Supplemental UV-B Radiation Research Articles

Photosynthesis performance in sweet almond [Prunus dulcis (Mill) D. Webb] exposed to supplemental UV-B radiation

Due to anthropogenic influences, solar UV-B irradiance at the earth's surface is increasing. To determine the effects of enhanced UV-B radiation on photosynthetic characteristics of Prunus dulcis, two-year-old seedlings of the species were submitted to four levels of UV-B stress, namely 0 (UV-Bc), 4.42 (UV-B1), 7.32 (UV-B2) and 9.36 (UV-B3) kJ m-2 d-1. Effects of UV-B stress on a range of chlorophyll (Chl) fluorescence parameters (FPs), Chl contents and photosynthetic gas-exchange parameters were investigated. UV-B stress promoted an increase in minimal fluorescence of dark-adapted state (F0) and F0/Fm, and a decrease in variable fluorescence (Fv, Fv/Fm, Fv/F0 and F0/Fm) due to its adverse effects on photosystem II (PSII) activity. No significant change was observed for maximal fluorescence of dark-adapted state (Fm). Enhanced UV-B radiation caused a significant inhibition of net photosynthetic rate (P N) at UV-B2 and UV-B3 levels and this was accompanied by a reduction in stomatal conductance (g s) and transpiration rate (E). The contents of Chl a, b, and total Chl content (a+b) were also significantly reduced at increased UV-B stress. In general, adverse UV-B effects became significant at the highest tested radiation dose 9.36 kJ m-2 d-1. The most sensitive indicators for UV-B stress were Fv/F0, Chl a content and P N. Significant P<0.05 alteration in these parameters was found indicating the drastic effect of UV-B radiation on P. dulcis.

Modification in growth, biomass and yield of radish under supplemental UV-B at different NPK levels

Growth, biomass, yield and quality characteristics of radish ( Raphanus sativus L. var. Pusa Himani) were investigated under supplemental UV-B (sUV-B; 280–320 nm; +7.2 kJ m −2 d −1) radiation at varying levels of soil NPK. Combinations of NPK were recommended, 1.5 times NPK, 1.5 times N and 1.5 times K. sUV-B radiation negatively affected the growth and economic yield with more reductions at 1.5 times recommended NPK, N and K compared to recommended NPK. Total biomass remained unaffected in plants at recommended NPK under sUV-B radiation. At 1.5 times NPK and N more partitioning of biomass to shoot led to reduction in root shoot ratio and consequently yield under sUV-B. Nutrients in edible part declined maximally at 1.5 times recommended K under sUV-B. The study suggests that higher than recommended NPK makes radish plants more sensitive to sUV-B in terms of yield by allocating less photosynthates towards roots compared to shoots.

Long-term effects of elevated UV-B radiation on photosynthesis and ultrastructure of Eriophorum russeolum and Warnstorfia exannulata

The depletion of stratospheric ozone above the Arctic regions may increase the amount of UV-B radiation to which the northern ecosystems are exposed. In this paper, we examine the hypothesis that supplemental UV-B radiation may affect the growth rate and photosynthesis of boreal peatland plants and could thereby affect the carbon uptake of these ecosystems. In this study, we report the effects of 3-year exposure to elevated UV-B radiation (46% above ambient) on the photosynthetic performance and ultrastructure of a boreal sedge Eriophorum russeolum and a moss Warnstorfia exannulata. The experiment was conducted on a natural fen ecosystem at Sodankylä in northern Finland. The effects of UV-B radiation on the light response of E. russeolum CO 2 assimilation and the maximal photochemical efficiency of photosystem II in a dark-adapted state ( F v/ F m) were measured in the field. In addition, the effect of supplemental UV-B radiation on organelles of photosynthetic cells was studied by electron microscopy. The UV-B treatment had no effect on the CO 2 assimilation rate of either species, nor did it affect the structure of the cell organelles. On chlorophyll fluorescence, the UV-B exposure had only a temporary effect during the third exposure year. Our results suggested that in a natural ecosystem, even long-term exposure to reasonably elevated UV-B radiation levels does not affect the photosynthesis of peatland plants.

