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The potential impact of an increase in solar ultraviolet-B (UV-B) radiation due to human activity on higher plants has been the subject of many studies. Little work has been carried out so far on cotton responses to enhanced UV-B radiation. The objective of this study was to determine whether or not the current and projected increases in UV-B levels affect cotton growth and development, and to quantify and develop UV-B radiation functional algorithms that can be used in simulation models. Two experiments were conducted during the summer of 2001 using sunlit plant growth chambers in a wide range of UV-B radiations under optimal growing conditions. Leaves exposed to UV-B radiation developed chlorotic and necrotic patches depending on the intensity and length of exposure. Along with changes in visible morphology, cotton canopy photosynthesis declined with increased UV-B radiation. The decline in canopy photosynthesis was partly due to loss of photosynthetic pigments and UV-B-induced decay of leaf-level photosynthetic efficiency (maximum photosynthesis) and capacity (quantum yield) as the leaves aged. The total leaf area was less due to smaller leaves and fewer leaves per plant. Less plant height was closely related to a shorter average internode length rather than a fewer mainstem nodes. The UV-B did not affect cotton major developmental events such as time taken to square, time to flower, and leaf addition rates on the mainstem. Lower biomass was closely related to both smaller leaf area and lower photosynthesis. The critical limit, defined as 90% of optimum or the control, for stem elongation was lower (8.7 kJ m−2 per day UV-B) than the critical limit for leaf expansion (11.2 kJ m−2 per day UV-B), indicating that stem elongation was more sensitive to UV-B than leaf expansion. The critical limits for canopy photosynthesis and total dry weight were 7 and 7.3 kJ m−2 per day, respectively. The identified UV-B-specific indices for stem and leaf growth and photosynthesis parameters may be incorporated into cotton simulation models such as GOSSYM to predict yields under present and future climatic conditions. [Copyright &y& Elsevier]

Summary 1. Despite the increasing occurrence of global environmental changes, including increases in ultraviolet-B radiation (UV-B; 280–320 nm), little is known about how factors such as UV-B affect animals physiologically. Amphibians provide a good model for examining physiological effects of UV-B exposure because studies documenting both lethal and sub-lethal effects have been completed on many species. 2. This study examines survival and the hormonal stress response, as measured by whole body levels of the glucocorticoid hormone corticosterone, of Cascades Frog, Rana cascadae , tadpoles exposed to ambient UV-B in the field for either 7 or 42 days. 3. There were no differences in corticosterone levels between UV-B exposed and non-exposed tadpoles at either time point. However, after 42 days, survival was significantly higher in tadpoles shielded from UV-B. 4. A stress response profile performed in the field demonstrated that young R. cascadae tadpoles are able to respond to confinement stress with increasing corticosterone. In addition, results from UV-B avoidance tests suggest that R. cascadae tadpoles do not avoid UV-B. 5. Thus, R. cascadae tadpoles may not perceive UV-B radiation, which would explain why they do not respond hormonally to exposure, despite the potential for exposure to result in mortality. [ABSTRACT FROM AUTHOR]

Two aquatic bryophytes, the moss Fontinalis antipyretica Hedw. and the foliose liverwort Jungermannia exsertifolia Steph. subsp. cordifolia (Dumort.) Váña were analysed with regard to their ultraviolet-B (UV-B) tolerance. The plants were cultivated in the laboratory for 36 days under three different radiation regimes which were set up using appropriate lamps and filters: P (PAR alone, control), PA (PAR + UV-A) and PAB (PAR + UV-A + UV-B). The two bryophytes responded differently to the enhancement in UV-B radiation, while UV-A radiation had only a slight biological effect. The samples of the moss which were irradiated with UV-B showed, with respect to the control, decreases in the chlorophyll and carotenoids concentration, the chlorophyll a/b quotient, the chlorophylls/phaeopigments ratios, the net photosynthesis rates, the light saturation point, the maximum quantum yield of photosystem II (F[sub v]/F[sub m]) and the apparent electron transport rate (ETR). They also showed increases in the sclerophylly index and the dark respiration rates. The majority of these changes are indicative of plant stress and some of them had been found previously in bryophytes exposed to enhanced UV-B radiation. However, the UV-B-irradiated samples of the liverwort only showed a decrease in F[sub v]/F[sub m], which might be the most sensitive physiological variable to UV-B, together with an increase in the concentration of UV-absorbing compounds. This defence mechanism, rarely described in bryophytes, would enable the liverwort to have a higher tolerance than the moss against UV-B radiation, at least under the specific experimental conditions used in this study. Also, the increment of UV-absorbing compounds in the liverwort, which seems to be a specific response to the enhancement in UV-B radiation, might be a useful ecophysiological tool in the bioindication of this phenomenon; long-term field research is needed to confirm this usefulness. The different UV-B tolerances of the species studied may be related, to a certain extent, with their different distribution and ecology. Given that UV-B tolerance depends on the species, bryophytes should not be grouped as a single functional type in the prediction of the consequences of a hypothetic enhancement in UV-B radiation. [ABSTRACT FROM AUTHOR]

We tested the effects of UV-B stressed algae on grazing rates of zooplankton. Four algal species (Chlamydomonas reinhardtii, Cryptomonas sp., Scenedesmus obliquus and Microcystis aeruginosa) were used as food and fed to three zooplankton species (Daphnia galeata, Bosmina longirostris and Brachionus calyciflorus), representing different taxonomic groups. The phytoplankton species were cultured under PAR conditions, and under PAR supplemented with UV-B radiation at two intensities (0.3 W m-2 and 0.7 W m-2, 6 hours per day). Ingestion and incorporation experiments were performed at two food levels (0.1 and 1.0 mg C l-1) using radiotracer techniques. The effect of food concentration on ingestion and incorporation rate was significant for all three zooplankton species, but the effect of UV-B radiation was more complex. The reactions of the zooplankton species to UV-B stressed algae were different. UV-B stressed algae did not affect Daphnia grazing rates. For Bosmina the rates increased when feeding on UV-B stressed Microcystis and decreased when feeding on UV-B stressed Chlamydomonas, compared with non-stressed algae. Brachionus grazing rates were increased when feeding on UV-B stressed Cryptomonas and UV-B stressed Scenedesmus, and decreased when feeding on UV-B stressed Microcystis, compared with non-stressed algae. These results suggest that on a short time scale UV-B radiation may result in increased grazing rates of zooplankton, but also in decreased grazing rates. Long term effects of UV-B radiation on phytoplankton and zooplankton communities are therefore difficult to predict. [ABSTRACT FROM AUTHOR]

Abstract We examined the influence of solar ultraviolet-B radiation (UV-B; 280–315 nm) on the growth of Colobanthus quitensis plants by placing them under contrasting UV-B filters at Palmer Station, along the Antarctic Peninsula. The filters reduced diurnal biologically effective UV-B (UV-BBE ) either by 83% (‘reduced UV-B’) or by 12% (‘near-ambient UV-B’) over the 63 day experiment (7 November 1998–8 January 1999). Ozone column depletion averaged 17% during the experiment. Relative growth and net assimilation rates of plants exposed to near-ambient UV-B were 30 and 20% lower, respectively, than those of plants exposed to reduced UV-B. The former plants produced 29% less total biomass, as a result of containing 54% less aboveground biomass. These reductions in aboveground biomass were mainly the result of a 45% reduction in shoot biomass, and a 31% reduction in reproductive biomass. Reductions in shoot biomass were owing to an 18% reduction in branch production by main shoots, while reductions in reproductive biomass were the result of a 19% reduction in individual capsule mass. Total plant leaf area was reduced by 19% under near-ambient UV-B, although total leaf biomass was unaffected because leaves had a greater specific leaf mass. The reduction in plant leaf area under near-ambient UV-B was attributable to: (1) production of 11% fewer leaves per main shoot system and plant, which resulted from an 18% reduction in branch production by main shoots. Leaf production per individual main shoot or branch was not affected; (2) shorter leaf longevity—main shoots contained 14% fewer green leaves at a given time; and (3) smaller individual leaves—leaf elongation rates were 14% slower and mature leaves were 13% shorter. [ABSTRACT FROM AUTHOR]

