Cell wall-bound ultraviolet-screening compounds explain the high ultraviolet tolerance of the Antarctic moss, Ceratodon purpureus

* Studies of ultraviolet (UV) light-induced DNA damage in three Antarctic moss species have shown Ceratodon purpureus to be the most UV tolerant, despite containing lower concentrations of methanol-soluble UV-screening compounds than the co-occurring Bryum pseudotriquetrum. * In this study, alkali extraction of cell wall-bound phenolics, combined with methanol extraction of soluble phenolics, was used to determine whether cell wall-bound UV screens explain the greater UV tolerance of C. purpureus. * The combined pool of UV screens was similar in B. pseudotriquetrum and C. purpureus, but whilst B. pseudotriquetrum had almost equal concentrations of MeOH-soluble and alkali-extractable cell wall-bound UV-screening compounds, in C. purpureus the concentration of cell wall-bound screening compounds was six times higher than the concentration of MeOH-soluble UV screens. The Antarctic endemic Schistidium antarctici possessed half the combined pool of UV screens of the other species but, as in C. purpureus, these were predominantly cell wall bound. Confocal microscopy confirmed the localization of UV screens in each species. * Greater investment in cell wall-bound UV screens offers C. purpureus a more spatially uniform, and potentially more effective, UV screen. Schistidium antarctici has the lowest UV-screening potential, indicating that this species may be disadvantaged under continuing springtime ozone depletion. Cell wall compounds have not previously been quantified in bryophytes but may be an important component of the UV defences of lower plants.     

Ultraviolet B screening potential is higher in two cosmopolitan moss species than in a co-occurring Antarctic endemic moss: implications of continuing ozone depletion

Concentrations of UVB (ultraviolet B) absorbing pigments and anthocyanins were measured in three moss species, over a summer growing season in Antarctica. Pigment concentrations were compared with a range of climatic variables to determine if there was evidence that pigments were induced by UVB radiation, or other environmental parameters, and secondly if there were differences between species in their pigment responses. Significant seasonal differences in the potential UVB screening pigments were found, with the two cosmopolitan species Bryum pseudotriquetrum and Ceratodon purpureus appearing better protected from the potentially damaging effects of ozone depletion than the Antarctic endemic Schistidium antarctici. B. pseudotriquetrum accumulated the highest concentration of UVB screening pigments and showed positive associations between UVB radiation and both UVB absorbing and anthocyanin pigments. The negative associations between water availability measures and UVB absorbing and anthocyanin pigments also suggest that B. pseudotriquetrum is well protected in the desiccated state. This could offer B. pseudotriquetrum an advantage over the other species when high UVB radiation coincides with low temperatures and low water availability, thus limiting physiological activity and consequently, active photoprotective and repair mechanisms. As these pigments could act as either direct UVB screens or antioxidants, the results suggest that B. pseudotriquetrum is best equipped to deal with the negative effects of increased exposure to UVB radiation due to ozone depletion. The most exposed species, C. purpureus, has intermediate and stable concentrations of UVB absorbing pigments suggesting it may rely on constitutive UVB screens. Anthocyanin pigments were more responsive in this species and could offer increased antioxidant protection during periods of high UVB radiation. S. antarctici appears poorly protected and showed no evidence of any UV photoprotective response, providing additional evidence that this endemic is more vulnerable to climate change.     

Moss Communities and Their Characteristics in Ice-free Areas of the Windmill Islands,Wilkes Land,Continental Antarctica

The moss vegetation of the Windmill Islands can be classified into the following seven communities: 1.Grimmia antarctici community; 2. G. antarctici-Ceratodon purpureus community; 3. Bryum pseudotriquetrum community; 4. G. antarctici-B, pseudotriquetrum community; 5. Ceratodon purpureus community; 6. C. purpureus-G, antarctici community; 7. C. purpureus B. pseudotriquetrum community. Communities 1, 3, 4 and 7 occur in wet habitats while communities 5 and 6 are found in rather dry habitats, community 2 distributes between dry and wet habitats. Microtopography governs the distribution of water supply and therefore community types. Stable moss communities usually form moss hummocks and hollows. Species composition, colour and height of the hummocks determines the vertical structure of the moss community layers with 2-3 strata being discernible and level structure forming mosaic. Other phytocoenological characteristics of the moss communities, such as the dense moss cushions, asexual reproduction (although antheridia or archegonia have been found in each of the species), abundance of epiphytic algae and lichens growing on the surface of moss hummocks, the colour change of some species in different habitats, appear related to moisture availability, light intensity, wind exposure and temperature.     

