Elevated UV-B Levels Research Articles

Effects of artificial ultraviolet B radiation on the macrophyte Lemna minor: a conceptual study for toxicity pathway characterization

Main conclusionUVB radiation caused irradiance-dependent and target-specific responses in non-UVB acclimated Lemna minor. Conceptual toxicity pathways were developed to propose causal relationships between UVB-mediated effects at multiple levels of biological organisation.Macrophytes inhabit waterways around the world and are used in hydroponics or aquaponics for different purposes such as feed and wastewater treatment and are thus exposed to elevated levels of UVB from natural and artificial sources. Although high UVB levels are harmful to macrophytes, mechanistic understanding of irradiance-dependent effects and associated modes of action in non-UVB acclimated plants still remains low. The present study was conducted to characterise the irradiance-dependent mechanisms of UVB leading to growth inhibition in Lemna minor as an aquatic macrophyte model. The L. minor were continuously exposed to UVB (0.008–4.2 W m−2) and constant UVA (4 W m−2) and photosynthetically active radiation, PAR (80 µmol m−2 s−1) for 7 days. A suite of bioassays was deployed to assess effects on oxidative stress, photosynthesis, DNA damage, and transcription of antioxidant biosynthesis, DNA repair, programmed cell death, pigment metabolism and respiration. The results showed that UVB triggered both irradiance-dependent and target-specific effects at multiple levels of biological organization, whereas exposure to UVA alone did not cause any effects. Inhibition of photosystem II and induction of carotenoids were observed at 0.23 W m−2, whereas growth inhibition, excessive reactive oxygen species, lipid peroxidation, cyclobutane pyrimidine dimer formation, mitochondrial membrane potential reduction and chlorophyll depletion were observed at 0.5–1 W m−2. Relationships between responses at different levels of biological organization were used to establish a putative network of toxicity pathways to improve our understanding of UVB effects in aquatic macrophytes under continuous UVB exposures. Additional studies under natural illuminations were proposed to assess whether these putative toxicity pathways may also be relevant for more ecologically relevant exposure scenarios.

Differential investment in pre- and post-mating male sexual traits in response to prolonged exposure to ambient UVB radiation in a fish

Increasing UVB radiation (UVB) reaching earth's surface following stratospheric ozone depletion is linked to serious consequences for organisms. While studies have focused on direct cytocidal and immunomodulatory effects of UVB, indirect consequences for fitness-related life-history traits are largely unexplored, although knowledge is needed to understand organismal responses to climate change. The present study investigates the effects of developmental exposure to enhanced, but ecologically relevant, ambient UVB levels on growth (body size), parental behavior (nest-building), fitness-enhancing traits of pre-mating (sexual ornamentation) and post-mating (sperm traits) sexual selection as well as their interrelationships in male three-spined sticklebacks (Gasterosteus aculeatus). Moreover, potentially underlying direct UVB effects, testicular antioxidant capacity (TAC) and testes and skin melanization were quantified. Juvenile full-siblings were split into two groups and raised until adulthood in a semi-natural set-up under light conditions including either natural (UVBnormal) or elevated UVB levels (UVBenhanced). When becoming reproductive, males were kept individually before data collection took place at their reproductive peak. The results showed that males from the UVBenhanced-group were smaller than their brothers from the UVBnormal-treatment, whereas no treatment differences were observed for sexual ornamentation, sperm traits and TAC. Moreover, UVB-stressed males built their nests faster and the relationship between body size and nest size was negative for UVBenhanced-males while being positive for UVBnormal-males. These results demonstrate that, depending on physical state, UVB-stressed males adjusted their behavior to some extent. Additionally, a trade-off between the investment in pre- and post-mating traits was found for UVBenhanced-males, i.e. the intensity of their breeding coloration was negatively correlated with sperm number whereas this relationship was reversed for UVBnormal-males, thus showing an interaction between pre- and post-mating traits regarding exposure treatment. The interaction provides first experimental evidence that differential allocation to energetically demanding pre- and post-mating components of male fitness is triggered by a key environmental stressor of climate change.