Abstract Stratospheric ozone depletion caused by the release of chlorofluorocarbons is most pronounced at high latitudes, especially in the Southern Hemisphere (including the so-called ‘ozone hole’). The consequent increase in solar ultraviolet-B radiation (UV-B, 280–315 nm) reaching the earth's surface may cause a variety of alterations in terrestrial ecosystems. Most effects might be expected to occur above-ground since sunlight does not penetrate effectively below-ground. Here, we demonstrate that solar UV-B radiation in a fen in Tierra del Fuego (Argentina), where the ozone hole passes overhead several times during the Austral spring, is causing large changes of below-ground processes of this ecosystem. During the third and fourth year of a manipulative field experiment, we investigated root systems in these plots and found that when the ambient solar UV-B radiation was substantially reduced, there was a 30% increase in summer root length production and as much as a threefold decrease in already low symbiotic mycorrhizal colonization frequency of the roots compared with plots receiving near-ambient solar UV-B. There was also an apparent shift toward older age classes of roots under reduced solar UV-B. Such large changes in root system behaviour may have decided effects on competition and other ecological interactions in this ecosystem. [ABSTRACT FROM AUTHOR]

UV-B radiation is absorbed effectively by nucleic acids and other sensitive targets, potentially causing harmful photochemical effects. Protection against UV-B radiation may be afforded by flavonoids and other phenolics, which absorb strongly in the UV region, but little is known about the role played by other compounds, such as terpenes. Grindelia chiloensis, native of Patagonia (Argentina), can accumulate as much as 25% resin (terpenes) in its leaves. The present investigation was carried out to test the effect of solar UV-B radiation on the allocation of photoassimilates to biomass and terpenes. Exposure to UV-B radiation reduced whole plant biomass, plant height and leaf area, and increased leaf thickness and resin accumulation in Grindelia chiloensis. Higher absorbance was found for refined resin in the UV-B waveband from plants grown under solar UV-B radiation than plants without UV-B radiation. These chemical and structural changes could protect the plant from UV radiation, and may help elucidate the importance of epicuticular resins for a species as G. chiloensis native to an environment with maximum daily integrated values of solar UV-B irradiance. [ABSTRACT FROM AUTHOR]

Seventeen herb, shrub and tree species of commercial and ecological importance in southern Africa were exposed at one location to ultraviolet‐B (UV‐B, 280–315 nm) radiation approx. 35 % above clear‐sky background (control). The aims were to assess how UV‐B affects canopy area, dry mass, and some biochemical and morphological properties of leaves, and to investigate whether differences between species are related to growth form of the plants. There was no pattern of response to UV‐B related to growth form. Leaves of trees had altered chlorophyll a and b, carotenoid and flavonoid concentrations, but those of shrubs or herbs did not. Non‐structural carbohydrates were unaffected. Smaller canopy areas and dry masses were observed under enhanced UV‐B, but these were not statistically different among growth forms. There was a general insensitivity of species to elevated UV‐B. Only five species had significantly altered leaf biochemical and morphological properties, canopy area and dry mass, the changes differing in magnitude. There was no consistent pattern of change in leaf thickness or biochemical composition with increased UV‐B. Correlation analyses did not support the view that growth is less negatively affected in species with thick leaves or in those where leaf thickness increases, or in species with naturally high leaf flavonoid contents or that are able to synthesize additional flavonoids in response to UV‐B enhancement. The analyses did not support the hypothesis that growth was inhibited by starch accumulation in leaves under elevated UV‐B. However, changes in leaf shape did correlate with canopy area and dry mass, showing the importance of photomorphogenetic changes caused by UV‐B which affect species’ performance. We conclude that generalizations on plant sensitivity to UV‐B based on growth form and functional type could be misleading, and that the great majority of economically important species of the region are likely to be insensitive to future UV‐B increases. Notable exceptions include the Colophospermum mopane tree ecotypes chota and leslie and the arable annual Vigna unguiculata, both of which are traditional sources of livelihood to rural African populations and of importance to African industry and agriculture. [ABSTRACT FROM PUBLISHER]

The responses of two amphibious species, Batrachium trichophyllum and Potamogeton alpinus to different UV-B environments were studied. Plant material from natural environments, as well as from outdoor treatments was examined. In long-term outdoor experiments plants were grown under three different levels of UV-B radiation: reduced and ambient UV-B levels, and a UV-B level simulating 17% ozone depletion. The following parameters were monitored: contents of total methanol soluble UV-absorbing compounds and chlorophyll a, terminal electron transport system (ETS) activity and optimal and effective quantum yield of photosystem II. No effect of the different UV-B levels on the measured parameters was observed. The amount of UV-B absorbing compounds seems to be saturated, since no differences were observed between treatments and no increase was found in peak season, when natural UV-B levels were the highest. Physiological measurements revealed no harmful effects; neither on potential and actual photochemical efficiency, nor on terminal ETS activity. The contents of UV-B absorbing compounds were examined also in plant material sampled in low and high altitude environments during the growth season. Both species exhibited no seasonal dynamics of production of UV-absorbing compounds. The contents were variable and showed no significant differences between high and low altitude populations. [Copyright &y& Elsevier]

The effect of enhanced UV-B radiation on buckwheat (Fagopyrum esculentum Moench. variety ‘Darja’), an important high elevation crop, was studied in order to estimate its vulnerability in changing UV-B environment. Plants were grown in outdoor experiments from July to October under reduced and ambient UV-B levels, and an UV-B level simulating 17% ozone depletion in Ljubljana. During the development the following parameters were monitored: light saturated photosynthetic activity, transpiration, potential and effective photochemical efficiencies of photosystem II, the contents of photosynthetic pigments and methanol soluble UV-B absorbing compounds. At the end of the experiment, growth rate and production of seeds were estimated. In the following growth season the seeds collected from plants exposed to different UV-B treatments were tested for germination capacity. Total UV-B absorbing compounds during plant development were increased by UV-B radiation, photosynthetic pigments (chlorophyll a and b and carotenoids) decreased. Photosynthetic rate was lowered in an early stage of development. UV-B treatment resulted in the increase in the transpiration rate and consequently the decrease in water use efficiency (WUE). The disturbances in water economy and in photosynthesis affected the reproduction potential negatively; the production of seeds in plants cultivated under ambient and enhanced UV-B was 57 and 39% of the production of specimens treated with reduced UV-B, respectively. The germination of seeds collected from treated plants revealed on average about 95% success, independently of the treatment, but the time needed for germination was the shortest for seeds developed under enhanced UV-B level treatment. Enhanced UV-B radiation affected water relations and production of buckwheat, but not the potential of seeds for germination. [Copyright &y& Elsevier]

