Evidence for the ultraviolet-B (280–320 nm) radiation induced structural reorganization and damage of photosystem II polypeptides in isolated chloroplasts

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 min 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 loss 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.     

Impacts of ambient solar uv (280-400 nm) radiation on three tropical legumes

Tropical regions receive the highest level of global solar ultraviolet (UV) radiation especially UV-B (280-320 nm). The average daily dose of the UV-B radiation in Madurai, South India (10°N) is 10 kJ m^-2. This is approximately 50% more than the average daily UV-B radiation in many European countries. A field study was conducted using selective filters to remove either the UV-B (< 320 nm) or UV-B/A (<400 nm) of the solar spectrum, and the effects were followed inCyamopsis tetragonoloba, Vigna mungo, andVigna radiata to determine their sensitivity to UV. When compared to ambient radiation, exclusion of solar UV-B increased the seedling height, leaf area, fresh weight and dry weight and the crop yield by 50% in the case ofCyamopsis, and the extent of such increase was slightly less under UV-B/A exclusion. InV. mungo a significant reduction was seen in solar UV excluded plants whileV. radiata was found to be unaffected.     

Ecological impact of solar ultraviolet-B (UV-B: 320-290 nm) radiation on Corynebacterium aquaticum and Xanthomonas sp. colonization on tea phyllosphere in relation to blister blight disease incidence in the field

AIMS: To assess the effects of solar UV-B radiation on phyllosphere bacteria of tea leaves in relation to blister blight disease in the field. METHODS AND RESULTS: The effects of UV-B radiation on the phyllosphere microbiology of tea (Camellia sinensis) were studied in contrasting wet and dry seasons at a tropical site. Wavelength-selective filters were used to separate the effects of UV-B from those of other factors. Bacterial populations were quantified in relation to the incidence of blister blight disease. Attenuation of UV-B increased the survival of Xanthomonas sp. when populations were not water limited, and increased the incidence of blister blight, but had no effect on Corynebacterium aquaticum. CONCLUSIONS: The effects of solar UV-B on phyllosphere bacteria were substantial but depended on both species and interactions with other environmental variables. Xanthomonas sp. was more sensitive to UV-B than C. aquaticum, but this did not result in differences in population density under high radiation conditions (dry season), but only in the wet season when other factors were not limiting. SIGNIFICANCE AND IMPACT OF THE STUDY: The role of UV-B on leaf surface microbiology in the tropics is marked but depends on other conditions, and the contrasting UV-B responses of different organisms can be masked by other limiting factors.     

On the interaction of UV-B radiation (280–315 nm) with water stress in crop plants

Cowpea ( Vigna unguiculata L. Walp.) seedlings (3-day-old) were subjected to 4 kinds of experimental treatments: (1) control without exposure to any stress (−D-UV), (2) moderate water stress with no UV-B irradiation (+D-UV), (3) no water stress but to UV-B radiation (−D+UV). and (4)moderate water stress and exposure to UV-B (+D+UV). UV-B and drought stress in the combined form elicited beneficial effects on the morphological and growth characteristics, and a few additive inhibitory effects in some functional processes. An increase in the specific leaf weight (SLW) was observed in the combination of stresses, which could be a defence mechanism against UV-B. The combination of stresses promoted the synthesis of anthocyanins and phenolic compounds. The responses of plants to the combination of stresses indicate that during simultaneous exposure of plants to multiple stresses, one form of stress could minimize the damage by the other. The enhancement of superoxide dismutase (SOD) and catalase activities appear to serve as acclimation mechanisms to scavenge the toxic, free radicals of oxygen produced under stress conditions. However, the inhibition in nitrate metabolism was greater in the combined stresses than in either of the stresses imposed separately. The results of this study illustrate that the interaction of stresses during simultaneous multiple stress conditions brings out certain beneficial effects.     

Response of sugar beet plants to ultraviolet-B (280–320 nm) radiation and Cercospom leaf spot disease

Sugar beet ( Beta vulgaris L.) plants injected with Cercospora beticota Sace. as well as non-infected 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 peraxidation in leaves also increased the most under the combined treatments. This was also true for ultraweak luminescence from both adaxial and abaxial 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 Cercospom leaf spot disease on sugar beet.     