The future of seaweed aquaculture in a rapidly changing world

Human activities are having increasingly negative impacts on the natural environment. The rapidly expanding human population has led to a shortage of resources and the ability to support the growing population sustainably is a major challenge for the future. Coastal environments, including natural seaweed communities, provide a range of important ecosystem services. Since seaweed aquaculture beds (SABs) provide many of the services associated with natural seaweed communities they have a potential role in providing solutions such as CO2 sequestration, provision of food and the supply of useful chemicals. However, the productivity of natural seaweed communities and SABs is under threat from the rapid changes in climate that the planet is experiencing. Here we examine the likely effects of global change, in particular elevated CO2 and ocean acidification, increased temperatures and elevated levels of UVB, on the performance of seaweeds. While it is clear that rising temperatures and elevated CO2 and their interactions with other environmental factors are likely to have profound effects on macroalgal production, such effects are likely to be species dependent. We also examine the fate of organic matter from seaweeds and the potential for using SAB productivity as a contributor to blue carbon as a strategy for amelioration of increases in anthropogenic CO2 emissions. There is considerable potential for increased drawdown of CO2 by SABs, though its effectiveness in amelioration of atmospheric CO2 increase will depend on the fate of the resulting biomass.

Enhanced ambient UVB light affects growth, body condition and the investment in innate and adaptive immunity in three-spined sticklebacks (Gasterosteus aculeatus)

With ongoing environmental change, ultraviolet-B radiation (UVB) reaching the Earth’s surface has increased over recent decades with consequences for terrestrial and also aquatic ecosystems. Despite evidence for direct physiological and immunological responses of aquatic animals following enhanced UVB exposure, studies investigating indirect impacts of ambient UVB radiation are scarce and mainly used only single doses and/or artificially high amounts of UVB. In the present study, the influence of chronic exposure to elevated UVB levels on growth, body condition and immune function was investigated in three-spined sticklebacks (Gasterosteus aculeatus). Fish were kept outdoors for 68 ± 2 days under two different spectral conditions; one group was exposed to natural solar radiation (UVB-normal), while the other group received additional UVB light for four hours daily (UVB-enhanced). Enhanced UVB radiation was within the range of UVB levels measured at the study site. Fish length and weight were determined at the beginning and end of the experiment to compare growth and body condition between the two treatment groups. At the end of the experiment, the splenosomatic index and the granulocyte-to-lymphocyte ratio were determined as immune parameters. Fish from the UVB-enhanced group showed a reduced growth and body condition as well as a lower splenosomatic index compared to the UVB-normal group. Furthermore, UVB-treated fish had a higher granulocyte-to-lymphocyte ratio representing a relatively higher activation of innate compared to adaptive immunity. Consequently, increased but ecologically relevant levels of ambient UVB negatively affect growth and body condition and have a considerable impact on immunity in three-spined sticklebacks.

Morphological, biochemical and physiological responses of Indian cress (Tropaeolum majus) to elevated UV-B radiation

Background and Purpose: UV-B radiation is an important environmental factor for many plants with remarkable influence on defence-related secondary metabolite biosynthesis. Possible consequences of UV-B radiation on plants have been widely reported, but its effect on secondary plant metabolites in ornamental and medicinal plants is poorly understood. The aim of the present research was to establish whether it is feasible to cultivate Tropaeolum majus under conditions of enhanced UV-B radiation to alter the content of total phenolic compounds in leaves and flowers and whether UV-B treatment affect plant physiological response. Material and Method: An outdoor study was conducted to examine the effect of different levels of UV-B radiation on the morphological (specific leaf area and length of internodes and petiole), biochemical (photosynthetic pigments, UV absorbing compounds) and physiological characteristics (photosynthesis, photochemical yield of PSII, transpiration rate, water use efficiency) of the widely cultivated annual herb Indian cress ( T. majus L.). Results and Conclusions: Enhanced UV-B radiation induced increased synthesis of total phenolic compounds in leaves, but not in flowers. Photosynthesis and photochemical yield of PSII were mostly unaffected by UV-B. Transpiration rate was higher at elevated UV-B levels in the beginning and peak of the season. Specific leaf area and length of internodes and petiole were unaffected. UV-B treated specimens of T. majus possessed enhanced amounts of total phenolic compounds, which are important for utilisation of T. majus herbs for human health. Enhanced UV-B treatment affected flowering of T. majus at the end of the growing season, which may have an important negative implication for success of this species in elevated UV-B radiation environments.