The increase in ground level UV-B radiation as a result of stratospheric ozone depletion may have major deleterious effects on crop photosynthesis and productivity. Plants are exposed to UV-B and other xenobiotics simultaneously in today''s industrialized world. The present studies were undertaken to see the effect of dual stress of UV-B and Cd2+ exposure on the growth of wheat (Triticum aestivum L.) seedlings. The plants grown in 0.25, 0.5, 1.0, 2.5, and 5.0 ppm Cd2+-supplemented medium were exposed to UV-B for 30 min (2 J, 0.4 mW/cm2) per day. After 5 and 10 days of treatment the combined stress of UV-B and Cd2+ resulted in reduction of biomass yield, growth, and chlorophyll content and changes in protein, free amino acid, starch, and total reducing sugar content. These data support the assumption that UV-B may have a regulatory role besides damaging effects and that an increased UV-B environment will likely increase this regulatory influence of UV-B radiation. The results also indicate that the adverse effects of one stress may be modulated in the presence of other stresses. [Copyright &y& Elsevier]

Continuous depletion of the stratospheric ozone layer has resulted in an increase in solar ultraviolet-B (UV-B; 280–315 nm) radiation reaching the Earth's surface. The consequences for aquatic phototrophic organisms of this small change in the solar spectrum are currently uncertain. UV radiation has been shown to adversely affect a number of photochemical and photobiological processes in a wide variety of aquatic organisms, such as cyanobacteria, phytoplankton and macroalgae. However, a number of photosynthetic organisms counteract the damaging effects of UV-B by synthesizing UV protective compounds such as mycosporine-like amino acids (MAAs) and the cyanobacterial sheath pigment, scytonemin. The aim of this contribution is to discuss the responses of algae and cyanobacteria to solar UV-B radiation and the role of photoprotective compounds in mitigating UV-B damage. [ABSTRACT FROM AUTHOR]

Impacts of UV-B radiation on the glutathione level were studied in mature Scots pine needles ( Pinus sylvestris L.) during the third season of a UV-B field experiment. Studies were made on 4-week-old (July) to 14-week-old (September) current-year needles and 3-year-old needles which had their third UV-B-exposure season in progress. Depending on the season and the year (1996–98), the supplemental UV-B dose varied from 0.92 to 5.09 kJ m −2 day −1 UV-B BE compared to 0.47–2.44 kJ m −2 day −1 UV-B BE under the ambient treatment. Fully grown UV-B-treated current-year needles showed lower total glutathione concentrations after the vegetation period in September, whereas in UV-B-treated 3-year-old needles the total glutathione content was significantly lower and the proportion of oxidized glutathione (GSSG%) 56% higher in July. The significant differences in total glutathione in current-year needles in September and in active 3-year-old needles in July seem to indicate that the effect of enhanced UV-B radiation on glutathione status could be cumulative. [ABSTRACT FROM AUTHOR]

AbstractThe performance of fifth generation offspring of a desert annual (Dimorphotheca sinuata DC.) were compared in the absence of UV-B, under variable atmospheric CO2 and nutrient supply, after four consecutive generations of concurrent exposure of their progenitors to UV-B at ambient (seasonal range: 2.55–8.85 kJ m–2 d–1) and enhanced (seasonal range: 4.70–11.41 kJ m–2 d–1) levels. Offspring of progenitors grown under elevated UV-B exhibited a diminished photosynthetic rate, a consequence of a reduced leaf density, and diminished foliar levels of carotenoids, polyphenolics and anthocyanins. Conversely, nonstructural carbohydrate and chlorophyll b levels were increased. Altered physiology was accompanied by reduced apical dominance and earlier flowering, features generally considered under photomorphogenic control, increased branching and inflorescence production and greater partitioning of biomass to reproductive structures, but diminished seed production. Many of these changes were magnified under nutrient limitation and intensified under atmospheric CO2 enriched conditions. The latter disagrees with current opinion that elevated CO2 may reduce detrimental UV-B effects, at least over the long-term. Observed correlations between seed production and polyphenolic, especially anthocyanin, levels in offspring, and indications of diminished lignification (thinner leaves, less robust stems and fewer lignified seeds set) all pointed to the involvement of the phenylpropanoid pathway in seed formation and plant structural development and its disruption during long-term UV-B exposure. Comparisons with earlier generations revealed trends with cumulative generations of enhanced UV-B exposure of increasing chlorophyll b and nonstructural carbohydrates, decreasing polyphenolics and biomass allocation to vegetative structures, and diminishing seed production despite increasing biomass allocation to reproductive structures. Notwithstanding some physiological compensation (increased chlorophyll b), the accumulation and persistence of these ostensibly inherited changes in physiological and reproductive performance suggest a greater impact of elevated UV-B on vegetation, primary production and regeneration over the long-term than presently envisaged. [ABSTRACT FROM AUTHOR]

ABSTRACTTreatment with supplementary UV-B resulted in decreases in transcripts of the photosynthetic genes Lhcb and psbA and concomitant increase in transcripts of two pathogen-related genes, PR-1 and PDF1·2, in Arabidopsis thaliana. UV-B exposure caused increases in jasmonic acid (JA) levels and ethylene production. UV-B treatment of jar1 and etr1-1 mutants, which are insensitive to JA and ethylene, respectively, showed that the increase in PR-1 transcripts was dependent on ethylene and PDF1·2 transcripts on both JA and ethylene. In contrast, the down-regulation of photosynthetic transcripts was independent of both compounds. Previous studies have indicated a role for reactive oxygen species (ROS) in the UV-B-induced down-regulation of the photosynthetic genes and up-regulation of PR-1 genes. Here we have shown that ROS are also required for the UV-B-induced up-regulation of PDF1·2 genes. The results indicate that the effects of UV-B on the three sets of genes are mediated through three distinct signal transduction pathways which are similar, but not identical, to pathways initiated in response to pathogen infection. In addition, the increased sensitivity of both jar1 and etr1-1 mutants to UV-B radiation, as compared with wild-type plants, indicated that intact JA and ethylene signal pathways are required for defence against UV-B damage. [ABSTRACT FROM AUTHOR]

Conifer needles are highly effective in screening ultraviolet-B radiation (280–320 nm). This ability is mainly attributed to the presence of flavonoids and hydroxycinnamic acids in the epidermal tissue. In two field cabinet experiments with two different clones of Norway spruce we assessed the seasonal accumulation of UV-B screening pigments under near-ambient, and close-to-zero UV-B irradiation. At the beginning of needle development, i.e. in June, kaempferol 3-O-glucoside was the dominant UV-B screening pigment. It was replaced during needle differentiation by the more effective diacylated flavonol glucosides, particulary kaempferol 3-O-(3",6"-O-di-p-coumaroyl)-glucoside, which reached highest concentrations in July. In addition to the soluble pool of diacylated flavonol glucoside derivatives, a cell wall-bound UV-B screen in the epidermal cell walls was formed during needle differentiation, consisting mainly of p-coumaric acid and kaempferol 3-O-glucoside. An effect of UV-B radiation on the accumulation of diacylated flavonol glucosides was only observed in 1996 with clone 2, when the concentrations of kaempferol 3-O-(3",6"-O-di-p-coumaroyl)-glucoside were significantly higher in July and August under field, and near-ambient than under close-to-zero UV-B irradiance. For wall-bound p-coumaric acid and kaempferol 3-O-glucoside UV-B radiation enhanced the concentrations of these compounds by approximately 20% in relation to the concentrations in close-to-zero UV-B-treated plants in both field cabinet experiments. [ABSTRACT FROM AUTHOR]