Responses to ultraviolet-B radiation (280–315 nm) of pea (Pisum sativum) lines differing in leaf surface wax

To test the hypothesis that leaf surface wax influences plant responses to UV-B, 6 lines of cultivated pea (Pisum sativum L.), selected as having more or less wax, were grown at 0 or 6.5 kJ m^-2 day^-1 plant-weighted UV-B against a background of 850–950 μmol m^-2 s^-1 photosynthetically active radiation. In the 4 lines with least leaf surface wax the amount of wax on adaxial and abaxial leaf surfaces was increased following exposure to 6.5 kJ m^-2 day^-1 UV-B, but UV-B decreased surface wax in Scout, which had the greatest wax deposits. On the adaxial leaf surface, UV-B radiation caused a shift in wax composition from alcohols to esters and hydrocarbons and the ratio of short to long chain length alkyl ester homologues was increased. There was no evidence of a shortening in carbon chain length of hydrocarbons, primary alcohols or fatty acids due to UV-B and no significant correlation between wax amount and UV reflectance from leaves. UV-B induced significant increases in UV-absorbing compounds in the expanded leaves and buds of most lines. UV-B reduced the growth of all lines. Foliage area (leaves plus stipules) declined by 5–30%, plant dry weight by 12–30%, and plant height by 24–38%. Reductions in growth occurred in the absence of any changes in chlorophyll fluorescence or photosynthetic rate. UV-B also had no major effect on carbon allocation patterns. The effects of UV-B on growth appeared to be due to changes in tissue extension and expansion. Indeed, many of the responses to UV-B observed in this study of pea appear more consistent with indirect effects being expressed in developing tissues rather than through the direct action of UV-B on mature tissues. There was no evidence that wax amount or biochemistry was associated with the sensitivity of the lines to UV-B radiation. Furthermore, induction of pigments was not correlated with changes in growth. However, lines with the greatest constitutive amounts of pigments in unexpanded bud tissues were most tolerant of elevated UV-B.     

Detoxification function of aldose/aldehyde reductase during drought and ultraviolet-B (280–320 nm) stresses

Plants may activate similar defence systems to reduce cellular damages caused by different stress conditions. In the present experiments, the formation of lipid peroxidation products [thiobarbituric acid reactive species (TBARS)] was significant during both drought and ultraviolet (UV)‐B stresses, whereas the formation of reactive oxygen species (ROS) was a more delayed response to UV‐B than to drought. H₂O₂ was detected during both stresses, whereas ^·OH radical production was a more characteristic response to drought. The present characterization of transgenic tobacco plants revealed a common role for aldose/aldehyde reductase (ALR) in the detoxification of lipid peroxidation products under water depletion and UV‐B irradiation. As the result of the increased synthesis of ALR enzyme, the transformed plants were more tolerant to both stress conditions, exhibiting reduced loss of photosynthetic function and decreased accumulation of TBARS and H₂O₂ as compared to control (SR1) plants. When plants had been exposed to mild, non‐lethal drought and were then watered again to recover, they were more tolerant to a subsequent stress by UV‐B. This was characteristic to both transgenic and wild‐type plants. However, this drought‐induced cross‐tolerance to UV‐B stress of SR1 tobacco did not reach the enhancement achieved by the overexpression of ALR.     

Impact of solar ultraviolet-B radiation (290-320 nm) upon marine microalgae

For years scientists and laymen alike have casually noted the impact of solar ultraviolet radiation upon the non-human component of the biosphere. It was not until recently, when human activities were thought to threaten the protective stratospheric ozone shield, that researchers undertook intensive studies into the biological stress caused by the previously neglected short-wavelength edge of the global solar spectrum. Stratospheric ozone functions effectively as an ultraviolet screen by filtering out solar radiation in the 220–320 nm waveband as it penetrates through the atmosphere, thus allowing only small amounts of the longer wavelengths of radiation in this waveband to leak through to the surface of the earth. Although this ultraviolet radiation (UV-B radiation, 290–320 nm) comprises only a small fraction (less than 1%) of the total solar spectrum, it can have a major impact on biological systems due to its actinic nature. Many organic molecules, most notably DNA and proteins, absorb UV-B radiation which can initiate photochemical reactions. It is life's ability, or lack thereof, to cope with enhanced levels of solar UV-B radiation that has generated the concern over the potential depletion of stratospheric ozone. The defense mechanisms that serve to protect both plants and animals from current levels of UV-B radiation are quite varied. Whether these mechanisms will suffice for marine microalgae under conditions of enhanced levels of UV-B radiation is the subject of this review.     

