UV-B Inhibition Research Articles

Analysis of limitations to CO2 assimilation on exposure of leaves of two Brassica napus cultivars to UV‐B

ABSTRACTApex and Bristol cultivars of oilseed rape (Brassica napus) were irradiated with 0.63 W m−2 of UV‐B over 5 d. Analyses of the response of net leaf carbon assimilation to intercellular CO2 concentration were used to examine the potential limitations imposed by stomata, carboxylation velocity and capacity for regeneration of ribulose 1,5‐bis‐phosphate on leaf photosynthesis. Simultaneous measurements of chlorophyll fluorescence were used to estimate the maximum quantum efficiency of photosystem II (PSII) photochemistry, the quantum efficiency of linear electron transport at steady‐state photosynthesis, and the light and CO2‐saturated rate of linear electron transport. Ribulose 1,5‐bisphosphate carboxylase/oxygenase (Rubisco) content and activities were assayed in vitro. In both cultivars the UV‐B treatment resulted in decreases in the light‐saturated rate of CO2 assimilation, which were accompanied by decreases in carboxylation velocity and Rubisco content and activity. No major effects of UV‐B were observed on end‐product inhibition and stomatal limitation of photosynthesis or the rate of photorespiration relative to CO2 assimilation. In the Bristol cultivar, photoinhibition of PSII and loss of linear electron transport activity were observed when CO2 assimilation was severely inhibited. However, the Apex cultivar exhibited no major inhibition of PSII photochemistry or linear electron transport as the rate of CO2 assimilation decreased. It is concluded that loss of Rubisco is a primary factor in UV‐B inhibition of CO2 assimilation.

UVB induced photooxidation of vitamin E.

The photochemistry of alpha-tocopherol (alpha-TH, vitamin E) may contribute to its inhibition of UVB (290-320 nm) photocarcinogenesis. Photochemical reactions of alpha-TH were studied by monitoring the fate of alpha-TH in UVB irradiated liposomes and solution. Soy phosphatidylcholine (SPC) and dioleoylphosphatidylcholine (DOPC) liposomes were supplemented with alpha-TH (1.0 mol % alpha-TH/phospholipid) and irradiated with UVB at a dose rate of 6.0 J m-2s-1 for up to 90 min. alpha-TH was rapidly depleted in UVB irradiated liposomes. Oxidative damage, assessed by monitoring lipid peroxidation, was suppressed in SPC liposomes until alpha-TH was depleted to 20% of initial levels. alpha-TH also was rapidly depleted by UVB irradiation in acetonitrile/H2O (4:1 v/v) solution. In SPC liposomes, products previously identified as marker products for peroxyl radical scavenging by alpha-TH were observed, including alpha-tocopherol quinone, 5,6-epoxy-alpha-tocopherol quinone, and 2,3-epoxy-alpha-tocopherol quinone. These products also were formed in DOPC liposomes, which are resistant to lipid peroxyl radical formation. In addition, an alpha-tocopherol dihydroxy dimer and several 8a-(hydroperoxy)epoxytocopherones were identified by HPLC and HPLC-MS. The dimer appears to result from recombination of photoinduced tocopheroxyl radicals. Products associated with peroxyl radical scavenging (quinones, epoxyquinones, 8a-(hydroperoxy)epoxytocopherones) and with UVB dependent production of tocopheroxyl radicals (dihydroxy dimer) also were found when alpha-TH was oxidized by UVB in acetonitrile. Because the acetonitrile contained no autoxidizable substrate, formation of peroxyl radical derived products may occur via intermediate tocopherone peroxyl radicals. These results indicate that alpha-TH photooxidation proceeds via competing reactions of UVB induced tocopheroxyl radicals.

Spectral Modeling of UV Inhibition of In Situ Antarctic Primary Production Using A Field‐Derived Biological Weighting Function