Ultraviolet-B-induced DNA damage and ultraviolet-B tolerance mechanisms in species with different functional groups coexisting in subalpine moorlands

High doses of ultraviolet-B (UV-B; 280–315 nm) radiation can have detrimental effects on plants, and especially damage their DNA. Plants have DNA repair and protection mechanisms to prevent UV-B damage. However, it remains unclear how DNA damage and tolerance mechanisms vary among field species. We studied DNA damage and tolerance mechanisms in 26 species with different functional groups coexisting in two moorlands at two elevations. We collected current-year leaves in July and August, and determined accumulation of cyclobutane pyrimidine dimer (CPD) as UV-B damage and photorepair activity (PRA) and concentrations of UV-absorbing compounds (UACs) and carotenoids (CARs) as UV-B tolerance mechanisms. DNA damage was greater in dicot than in monocot species, and higher in herbaceous than in woody species. Evergreen species accumulated more CPDs than deciduous species. PRA was higher in Poaceae than in species of other families. UACs were significantly higher in woody than in herbaceous species. The CPD level was not explained by the mechanisms across species, but was significantly related to PRA and UACs when we ignored species with low CPD, PRA and UACs, implying the presence of another effective tolerance mechanism. UACs were correlated negatively with PRA and positively with CARs. Our results revealed that UV-induced DNA damage significantly varies among native species, and this variation is related to functional groups. DNA repair, rather than UV-B protection, dominates in UV-B tolerance in the field. Our findings also suggest that UV-B tolerance mechanisms vary among species under evolutionary trade-off and synergism.     

Susceptibility of pollen to UV-B radiation: an assay of 34 taxa

Much of the ultraviolet-B radiation (UV-B) research on plants has concentrated on vegetative plant parts, and only a small fraction has dealt with the reproductive system. The present study analyzed pollen grains of 34 taxa germinated and grown under two levels of UV-B radiation (187 and 460 mW/m2) or no UV-B (control group). Visible radiation at 260 mmol/m/s was present in all treatments. Taxa included those with binucleate and trinucleate pollen types. We detected differences among species. A significant reduction in pollen germination occurred in only five species. Pollen tubes of >50% of the species showed significant reduction in length. Trinucleate pollen types were more likely to exhibit tube length reduction than the binucleate types. Proportionately more monocotyledonous species were sensitive to UV-B treatment than dicotyledonous species, and proportionately more wild species were sensitive than cultivated species and pollen collected from plants growing in the field were somewhat more sensitive than pollen collected from plants grown in the greenhouse. Species in which pollination occurred earlier in the season were more likely to be susceptible to UV-B radiation than those for which anthesis took place later in the season, suggesting a possible adaptation to UV-B radiation.     

Effects of spring and summer levels of UV-B radiation on the growth and reproduction of a temperate perennial forb

Narrow-leaved plantain (Plantago lanceolata L.), a perennial forb, flowers for virtually the full length of the growing season in temperate latitudes and as a result it is exposed to widely variable intensities of shortwave (UV-B) radiation. In order to determine effects of spring and summer levels of UV-B exposure on growth and development, representatives of 42 maternal families were grown for 85 days at 3.2 and 6.4 kJ m^–2 day^–1 BE₃₀₀, levels corresponding to early spring and mid-summer in central Illinois. Impacts on early vegetative stages were most pronounced; early vegetative growth was decreased by higher levels of UV-B and both leaf angle (a measure of erectness) and leaf hair density were increased. At harvest, vegetative growth was significantly affected by higher levels of UV-B as well; the mass of senescent leaves and crown tissue were both decreased. Although exposure to higher levels of UV-B decreased inflorescence number by nearly 15%, it did not significantly alter reproductive biomass. Significant variation attributable to maternal families was present in nearly all measurements and the range of variation among families was wider than among UV-B treatments. A marginally significant (p=0.07) maternal family by UV-B interaction was found for the number of inflorescences, suggesting that, within populations of this plant, some small amount of genetic variation exists to allow for differential reproductive performance under a regime simulating spring and summer differences in UV-B exposure. For the most part, however, in this cosmopolitan species the level of adaptation to natural levels of variation in UV-B radiation does not differ dramatically among maternal families.     

