in C. crispus, one for blue light and one for UV-A, which can act synergistically. This system would predispose C. crispus to efficiently synthesize UV protective compounds when radiation levels are rising, for example, on a seasonal basis. However, because the UV-B increase associated with artificial ozone reduction will not be accompanied by an increase in blue light, this triggering mechanism will have little additional adaptive value in the face of global change unless a global UV-B increase positively affects water column clarity.     

Effect of UV radiation on photoinhibition of marine macrophytes in culture systems

The present study examined the effect of UV andphotosynthetically active radiation (PAR) onphotoinhibition and recovery in the Phaeophyte Macrocystis pyrifera, the Rhodophyte Chondruscrispus and the Chlorophyte Ulva lactuca underoutdoor culture conditions. There was an increase inphotoinhibition as a consequence of high exposure toUV-B radiation in M. pyrifera, however, highlevels of PAR accounted for most of thephotoinhibition in C. crispus and U.lactuca. Photodamage by UV-A, UV-B and PAR wascompletely repaired within 5 h and effective quantumyield reached pretreatment values in the three speciesstudied. Species were less susceptible tophotoinhibition after being incubated for 5 d underhigh exposures of natural irradiance suggesting aphotoadaptive process. The recovery of the effectivequantum yield was impaired by long exposure to highlevels of UV-B in C. crispus and UV-A, UV-B andPAR in M. pyrifera. This suggests a differentkind of damage by UV-A and PAR radiation, one to thephotosynthetic apparatus and another which affects therepair mechanism of some species. There was anincrease in UV-absorption ( 330 nm) in M. pyrifera and C. crispus within four days ofthe initiation of the experiment suggesting that thesespecies photoprotect their photosynthetic system whenexposed to elevated UV and PAR levels.