Morphological, biochemical and physiological responses of Indian cress (Tropaeolum majus) to elevated UV-B radiation

Background and Purpose: UV-B radiation is an important environmental factor for many plants with remarkable influence on defence-related secondary metabolite biosynthesis. Possible consequences of UV-B radiation on plants have been widely reported, but its effect on secondary plant metabolites in ornamental and medicinal plants is poorly understood. The aim of the present research was to establish whether it is feasible to cultivate Tropaeolum majus under conditions of enhanced UV-B radiation to alter the content of total phenolic compounds in leaves and flowers and whether UV-B treatment affect plant physiological response. Material and Method: An outdoor study was conducted to examine the effect of different levels of UV-B radiation on the morphological (specific leaf area and length of internodes and petiole), biochemical (photosynthetic pigments, UV absorbing compounds) and physiological characteristics (photosynthesis, photochemical yield of PSII, transpiration rate, water use efficiency) of the widely cultivated annual herb Indian cress ( T. majus L.). Results and Conclusions : Enhanced UV-B radiation induced increased synthesis of total phenolic compounds in leaves, but not in flowers. Photosynthesis and photochemical yield of PSII were mostly unaffected by UV-B. Transpiration rate was higher at elevated UV-B levels in the beginning and peak of the season. Specific leaf area and length of internodes and petiole were unaffected. UV-B treated specimens of T. majus possessed enhanced amounts of total phenolic compounds, which are important for utilisation of T. majus herbs for human health. Enhanced UV-B treatment affected flowering of T. majus at the end of the growing season, which may have an important negative implication for success of this species in elevated UV-B radiation environments.

Underlying genetic variation in the response of cultivated and wild soybean to enhanced ultraviolet-B radiation

Reductions in stratospheric ozone have resulted in increased levels of ultraviolet-B (UV-B, 280–315 nm) radiation reaching the earth’s surface. UV-B radiation can damage plant DNA and photosynthetic machinery. Therefore, enhanced UV-B levels would seriously affect crop production in several regions of the world. Here, we provide a comprehensive report of intraspecific variability in the response of soybean to UV-B radiation and the genetic variation underlying UV-B resistance. We evaluated 140 genotypes, including 94 Glycine max and 46 G. soja accessions, for their sensitivity to supplemental UV-B radiation. Differences in responses to elevated UV-B levels, such as changes in leaf area and aerial biomass, were observed among soybean genotypes, supporting the notion that there is intraspecific variability in this response in G. soja as well as G. max. To explore how genetic variation contributes to differences in UV-B resistance, we sequenced the most UV-B resistant (IT162669) and sensitive (Cheongjakong 3) genotypes at the whole-genome level. Based on their homology and functional annotation, 137 genes were determined to be “UV-B-related genes” in soybean, harboring almost 100 high-confidence single nucleotide polymorphisms between the two genotypes. Interestingly, we identified four genes with non-synonymous mutations that are related to plant protection mechanisms, such as the UV-protection, DNA damage repair and DNA damage tolerance pathways. Our results provide valuable information about UV-B-resistant soybean genotypes, UV-B-related soybean genes, and sequence variations between resistant and sensitive genotypes.

The UV-B photoreceptor UVR8 promotes photosynthetic efficiency in Arabidopsis thaliana exposed to elevated levels of UV-B

The UV-B photoreceptor UVR8 regulates expression of genes in response to UV-B, some encoding chloroplast proteins, but the importance of UVR8 in maintaining photosynthetic competence is unknown. The maximum quantum yield of PSII (F (v)/F(m)) and the operating efficiency of PSII (Φ(PSII)) were measured in wild-type and uvr8 mutant Arabidopsis thaliana. The importance of specific UVR8-regulated genes in maintaining photosynthetic competence was examined using mutants. Both F (v)/F(m) and Φ(PSII) decreased when plants were exposed to elevated UV-B, in general more so in uvr8 mutant plants than wild-type. UV-B increased the level of psbD-BLRP (blue light responsive promoter) transcripts, encoding the PSII D2 protein. This increase was mediated by the UVR8-regulated chloroplast RNA polymerase sigma factor SIG5, but SIG5 was not required to maintain photosynthetic efficiency at elevated UV-B. Levels of the D1 protein of PSII decreased markedly when plants were exposed to elevated UV-B, but there was no significant difference between wild-type and uvr8 under conditions where the mutant showed increased photoinhibition. The results show that UVR8 promotes photosynthetic efficiency at elevated levels of UV-B. Loss of the DI polypeptide is probably important in causing photoinhibition, but does not entirely explain the reduced photosynthetic efficiency of the uvr8 mutant compared to wild-type.