Intraspecific variation in sensitivity to ultraviolet-B radiation in endogenous hormones and photosynthetic characteristics of 10 wheat cultivars grown under field conditions

Field studies were conducted to determine the potential of altering endogenous hormones and photosynthetic characteristics and intraspecific variation in sensitivity of 10 wheat ( Triticum aestivum) cultivars (four tolerant, two middle sensitive and four sensitive) to enhanced ultraviolet-B (UV-B, 280–315 nm) radiation under field conditions. The supplemental UV-B radiation was 5.00 kJ m − 2, simulating a depletion of 20% stratospheric ozone. Responses were cultivar-specific. Out of the 10 tested wheat cultivars, six showed significant decrease in IAA content. UV-B radiation significantly increased ZR content in two wheat cultivars and significantly decreased in five cultivars. ABA content of three wheat cultivars was increased significantly, while that of five cultivars was decreased significantly. UV-B radiation significantly increased the stomatal conductance of three cultivars, and significantly decreased that of four cultivars. Intercellular CO 2 concentrations were significantly increased in five cultivars and significantly decreased in one cultivar (Mianyang 20). Transpiration rate of three cultivars significantly increased, while that of three cultivars significantly decreased. UV-B radiation significantly decreased the net photosynthetic rate of six cultivars. Intraspecific differences were found for the different measured parameters. For seven measured parameters, UV-B radiation had significant effects on five wheat cultivars, while no effect on the others. Significant correlations were observed between net photosynthetic rate and stomatal conductance, intercellular CO 2 concentrations and transpiration rate in eight cultivars. UV-B radiation might change stomatal conductance, intercellular CO 2 concentrations and transpiration rate, thus resulting in changes in net photosynthetic rate.

Responses in the morphology, physiology and biochemistry of Taxus chinensis var. mairei grown under supplementary UV-B radiation

The effects of supplemental UV-B radiation on Taxus chinensis var. mairei were studied. Leaf traits, gas exchange parameters and the concentrations of photosynthetic pigments, cellular defense system products, secondary metabolites and ultrastructure were determined. UV-B radiation significantly decreased leaf area (p<0.05). Leaf number, secondary branch number, leaf weight per plant and leaf moisture all increased dramatically (p<0.05). Neither the leaf weight nor the specific leaf weight (SLW) exhibited significant differences between ambient and enhanced UV-B radiation. Gas exchange parameters were all dramatically reduced by enhanced UV-B radiation (p<0.05). The contents of chlorophyll and the chlorophyll a/b ratio were not distinctly affected by UV-B radiation, while carotenoids content significantly decreased (p<0.05). Supplemental UV-B treatment induced significant flavonoid accumulation (p<0.05), which was able to protect plant from radiation damage. Meanwhile, the appendage content, abaxial stomatal density, papilla density and particulate matter content in substomatic chambers increased noticeably by supplemental UV-B radiation, whereas the aperture size of single stomata was diminished. The number and area of plastoglobuli were apparently reduced by UV-B radiation, but stroma and grana lamellae were not destroyed. Our results demonstrated that T. chinensis var. mairei can activate several defense mechanisms against oxidative stress injury caused by supplemental UV-B radiation.

Arabidopsis RADICAL-INDUCED CELL DEATH1 is involved in UV-B signaling

The Arabidopsis radical-induced cell death1 (rcd1) mutant is sensitive to ozone fumigation and apoplastic superoxide, but tolerant to methyl viologen. In the present article, we report that the rcd1 mutant is also tolerant to supplementary UV-B radiation. The rcd1-1 mutant exhibits less accumulation of TT dimers, increased hypocotyl growth inhibition and higher accumulation of flavonoids under supplemental UV-B radiation. Moreover, the expression of HY5 (elongated hypocotyl5) is increased in the mutant after UV-B treatment. Gene expression downstream of UV-B signaling reveals that COP1 (constitutively photomorphogenic1)-regulated genes have an elevated expression in rcd1-1 mutant under UV-B radiation, while expression of UVR8 (UV resistance locus 8)-regulated and HY5-independent genes are not changed. Interestingly, the expression of RCD1 genes is not significantly changed by UV-B radiation. Previous study has shown that STO protein is interacting with RCD1 in vitro. Here, we found the mRNA level of STO (salt tolerance) is greatly increased in rcd1-1 mutant after UV-B radiation. However, UV-B-induced HY5 and CHS expression is partially inhibited in sto mutant. Based on the above results, it is deduced that the RCD1, working together with STO, is involved in Arabidopsis UV-B signaling.

