Effects Of ultraviolet-B Research Articles

Regulation of Ultraviolet B-induced Phosphorylation of Histone H3 at Serine 10 by Fyn Kinase

Ultraviolet B (UVB) induces phosphorylation of histone H3 at serine 10, and mitogen-activated protein kinases are involved in this signal transduction pathway. Here we provide evidence that Fyn kinase, a member of the Src kinase family, is involved in the UVB-induced phosphorylation of histone H3 at serine 10. UVB distinctly increased Fyn kinase activity and phosphorylation. Fyn kinase inhibitors 4-amino-5-(4-chlorophenyl)-7(t-butyl)pyrazol(3,4-d)pyramide and leflunomide, an Src kinase inhibitor, suppressed both UVB-induced phosphorylation of histone H3 at serine 10 and Fyn kinase activity and phosphorylation. UVB-induced phosphorylation of histone H3 at serine 10 was blocked by either a dominant-negative mutant of Fyn (DNM-Fyn) kinase or small interfering RNA of Fyn kinase. UVB-induced phosphorylation and activities of ERKs and protein kinase B/Akt were markedly inhibited by DNM-Fyn kinase. However, DNM-Fyn kinase did not inhibit UVB-induced phosphorylation of p38 MAPK or c-Jun N-terminal kinases. Active Fyn kinase phosphorylated histone H3 at serine 10 in vitro, and the phosphorylated Fyn kinase could translocate into the nucleus of HaCaT cells. These results indicate that Fyn kinase plays a key role in the UVB-induced phosphorylation of histone H3 at serine 10.

Effects of calcium concentration on the SOD activity and UVB‐induced cytotoxicity in cultured human keratinocytes

Cellular differentiation due to the extracellular calcium (Ca(2+)) concentration affects the level of several antioxidant enzymes in cultured human keratinocytes. Because the epidermis includes well- and un-differentiated keratinocytes, we expected that keratinocytes possess different antioxidant capacity and sensitivity to damaging effects of ultraviolet-B (UVB) depending on the differentiation. We examined the effects of Ca(2+) concentration of culture medium (DMEM (Dulbecco's modified Eagle's medium)) on the superoxide dismutase (SOD) activity and UVB-induced cytotoxicity in cultured human keratinocytes in order to investigate the relationship between cell differentiation and antioxidant defense. Human keratinocytes (HaCaT cells) were incubated in high Ca(2+) (>1 mM) or low Ca(2+) (<0.1 mM) concentration DMEM for 24 h at 37 degrees C in 5% CO(2). Then, we measured total SOD activity and also individual Cu,Zn- and Mn-SOD activities in keratinocytes. Furthermore, after incubation in high or low Ca(2+) concentration DMEM, human keratinocytes were irradiated with 10, 20 or 30 mJ/cm(2) UVB. The quantity of lactate dehydrogenase (LDH) leaked in the supernatant from damaged keratinocytes, cell viability and TdT-mediated dUTP nick end labelings (TUNEL) positive keratinocytes were measured at 24 h after UVB irradiation. Total SOD activity and Cu,Zn-SOD activity in human keratinocytes cultured in low Ca(2+) were significantly lower than in keratinocytes cultured in high Ca(2+) concentration DMEM. In contrast, Mn-SOD activity was not affected. LDH leakage in the supernatant from keratinocytes cultured in low Ca(2+) concentration was significantly higher than that from keratinocytes cultured in high Ca(2+) concentration DMEM after UVB irradiation. The cell viability of keratinocytes cultured in low Ca(2+) concentration DMEM was significantly decreased compared to that of keratinocytes cultured in high Ca(2+) concentration DMEM after UVB irradiation. Furthermore, UVB-induced apoptosis was increased in keratinocytes cultured in low Ca(2+) concentration DMEM by the TUNEL method. These results suggest that cellular differentiation due to the change of Ca(2+) concentration of culture medium affects the Cu,Zn-SOD activity and UVB-induced cytotoxicity in cultured human keratinocytes.