Impact of elevated UVB radiation on marine biota: a meta‐analysis

AbstractAimThe emission of chlorofluorocarbon compounds eroded the ozone layer, raising incident ultraviolet B radiation to levels that affect biota. However, the role of UVB radiation (280–315 nm), which remains elevated to date, as a possible driver of the widespread global deterioration of marine ecosystems has not yet been fully quantified. In this paper we assess the magnitude of the impacts of elevated UVB radiation and evaluate the relative sensitivity to UVB across marine taxa and processes.LocationThe analyses presented are based on 1784 experimental assessments of the impacts of UVB performed with natural radiation and organisms from different geographical areas, as well as with artificial radiation and cultured organisms at many laboratories around the world.MethodsFirst we compiled the published literature concerning experimental evaluation of the impacts of UVB on marine biota. Then a meta‐analysis was conducted with the data set obtained to evaluate the responses of marine organisms and processes to enhanced and reduced UVB levels.ResultsIncreased UVB radiation leads to a sharp increase in mortality rates across marine taxa, with protists, corals, crustaceans and fish eggs and larvae being most sensitive. A general relationship between relative changes in UVB doses and mortality rates was developed. This relationship can help assess the effects of changes in incident UVB radiation (past, present or future) on marine organisms.Main conclusionsThis meta‐analysis demonstrates that mortality rates of marine biota increase rapidly in response to elevated UVB radiation. The enhanced mortality rates associated with currently elevated UVB levels may represent a major threat to marine biota, possibly underlying recent widespread declines in the abundance of marine organisms ranging from corals to fish and krill.

Elevated Ultraviolet-B Radiation Reduces Concentrations of Isoflavones and Phenolic Compounds in Soybean Seeds

The gradual disruption of the ozone layer in the stratosphere has resulted in increased exposure of plants to ultraviolet-B (UV-B, 280–315 nm) radiation. UV-B radiation is known to affect crop growth and quality negatively. A study was conducted to determine the impact of elevated UV-B radiation levels on the isoflavones and phenolic compound concentrations of seven soybean varieties. UV-B radiation significantly reduced the concentration of most isoflavones and phenolic compounds in soybean seeds. Exposure to elevated UV-B levels overall resulted in 35 % reduction in total isoflavones and 31 % in phenolic compounds concentrations. The effect on individual isoflavones and phenolic compounds depended on the compound and variety, but UV-B overwhelmingly reduced concentrations. This study suggests that increased UV-B radiation negatively impacted soybean quality by reducing the concentration of compounds that have health-beneficial properties.

Altered kelp (Laminariales) phlorotannins and growth under elevated carbon dioxide and ultraviolet‐B treatments can influence associated intertidal food webs

AbstractDue to the importance of brown algae, such as kelp (Laminariales, Phaeophyta), within most cool nearshore environments, any direct responses of kelp to multiple global changes could alter the integrity of future coastal marine systems. Fifty‐five‐day manipulation of carbon dioxide (CO2) and ultraviolet light (UVB) within outdoor sea‐tanks, approximating past, present and two predicted future levels, examined the direct influences on Saccharina latissima (=Laminaria saccharina) and Nereocystis luetkeana development and biochemistry, as well as the indirect influences on a marine herbivore (Tegula funebralis; Gastropoda, Mollusca) and on naturally occurring intertidal detritivores. Kelp species displayed variable directional (negative and positive growth) and scale responses to CO2 and UVB manipulations, which was influenced by interactions. Kelp phlorotannin (phenolic) production in blade tissues was induced by elevated UVB levels, and especially enhanced (additively) by elevated CO2, further suggesting that some actively growing kelp species are carbon limited in typical nearshore environments. Negative indirect effects upon detritivore consumers fed CO2‐manipulated kelp blade tissues were detected, however, no statistical relationships existed among UVB‐treated tissues, and test herbivores did not distinguish between phlorotannin‐altered CO2: UVB‐treated kelp blade tissues. Results suggest that past and future conditions differentially benefit these kelp species, which implies a potential for shifts in species abundance and community composition. Higher CO2 conditions can indirectly impede marine decay processes delaying access to recycled trace nutrients, which may be disruptive to the seasonal regrowth of algae and/or higher trophic levels of nearshore ecosystems.