紫外-B辐射对番茄幼苗生长、POD和IAA氧化酶活性的影响

模拟兰州地区夏季臭氧层减薄12%和20%时增强的UV-B辐射2.54kJ·m-2·d-1 T1 和4.25kJ·m-2·d-1 T2 ,以自然辐射为对照,研究2种辐射对番茄幼苗生长、叶绿素和可溶性蛋白质含量以及过氧化物酶 POD 、吲哚乙酸氧化酶 IAA氧化酶 活性的影响.结果表明:低强度、短时间的辐射 T1,10d 对番茄幼苗生长没有影响,而高强度、长时间的辐射可减少生物量累积,降低株高、壮苗指数、叶绿素a、b和总叶绿素的含量,对叶绿素a/b无显著影响;与对照相比,可溶性蛋白质含量在辐射处理10d和20d时均减低,40d时升高;T2处理使POD活性一直显著高于对照;IAA氧化酶活性在辐射10d和20d时增加,40d时显著低于对照.说明在UV-B辐射下光合下降、IAA分解抑制了番茄幼苗的生长.     

UV-B辐射增强对水稻生长发育及其产量形成的影响

在盆载条件下,研究UV-B辐射（280-320nm）增强对3个不同类型水稻品种（组合）的生长发育及其产量构成的影响。结果表明,UV-B辐射增强明显抑制水稻生长,使株高变矮、分蘖数减少、叶面积和干物质量下降,但其抑制程度依品种、水稻所处的生长阶段的不同而不同;株高在苗期下降幅度最大,为9.4%-12.2%,干物质量在分蘖期下降幅度最大,地下部和地上部干物质量分别下降45.3%-59.8%、54.9%-59.0%,增强的UV-B辐射使水稻主茎不同叶位的出叶时间延迟,生育期延长,汕优63、南川、IR65600-85的抽穗时间分别比对照延迟2d、3d和7d,成熟期分别推迟3d、4d和9d,UV-B辐射增强明显降低水稻叶片的叶绿素和类胡萝卜素含量,叶片叶绿素a荧光诱导动力学参数Fv、Fv/Fm、Fv/Fo下降,与对照相比,汕优6.3、南川、IR65600-85叶片的净光合速率分别下降了11.9%、12.8%、29.7%,UV-B辐射增强使水稻每株有效穗、每穗总粒数、结实率、千粒重下降,最终导致水稻籽粒产量下降25.2%-31.1%。     

Responses in ion leakage of wild sugarcane (Saccharum spontaneum L.) clones to enhanced ultraviolet-B radiation under field conditions

Field studies were conducted to determine the potential for alterations in ion leakage and the intraspecific variation in ion leakage sensitivity of 20 wild sugarcane clones (Saccharum spontaneum L.) to enhanced ultraviolet-B (UV-B, 280–315nm) radiation in two consecutive years. The clones were collected from original sites with different altitude (from 0 to 1650 m) and latitude (from 18–37 °N). The supplemental UV-B radiation was 5.00 kJ m⁻², simulating a depletion of 20 % stratospheric ozone. Across all clones tested in the present study, a significant change (P<0.01 or 0.05) in ion leakage for 11 in tillering, 14 in elongation and 15 in flowering in 2003, and for 9 in tillering, 5 in elongation and 5 in floweing in 2004 were observed. In general, intraspecific responses of ion leakage of wild sugarcane clones to enhanced UV-B radiation existed under field conditions for two consecutive years, although intraspecific difference in 2003 was more obvious than that in 2004. Wild sugarcane clones originating from lower latitude or high elevation were not necessarily the more tolerant to enhanced UV-B radiation.     