Neutrophils infiltrating ultraviolet B-irradiated normal human skin display high IL-10 expression

Exposure to an erythemal dose of ultraviolet B (UVB) is known to induce interleukin (IL-10) expression in human skin. It is generally believed that this IL-10 is predominantly expressed by CD11b+ HLA-DR+ macrophages that infiltrate the UVB-exposed skin. This cytokine is presumed to contribute to the immunosuppressive effects of UVB by inhibiting cell-mediated immune responses. We recently demonstrated that neutrophils, which also invade UVB-irradiated skin, express CD11b and HLA-DR as well. In addition, we showed that the presence of these neutrophils affects T-cell responses in primary T-cell cultures derived from UVB-exposed skin. Since neutrophils invade UVB-exposed skin and, like macrophages, express CD11b and HLA-DR, we sought to determine whether neutrophils represent another source of IL-10. Skin biopsies were obtained from four healthy volunteers before and 2 days after exposure to four minimal erythema doses of UVB. A series of immunohistochemical double-staining procedures using the following markers was performed: IL-10, CD11b, HLA-DR, CD36, neutrophil elastase, and CD66b. As expected IL-10 could be detected in CD11b+ HLA-DR+ CD36+ macrophages in the epidermis and dermis of UVB-exposed skin. Surprisingly, the majority of the abundant IL-10 expression was found in CD11b+ HLA-DR+ elastase+ CD66b+ neutrophils. Cytospin preparations from dermal cell suspensions confirmed the IL-10 expression by neutrophils displaying characteristic multilobular nuclei. Thus, neutrophils in UVB-exposed skin express IL-10 and should be recognized as active coplayers in the creation of the UVB-induced immunosuppressive microenvironment.

Effects of nitric oxide on growth of maize seedling leaves in the presence or absence of ultraviolet-B radiation

The leaves of maize seedlings were used to measure leaf biomass including leaf length, width and weight, and to examine the relationship between nitric oxide (NO) synthase activity in microsomes and cytosol to the exo- and endo-beta-glucanase activity during growth. It was found that ultraviolet-B radiation (UV-B radiation) strongly induced nitric oxide synthase (NOS) activity but caused both a decrease of leaf biomass and exo- or endo-beta-glucanase activity. In contrast, the NOS inhibitor and NO donor largely decreased the activity of NOS in non-irradiated seedlings. The inhibitor also reduced exo- and endo-beta-glucanase activity and leaf biomass while the donor increased the enzyme activity and leaf biomass under normal conditions. Alternatively, under ultraviolet-B, the additional inhibitor of NOS and NO donor appeared to compromise the effects of ultraviolet-B on glucanase activity and leaf biomass, making the relationship between NOS activity and glucanase activity negatively correlated. This suggests that the changes of NOS activity showed a positive correlation to glucanase activity and leaf biomass in the absence of ultraviolet-B, but a negative correlation to ultraviolet-B irradiation and NO donor treatment alone. It is assumed that exo- and endogenous NO is responsible for the up-regulation of regular growth and development without ultraviolet-B. Under UV-B radiation, however, it might function as a signaling molecule of ultraviolet-B inhibiting leaf growth of maize seedlings to carry out stress-signaling transduction.

Solar UV-B radiation on growth, photosynthesis and the xanthophyll cycle in tropical acacias and eucalyptus

The effects of solar ultraviolet-B (UV-B) radiation on the growth, photosynthesis and the xanthophyll cycle of four acacia and two eucalyptus species in Darwin, Australia, were examined. Solar UV-B radiation seemed to delay plant growth in all species examined even though it did not affect photosynthetic activity significantly. Under solar UV-B radiation, a reduced specific leaf area (SLA), and an increased leaf thickness and size of epidermis were observed in plants. Chlorophyll content decreased but leaf soluble protein content increased in plants under solar UV-B radiation. Solar UV-B radiation apparently had a strong effect on chlorophyll degradation rather than the size of the xanthophyll cycle pool. It seemed that xanthophyll formation was more strongly affected by photosynthetic photon flux density (PPFD) than by solar UV-B radiation. The contents of sugar, sugar alcohol and total sugar did vary dramatically between treatments in all the species examined. Overall, the effect of solar UV-B on plant growth and form was much less detrimental than reported for indoor experiments where high UV-B dosage (relative to growth irradiance) was used. Solar UV-B radiation did not cause any photodamage in acacia and eucalyptus plants examined. It had a significant effect on growth and metabolism such as plant height, canopy, leaf thickness and the contents of chlorophyll and soluble protein.