Limited effects of Antarctic ozone depletion on sea urchin development

The sea urchin, Sterechinus neumayeri ,h as a circumpolar distribution and is an abundant species in benthic communities of the Antarctic. Reproduction occurs during austral spring, when ozone concentrations over the past 25 years have been reduced by 50% or more, potentially exposing the planktonic embryos and larvae to elevated levels of UVB. During spring of 1996, cultures of S. neumayeri embryos incubated under ambient and partitioned sunlight (minus UVB) at static depths between 0 and 7 m were analyzed for DNA damage (cyclobutane pyrimidine dimers (CPDs)) and morphological abnormalities. At 0-m and 1-m depths, nearly 100% of embryos developed abnormally, even under UVB-shielded conditions where little or no DNA damage accumulated. At depths >3 m, reduced or no abnormality was evident and DNA damage was negli- gible. Although UVB contributed to 0-65% of solar- induced abnormalities, the mean contribution was 11±17% and UVB was not primarily responsible for observed defects in urchin development. Moreover, developmental responses were not linearly related to ambient UVB gradients as might be expected, but are better characterized relative to threshold levels of total UVB exposure. Accumulated exposures of £ 25 kJ m )2 ambient UVB caused minimal DNA damage and al- lowed normal embryological development to proceed. Higher UVB exposures (especially ‡80 kJ m )2 ) pre- cluded normal development. An ancillary threshold limit of 17 CPDs mb )1 has been identified as the level of DNA damage that proscribes abnormal development. While higher wavelengths of UVA and visible light are not affected by ozone concentration and do not initiate significant CPD DNA damage, they did interfere sig- nificantly with the embryological development of S. neumayeri. It is concluded that exposure to increased UVB during recent Antarctic ozone-depletion cycles probably has only a small degree of impact relative to the magnitude of other solar effects on the develop- mental success of Sterechinus embryos, or compared to spawning seasons before ozone depletion (i.e., years prior to 1978).

Contribution of phenolic compounds to the UV-B screening capacity of developing barley primary leaves in relation to DNA damage and repair under elevated UV-B levels

Epidermally located UV-absorbing hydroxycinnamic acid conjugates and flavonoid glycosides are known to be efficient UV-B protectants in higher plants, although important biological molecules are not always fully protected. However, repair mechanisms also exist, such as repair of damaged DNA by photolyases. To distinguish between the relative importance of the phenolic compounds and of DNA repair, developing primary leaves of two barley lines, mutant ant 30-310, deficient in flavonoids, and its parent line Ca 33787, were grown under relatively high visible light (650–700 μmol m−2 s−1 max for 6 h in a 13 h photoperiod) and supplemented with (+ UV-B) or without (−UV-B) 12 kJ m−2 UV-BBE for 6 h daily. UV-B screening capacity of the leaf phenolics was determined at 315 nm during leaf development and compared with thymine dimers (TD) accumulation, as an indicator of UV-B-induced DNA damage and potential subsequent repair. The degree of damage was related to the phenolic contents of the leaves. UV-B screening capacity was increased ca. 4-fold in the parent line (+ UV-B), mainly due to UV-induced flavonoid (saponarin, lutonarin) accumulation in epidermal and subepidermal mesophyll tissue, relative to the flavonoid-deficient mutant. Nevertheless, in the parent line an 8-fold increase in TD levels occurred over the growth period of 18 days, whereas the mutant accumulated additional DNA damage, with 6- to 9-fold higher TD amounts. Surprisingly, under the high UV-B irradiation, growth and development of the primary leaves in both lines were only slightly reduced.