紫外线-B辐射对植物DNA及蛋白质的影响

大气平流层中的臭氧衰减,导致太阳辐射中的紫外辐射量有明显的增加,其中UV-B辐射对植物会产生不同程度的影响。分子生态学理论认为,UV-B辐射对植物造成的损伤,首先伤害植物的生物大分子,即进行光化学修饰。本文就臭氧衰减对生态环境和植物的影响途径进行了讨论,重点论述了UV-B辐射对植物蛋白质合成的抑制和DNA的损伤修复途径。并应用分子生物学技术研究植物对UV-B辐射的抗性机理和DNA修复技术的前景进行了展望。     

Effects of ultraviolet-B radiation on growth, mycorrhizas and mineral nutrition of silver birch (Betula pendula Roth) seedlings grown in low-nutrient conditions

The effects of increased ultraviolet‐B (UV‐B) radiation on the growth, mycorrhizas and mineral nutrition of silver birch (Betula pendula Roth) seedlings were studied in greenhouse conditions. Seedlings—planted in a birch‐forest top soil and sand substrate—were grown without additional nutrient supply. Ultraviolet treatment started immediately after the seedlings emerged and the daily integrated biologically effective UV‐B irradiance on the UV‐B‐treated plants was equivalent to a 25% depletion of stratospheric ozone under clear sky conditions. Visible symptoms of UV‐B damage or nutrient deficiency were not observed throughout the experiment. Seedling height and dry weight (DW) (measured after 58 days and 76 days of treatment) were not affected by increased UV‐B. However, a significant shift in DW allocation toward roots resulted in a lower shoot/root ratio and leaf area ratio in UV‐B‐treated plants compared to control plants. At the first harvest (after 58 days of treatment), the percentage of various mycorrhizal morphotypes and the number of short roots per unit of root length or weight were not affected by increased UV‐B despite significantly increased DW allocation toward roots. Subtle reduction in the allocation of nitrogen (N) to leaves and increased allocation of phosphorus (P) to roots may suggest cumulative effects that could affect the plant performance over the long‐term.     

Differential response of leaf gas exchange to enhanced ultraviolet-B (UV-B) radiation in three species of herbaceous climbingplants

We measured diurnal changes in photosynthetic rate, transpiration rate, stomatal conductance and water use efficiency in three species of herbaceous climbing plants (Luffa cylindrica, Trichosanthes kirilowii and Dioscorea opposita) exposed to two intensities of UV-B radiation: 3.0 μw cm^?2 (R1) and 8.0 μw cm^?2 UV-B (R2) radiation under ambient growth conditions. Responses differed per species and per treatment. In Luffa all values increased compared to the Control in both treatments, except for stomatal conductance in R2. In Trichosanthes photosynthetic rates and water use efficiency increased, while the transpiration rates decreased under both treatments, and stomatal conductance was lower in R1. In Dioscorea photosynthetic rates and water use efficiency decreased under both treatments, while the transpiration rates and stomatal conductance increased. The results suggested that to some extent increased UV-B radiation was beneficial to the growth of L. cylindrica and T. kirilowii, but detrimental to D. opposita.     

Membrane- and cell wall-associated heat shock proteins in two genotypes of barley seedlings

ABSTRACT

The heat-shock (HS) response of two genotypes (cv. Onice, winter type, and cv. Georgie, spring type) of barley (Hordeum vulgare L.) was compared. Protein synthesis was markedly reduced by HS in roots and coleoptiles of both genotypes. The reduction in cv. Onice was higher than in cv. Georgie. The pattern of cytosolic, membrane and cell wall HSPs was analysed by SDS-PAGE in coleoptiles and roots of the two genotypes. Differences and similarities in coleoptiles and roots of the same genotype and between the two genotypes were observed. In roots of the genotype Onice, LMW and HMW HSPs isolated from the cytosol, membranes and cell walls were resolved into a diverse array of polypeptides by two-dimensional gel electrophoresis. Present results confirm the de novo synthesis of cytosolic and membrane HSPs, and demonstrate, for the first time, their presence in cell walls.     