P38 Mitogen-Activated Protein Kinase Mediates Dual Role of Ultraviolet B Radiation in Induction of Maturation and Apoptosis of Monocyte-Derived Dendritic Cells

Although ultraviolet B (UVB) induces apoptosis and functional perturbations in dendritic cells (DC), for example, Langerhans cells (LC), it also stimulates some LC into maturation after irradiation in vivo. To analyze its reciprocal effects on DC, we elucidated the direct effect of UVB on DC in vitro using human monocyte-derived DC (MoDC). UVB from 50 to 200 J per m2 stimulated the maturation of MoDC with (1) augmented expression of CD86 and HLA-DR, (2) enhanced production of IL-1beta, IL-6, IL-8, and TNF-alpha at both the mRNA and protein levels, and (3) enhanced allostimulatory capacity on a per-cell basis, whereas the exceeded doses induced apoptotic cell death. Western-blot analysis of MoDC after UVB demonstrated a concentration-dependent phosphorylation of p38- and c-JUN N-terminal kinase (JNK)-mitogen-activated protein kinases (MAPK), but not that of extracellular signal-regulated kinases. p38 MAPK-inhibitor, SB203580, inhibited both UVB-induced maturation and apoptosis of MoDC. Interestingly, MoDC that had undergone apoptosis exhibited an augmented expression of HLA-DR without upregulation of CD86 antigen, suggesting their tolerogenic phenotype. Thus, our study revealed a dual effect of UVB, to stimulate maturation or to induce apoptosis in MoDC, depending on the dosage, via p38 MAPK pathway.

Ultraviolet-B effects on stomatal density, water-use efficiency, and stable carbon isotope discrimination in four glasshouse-grown soybean ( Glyicine max) cultivars

Interactions between UV-B radiation and drought stress have been studied but the underlying mechanisms have not been thoroughly investigated. We hypothesized that UV-B radiation would improve water-use efficiency (WUE) by its effects on epidermal development, specifically stomatal density, and leaf gas exchange. Four lines of soybean ( Glycine max: Essex, Williams, OX921, and OX922) were grown for 28 days in a glasshouse with and without supplemental UV-B radiation levels of 13 kJ m −2 biologically effective radiation. UV-B radiation increased phenolic content of leaves in all lines and reduced leaf area in all lines except Williams. Adaxial stomatal density was reduced in Essex, OX921, and OX922 but abaxial stomatal density was reduced only in OX922. Stomatal conductance was reduced in concert with stomatal density as was internal CO 2 concentration ( C i). Instantaneous WUE was increased in Essex, Williams, and OX922. Stable carbon isotope analysis showed similar trends in long-term WUE, but these were only statistically significant in OX922. These results suggest that some soybean cultivars may respond to increased levels of UV-B by increasing WUE and that this response could be manifested through changes in stomatal development and functioning. Field studies are needed to test this hypothesis and to further evaluate the role of the K9 gene in this response.

Response of maize (Zea mays L.) nitrate reductase to UV-B radiation

The effect of ultraviolet-B (UV-B) radiation on plant growth, photosynthetic competence, nitrate content, and extractable activity of nitrate reductase (NR; EC 1.6.6.1) was evaluated in maize (Zea mays L.) seedlings during a 13 days growth period. The results showed that under our experimental conditions UV-B radiation decreased the O2 evolution rates and the sugar contents of maize leaves without affecting the chlorophyll content, the chloroplast ultrastructure or the plant weight. Furthermore, this treatment induced significant decreases of leaf and root NR activities and of nitrate content. In addition, the analysis of the transcript accumulation of genes encoding maize NR and of a gene encoding a maize 14-3-3 protein did not show significant differences between tissues of control and UV-B treated seedlings. Conversely, the activation state of NR was slightly lower in treated plants with respect to the control. Our results indicate that the effect of UV-B radiation on NR activity do not involve a transcriptional mechanism of regulation of the NR isoforms considered in this research, but may implicate its modulation at a post-transcriptional level or may require the involvement of the transcriptional regulation of some other NR isoform.