Flavonoid concentrations in three grass species and a sedge grown in the field and under controlled environment conditions in response to enhanced UV-B radiation

An investigation was carried out to find whether enhanced levels of UV-B radiation induce increased concentrations of flavonoids in the leaves of the grass species Deschampsia antarctica, Deschampsia borealis and Calamagrostis epigeios and the sedge Carex arenaria. Whether the enhanced levels of UV-B influenced the proportions of the various flavonoids in the leaves was also studied. Increased flavonoid concentrations would improve the UV-B shielding of UV-B susceptible tissues. Using HPLC analysis the flavonoids orientin and luteolin were identified in D. antarctica, orientin in D. borealis and tricin in C. arenaria. Neither flavonoid concentrations nor the proportion of the various flavonoids in climate room grown D. antarctica and D. borealis plants differed between individuals grown under 0, ambient or elevated UV-B levels. After 12 weeks of growth biomass production and shoot-to-root ratios of D. antarctica were not affected by elevated UV-B radiation. Greenhouse grown C. epigeios plants contained higher concentrations and different proportions of flavonoids grown under elevated levels of UV-B than when grown under ambient or 0 UV-B. In C. epigeios plants grown in their natural habitat in the field under ambient or elevated levels of UV-B, flavonoid concentrations and proportions were the same in plants from both treatments. In the leaves of the sedge C. arenaria grown in a greenhouse flavonoid concentrations and proportions were not affected by UV-B radiation. Leaves were harvested four times during the growing season from C. arenaria plants grown in their natural habitat in the field under ambient or elevated levels of UV-B. Leaves harvested in January contained higher concentrations of flavonoids when grown under elevated UV-B than when grown under ambient UV-B radiation. In leaves harvested in May, September and December flavonoid concentrations were the same in plants grown under ambient or elevated UV-B. The proportion of the different flavonoids was the same for both treatments in all months. These results indicate that constitutive levels of flavonoids in these grass and sedge species are adequately high to protect them against ambient and elevated levels of solar UV-B radiation.

The influence of UV-B radiation on the reproductive cells of the intertidal macroalga, Enteromorpha intestinalis

The relative sensitivity of the reproductive cells of the seaweed Enteromorpha intestinalis to UV-B was assessed by measuring in vivo chlorophyll fluorescence ( F v variable fluorescence), germination success and growth rates. Zoospores (the asexual reproductive cells) exhibit up to a 6 fold higher sensitivity to UV-B exposure than the mature thalli (measured as chlorophyll fluorescence, F v), and differences in growth rates were also found. Consistent patterns emerged throughout these experiments in variable fluorescence, germination success and growth rates, indicating a greater sensitivity in the sexual reproductive phase of the life cycle compared with the asexual phase. Inhibition of germination success (up to 50%) and growth rates (up to 16.4%) of settled gametes and zoospores after 1-h exposures to elevated levels of UV-B (equivalent to 27 and 31% ozone depletion) showed that damage to the reproductive cells was irreversible. In conclusion, the ecological significance of elevated UV-B exposure in the marine environment may be seriously under-estimated if effects on the early lifestages of algae are not considered.

Histopathology of the skin of UV-B irradiated sole ( Solea solea) and turbot ( Scophthalmus maximus) larvae

Larval stages of two economically important flatfish, the sole ( Solea solea) and turbot ( Scophthalmus maximus) were exposed to ambient and elevated levels of UV-B. Sole larvae, which naturally occur in the plankton in early spring, demonstrated skin lesions at elevated levels of UV-B. Histopathology of the sole revealed cellular changes in the integument, characteristic of sunburn damage, with a reduction in the size of mucus-secreting cells and an increased epidermal thickening, especially at the highest doses of UV-B (2.15 KJ bio eff/m 2). Pigmentation in the sole is restricted to a few isolated melanocytes. The integrity of the heavily pigmented skin of turbot appeared to be unaffected by comparable doses of UV-B. Both species have protective mechanisms, which minimize the effects of naturally-occurring levels of UV-B. However, sole appear to be poorly adapted to accommodate any further increase in solar radiation.

Impact of increased UV-B on plant ecosystems

Elevated levels of UV-B are known to inhibit plant growth, development and physiological processes, although sensitivity to UV-B varies within plant species. Conifers and many evergreens are long-lived with long generation times, and damage by UV-B is likely to accumulate over the years. Three pilot experiments in closed chambers and greenhouses have revealed variable responses in subarctic forest plants and trees to enhanced UV-B radiation. These responses have been identified as changes in morphology and growth. Our current project seeks to identify and quantify the effects of UV-B radiation on the growth and development of Scots pine needles under field conditions.