Variations in the amount of solar ultraviolet-B radiation (UV-B) reaching the biosphere may alter productivity in non-agricultural plants. We examined how ambient levels of UV-B modify the biomass of seven temperate-zone species including three grass species (Echinochloa crusgalli, Setaria faberi, Elymus virginicus), three forbs (Verbascum blattaria, Lactuca biennis, Oenothera parviflora), and one tree species (Quercus rubra). Plants were grown outside in enclosures near Morgantown, WV, USA (39° N, 79° W) for one season under near-ambient or no UV-B conditions. The different levels of UV-B were achieved using filters which differentially transmit UV-B irradiance. There was a trend towards reduced above-ground biomass in L. biennis (14%) and significantly increased above-ground biomass in O. parviflora (10.2%) under ambient UV-B. The partitioning of biomass between individual plant parts was altered by ambient UV-B in O. parviflora. Leaf biomass was significantly increased (18%), and there were trends toward increased stem (6.7%) and reproductive (9%) biomass. In addition to biomass stimulations, O. parviflora grew significantly taller (5.3%) under ambient UV-B. This study provides evidence that some non-agricultural plants exhibit species-specific growth responses to variable UV-B, with short-lived forbs appearing to be the most sensitive. If the biomass and morphological alterations observed for the forbs in this study were to persist over several years, they might modify population dynamics, competitive interactions, and productivity in ecosystems as UV-B levels fluctuate in the future. [ABSTRACT FROM AUTHOR]

Abstract Heather plants (Calluna vulgaris (L.) Hull) were grown for two years (April 1994–October 1996) under ambient or enhanced ultraviolet-B radiation (UVB: 280–315 nm), provided as a modulated treatment simulating a 15% ozone depletion (seasonally adjusted). Populations of the psyllid (Strophingia ericae (Curtis)) were measured before treatment and at yearly intervals thereafter. Before treatment there was no significant difference in the psyllid populations between treatments, or between the experimental and source populations. Enhanced UVB progressively produced a reduction in S. ericae populations compared with controls over 27 months. Analyses of C, N, total water soluble phenolics, total free amino acids and measurements of leaf angles and distances between leaflets demonstrated no effects of UVB treatment. However, concentrations of the amino acid isoleucine were lower (28%) in C. vulgaris exposed to the enhanced UVB treatment. Over the duration of the experiment the psyllid population structure at Lancaster changed from that typical of the upland site of origin (two-year cycle with overlapping cohorts) to a one-year life cycle typical of lowland sites, but this was independent of UVB treatments. Reduced isoleucine might explain the negative effects of elevated UVB on psyllid population numbers, but the precise effects of UVB on host chemistry and morphology are unknown. The problem of interpreting herbivore responses to enhanced UVB treatments in the field is discussed. [ABSTRACT FROM AUTHOR]

Gene expression is known to change in response to UV-B radiation. In this paper, we have investigated three factors in Arabidopsis leaves that are likely to influence these changes: development, protective pigments and the ‘chloroplast signal’. During late leaf development the major change in pigment composition, after exposure to UV-B radiation, is an increase in UV-absorbing pigments. Chl and Chl a/b ratio do not change substantially. Similarly Chl fluorescence is not altered. In contrast, RNA transcripts for photosynthetic proteins are reduced more in older leaves than in young leaves. To determine the role of flavonoids in UV-B protection, plants of Arabidopsis mutant tt-5, which have reduced flavonoids and sinapic esters, were exposed to UV-B and RNA transcript levels determined. The tt-mutants were more sensitive to UV-B radiation than wild-type. To examine the role of the chloroplast signal in regulating UV-B-induced changes in gene expression, Arabidopsis gun mutants (genome uncoupled) have been used. The results show that UV-B-induced down-regulation still takes place in gun mutants and strongly suggests that the chloroplast signal is not required. Overall, this study clearly demonstrates that UV-B-induced changes in gene expression are influenced by both developmental and cellular factors but not chloroplastic factors. [ABSTRACT FROM AUTHOR]

The effects of ultraviolet-B radiation (UV-B) on accessory function of human blood adherent mononuclear cells (ADH) for antigen and mitogen-induced responses, and production by ADH of the amplifying cytokine interleukin I (lL-1) were examined. Responder lymphocytes were rendered accessory cell dependent by treatment of nunadherent cells with OKIal+comptenient. UV-B depressed accessory function of ADH in a dose-dependent manner. UV-B at 5 ml/cm² decreased accessor) function of 2 × 104 ADH for tetanus toxoid-induced responses (measured as incorporation of ³H-thymidine) by 84% (P<0.001, n =6) and phytohaemagglutinin-induced responses 91% (P<0.001. n = 4). UV-B also decreased accessory activity of peripheral blood mononuclear cells but not Epstein-Barr virus-transformed B cells for a PPD-reactive T cell line. Viability was approximately 90% 0-72 h after exposure of ADH to 5 mJ/cm² of UV-B. Interleukin I (IL-1) activity of supernatants of ADH was assayed on C3H/HeJ mouse thymocytes. Pretreatment of ADH with 5 mJ/cm- UV-B decreased lipopolysaccharide-stimulated IL-1 activity from 169±34 (mean U/ml±s.e.) to4±1 (P<0.01, n = 4). Lysates of UV-B irradiated, LPS-stimulated ADH had no discernible IL-1 activity. Addition of I L-l partially restored accessory activity of UV-B irradiated ADH for lymphocyte responses to TT. Exposure of ADH to TT or PHA for 30 min before irradiation blocked the inhibitory effect of UV-B on accessory activity. Thus, low doses of UV-B are deleterious to accessory function and to production of lL-1 by ADH. Interference with production of cytokines and with initial interactions of accessory ceils with antigen and mitogen may be critical to the effects of UV-B on immunoregulatory function of ADH. [ABSTRACT FROM AUTHOR]

Light-harvesting complex II chlorophyll a/b-binding protein (Lhcb) mRNA levels are differentially affected by ultraviolet-B radiation (UV-B, 280-320 nm) at different stages of development of pea (Pisum sativum L. cv. Feltham first) seedlings. Addition of UV-B radiation to the light periods of diurnal cycles of white light resulted in reduction of total Lhcb mRNAs in green leaves but a transient increase in etiolated buds. The aims of this study were to determine the stage during de-etiolation at which supplementary UV-B began to inhibit Lhcb gene expression, and to determine whether differential regulation of individual Lhcb genes could explain the differential response to supplementary UV-B at different developmental stages. All seven Lhcb mRNAs were shown to increase in etiolated buds transferred to a diurnal cycle with supplementary UV-B during the light periods, but were greatly reduced in green leaves given the same treatment. Therefore, the different responses of total Lhcb mRNA levels to UV-B radiation in green leaves and etiolated buds are not primarily due to the expression of different members of the Lhcb gene family at different developmental stages. However, the Lhcb genes could be divided into two groups based on their sensitivities to UV-B. Transcripts from the three genes, Lhcb1*2, Lhcb1*3 and Lhcb1*5, which were undetectable in dark-grown etiolated buds, exhibited stronger responses to supplementary UV-B in green leaves than the four genes, Lhcb1*1, Lhcb1*4, Lhcb2*1 and Lhcb3*1, which showed low levels of initial transcript accumulation in dark-grown etiolated buds. The effect of UV-B on Lhcb mRNA levels were, however, correlated with chlorophyll content, suggesting that the developmental stage of chloroplasts may be important in determining the responses of the Lhcb genes to supplementary UV-B radiation in pea seedlings. [ABSTRACT FROM AUTHOR]