Response of maize ( Zea mays L.) nitrate reductase to UV-B radiation

The effect of ultraviolet-B (UV-B) radiation on plant growth, photosynthetic competence, nitrate content, and extractable activity of nitrate reductase (NR; EC 1.6.6.1) was evaluated in maize ( Zea mays L.) seedlings during a 13 days growth period. The results showed that under our experimental conditions UV-B radiation decreased the O 2 evolution rates and the sugar contents of maize leaves without affecting the chlorophyll content, the chloroplast ultrastructure or the plant weight. Furthermore, this treatment induced significant decreases of leaf and root NR activities and of nitrate content. In addition, the analysis of the transcript accumulation of genes encoding maize NR and of a gene encoding a maize 14-3-3 protein did not show significant differences between tissues of control and UV-B treated seedlings. Conversely, the activation state of NR was slightly lower in treated plants with respect to the control. Our results indicate that the effect of UV-B radiation on NR activity do not involve a transcriptional mechanism of regulation of the NR isoforms considered in this research, but may implicate its modulation at a post-transcriptional level or may require the involvement of the transcriptional regulation of some other NR isoform.

Two 14-epi analogues of 1,25-dihydroxyvitamin D3 protect human keratinocytes against the effects of UVB.

In search of photoprotective agents, we recently demonstrated a protective effect of 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] against different events mediated by ultraviolet B (UVB) in human keratinocytes. Pharmacological doses of 1,25(OH)(2)D(3) were required to obtain significant UVB protection; however, these doses cannot be used in vivo due to the calcemic properties of 1,25(OH)(2)D(3). Therefore, we evaluated the photoprotective capacities of two low-calcemic 14-epi analogues of 1,25(OH)(2)D(3), 19-nor-14-epi-23-yne-1,25(OH)(2)D(3) (TX 522) and 19-nor-14,20-bisepi-23-yne-1,25(OH)(2)D(3) (TX 527). Using cultured human keratinocytes, we investigated the influence of TX 522 and TX 527 on two hallmark events in UVB-irradiated keratinocytes: the induction of apoptosis and the production of interleukin-6 (IL-6). Treatment of the keratinocytes with TX 522 or TX 527, 24 h before irradiation, resulted in a significant and dose-dependent reduction of both UVB-induced apoptosis and IL-6 production. Both analogues were equally efficient in their anti-UVB effects and at least 100 times more potent than 1,25(OH)(2)D(3). We further demonstrated that metallothionein (MT) mRNA expression was clearly induced by 1,25(OH)(2)D(3) and both analogues. MT acts as a radical scavenger in oxygen-mediated UVB injury and its induction may therefore be relevant for the anti-UVB effects of 1,25(OH)(2)D(3) and both analogues. Taken together, these findings create new perspectives for the use of active vitamin D analogues as photoprotective agents.

T cells in psoriatic lesional skin that survive conventional therapy with NB-UVB radiation display reduced IFN-gamma expression.