Current and elevated levels of UV-B radiation have few impacts on yields of perennial forage crops

Two experiments assessed the effect of current and elevated levels of ultraviolet_B (UV-B) radiation on forage crop production. The effect of current levels of UV-B radiation was assessed by comparing the growth of eight cultivars of four legume and four grass species for three growing seasons (1994–96) under simulated sward conditions in the field. An exclusion system using cellulose diacetate or polyester covers provided comparable growing conditions for the plants, except for the presence or absence of ambient UV-B radiation, respectively. The second experiment studied the effect of elevated levels of UV-B on eight cultivars of two legume and two grass species in the greenhouse under simulated sward conditions. Natural lighting with sufficient supplemental light was used to provide ideal growing conditions for a 16-h day length. Separate sets of UV lights were installed to provide UV-B levels at approximately the same intensity as would be found in mid summer, and 33% and 66% more than this value. A fourth treatment consisted of removing UV-B radiation by using a polyester filter. Plant production was measured in both experiments. In 1994, field herbage yields from all young grass and legume seedlings were not significantly affected by the exclusion of ambient UV-B radiation, with the exception of alfalfa. Intra-specific variations with alfalfa yields were found for reduced levels of UV-B radiation. In general, these trends persisted as stands matured during two post seeding years. In the second experiment, no significant differences were observed for all tested species with increasing levels of UV-B radiation, except with some alfalfa cultivars and one birdsfoot trefoil cultivar. Collectively, these results demonstrate that in the northern latitudes young and mature plants of the studied species are resistant to current and potentially higher levels of solar UV-B radiation, with the exception of some alfalfa cultivars. The yield of these cultivars increased under enhanced levels of UV-B radiation in the greenhouse and decreased when UV-B was excluded in the field.     

The effects of solar ultraviolet-B radiation on the growth and yield of barley are accompanied by increased DNA damage and antioxidant responses

There is limited information on the impacts of present-day solar ultraviolet-B radiation (UV-B) on biomass and grain yield of field crops and on the mechanisms that confer tolerance to UV-B radiation under field conditions. We investigated the effects of solar UV-B on aspects of the biochemistry, growth and yield of barley crops using replicated field plots and two barley strains, a catalase (CAT)-deficient mutant (RPr 79/4) and its wild-type mother line (Maris Mink). Solar UV-B reduced biomass accumulation and grain yield in both strains. The effects on crop biomass accumulation tended to be more severe in RPr 79/4 (≈ 32% reduction) than in the mother line (≈ 20% reduction). Solar UV-B caused measurable DNA damage in leaf tissue, in spite of inducing a significant increase in UV-absorbing sunscreens in the two lines. Maris Mink responded to solar UV-B with increased CAT and ascorbate peroxidase (APx) activity. No effects of UV-B on total superoxide dismutase (SOD) activity were detected. Compared with the wild type, RPr 79/4 had lower CAT activity, as expected, but higher APx activity. Neither of these activities increased in response to UV-B in RPr 79/4. These results suggest that growth inhibition by solar UV-B involves DNA damage and oxidative stress, and that constitutive and UV-B-induced antioxidant capacity may play an important role in UV-B tolerance.     