An application of stable carbon isotope analysis to the mechanistic interpretation of ultraviolet-B (UV-B) effects on growth inhibition is described that is particularly useful for small plants such as Arabidopsis thaliana that are not well suited for gas exchange studies. Many investigators use tissue δ13C, relative abundance of 13C and 12C, as a proxy for water use efficiency and as an indicator of environmental effects on stomatal behaviour and on photosynthesis during growth. Discrimination against 13C is enhanced by both high stomatal conductance and damage to photosynthetic machinery. Because the thinning of the stratospheric ozone layer is permitting more UV-B to enter the biosphere, the mechanisms of action of UV-B radiation on plants are of particular current interest. Arabidopsis thaliana wild-type Landsberg erecta (Ler) and the UV-B-sensitive mutant fah1 , deficient in UV-absorbing sinapate esters, were grown in a controlled environment and exposed to UV-BBE doses of 0 or 6-7 kJ m-2 day-1. UV-B exposure decreased dry matter production and δ 13C in both genotypes, but growth inhibition was generally greater in fah1 than in Ler. The fah1 mutant also had less leaf greenness than Ler. Changes in leaf tissue δ 13C were detected before growth inhibition and were evident in treatments of both genotypes that did not cause marked growth effects. This suggests that the effects of UV-B contributing to increased carbon isotope discrimination in Ler may have been primarily associated with high stomatal conductance, and in fah1 with both high stomatal conductance and damage to photosynthetic machinery. [ABSTRACT FROM AUTHOR]

Different species of microalgae show a wide range of susceptibility to ultraviolet-B (UV-B) radiation. To identify factors responsible for the UV-B tolerance of some of the algae, we compared 8 species that are highly tolerant to UV-B to 8 species that are highly susceptible. The tolerant species contained substantial amounts of an acetolysis-resistant residue. The residue consists of sporopollenin, a biopolymer of variable chemical composition that occurs in the algal cell walls and absorbs UV-B radiation. The susceptible species contained little or no sporopollenin. We propose that sporopollenin provides protection to the tolerant species by screening the incident UV-B radiation. Previous studies showed that the mycosporine-like amino acids (MAA) also act as effective UV-B screens. Our data indicate that sporopollenin provides a constant protection while MAA are induced by radiation stress and occur with some delay. The tolerant species also differ from the susceptible species in their capacity to repair the reaction centers damaged by UV-B. The tolerant algae became vulnerable to UV-B when protein synthesis needed for repair was blocked by streptomycin. In the susceptible species, streptomycin had no effect during the UV-B stress. The repair deficiency in the susceptible species can be explained either by relatively less effective protein synthesis or by an inhibition of the protein synthesis by UV-B. In the tolerant species, the structures needed for protein synthesis are protected by UV -B screening of sporopollenin and MAA. [ABSTRACT FROM AUTHOR]

The interactive effects of increased carbon dioxide (CO2) concentration and ultraviolet-B (UV-B, 280-320 nm) radiation on Acacia karroo Hayne, a C3 tree, and Themeda triandra Forsk., a C4 grass, were investigated. We tested the hypothesis that A. karroo would show greater CO2-induced growth stimulation than T. triandra, which would partially explain current encroachment of A. karroo into C4 grasslands, but that increased UV-B could mitigate this advantage. Seedlings were grown in open-top chambers in a greenhouse in ambient (360 μmol mol-1) and elevated (650 μmol mol-1) CO2, combined with ambient (1.56 to 8.66 kJ m-2 day-1) or increased (2.22 to 11.93 kJ m-2 day-1) biologically effective (weighted) UV-B irradiances. After 30 weeks, elevated CO2 had no effect on biomass of A. karroo, despite increased net CO2 assimilation rates. Interaction between UV-B and CO2 on stomatal conductance was found, with conductances decreasing only where elevated CO2 and UV-B were supplied separately. Increases in water use efficiencies, foliar starch concentrations, root nodule numbers and total nodule mass were measured in elevated CO2. Elevated UV-B caused only an increase in foliar carbon concentrations. In T. triandra, net CO2 assimilation rates were unaffected in elevated CO2, but stomatal conductances and foliar nitrogen concentrations decreased, and water use efficiencies increased. Biomass of all vegetative fractions, particularly leaf sheaths, was increased in elevated CO2, and was accompanied by increased leaf blade lengths and individual leaf and leaf sheath masses. However, tiller numbers were reduced in elevated CO2. Significantly moderating effects of elevated UV-B were apparent only in individual masses of leaf blades and sheaths, and in total sheath and shoot biomass. The direct CO2-induced growth responses of the species therefore do not support the hypothesis of CO2-driven woody encroachment of C4 grasslands. Rather, differential changes in resource use efficiency between grass and woody species, or morphological responses of grass species, could alter the competitive balance. Increased UV-B radiation is unlikely to substantially alter the CO2 response of these species. [ABSTRACT FROM AUTHOR]

Predicted increase in ultraviolet-B (UV-B: 280-320 nm) radiation may have adverse impacts on growth and yield of rice (Oryza sativa L.), as has been found in studies hitherto. However, most of the studies were conducted in growth chambers or greenhouses where the plants are generally more sensitive to UV-B than in the field, presumably because of the distorted balance between UV-B and ultraviolet-A as well as PAR. This study was conducted to address the effects of enhanced UV-B on growth and yield of rice under a realistic spectral balance in the field. Three cultivars, 'Koshihikari', 'IR 45' and 'IR 74' were pot-grown and irradiated with enhanced UV-B for most of the growing season in the field at Tsukuba, Japan (36°01'N, 140°07'E). The UV-B enhancement simulated ca 38% depletion of stratospheric ozone at Tsukuba. The results showed no UV-B effects on plant height, numbers of tillers and panicles, dry weight of the plant parts or the grain yield for any of the 3 cultivars. Natural abundance of 13 C in the flag leaves was not altered by the UV-B enhancement either. While UV-absorbing compounds showed no response to the UV-B enhancement, chlorophyll contents decreased with enhanced UV-B. However, the decrease of chlorophyll was limited to an early growth stage with no effect later. We thus found no extraordinary impact of the nearly doubled UV-B radiation on rice in the field, and it would appear that a reliable prediction of the effects of UV-B will require experiments carried out over a number of years under various climatic and solar UV-B regimes. [ABSTRACT FROM AUTHOR]

Sugar beet (Beta vulgaris L.) plants injected with Cercospora beticola Sacc. as well as non-infeded plants were grown under visible light with or without ultraviolet-B (UV-B, 280-320 nm) radiation for 40 days. An interaction between UV-B radiation and Cercospora leaf spot disease was observed, resulting in a large reduction in leaf chlorophyll content, dry weight of leaf laminae, petioles and storage roots. Lipid. peroxidation in leaves also increased the most under the combined treatments. This was also true for ultraweak luminescence from both adaxial and adaxial leaf surfaces. However, no correlation between lipid peroxidation and ultraweak luminescence was observed. Ultraviolet-B radiation given alone appeared to have either a stimulating effect, giving an increase in dry weight of laminae and reducing lipid peroxidation. or no effect. This lack of effect was seen in the absence of change in dry weight of storage roots and chlorophyll content relative to! controls. The study demonstrated a harmful interaction between UV-B radiation and Cercospora leaf spot disease on sugar beet. [ABSTRACT FROM AUTHOR]