The type 1 T cell-derived cytokine interferon gamma (IFN-gamma) is overexpressed in psoriatic lesional skin. Recently, we have shown that a single high erythemal dose of broad-band ultraviolet B (UVB) irradiation reduces type 1 and favors type 2, i.e. interleukin-4 (IL-4), cytokine expression in normal and psoriatic skin. In this study, we wanted to see whether conventional narrow-band UVB (NB-UVB) therapy (i.e. repeated exposure to nonerythemal doses) also affects type 1/type 2 cytokine expression of T cells present in chronic plaque type psoriatic lesions. Staining of cryostat sections showed decreased expression of both IFN-gamma and IL-4 in situ after NB-UVB therapy. CD4(+) dermal T cell lines, derived from psoriatic lesional skin, displayed significantly decreased intracellular IFN-gamma expression during and after NB-UVB therapy as compared to pretreatment values. Intracellular IL-4 expression was increased in most patients after therapy. Analysis of the supernatants of these stimulated dermal T cells revealed that IFN-gamma production decreased significantly following NB-UVB therapy, whereas IL-4 expression increased in the T cell supernatants from most patients, confirming the intracellular determinations. In addition, IL-10 and transforming growth factor-beta levels in the supernatants appeared to be increased in the majority of patients following UVB therapy. Apart from the well-known killing effect of UVB on T cells, our results show that the improvement in psoriatic skin following NB-UVB therapy is also due to a reduced capacity of the surviving dermal T cells to express the proinflammatory cytokine IFN-gamma.

Solar UV-B radiation affects leaf quality and insect herbivory in the southern beech tree Nothofagus antarctica.

We examined the effects of solar ultraviolet-B (UV-B) radiation on plant-insect interactions in Tierra del Fuego (55 degrees S), Argentina, an area strongly affected by ozone depletion because of its proximity to Antarctica. Solar UV-B under Nothofagus antarctica branches was manipulated using a polyester plastic film to attenuate UV-B (uvb-) and an Aclar film to provide near-ambient UV-B (uvb+). The plastic films were placed on both north-facing (i.e., high solar radiation in the Southern Hemisphere) and south-facing branches. Insects consumed 40% less leaf area from north- than from south-facing branches, and at least 30% less area from uvb+ branches than from uvb- branches. The reduced herbivory on leaves from uvb+ branches occurred for both branch orientations. Leaf mass per area increased and relative water content decreased on north- versus south-facing branches, while no differences were apparent between the UV-B treatments. Solar UV-B did lead to lower gallic acid concentration and higher flavonoid aglycone concentration in uvb+ leaves relative to uvb- leaves. Both the flavonoid aglycone and quercetin-3-arabinopyranoside were higher on north-facing branches. In laboratory preference experiments, larvae of the dominant insect in the natural community, Geometridae "Brown" (Lepidoptera), consumed less area from field-grown uvb+ leaves than from uvb- leaves in 1996-97, but not in 1997-98. Correlation analyses suggested that the reduction in insect herbivory in the field under solar UV-B may be mediated in part by the UV-B effects on gallic acid and flavonoid aglycone.

Effects of supplementary ultraviolet-B irradiance on maize yield and qualities: a field experiment.

Stratospheric ozone depletion has caused an increase in the amount of ultraviolet-B (UV-B) radiation reaching the earth's surface. Numerous investigations have demonstrated that the effect of UV-B enhancements on plants includes reduction in grain yield, alteration in species competition, susceptibility to disease and changes in plant structure and pigmentation. Many experiments examining UV-B radiation effects on plants have been conducted in growth chambers or greenhouses. It has been questioned whether the effect of UV-B radiation on plants can be extrapolated to field responses from indoor studies because of the unnaturally high ratios of UV-B/ultraviolet-A radiation (320-400 nm) and UV-B/photosynthetically active radiation (PAR) in many indoor studies. Field studies on UV-B radiation effect on plants have been recommended to use the UV and PAR irradiance provided by natural light. This study reports the growth and yield responses of a maize crop exposed to enhanced UV-B radiation and the UV-B effects on maize seed qualities under field conditions. Enhanced UV-B radiation caused a significant reduction in the dry matter accumulation and the maize yield in turn was affected. With increased UV-B radiation the flavonoid accumulation in maize leaves increased and the contents of chlorophyll a, b and (a + b) of maize leaves were reduced. The levels of protein, sugar and starch of maize seed decreased with enhanced UV-B radiation, whereas the level of lysine increased with enhanced UV-B radiation.