Elevational differences in trait response to UV-B radiation by long-toed salamander populations

Amphibian species capable of optimizing trait response to environmental stressors may develop complex strategies for defending against rapid environmental change. Trait responses may differ between populations, particularly if stressor strength varies across spatial or temporal gradients. Ultraviolet-B (UV-B) radiation is one such stressor that poses a significant threat to amphibian species. We examined the ability of long-toed salamanders (Ambystoma macrodactylum) at high- and low-elevation breeding sites to cooperatively employ behavioral and physiological trait responses to mediate UV-B damage. We performed a microhabitat survey to examine differences in oviposition behavior and UV-B conditions among breeding populations at high- (n = 3; >1,500 m) and low-elevation (n = 3; <100 m) sites. We found significant differences in oviposition behavior across populations, with females at high-elevation sites selecting oviposition substrates in UV-B protected microhabitats. We also collected eggs (n = 633) from each of the breeding sites for analysis of photolyase activity, a photoreactivating enzyme that repairs UV-B damage to the DNA, using a photoproduct immunoassay. Our results revealed no significant differences in photolyase activity between long-toed salamander populations at high and low elevations. For high-elevation salamander populations, relatively low physiological repair capabilities in embryos appear to be buffered by extensive behavioral modifications to reduce UV-B exposure and standardize developmental temperatures. This study provides valuable insight into environmental stress responses via the assessment of multiple traits in allowing sensitive species to persist in rapidly changing landscapes.     

Artificial and solar UV radiation induces strand breaks and cyclobutane pyrimidine dimers in Bacillus subtilis spore DNA

The loss of stratospheric ozone and the accompanying increase in solar UV flux have led to concerns regarding decreases in global microbial productivity. Central to understanding this process is determining the types and amounts of DNA damage in microbes caused by solar UV irradiation. While UV irradiation of dormant Bacillus subtilis endospores results mainly in formation of the "spore photoproduct" 5-thyminyl-5,6-dihydrothymine, genetic evidence indicates that an additional DNA photoproduct(s) may be formed in spores exposed to solar UV-B and UV-A radiation (Y. Xue and W. L. Nicholson, Appl. Environ. Microbiol. 62:2221-2227, 1996). We examined the occurrence of double-strand breaks, single-strand breaks, cyclobutane pyrimidine dimers, and apurinic-apyrimidinic sites in spore DNA under several UV irradiation conditions by using enzymatic probes and neutral or alkaline agarose gel electrophoresis. DNA from spores irradiated with artificial 254-nm UV-C radiation accumulated single-strand breaks, double-strand breaks, and cyclobutane pyrimidine dimers, while DNA from spores exposed to artificial UV-B radiation (wavelengths, 290 to 310 nm) accumulated only cyclobutane pyrimidine dimers. DNA from spores exposed to full-spectrum sunlight (UV-B and UV-A radiation) accumulated single-strand breaks, double-strand breaks, and cyclobutane pyrimidine dimers, whereas DNA from spores exposed to sunlight from which the UV-B component had been removed with a filter ("UV-A sunlight") accumulated only single-strand breaks and double-strand breaks. Apurinic-apyrimidinic sites were not detected in spore DNA under any of the irradiation conditions used. Our data indicate that there is a complex spectrum of UV photoproducts in DNA of bacterial spores exposed to solar UV irradiation in the environment.     

Increased DNA repair in <Emphasis Type="Italic">Arabidopsis</Emphasis> plants overexpressing CPD photolyase

Ultraviolet-B (UV-B, 280–320 nm) radiation may have severe negative effects on plants including damage to their genetic information. UV protection and DNA-repair mechanisms have evolved to either avoid or repair such damage. Since autotrophic plants are dependent on sunlight for their energy supply, an increase in the amount of UV-B reaching the earth’s surface may affect the integrity of their genetic information if DNA damage is not repaired efficiently and rapidly. Here we show that overexpression of cyclobutane pyrimidine dimer (CPD) photolyase (EC 4.1.99.3) in Arabidopsis thaliana (L.), which catalyses the reversion of the major UV-B photoproduct in DNA (CPDs), strongly enhances the repair of CPDs and results in a moderate increase of biomass production under elevated UV-B.     