Abstract— Our major aim is to illustrate an approach for hindcasting or forecasting UV radiation (UVR, 280–400 nm) effects on in situ rates of aquatic primary production when field measurements do not include estimates of UVR effects. A composite of spectral field measurements is employed to model UVR‐dependent rates of photosynthesis in diatomdominated waters in a coastal region of the Southern Ocean. Assumptions, caveats and limitations of the modeling are discussed. Calculations begin with the 1991 Palmer Long Term Ecological Research (LTER) primary production and optical databases, from which daily integrated rates of carbon fixation in the absence of UVR are calculated as a function of depth for a 140 km transect line sampled between dawn and dusk of a single day (14 November 1991). The UVR measurements from the nearby NSF/OPP Polar Network at Palmer Station are used to determine ozone (O3) concentration on the day of the transect, which is then employed in Madronich's (In UV‐B Radiation and Ozone Depletion (Edited by M. Tevini), pp. 17–68. Lewis, Boca Raton, FL, 1993) spectral code to model daytime variations in surface spectral irradiances under clear sky conditions. These data are corrected for cloudiness and then combined with estimates of in‐water UVR spectral attenuation coefficients, derived from Icecolors 90 data, to estimate in situ light exposure for phytoplankton collected at different depths and locations. An absolute chlorophyllspecific biological weighting function (BWF), determined under natural solar light fields for Antarctic diatom communities and shown to be reproducible while differing from a laboratory diatom BWF and other in situ BWF determined for other phytoplankton assemblages, is combined with estimates of in situ UVR exposure to derive in situ estimates of chlorophyllspecific losses of carbon fixation due to UVR inhibittion. By repeating calculations for every sampling site along the transect, we derive a spatial map of estimated UVR effects on primary production across the region. We repeat calculations for different O, concentrations expected during the austral spring over Antarctica and illustrate the O, dependency of UVB (280–320 nm) inhibition effects in near surface waters. We estimate ambient UVR reduced carbon fixation rates up to 65% in surface waters, depending upon location, down to undetectable levels at 36 m. Reducing stratospheric O3 concentrations by 50% further inhibits near surface primary production by 8% and integrated primary production by 5%. Primary production was forced to subsurface maxima across the entire transect line in the presence of UVR.

GROWTH OF ANTARCTIC CYANOBACTERIA UNDER ULTRAVIOLET RADIATION: UVA COUNTERACTS UVB INHIBITION1

ABSTRACT A mat‐forming cyanobacterium (Phormidium mur‐rayi West and West) isolated from an ice‐shelf pond in Antarctica was grown under white light combined with a range of UVA and UVB irradiances. The 4‐day growth rate decreased under increasing ultraviolet (UV) radiation, with a ninefold greater response to UVB relative to UVA. In vivo absorbance spectra showed that UVA and to a greater extent UVB caused a decrease in phycocyanin/ chlorophyll a and an increase in carotenoids/chlorophyll a. The phycocyanin/chlorophyll a ratio was closely and positively correlated to the UVB‐inhibited growth rate. Under fixed spectral gradients of UV radiation, the growth inhibition effect was dominated by UVB. However, at specific UVB irradiances the inhibition of growth depended on the ratio of UVB to UVA, and growth rates increased linearly with increasing UVA. These results are consistent with the view that UVB inhibition represents the balance between damage and repair processes that are each controlled by separate wavebands. They also underscore the need to consider UV spectral balance in laboratory and field assays of UVB toxicity.

Ozone depletion: ultraviolet radiation and phytoplankton biology in antarctic waters.

The springtime stratospheric ozone (O3) layer over the Antarctic is thinning by as much as 50 percent, resulting in increased midultraviolet (UVB) radiation reaching the surface of the Southern Ocean. There is concern that phytoplankton communities confined to near-surface waters of the marginal ice zone will be harmed by increased UVB irradiance penetrating the ocean surface, thereby altering the dynamics of Antarctic marine ecosystems. Results from a 6-week cruise (Icecolors) in the marginal ice zone of the Bellingshausen Sea in austral spring of 1990 indicated that as the O3 layer thinned: (i) sea surface- and depth-dependent ratios of UVB irradiance (280 to 320 nanometers) to total irradiance (280 to 700 nanometers) increased and (ii) UVB inhibition of photosynthesis increased. These and other Icecolors findings suggest that O3-dependent shifts of in-water spectral irradiances alter the balance of spectrally dependent phytoplankton processes, including photoinhibition, photoreactivation, photoprotection, and photosynthesis. A minimum 6 to 12 percent reduction in primary production associated with O3 depletion was estimated for the duration of the cruise.

Effect of indomethacin on UVB— and UVA—induced erythema in polymorphic light eruption

The effect of topical indomethacin on the intensity of UVB— and UVA—induced erythema was measured by reflectance spectrophotometry in 23 patients with polymorphic light eruption. Thirteen patients showed a response to indomethacin that was identical to that seen in normal subjects (i.e., inhibition of UVB—induced erythema; no effect on the intensity of UVA—induced erythema). In the remaining 10 patients indomethacin caused abnormal augmentation of UVB— and UVA—induced erythema. These results show for the first time that the term polymorphic light eruption embraces at least two disease states that have different mechanisms of photosensitivity and that can be distinguished on the basis of clinical features and results of photobiologic investigation.