Ultraviolet B-Induced Skin Angiogenesis Is Associated with a Switch in the Balance of Vascular Endothelial Growth Factor and Thrombospondin-1 Expression

We have previously shown that targeted overexpression of the endogenous angiogenesis inhibitor thrombospondin-1 (TSP-1) in the epidermis prevented chronic ultraviolet B (UVB)-induced angiogenesis and cutaneous photodamage in mice, suggesting that angiogenesis plays a critical role in the mediation of UVB effects on the skin. Nevertheless, the molecular regulation of angiogenesis factors and inhibitors by acute UVB irradiation still remains to be elucidated. We performed quantitative analyses of cutaneous vascularity and of vascular endothelial growth factor (VEGF) and TSP-1 expression after acute UVB irradiation of mouse skin. Skin vascularity in the upper dermis was greatly increased until day 8 after a single dose of UVB irradiation (200 mJ per cm2) and associated with upregulation of VEGF mRNA expression, with downregulation of TSP-1 mRNA, and with protein expression in the hyperplastic epidermis. After 13 days, skin vascularity was normalized with downregulation of VEGF mRNA expression and upregulation of TSP-1 mRNA expression to the levels observed in non-UVB-irradiated skin. In contrast, the angiogenic UVB response was prolonged in TSP-1-deficient mice. We found a pronounced induction of the TSP-1 receptor CD36 in CD31-positive vessels on day 8 after UVB irradiation, associated with vascular endothelial cell apoptosis. These results demonstrate that acute UVB irradiation leads to a shift toward a proangiogenic environment and they suggest that the balance between VEGF and TSP-1 plays a critical role in the control of angiogenesis and vascular regression induced by acute UVB irradiation.

Thylakoid-associated Polyamines Adjust the UV-B Sensitivity of the Photosynthetic Apparatus by Means of Light-harvesting Complex II Changes¶

The sensitivity of the photosynthetic apparatus to ultraviolet-B (UV-B) irradiation was studied in cultures of unicellular green alga Scenedesmus obliquus incubated in low light (low photosynthetically active radiation intensity [LL]) and high light (high photosynthetically active radiation intensity [HL]) conditions, treated or not with exogenous polyamines. Biochemical and physicochemical measurements showed that UV-B radiation induces a decrease in the thylakoid-associated putrescine (Put) and an increase in spermine (Spm), so that the reduction of Put/Spm ratio leads to the increase of light-harvesting complex II (LHCII) size per active reaction center and, consequently, the amplification of UV-B effects on the photosynthetic apparatus. The separation of oligomeric and monomeric forms of LHCII from isolated thylakoids showed that UV-B induces an increase in the oligomeric forms of LHCII, which was more intense in LL than in HL. By manipulating the LHCII size with exogenous polyamines, the sensitivity degree of the photosynthetic apparatus to UV-B changed significantly. Specifically, the addition of Put decreased highly the sensitivity of LL culture to UV-B because of the inhibitory effect of Put on the LHCII size increasing, whereas the addition of Spm enhanced the UV-B injury induced in HL culture because of the increasing of LHCII size. The ability of the photosynthetic apparatus to recover the UV-B induced changes was also investigated.

Photoinduced lethal and sublethal toxicity of retene, a polycyclic aromatic hydrocarbon derived from resin acid, to coregonid larvae.

A comparative investigation on the acute phototoxicity of retene to vendace (Coregonus albula) and whitefish (C. lavaretus), both having pelagial larvae in spring, was conducted. To test the concept of early warning of sublethal biomarkers in relation to lethality to posthatch stages, we examined the effects of ultraviolet-B (UV-B) and retene on the levels of cytochrome P4501A (CYP1A) and heat shock protein 70 (HSP70) by exposing the animals to elevated levels of these factors for 48 and 72 h, respectively. Whereas UV-B and retene on their own were not lethal, simultaneous retene and UV-B exposure caused very high mortality to both species. The median lethal concentration (LC50; i.e., the concentration at which half of the larvae died) of retene as a precursor was 41 g/L for vendace and 15 to 16 microg/L, depending on the UV-B dose, for whitefish. Retene evoked substantial induction of CYP1A in larvae of both species, and UV-B induced CYP1A in whitefish. In vendace, no effect on HSP70 levels by any factor was observed. In whitefish, however, UV-B radiation and water retene alone upregulated HSP70, but no additive response was detected. The CYPIA is a biomarker of exposure to retene in both species. The HSP70 is an early warning signal of UV-B exposure in whitefish. As a species, vendace appears to be more resistant than whitefish to the phototoxicity of retene, as indicated by the higher tolerance.