Direct evidence for the possible loss of photosystem II (PS II) activity in chloroplasts of Vigna sinensis L. cv. Walp after ultraviolet-B (UV-B, 280-320 nm) radiation treatment was provided by polyacrylamide gel electrophoretic analysis of PS ii polypeptides. A 30 mm UV-B treatment of chloroplasts caused a 50% loss of PS II activity. The artificial electron donor, Mn2+ failed to restore UV-B radiation induced loss of PS II activity, while diphenyl carbazide (DPC) and NH2OH only partially restored activity. Such a toss in PS II activity was found to be primarily due to a loss of 23 and 33 kDa extrinsic polypeptides. UV-B treatment induced the synthesis of a few polypeptides and a 29 kDa light-harvesting chlorophyll protein. [ABSTRACT FROM AUTHOR]

Cucumber (Cucumis sativus L. cv. Natsusairaku 3) seedlings were grown in a growth cabinet under UV-B (290-320 nm) irradiation (equivalent to the UV-B radiation normally incident at Tokyo, 36°N latitude, during clear sky conditions in mid-april on a weighted daily fluence basis) and a UV-B-free control condition. UV-B irradiation inhibited the growth of the cotyledons, i.e. the increase in area, and increase in fresh and dry weights of the cotyledons. The greatest inhibition rate was observed in the increase in area, causing a significant increase in specific leaf weight (the ratio of weight to area). UV-B irradiation had no significant effect on DNA and RNA contents in the cotyledons, but decreased protein content slightly. In contrast, the irradiation reduced the amounts of organic acids and soluble sugars, indicating that primary carbon metabolism was very sensitive to UV-B radiation. UV-B irradiation lowered the photosynthetic activity in the cotyledons without any effect on chlorophyll content and respiratory activity. These results indicate that UV-B radiation at the ambient level may act as a physiological stress in some UV-sensitive plants. [ABSTRACT FROM AUTHOR]

Supplementary UV-B was shown to lead to a decrease in transcripts encoding the photosynthetic genes Lhcb and psbA and a concomitant increase in transcripts encoding three acid-type pathogenesis-related proteins, PR-1, PR-2 and PR-5, in Arabidopsis thaliana. UV-B radiation has been reported to lead to the generation of reactive oxygen species (ROS). Here we report that ROS are required for UV-B-induced down-regulation of the photosynthetic genes and up-regulation of PR genes, as the addition of antioxidants before UV-B treatment resulted in a marked reduction in the effect of UV-B on both sets of genes. Rises in ROS are frequently accompanied by increases in salicylic acid (SA) accumulation. UV-B treatment of transgenic NahG Arabidopsis plants, which are unable to accumulate SA, showed that the increase in PR transcripts, but not the decrease in photosynthetic transcripts, was dependent on the increase in SA. In addition, a 3 d exposure to UV-B radiation resulted in a 7-fold increase in SA levels. Oxidant treatment of NahG plants indicated that ROS could not up-regulate PR genes in the absence of SA accumulation; however, the down-regulation of photosynthetic transcripts was unchanged from that in wild-type plants. The results indicate that the effects of UV-B on the two sets of genes are mediated through two distinct signal tranduction pathways. One pathway is ROS-dependent but SA-independent and mediates the down-regulation of photosynthetic genes. The other is SA- and ROS-dependent and mediates the up-regulation of the acidic-type PR genes. [ABSTRACT FROM AUTHOR]

Transmittance spectra of isolated plant cuticles were measured in the wavelength range from 270 to 600 nm. The cuticles were enzymatically isolated from the leaves of 27 species (26 evergreen or deciduous woody, one succulent herbaceous) and from four species of fruits. With the exception of subtropical and tropical species all plants were cultivated in the field. The cuticles of the species studied strongly attenuated ultraviolet (UV) radiation at wavelengths < 400 nm while they were practically translucent in the visible range. Relatively broad transmittance minima occurred at wavelengths from 280 to 320 nm (UV-B). Spectral transmittances at 300 nm ranged from 0.004 (Ilex aquifolium) to 0.50 (Prunus avium) for leaf cuticles and from 0.00023 (Cydonia oblonga) to 0.005 (Malus domestica) for fruit cuticles. The constitutive UV protection by cuticular pigments may be supplemented, to varying degrees, by pigments located in the epidermal cell wall and protoplast. Thus, it is concluded that only a small fraction of incident UV-B radiation may actually reach the sensitive tissues of the leaves of non-herbaceous species and of fruits. [ABSTRACT FROM AUTHOR]

Data of ultraviolet-B radiation in 1996 at Northridge were derived from a Yankee Environmental Systems UVB-1 pyranometer. Hourly and seasonal variations of ultraviolet-B radiation were presented. Statistical analyses between ultraviolet-B radiation and other weather variables including air temperature, relative humidity, visible-infrared radiation, and precipitation were conducted. [ABSTRACT FROM AUTHOR]

A South African winter ephemeral D. pluvialis was exposed, under low and high nutrient conditions, to four different daily doses of biologically effective UV-B radiation. These simulated different depletions (range 0-30%) in the ozone layer at the southerly distribution limit (33° 56'S) of this species, and included daily UV-B doses received at the northerly distribution limit (26° 38'S) without ozone depletion. Growth inhibition by increased UV-B radiation was observed during early vegetative stages, but only under low nutrient conditions. Thereafter, net CO2 assimilation rate, growth and reproduction were stimulated by an increase in UV-B radiation, though doses above those approximating a 20% ozone depletion appeared to be inhibitory. Differential stimulation occurred in the two nutrient treatments. Under high nutrient conditions, photosynthesis (specifically carboxylation efficiency), and numbers of leaves, inflorescences and diaspores per plant, and leaf areas increased, but leaf thickness decreased with increased UV-B radiation. Under low nutrient conditions, dry masses of leaves, stems, inflorescences and diaspores, and total above-ground dry masses increased with increased UV-B radiation. Foliar organic carbon and nitrogen concentrations and foliar concentrations of UV-B absorbing compounds were unaffected by increased UV-B radiation, but foliar P concentrations declined. Diaspore viability declined with increased UV-B radiation. The net effect was a 35 to 43% reduction in viable diaspore production under high nutrient conditions at UV-B doses equivalent to those currently received at the northerly distribution limit during the reproductive phase. It is concluded that anticipated increases in UV-B radiation could reduce regeneration success, and seedling survival in areas of low soil fertility, particularly at lower latitudes, and consequently increase the risk of localized population extinctions from stochastic causes. [ABSTRACT FROM AUTHOR]

To evaluate the effect of different naturally occurring irradiation conditions on the sensitivity of bean (Phaseolus vulgaris cv. Label) to increased UV-B levels, plants were grown under six different light treatments. In the control series (at ambient levels of UV-B), UV-B and visible light were decreased in parallel, resulting in three different total irradiation treatments with the same UV-B/PAR ratio. A second series with a 15% increase in UV-B irradiation at each PAR level was used to investigate the effect of UV-B under the varying total irradiance levels. The different total irradiance levels resulted in large differences in total dry weight, specific leaf weight, photosynthesis-light response and pigment concentrations. Nevertheless, the 1S% increase in UV-B resulted In equal reductions in total dry weight (from 245 to 343%) and effective photosynthesis for all light levels. The accumulation of protective pigments in the primary bean leaves was strongly correlated to the total irradiance level (200% increase from the lowest to the highest light level), but was not influenced by increasing UV-B levels. As the UV-BIPAR ratio outside increases with decreasing total irradiance (when induced by cloud cover) this implies that low radiation levels are potentially dangerous to some plants, even though the UV-B levels may seem negligible. [ABSTRACT FROM AUTHOR]