UV damage to plant life in a photobiologically dynamic environment: the case of marine phytoplankton

The effect of UV-B radiation on growth of marine phytoplankton was investigated in relation to DNA damage induced by a range of biologically effective doses (BEDs). Emiliania huxleyi (Prymnesiophyceae) was chosen as a model organism of the ocean's phytoplankton because of its importance in global biogeochemical cycling of carbon and sulphur, elements that influence the world's climate as components of the trace gases carbon dioxide (CO₂) and dimethylsulfide (DMS). A marine diatom, Cyclotella, was studied for its capacity to repair the DNA damage, quantified as thymine dimers by the application of a monoclonal antibody against these photoproducts. DNA repair was shown to be complete after just a few hours of exposure to visible light; the repair rate increased with PAR intensity. E. huxleyi appeared to be most sensitive to UV-B radiation: growth was already affected above a dose of 100 J m^-2 d^-1 (biologically effective radiation, weighted with Setlow's DNA action spectrum), probably through effects on the cell cycle related to damage to nuclear DNA: mean specific growth rates were inversely correlated with thymine dimer contents in cells. Near the ocean's surface UV-B radiation conditions that induce the changes observed by us in cultures can be expected during the growing season of phytoplankton, not only in the tropics but also at higher latitudes. Nevertheles, blooms of species such as E. huxleyi are often excessive in the field. It is suggested that exposure duration of cells near the surface of the ocean can be shorter than our artificial 3 h in the laboratory due to vertical mixing, a phenomenon that is typical for the ocean's upper 50–100 m. When mixing reaches depths greater than the layer where most UV-B is attenuated, negative effects on cells through UV-A-induced inhibition of photosynthesis may prevail over DNA damage, the action spectrum of which has been shown to be limited to the UV-B part of the spectrum. Moreover, the radiation wavelengths that induce DNA damage repair (UV-A and visible) are attenuated vertically much less than UV-B. The photobiological situation in the upper ocean is much more complicated than on land, and effects of UV radiation on plankton biota can only be modelled realistically here when both the spectrally differential attenuation in the UV and visual part of the spectrum and the rate of vertical mixing are taken into account. Action spectra of both damage and repair of DNA and of photosynthesis inhibition of representative microalgal species are the second conditio sine qua non if we want to predict the effect of stratospheric ozone depletion on marine phytoplankton performance.     

UV-B-Induced PR-1 Accumulation Is Mediated by Active Oxygen Species

Depletion of the stratospheric ozone layer may result in an increase in the levels of potentially harmful UV-B radiation reaching the surface of the earth. We have found that UV-B is a potent inducer of the plant pathogenesis-related protein PR-1 in tobacco leaves. UV-B fluences required for PR-1 accumulation are similar to those of other UV-B-induced responses. The UV-B-induced PR-1 accumulation was confined precisely to the irradiated area of the leaf but displayed no leaf tissue specificity. A study of some of the possible components of the signal transduction pathway between UV-B and PR-1 induction showed that photosynthetic processes are not essential, and photoreversible DNA damage is not involved. Antioxidants and cycloheximide were able to block the induction of PR-1 by UV-B, and treatment of leaves with a generator of reactive oxygen resulted in the accumulation of PR-1 protein. These results demonstrate an absolute requirement for active oxygen species and protein synthesis in this UV-B signal transduction pathway. In contrast, we also show that other elicitors, notably salicylic acid, are able to elicit PR-1 via nonreactive oxygen species-requiring pathways.     

The two major spore DNA repair pathways, nucleotide excision repair and spore photoproduct lyase, are sufficient for the resistance of Bacillus subtilis spores to artificial UV-C and UV-B but not to solar radiation.