Field crop responses to ultraviolet-B radiation: a review

This paper provides an overview of existing literature on the ultraviolet-B (UV-B) radiation effects on field crops. Earlier reviews on field crop responses to UV-B considered few physiological processes or crops. For this review, we easily located about 129 studies on 35 crop species published since 1975. Here, we report the effects of UV-B radiation on visual symptoms, leaf ultrastructure and anatomy, photosynthetic pigments, UV-B absorbing compounds, photosynthesis, growth and development, yield, genotypic differences, and finally, interactions of UV-B with abiotic and biotic factors of crop plants. Experiments conducted in glasshouses, in closed and open top chambers, and under field conditions, with varying source (solar or artificial) and intensity of photosynthetically active radiation (PAR, 50–1800 μmol m −2 s −1) and UV-B (0–50 kJ m −2 per day) are included. It is easy to conduct experiments that purport to evaluate the effects of projected UV-B intensities on crop species by providing supplemental irradiance with lamps or by reducing UV-B with filters; however, it is very difficult to simulate UV-B irradiance spectral changes that are likely to occur in nature. Collated results for each process are presented as percent change from control along with the experimental conditions in tabular format. Many of the studies showed dramatic effects of UV-B radiation, but under conditions with supplemental UV-B irradiance that was higher than would ever occur outside experimental conditions or in which the longer wavelengths in the PAR and UV-A, which moderate UV-B effects, were greatly reduced. Only 25 of the studies reviewed used experimental conditions and supplemental UV-B irradiance that approached realism. However, unrealistic the experimental conditions might be, an increase in understanding of basic plant physiology was gained from most of the studies. Visual symptoms consisting of chlorotic or necrotic patches on leaves exposed to UV-B were not unique. Both vegetative and reproductive morphology were altered by UV-B radiation. Leaf anatomy was altered due to changes in thickness of epidermal, palisade, and mesophyll layers. Enhanced UV-B generally decreased chlorophyll content (10–70%), whereas it increased UV-B absorbing compounds (10–300%) in many crops. Decrease in photosynthesis (3–90%), particularly at higher UV-B doses, was due to both direct (effect on photosystem) and indirect (decrease in pigments and leaf area) effects. The decreases in chlorophyll pigments and photosynthesis resulted in lower biomass and yield of most crop plants. Genotypes of crop species exhibited variability in leaf wax layer thickness, loss of chlorophyll, and increase in phenolics as mechanisms of tolerance to enhanced UV-B radiation resulting in changes in biomass/yield. Results from the few studies on interaction of UV-B with other abiotic and biotic factors did not lead to useful conclusions. Studies are needed to quantify the effects of UV-B radiation on crops in order to develop dose response functions that can facilitate development of dynamic simulation models for use in UV-B and other environmental impact assessments.

DOES JELLY ENVELOPE PROTECT THE COMMON FROG RANA TEMPORARIA EMBRYOS FROM UV-B RADIATION?

Animals have evolved a number of ways to protect themselves from the harmful effects of ultraviolet-B (UV-B) radiation, but little is known about the relative importance of different mechanisms protecting amphibian embryos from UV-B radiation. Using enzymatic removal of gelatinous coats (jelly envelope) surrounding the eggs of Rana temporaria, we tested the hypothesis that the jelly envelope acts as a sunscreen that protects embryos from harmful effects of UV-B radiation. We conducted two independent factorial laboratory experiments employing three different UV-B (no UV-B, normal, and enhanced) levels and jelly removal (control, modified, and completely removed) treatments. We found no UV-B × jelly removal treatment interactions in survival rates or in frequency of abnormal individuals, suggesting that jelly removal did not increase susceptibility of embryos to UV-B radiation. These results support the contention that the jelly envelope is not the most important means of protecting R. temporaria embryos f...