Differential response of leaf gas exchange to enhanced ultraviolet-B (UV-B) radiation in three species of herbaceous climbingplants

We measured diurnal changes in photosynthetic rate, transpiration rate, stomatal conductance and water use efficiency in three species of herbaceous climbing plants (Luffa cylindrica, Trichosanthes kirilowii and Dioscorea opposita) exposed to two intensities of UV-B radiation: 3.0 μw cm^?2 (R1) and 8.0 μw cm^?2 UV-B (R2) radiation under ambient growth conditions. Responses differed per species and per treatment. In Luffa all values increased compared to the Control in both treatments, except for stomatal conductance in R2. In Trichosanthes photosynthetic rates and water use efficiency increased, while the transpiration rates decreased under both treatments, and stomatal conductance was lower in R1. In Dioscorea photosynthetic rates and water use efficiency decreased under both treatments, while the transpiration rates and stomatal conductance increased. The results suggested that to some extent increased UV-B radiation was beneficial to the growth of L. cylindrica and T. kirilowii, but detrimental to D. opposita.     

Effect of enhanced UV-B radiation on pollen quantity,quality, and seed yield in Brassica rapa (Brassicaceae)

We conducted three experiments to examine the influence of ultraviolet-B radiation (UV-B; 280–320 nm) exposure on reproduction in Brassica rapa (Brassicaceae). Plants were grown in a greenhouse under three biologically effective UV-B levels that simulated either an ambient stratospheric ozone level (control), 16% (“low enhanced”), or 32% (“high enhanced”) ozone depletion levels at Morgantown, WV, USA in mid-March. In the first experiment, we examined whether UV-B level during plant growth influenced in vivo pollen production and viability, and flower production. Pollen production and viability per flower were reduced by ≈50% under both enhanced UV-B levels relative to ambient controls. While plants under high-enhanced UV-B produced over 40% more flowers than plants under the two lower UV-B treatments, whole-plant production of viable pollen was reduced under high-enhanced UV-B to 17% of that of ambient controls. Whole-plant production of viable pollen was reduced under low-enhanced UV-B to 34% of ambient controls. In the second experiment, we collected pollen from plants under the three UV-B levels and examined whether source-plant UV-B exposure influenced in vitro pollen germination and viability. Pollen from plants under both enhanced-UV-B treatments had initially lower germination and viability than pollen from the ambient level. After in vitro exposure to the high-enhanced UV-B levels for 6 h, viability of the pollen from plants grown under ambient UV-B was reduced from 65 to 18%. In contrast, viability of the pollen from plants grown under both enhanced UV-B treatments was reduced to a much lesser extent: only from ≈43 to 22%. Thus, ambient source-plant pollen was more sensitive to enhanced UV-B exposure. In the third experiment, we used pollen collected from source plants under the three UV-B levels to fertilize plants growing under ambient-UV-B levels, and assessed subsequent seed production and germination. Seed abortion rates were higher in plants pollinated with pollen from the enhanced UV-B treatments, than from ambient UV-B. Despite this, seed yield (number and mass) per plant was similar, regardless of the UV-B exposure of their pollen source. Our findings demonstrate that enhanced UV-B levels associated with springtime ozone depletion events have the capacity to substantially reduce viable pollen production, and could ultimately reduce reproductive success of B. rapa.     

Responses in growth,physiology and nitrogen nutrition of dragon spruce (<Emphasis Type="Italic">Picea asperata</Emphasis>) seedlings of different ages to enhanced ultraviolet-B

In the southeast of the Qinghai–Tibetan Plateau of China, dragon spruce (Picea asperata) is a key species and widely used in reforestation processes in the area. The paper mainly studied the effects of ultraviolet-B (UV-B) on growth, physiology and nitrogen nutrition of 3- and 6-year-old dragon spruce seedlings. The experimental design included ambient UV-B (control) and enhanced UV-B (+UV-B, a 30% increase). Enhanced UV-B significantly decreased growth, needle and root nitrogen concentration, needle nitrate reductase activity and increased UV-B absorbing compounds and malondialdehyde (MDA) content of two old dragon spruce seedlings. Glutamine synthetase activity was not affected by enhanced UV-B in two old dragon spruce seedlings. On the other hand, different old seedlings also exhibited different physiological responses to enhanced UV-B radiation. Chlorophyll content, carotenoids content and soluble protein content in 3-year-old seedlings significantly reduced by enhanced UV-B, but those in 6-year-old seedlings were not affected by enhanced UV-B. Proline content of 6-year-old seedlings were increased by enhanced UV-B. Compared with the 3-year-old seedlings, the 6-year-old seedlings showed lower reduction of growth and MDA content, and accumulated more proline and UV-B absorbing compounds for protecting seedlings under enhanced UV-B. The results implicated that 3-year-old seedlings were more sensitivity to enhanced UV-B than 6-year-old seedlings.     