Bacterial endospores are 1 to 2 orders of magnitude more resistant to 254-nm UV (UV-C) radiation than are exponentially growing cells of the same strain. This high UV resistance is due to two related phenomena: (i) DNA of dormant spores irradiated with 254-nm UV accumulates mainly a unique thymine dimer called the spore photoproduct (SP), and (ii) SP is corrected during spore germination by two major DNA repair pathways, nucleotide excision repair (NER) and an SP-specific enzyme called SP lyase. To date, it has been assumed that these two factors also account for resistance of bacterial spores to solar UV in the environment, despite the fact that sunlight at the Earth's surface consists of UV-B, UV-A, visible, and infrared wavelengths of approximately 290 nm and longer. To test this assumption, isogenic strains of Bacillus subtilis lacking either the NER or SP lyase DNA repair pathway were assayed for their relative resistance to radiation at a number of UV wavelengths, including UV-C (254 nm), UV-B (290 to 320 nm), full-spectrum sunlight, and sunlight from which the UV-B portion had been removed. For purposes of direct comparison, spore UV resistance levels were determined with respect to a calibrated biological dosimeter consisting of a mixture of wild-type spores and spores lacking both DNA repair systems. It was observed that the relative contributions of the two pathways to spore UV resistance change depending on the UV wavelengths used in a manner suggesting that spores irradiated with light at environmentally relevant UV wavelengths may accumulate significant amounts of one or more DNA photoproducts in addition to SP. Furthermore, it was noted that upon exposure to increasing wavelengths, wild-type spores decreased in their UV resistance from 33-fold (UV-C) to 12-fold (UV-B plus UV-A sunlight) to 6-fold (UV-A sunlight alone) more resistant than mutants lacking both DNA repair systems, suggesting that at increasing solar UV wavelengths, spores are inactivated either by DNA damage not reparable by the NER or SP lyase system, damage caused to photosensitive molecules other than DNA, or both.     

Is UV-B radiation affecting charophycean algae in shallow freshwater systems?

The objective of this study was to determine the effects of UV-B radiation on charophycean algae under natural conditions, since charophytes enhance water transparency in freshwater systems and levels of UV-B radiation have increased by ozone depletion. Potential and actual UV-B effects were studied by combining a glasshouse experiment in which plants were exposed to various levels of UV-B radiation and field measurements in two freshwater systems dominated by charophytes in the Netherlands. The glasshouse experiment showed that charophytes were sensitive to UV-B radiation. UV-B radiation negatively affected growth, while it increased levels of DNA damage in Chara aspera. Moreover, the charophytes did not seem to develop UV-B screens to protect against UV-B radiation since no increase in UV-B absorbing compounds was found. At field conditions, both spectroradiometrical measurements and DNA dosimeters showed that UV-B radiation was attenuated quickly in both freshwater systems, indicating that UV-B does not reach the submerged charophyte vegetation. However, specific conditions, like fluctuating water tables, may result in UV-B exposure to charophytes for certain periods annually.     

Reviewing the Technical Designs for Experiments with Ultraviolet-B Radiation and Impact on Photosynthesis, DNA and Secondary Metabolism

The ultraviolet-B (UV-B) portion of sunlight has received much attention in the last three decades, because radiation from this spectral region increases due to the stratospheric ozone depletion, which results from increases of chlorofluorocarbons in the atmosphere. Plant responses to UV-B exposure vary greatly and the interpretation of and comparison between studies is hindered, mainly by the contrasting experimental conditions used and interactive factors such as low light levels and possible artifacts due to the artificial experimental conditions. It seems likely that increases in solar UV-B radiation of the magnitude anticipated under current stratospheric ozone projections will not significantly inhibit photosynthesis and cause DNA damage in plants. This is in part due to the well-evolved protection mechanisms present in most plant species. One of the significant plant responses to UV-B is changes in foliar secondary chemistry, which could be translated into significant effects at higher trophic levels through plant-herbivore interactions and decomposition. Enhanced UV-B radiation due to stratospheric ozone depletion could also cause morphological changes that would affect competitive interactions, especially if contrasting UV-B sensitivity exists among the competitors.     