The effects of ultraviolet-B (UV-B between 290 and 320 nm) on photosynthesis and growth characteristics were investigated in field grown cassava (Manihot esculentum Crantz). Plants were grown at ambient and ambient plus a 5.5 kJ m-2 d-1 supplementation of UV-B radiation for 95 d. The supplemental UV-B fluence used in this experiment simulated a 15% depletion in stratospheric ozone at the equator (0°N). Carbon dioxide exchange, oxygen evolution, and the ratio of variable to maximum fluorescence (Fv/Fm) were determined for fully expanded leaves after 64-76d of UV-B exposure. All plants were harvested after 95d of UV-B exposure, assayed for chlorophyll and UV-B absorbing compounds, and separated into leaves, petioles, stems and roots. Exposure to UV-B radiation had no effect on in situ rates of photosynthesis or dark respiration. No difference in the concentration of UV-B absorbing compounds was observed between treatments. A 2-d daytime diurnal comparison of Fv to Fm ratios indicated a significant decline in Fv/Fm ratios and a subsequent increase in photoinhibition under enhanced UV-B radiation if temperature or PPF exceeded 35°C or 1800 μmol m-2 s-1, respectively. However, UV-B effects on fluorescence kinetics appeared to be temporal since maximal photosynthetic rates as determined by oxygen evolution at saturated CO2 and PPF remained unchanged. Although total biomass was unaltered with UV-B exposure, alterations in the growth characteristics of cassava grown with supplemental UV-B radiation are consistent with auxin destruction and reduced apical dominance. Changes in growth included an alteration of biomass partitioning with a significant increase in shoot/root ratio noted for plants receiving supplemental UV-B radiation, The increase in shoot/root ratio was due primarily to a significant decrease in root weight (-32%) with UV-B exposure. Because root production determines the harvestable portion of cassava, UV-B radiation may still influence. [ABSTRACT FROM AUTHOR]

Relationships between leaf wettability and surface physicochemical characteristics were examined in two genotypes of tobacco (Nicotiana tabacum L. cv. Samsun) grown under controlled conditions at three different levels of biologically effective ultraviolet-B (UV-BBE; 280–320 nm) radiation; 0 (control), 4.54 and 5.66 kJ m−2d−1. Leaf wettability, assessed by measuring leaf-water droplet contact angles, was positively correlated with epicuticular wax chemical composition and trichome density, but not the amount of wax on the surface of leaves. Tobacco wax comprised a mixture of C19–C33 n-alkanes (≈59%) with homologues containing an odd number of carbon atoms predominating, C28–C32 br-alkanes (≈38%), and a small quantity (≈3%) of free Cl6–C18 fatty acids. Significant effects of UV-B radiation upon wax production and chemical composition were restricted to the adaxial surface of leaves. Enhanced UV-B radiation reduced the quantity of epicuticular wax in the more sensitive genotype [GR32-3], assessed from effects on dry matter accumulation, partitioning and changes in leaf morphology, and resulted in marked changes in wax composition and homologue distributions in both genotypes. UV-B-induced increases in branching, and shifts toward the synthesis of shorter-chain homologues provided evidence for a fundamental effect of UV-B radiation on wax biosynthesis, with the observed effects consistent with a highly specific and direct effect of UV-B radiation on microsomal-based elongases in the epidermis. UV-B radiation also reduced the density of trichomes on the adaxial leaf surface, whilst increasing the number of trichomes on the abaxial leaf surface. Changes in wax composition and trichome density induced by UV-B radiation were associated with increases in leaf surface wettability which were particularly pronounced on the adaxial surface. The subtle, though possibly far-reaching, physiological consequences of such UV-B-induced changes in surface wettability are discussed in the light of other recent findings. [ABSTRACT FROM PUBLISHER]

Mixtures and monocultures of wheat (Triticum aestivum) and wild oat (Avena fatua), a common weedy competitor of wheat, were exposed to enhanced solar UV-B radiation simulating a 20% reduction in stratospheric ozone to assess the timing and seasonal development of the UV-B effects on light competition in these species. Results from two years of field study revealed that UV-B enhancement had no detectable effect on the magnitude or timing of seedling emergence in either species. End-of-season measurements showed significant UV-B inhibition of leaf insertion height in wild oat in mixture and monoculture in the second year (irrigated year) but not in the first year (drought year). Leaf insertion height of wheat was not affected by UV-B in either year. The UV-B treatment had no detectable effect on monoculture or total (combined species) mixture LAI but did significantly increase (5-7%) the fractional contribution of wheat to the mixture LAI after four weeks of growth in both years. In addition, the UV-B treatment had subtle effects on LAI height profiles with early season mixtures showing significant reductions in wild oat LAI in lower canopy layers in both years while mid-season Year 2 mixtures showed significant reductions in wild oat LAI in upper canopy layers. The changes in canopy structure were found to significantly increase (6-7%) the proportional simulated clear sky canopy photosynthesis and light interception of wheat in mixture. These findings, and others, indicate that the effects of UV-B enhancement on competition are realized very early in canopy development and provide additional support for the hypothesis that UV-B enhancement may shift the balance of competition between these species indirectly by altering competitive interactions for light. [ABSTRACT FROM AUTHOR]

A growth chamber experiment was conducted to examine the effects of UV-B exposure (4.9 kJ m−2 day−1 of biologically effective UV-B, 280–320 nm) on shoot growth and secondary metabolite production in Betula pendula (Roth) and B. resinifera (Britt.) seedlings originating from environments in Finland, Germany and Alaska differing in solar UV-B radiation and climate. Neither shoot growth nor the composition of secondary metabolites was affected by UV-B irradiance, but the treatment induced significant changes in the amounts of individual secondary metabolites in leaves. Leaves of seedlings exposed to UV-B radiation contained higher concentrations of several flavonoids, condensed tannins and some hydroxycinnamic acids than leaves of control seedlings that received no UV-B radiation. At the population level, there was considerable variation in secondary metabolite responses to UV-B radiation: among populations, the induced response was most prominent in Alaskan populations, which were adapted to the lowest ambient UV-B radiation environment. I conclude that solar UV-B radiation plays an important role in the formation of secondary chemical characteristics in birch trees. [ABSTRACT FROM PUBLISHER]