增强UV-B辐射对大豆胚轴DNA损伤、修复和蛋白质含量的影响

大气平流层臭氧层减薄引起到达地表的 UV- B辐射增强。为探讨在增强 UV- B辐射下植物细胞 DNA的损伤修复和蛋白质含量的关系 ,利用 3H- Td R掺入法 ,研究了在 8.2 2 k J/(m2 d)和 12 .4 2 k J/(m2 d) U V- B辐射 (相当于兰州地区大气平流层臭氧减薄约 12 %和 2 0 % )胁迫下 ,大豆胚轴细胞 DNA合成和非按期合成 (UDS)变化 ,并测定了胚轴蛋白质含量变化 ,结果显示 ,UV- B辐射导致 DNA损伤 ,并诱导了 DNA损伤的修复 ,胚轴细胞 UDS效应增强 ,U DS指数增大。低 UV- B辐射强度下 ,胚轴蛋白质含量增加 ,可能是 U V- B诱导了一些与抗性有关的基因表达 ,导致一些新的与抗性有关的蛋白质合成 ;在高强度 UV- B辐射下 ,U DS指数与低强度辐射下无显著差异 (P=0 .0 5 ) ,但蛋白含量较低强度辐射下显著下降 (P=0 .0 5 ) ,说明高强度 UV- B辐射加重了 DNA损伤 ,而修复并未加强 ,并且高强度辐射抑制基因的正常表达和蛋白质合成。这些蛋白质的合成可能与大豆对 UV- B辐射的抗性有关。     

DNA damage and photosynthesis in Antarctic and Arctic Sanionia uncinata (Hedw.) Loeske under ambient and enhanced levels of UV-B radiation

The response of the bipolar moss Sanionia uncinata (Hedw.) Loeske to ambient and enhanced UV‐B radiation was investigated at an Antarctic (Léonie Island, 67°35′ S, 68°20′ W) and an Arctic (Ny‐Alesund, 78°55′ N, 11°56′ E) site, which differed in ambient UV‐B radiation (UV‐BR: 280–320 nm) levels. The UV‐BR effects on DNA damage and photosynthesis were investigated in two types of outdoor experiments. First of all, sections of turf of S. uncinata were collected in an Arctic and Antarctic field site and exposed outdoors to ambient and enhanced UV‐BR for 2 d using UV‐B Mini‐lamps. During these experiments, chlorophyll a fluorescence, chlorophyll concentration and cyclobutyl pyrimidine dimer (CPD) formation were measured. Secondly, at the Antarctic site, a long‐term filter experiment was conducted to study the effect of ambient UV‐BR on growth and biomass production. Additionally, sections of moss turf collected at both the Antarctic and the Arctic site were exposed to UV‐BR in a growth chamber to study induction and repair of CPDs under controlled conditions. At the Antarctic site, a summer midday maximum of 2·1 W m^?2 of UV‐BR did not significantly affect effective quantum yield (ΔF/F_m′) and the ratio of variable to maximal fluorescence (F_v/F_m). The same was found for samples of S. uncinata exposed at the Arctic site, where summer midday maxima of UV‐BR were about 50% lower than at the Antarctic site. Exposure to natural UV‐BR in summer did not increase CPD values significantly at both sites. Although the photosynthetic activity remained largely unaffected by UV‐B enhancement, DNA damage clearly increased as a result of UV‐B enhancement at both sites. However, DNA damage induced during the day by UV‐B enhancement was repaired overnight at both sites. Results from the long‐term filter experiment at the Antarctic site indicated that branching of S. uncinata was reduced by reduction of ambient summer levels of UV‐BR, whereas biomass production was not affected. Exposure of specimens collected from both sites to UV‐BR in a growth chamber indicated that Antarctic and Arctic S. uncinata did not differ in UV‐BR‐induced DNA damage. It was concluded that S. uncinata from both the Antarctic and the Arctic site is well adapted to ambient levels of UV‐BR.