一株北极小球藻Chlorella sp.的分离鉴定及其对增强的UV-B辐射的生理生化响应

为了了解微藻对UV-B辐射增强效应的响应,以一种分离纯化于北极冰川融水的淡水微藻（Chlorella sp.）为实验材料,在不同强度UV-B辐射下对其生长、生化组分和细胞超微结构等进行了研究。研究结果显示：3种不同强度的UV-B（22μW/cm2,45μW/cm2,70μW/cm2）辐射均可导致藻的比生长速率及色素含量下降,且辐射强度越强,两者的下降越明显;而MDA含量和SOD活性会随辐照强度的增强而提高。表明辐射强度增强,UV-B对藻的伤害程度加大,而该小球藻SOD活性随UV-B强度增强而提高,表明其对上升的UV-B辐射有一定的适应能力。     

Protective mechanisms and acclimation to solar ultraviolet-B radiation in Oenothera stricta

Abstract Mechanisms of plant protection and acclimation to potentially damaging solar ultraviolet-B (UV-B, 280–320 nm) radiation incident on the Earth's surface were examined in Oenothera stricta. Attenuation of this radiation in the upper leaf epidermis reduces the penetration of UV-B radiation to the mesophyll where damage to physiologically sensitive targets can occur. The epidermis is a highly selective radiation filter that can attenuate up to 95% of the incident UV-B radiation and yet transmit between 70% and 80% of the visible radiation. Exposure to UV-B radiation significantly reduced the degree of epidermal UV-B transmittance by as much as 33%. No significant reduction in epidermal transmittance of visible radiation was observed as a result of UV-B exposure. The plasticity in epidermal UV-B transmittance results from production of flavonoid and related phenolic compounds in the tissue. Absorbance of UV-B radiation in llavonoid extract solutions from epidermal and mesophyll tissues significantly increased by as much as 100% and 35%, respectively, after exposure to UV-B radiation. Photosynthetic rates of leaves exposed to UV-B radiation were not significantly reduced at dose rates representative of the radiation flux found in the habitat of this species, but significant photosynthetic depression was observed at dose rates that exceed the field UV-B flux. The phenotypic plasticity in epidermal UV-B transmittance resulting in decreased penetration of damaging UV-B radiation to the mesophyll may reduce the rate of damage to a level where repair mechanisms can keep pace with reduced injury.     

Impact of ultraviolet radiation on cell structure, UV-absorbing compounds, photosynthesis, DNA damage, and germination in zoospores of Arctic Saccorhiza dermatodea

Stratospheric ozone depletion leads to enhanced UV-B radiation. Therefore, the capacity of reproductive cells to cope with different spectral irradiance was investigated in the laboratory. Zoospores of the upper sublittoral kelp Saccorhiza dermatodea were exposed to varying fluence of spectral irradiance consisting of photosynthetically active radiation (PAR, 400-700 nm; =P), PAR+UV-A radiation (UV-A, 320-400 nm; =PA), and PAR+UV-A+UV-B radiation (UV-B, 280-320 nm; =PAB). Structural changes, localization of phlorotannin-containing physodes, accumulation of UV-absorbing phlorotannins, and physiological responses of zoospores were measured after exposure treatments as well as after 2-6 d recovery in dim white light (8 mumol photon m(-2) s(-1)). Physodes increased in size under PAB treatment. Extrusion of phlorotannins into the medium and accumulation of physodes was induced not only under UVR treatment but also under PAR. UV-B radiation caused photodestruction indicated by a loss of pigmentation. Photosynthetic efficiency of spores was photoinhibited after 8 h exposure to 22 and 30 mumol photon m(-2) s(-1) of PAR, while supplement of UVR had a significant additional effect on photoinhibition. A relatively low recovery of photosystem II function was observed after 2 d recovery in spores exposed to 1.7 x 10(4) J m(-2) of UV-B, with a germination rate of only 49% of P treatment after 6 d recovery. The amount of UV-B-induced DNA damage measured as cyclobutane-pyrimidine dimers (CPDs) increased with the biologically effective UV-B dose (BED(DNA)). Significant removal of CPDs indicating repair of DNA damage was observed after 2 d in low white light. The protective function of phlorotannins has restricted efficiency for a single cell. Within a plume of zoospores, however, each cell can buffer each other and protect the lower layer of spores from excessive radiation. Exudation of phlorotannins into the water can also reduce the impact of UV-B radiation on UV-sensitive spores. The results of this study showed that the impact of UVR on reproductive cells can be mitigated by protective and repair mechanisms.