Competition for light among species in a mixed canopy can be assessed quantitatively by a simulation model which evaluates the importance of different morphological and photosynthetic characteristics of each species. A model was developed that simulates how the foliage of all species attenuate radiation in the canopy and how much radiation is received by foliage of each species. The model can account for different kinds of foliage (leaf blades, stems, etc.) for each species. The photosynthesis and transpiration for sunlit and shaded foliage of each species is also computed for different layers in the canopy. The model is an extension of previously described single-species canopy photosynthesis simulation models. Model predictions of the fraction of foliage sunlit and interception of light by sunlit and shaded foliage for monoculture and mixed canopies of wheat ( Triticum aestivum) and wild oat ( Avena fatua) in the field compared very well with measured values. The model was used to calculate light interception and canopy photosynthesis for both species of wheat/wild oat mixtures grown under normal solar and enhanced ultraviolet-B (290-320 nm) radiation (UV-B) in a glasshouse experiment with no root competition. In these experiments, measurements showed that the mixtures receiving enhanced UV-B radiation had a greater proportion of the total foliage area composed of wheat compared to mixtures in the control treatments. The difference in species foliage area and its position in the canopy resulted in a calculated increase in the portion of total canopy radiation interception and photosynthesis by wheat. This, in turn, is consistent with greater canopy biomass of wheat reported in canopies irradiated with supplemental UV-B. [ABSTRACT FROM AUTHOR]

Studies of the effects of increases in ultraviolet-B (UV-B) radiation on plants and terrestrial ecosystems have been undertaken using a variety of methods including: controlled-environment cabinets, glasshouses, outdoor filtration and outdoor supplementation using fluorescent UV-B lamps. Outdoor supplementation systems provide a method of study which creates only small alterations to the microclimate and the number of such studies has increased during the past 3 years. These supplementation systems differ in their methods of operation, equipment, UV-B exposure regime and experimental design. This essay surveys the systems currently developed, considers problems associated with their use and discusses these in relation to the interpretation of biological effects. [ABSTRACT FROM AUTHOR]

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Ultraviolet-B (UV-B) irradiation of DNA generates mutagenic photoproducts such as cyclobutane pyrimidine dimers (CPDs) which can affect the growth and development of amphibian embryos. Differential ability to repair UV-B-induced DNA damage may be␣responsible for differences in population stability between␣some amphibian species. Photoreactivation via the enzyme photolyase is a major mechanism used to remove CPDs from DNA. The aim of this study was to determine if photolyase activity differed in three sympatric Australian amphibian species, one of which has suffered marked population declines ( Litoria aurea) and two whose populations do not appear to be in decline ( L. dentata and L. peronii). The specific activity of photolyase was measured in each species and compared to the hatching success of their eggs under unfiltered summer sunlight. The mean specific activities of photolyase were 1.10 ± 0.18 × 1011, 5.76 ± 1.01 × 1011, and 2.66 ± 0.15 × 1011 CPDs repaired per hour per microgram of egg protein extract, for L. aurea, L. dentata and L. peronii, respectively. When intrinsic differences in hatching success between species were controlled for, the relative percentage hatching success under unfiltered sunlight of L. aurea (77%) was lower than that of L.␣peronii (91%) and L. dentata (98%); however, these values did not differ significantly. L. aurea had the lowest photolyase activity of the three species and showed a non-significant trend of reduced hatching success under UV-B exposure. [ABSTRACT FROM AUTHOR]

Bryophytes are the dominant vegetation in the Antarctic continent. They have suffered more unpleasant ultraviolet radiation due to the Antarctic ozone layer destruction. However, it remains unclear about the molecular mechanism of Antarctic moss acclimation to UV-B light. Here, the transcriptomics and metabolomics approaches were conducted to uncover transcriptional and metabolic profiling of the Antarctic moss Leptobryum pyriforme under UV-B radiation. Totally, 67,290 unigenes with N 50 length of 2,055 bp were assembled. Of them, 1,594 unigenes were significantly up-regulated and 3353 unigenes were markedly down-regulated under UV-B radiation. These differentially expressed genes (DEGs) involved in UV-B signaling, flavonoid biosynthesis, ROS scavenging, and DNA repair. In addition, a total of 531 metabolites were detected, while flavonoids and anthocyanins accounted for 10.36% of the total compounds. There were 49 upregulated metabolites and 41 downregulated metabolites under UV-B radiation. Flavonoids were the most significantly changed metabolites. qPCR analysis showed that UVR8-COP1-HY5 signaling pathway genes and photolyase genes (i.e., LpUVR3 , LpPHR1 , and LpDPL ) were significantly up-regulated under UV-B light. In addition, the expression levels of JA signaling pathway-related genes (i.e., OPR and JAZ ) and flavonoid biosynthesis-related genes were also significantly increased under UV-B radiation. The integrative data analysis showed that UVR8-mediated signaling, jasmonate signaling, flavonoid biosynthesis pathway and DNA repair system might contribute to L. pyriforme acclimating to UV-B radiation. Therefore, these findings present a novel knowledge for understanding the adaption of Antarctic moss to polar environments and provide a foundation for assessing the impact of global climate change on Antarctic land plants., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Liu, Fang, Liu, Zhao, Cong and Zhang.)

The solar ultraviolet-B radiation (UVB) is increasingly affecting the aquatic ecosystems due to the long-term antropic damage to the stratospheric ozone. The distrupted interspecies competition is one of the primary causes driving the plankton community composition shifts under UVB stress. To reveal the competitive responses to enhanced UVB radiation, we grew two green algae Scenedesmus obliquus and Chlorella pyrenoidosa , and the unicellular cyanobacterium Microcystis aeruginosa in monocultures and in cocultures under differerent UVB intensities (0, 0.3 and 0.7 W m−2), respectively. Results showed that elevated UVB radiation consistently decreased the population carrying capacies and the photosynthesis of the three species in monocultures. While cocultivated, C. pyrenoidosa was competively excluded by the presence of S. obliquus , and the competitive outcome was not affected by UVB exposure. By contrast, unicellular M. aeruginosa overwhelmingly suppressed the population growth of S. obliquus under no UVB, yet S. obliquus tended to be a better competitor under 0.3–0.7 W m−2 UVB exposure. The species-specific photosynthesis sensitivity to UVB can partly explain the different tolerance of the algae to UVB and the change of competition outcome under elevated UVB. The present study elucidated the potential role of increased UVB radiation in determining the competitions between phytoplankton species, contributing to the understanding of phytoplankton community shifts under enhanced UVB stress. • High UVB does not alter the competitive superiority of Scenedesmus over Chlorella. • Unicellular Microcystis suppresses population growth of Scenedesmus under no UVB. • Scenedesmus is a better competitor compared to unicellular Microcystis under UVB. • UVB reverses competitive outcome between unicellular Microcystis and Scenedesmus. [ABSTRACT FROM AUTHOR]

Prolonged exposure to harmful ultraviolet radiation (UVR) can induce many chronic or acute skin disorders in humans. To protect themselves, many people have started to apply cosmetic products containing UV-screening chemicals alone or together with physical sunblocks, mainly based on titanium–dioxide (TiO2) or zinc-oxide (ZnO2). However, it has now been shown that the use of chemical and physical sunblocks is not safe for long-term application, so searches for the novel, natural UV-screening compounds derived from plants or bacteria are gaining attention. Certain photosynthetic organisms such as algae and cyanobacteria have evolved to cope with exposure to UVR by producing mycosporine-like amino acids (MAAs). These are promising substitutes for chemical sunscreens containing commercially available sunblock filters. The use of biopolymers such as chitosan for joining MAAs together or with MAA-Np (nanoparticles) conjugates will provide stability to MAAs similar to the mixing of chemical and physical sunscreens. This review critically describes UV-induced skin damage, problems associated with the use of chemical and physical sunscreens, cyanobacteria as a source of MAAs, the abundance of MAAs and their biotechnological applications. We also narrate the effectiveness and application of MAAs and MAA conjugates on skin cell lines. [ABSTRACT FROM AUTHOR]