Photoinduction of UV-absorbing compounds in Antarctic diatoms and Phaeocystis antarctica

The presence of UV-absorbing compounds, thelr In vivo absorption and photoinducibility were investigated in the colonial form of the prymnesiophyte Phaeocystis antarctica and 11 species of Antarctic diatoms. High in vivo phytoplankton absorption peaks centered at 315-320 nm wavelengths indicated the presence of UV-absorbing compounds in all cultures of P. antarctica examined. UVabsorbing compounds, detected as in vivo absorption peaks at slightly longer wavelengths (330-333 nm), were present in most diatom species, but were absent or present in minute amounts in smaller species. UV-absorbing compounds were identified in the diatoms as the mycosporine-like amino acids (MAAs) porphyra-334, mycosporine-glycine, and shinorine but could not be positively identified in P. antarctica. Accumulation of MAAs was induced by llght in the blue/UV portion of the spectrum (305-460 nm). For diatoms, induction was most effective at wavelengths between 370 and 460 nm with very little response observed at ultraviolet-B (UVB, 280-320 nm) wavelengths. In contrast, induction was maximal at 340 nrn and significant at wavelengths down to 305 nrn in colonies of P. antarctica. Induction requlred continuous illurn~nation and exhibited an Initial lag phase (10 to 24 h in length), an exponential phase (60 to 120 h in length), and a steady-state phase. The cellular content of MAAs was stable in viable cells kept for over 2 mo In prolonged darkness and became d~luted through successive cell divisions in cells grown in non-inducing ~llumination (red and yellow light). The potential sunscreening effect (S) of MAAs, calculated as the fractional reduction of UV exposure to a centrally-located cellular target, ranged from 0.03 to 0.50 for diatom cells and from 0.65 and 0.72 for P, antarctica colonies. S exhibited an increasing trend with cell and colony size. Whereas MAAs are thought to be intracellularly located, the large values of S determined for P. antarctica suggests that UV-absorbing compounds associated with the colonies are located within the extracellular colonial matrix. The results indicate that most phytoplankton species examined have the potential to respond to an increase in UVR resulting from seasonal changes in solar zenith angle by increasing their MAA content. A similar response to elevated UVB levels resulting from depleted ozone concentrations may be limited to P. antarctica.

Paclobutrazol enhances tolerance to increased levels of UV-B radiation in soybean (Glycine max) seedlings

Paclobutrazol increased the tolerance of soybean (Glycine max L. cv. AC Bravor) seedlings to elevated levels of ultraviolet-B radiation (UV-B: 280 – 320 nm). Within 3 days of exposure to elevated UV-B, nontreated seedlings were photosynthetically less efficient, and by the end of the 11th day of treatment their leaf areas and fresh masses were, respectively, 55 and 44% smaller than their counterparts not irradiated with UV-B. In contrast, paclobutrazol-treated seedlings maintained full photosynthetic efficiency throughout the duration of UV-B irradiation, and their leaf areas and fresh masses were only reduced by 30 and 22%, respectively. No changes in flavonoid composition occurred as a result of the paclobutrazol or UV-B radiation treatments, but UV-B tended to increase the quantities of photosynthetic pigments. The leaves from paclobutrazol-treated seedlings were significantly thicker than those of nontreated seedlings, and SEM analyses showed that the treated seedlings had much denser epicuticular wax layers than the controls. No significant changes in protein expression occurred during exposure to elevated UV-B and (or) paclobutrazol. Further studies with paclobutrazol-treated plants may lead to a better understanding of one or more of the mechanisms employed by plants to tolerate elevated levels of UV-B. Key words: leaf fluorescence, paclobutrazol, pigments, protein synthesis, SEM, UV-B radiation.

The effect of elevated UV-B (280–320 nm) radiation levels on Silene vulgaris: A comparison between a highland and a lowland population

Highland (altitude 1600 m above sea level) and lowland (altitude −2 m below sea level) populations of the perennial herb Silene vulgaris (Moench) Garcke, were tested on their response to elevated levels of UV-B radiation. Highland populations typically receive high natural UV-B fluxes, whereas lowland populations receive a lower natural UV-B dose. Adaptation to high UV-B levels of the highland population is to be expected. Experimental comparison of growth rates, gas exchange rates, transpiration and biochemical parameters using adult plants as well as seedlings did not show a difference in the response to elevated UV-B levels between the two populations. Individuals of both populations were relatively insensitive to elevated UV-B radiation. The response of alpine and lowland populations of Silene vulgaris is discussed in relation to the dispersal of this species after the last ice age.