COMBINED EFFECTS OF UV-B RADIATION AND NITRATE FERTILIZER ON LARVAL AMPHIBIANS

As part of a global loss of biodiversity, amphibian populations are declining worldwide. Numerous factors may be involved in these declines, including environmental changes and the spread of contaminants. Ultraviolet-B (UV-B) radiation (280–315 nm) and chemical pollution are two factors that have become increasingly important as contributing to amphibian mortality and, perhaps, to amphibian population declines. Therefore, we studied the combined effects of ambient UV-B radiation and nitrate fertilizer pollution on larval amphibians in outdoor experiments at low and high elevation sites in Oregon. Recent studies have shown that UV-B radiation and nitrate fertilizer pollution have differential effects on amphibians. Some species are more susceptible than others when exposed to either UV-B or to nitrate. Moreover, depending upon other environmental conditions, UV-B levels are often greater in intensity at higher elevation sites compared to lower elevation sites. Therefore, our experiments were designed to include amphibians from both low and high elevations. Very little is known about interpopulational variation regarding the effects of environmental stressors. We tested the combined effects of UV-B and nitrate on larval survival, mass, and length in Pacific treefrogs (Hyla regilla) and long-toed salamanders (Ambystoma macrodactylum). In the low elevation experiment, we found that UV-B and nitrate together reduced the mass of larval H. regilla. In the high elevation experiment, we found that UV-B and nitrate together reduced the survival of larval H. regilla. In both the low elevation and the high elevation experiment, nitrate increased the mass of larval A. macrodactylum. However, in the high elevation experiment, this result occurred only when UV-B was blocked. This result indicates that the effects of nitrate addition could depend upon the presence of other factors such as UV-B. Results emphasize the importance of considering the effects of multiple stressors.

Interactions between willows and insect herbivores under enhanced ultraviolet-B radiation.

We studied the effects of elevated ultraviolet-B radiation on interactions between insect herbivores and their host plants by exposing two species of phytochemically different willows, Salix myrsinifolia and S. phylicifolia, to a modulated increase in ultraviolet radiation in an outdoor experiment and monitoring the colonisation of insect herbivores on these willows. We examined the effect of increased ultraviolet-B (UV-B) radiation on (1) the quality of willow leaves, (2) the distribution and abundance of insect herbivores feeding on these willows, (3) the resulting amount of damage, and (4) the performance of insect larvae feeding on the exposed plant tissue. Six clones of each of the two willow species were grown in eight blocks for 12 weeks in the UV-B irradiation field. The clones were exposed to a constant 50% increase in UV-B radiation (simulating 20-25% ozone depletion), to a small increase in UV-A radiation or to ambient solar irradiation. We allowed colonisation on the willows by naturally occurring insects, but also introduced adults of a leaf beetle, Phratora vitellinae, a specialist herbivore on S. myrsinifolia. Increased UV-B radiation did not affect any of the measured indices of plant quality. However, numbers of P. vitellinae on S. myrsinifolia were higher in plants with UV-B treatment compared with UV-A and shade controls. In laboratory tests, growth of the second-instar larva of P. vitellinae was not affected by UV-B treatment of S. myrsinifolia, but was retarded on UV-B treated leaves of S. phylicifolia. In addition, naturally occurring insect herbivores were more abundant on willows exposed to elevated UV-B radiation compared to those grown under control treatments. In spite of the increased abundance of insect herbivores, willows treated with elevated UV-B did not suffer more herbivore damage than willows exposed to ambient solar radiation (shade control). The observed effects of UV-B on herbivore abundance, feeding and growth varied significantly due to spatial variation in environment quality, as indicated by the UV-treatment x block interaction. The results suggest that (1) environmental variation modifies the effects of UV-B radiation on plant-insect interactions and (2) specialist herbivores might be more sensitive to chemical changes in their secondary host plants (S. phylicifolia) than to changes in their primary hosts (S. myrsinifolia).