Ultraviolet-B inhibition of motility in green and dark bleachedEuglena gracilis

The motility of green and dark bleachedEuglena gracilis was studied under artificial and solar UV-B radiation. The percentage of motile cells in the population was drastically impaired after exposure to unfiltered sunlight for only a few hours. Dark bleached cells were even more affected than green organisms. The effect is caused mainly by the solar UV-B component, since filtering the sunlight by either a layer of ozone or a UV-B-absorbing filter substantially increased the survival rate. Addition ofp-quinone, a scavenger of free radicals produced in a type I photodynamic reaction, did not relieve the UV-B effects, but was cytotoxic at higher concentrations. Likewise, 1,4-diazobicyclo[2,2,2]octane and imidazole, which quench singlet oxygen (1O2) generated in a type II photodynamic reaction, did not prolong the survival in UV-B irradiation. D2O, which, in contrast, prolongs the lifetime of1O2, is tolerated by the cells but does not aggravate the UV-B inhibition. Thus, photodynamic processes of both type I and II can be ruled out as possible mechanisms of UV-B inhibition of motility inEuglena gracilis.

Inhibition of motility and phototaxis in the green flagellate, Euglena gracilis, by UV-B radiation

The effects of solar UV-B radiation on the green flagellate, Euglena gracilis, are measured under controlled conditions. Both photoorientation and motility are drastically impaired even after short exposure times of a few hours to sunlight not filtered by an ozone cuvette. Phototactic orientation starts to deteriorate after about 90 min and is completely lost after about 5 h. The percentage of motile cells in a population decreases likewise after an exposure of about 2 h and the velocity distributions shows a reduced speed of movement after an initial photokinetic increase. The damage is irreversible: in populations exposed for >2 h no living cell was found 24 h later. The UV-B sensitivity seems to be independent of the culture age at least over three weeks: While the percentage of motile cells changes with a peak at about 8 d, the relative UV-B induced inhibition is constant and depends only on the UV dose. DNA seems not to be the primary UV-B target since UV-B inhibition could not be repaired during subsequent dark or moderate light conditions even after low doses.

Inhibition of DNA repair synthesis by sunlight.

Abstract— DNA repair synthesis as determined by thymidine incorporation in the presence of hydroxyurea reached a much lower maximum level after solar compared with UVC exposure in five human melanoma cell lines, in HeLa cells, and in two human fibroblast strains. This finding was confirmed by determination of unscheduled DNA synthesis where both the number of labelled nuclei and grain count per nucleus were lower in sun‐exposed cells. In a cloned human melanoma line (MM253cl), glass‐filtered sunlight inhibited UVC repair synthesis, and solar UVB alone induced a higher level of repair synthesis than either complete sun or solar UVA plus solar UVB. The fluence response of filtered sunlight for inhibition of UVB (sunlamp) and UVC showed that most inhibition was obtained at low fluences (5‐10 min), further exposure giving a plateau at 40% of the original level. Ultraviolet C and sunlight inactivated adenovirus 5 giving F0 values for virus survival 40‐fold higher than for cell survival. Replication of either UVC‐ or solar‐irradiated virus was not affected by prior irradiation of cells with glass‐filtered sunlight. Stathmokinetic analysis of cell cycle progression by DNA flow cytometry showed that UVC and sunlamp UVB retarded cell movement from the G1 and S phases whereas equitoxic sunlight and glass‐filtered sunlight (nontoxic) had no effect. These results indicate that solar UVA at low environmental fluences partially inhibits UVB repair synthesis in a range of human cell types but does not affect the replication of a UVB‐ or UVC‐damaged virus when applied to the genome alone or to the host cell.

Evaluation of Seven Sunscreens on Hairless Mouse Skin

The ability of seven sunscreens to protect against ultraviolet (UV)--induced inhibition of epidermal DNA synthesis was evaluated in vivo using a hairless mouse model. There were statistically significant differences among sunscreens in their ability to prevent UV-B (290 to 320 nm) inhibition of DNA synthesis. The protective factor (PF) of a sunscreen was arbitrarily defined as the ratio of the dose required to inhibit DNA synthesis by 50% with and without a sunscreen. The following PF values were determined: Coppertone 4, 4.4; Sundown Extra Protection, 8.4; Supershade 15, 21.0; Eclipse 15, 22.2; Blockout 15, 22.4; and Bain de Soleil 15, 27.6. Zinc oxide ointment protected against any significant suppression of DNA synthesis at all UV-B doses used. There was a relatively good correlation between the PF and the sun protection factor (SPF) claimed for each sunscreen by the manufacturer. However, the PF values determined in mouse skin were generally higher than the SPF values measured in human skin. Further studies are needed to determine if sunscreen substantivity (resistance to removal by water) can be evaluated by this technique.