Supplemental ultraviolet-B induced changes in essential oil composition and total phenolics of Acorus calamus L. (sweet flag)

The effect of supplemental UV-B radiation (sUV-B) was evaluated on the essential oil contents of sweet flag ( Acorus calamus L.), a medicinal plant grown under natural field conditions. After the emergence of two leaves, plants were exposed to sUV-B radiation of 1.8 kJ m −2 above the ambient level of UV-B. The level of essential oil and phenol contents increased with exposure to sUV-B. Exposure of sUV-B resulted in significant increase in p-cymene and carvacrol contents of essential oil. Decrease in the level of major component beta-asarone due to sUV-B treatment is of prime importance, because of its toxicological concern to human health.

Responses of leaf traits of European beech ( Fagus sylvatica L.) saplings to supplemental UV-B radiation and UV-B exclusion

This study describes the physiological responses of European beech saplings to varying levels of UV-B radiation under natural environmental conditions and in an outdoor pot experiment. In the field experiment, saplings at a forest site regenerating naturally in a clear-cut area were subjected to UV-B exclusion (UV-B exc). In the outdoor pot experiment saplings were subjected to both exclusion of UV-B and enhanced UV-B (UV-B enh). Three months after the start of the treatments, plants were monitored for photosynthetic pigment composition, chlorophyll fluorescence parameters, specific leaf mass (SLM, dry matter per unit leaf area) and UV-B absorbing pigments (flavonoids). In the forest site the leaves which developed under UV-B exclusion had significantly higher chlorophyll and water contents than under ambient UV-B (UV-B amb). Both the total pool size of xanthophyll cycle pigments (violaxanthin + antheraxanthin + zeaxanthin: VAZ-pool) and the de-epoxidation state (DEEPS) of these pigments and the flavonoid content and specific leaf mass were significantly lower at midday under UV-B exc. UV-B exclusion moderated the changes in several leaf traits between morning and midday. In the pot experiment UV-B exc caused smaller, but significant, increase in chlorophyll content and decrease in flavonoid content as compared to UV-B amb than in forest site experiment. UV-B enh resulted in activation of photoprotective mechanisms (significant increases of total carotenoid content, VAZ-pool, DEEPS, flavonoid accumulation), but significantly lowered the concentration of chlorophylls and water content, while it slightly increased the Chl_a/b ratio as compared to the UV-B amb. Larger midday reductions of potential photochemical activity of PSII assessed as F v/ F m, ratio of variable chlorophyll fluorescence ( F v, difference between maximum ( F m) and ground fluorescence F 0) to maximum fluorescence ( F m) in dark-adapted leaves and leaf water content occurred under UV-B enh than under UV-B amb and UV-B exc. Altogether, greater differences were observed in leaf traits between UV-B amb and UV-B exc than between UV-B enh and UV-B amb. Chlorophyll fluorescence parameters of dark adapted leaves were not significantly affected by UV-B exc in either experiment. UV-B absorbing compounds responded most to the different UV-B levels, which may explain the practically unaltered photosynthetic activity as these compounds, due to their antioxidant properties, can reduce lipid peroxidation and damage to the photosynthetic apparatus.

Solar and Supplemental UV‐B Radiation Effects in Lemon Peel UV‐B‐absorbing Compound Content—Seasonal Variations

Effects of solar and supplemental UV-B radiation on UV-B-absorbing compounds and malondialdehyde (MDA) accumulations in the peel of lemons collected in summer and winter were analyzed. UV-B-absorbing compounds were higher in flavedo than in albedo tissue in both seasons; however, the highest values were observed in summer. These compounds were also higher in outer than in inner flavedo surface. Lemons were categorized as sun-, semisun- and shaded-lemon according to localization inside the tree canopy. Depending on-tree localization UV-B-absorbing compounds were higher in flavedo of sun-lemon than in semisun- and shaded-lemon. Supplementary UV-B radiation (22 kJ m(-2) day(-1) UV-BBE) induced UV-B-absorbing compound synthesis in on-tree and postharvest lemons. Two minutes of supplemental UV-B irradiation in summer lemons produced a strong increment (300%) of UV-B-absorbing compound content, whereas in winter lemons a slight increase (30%) was observed only after 3 min of irradiation. By contrast, UV-B-absorbing compound accumulation was not observed in albedo. MDA accumulation showed approximately a similar trend of UV-B-absorbing compounds. According to our results, solar UV-B was not required for UV-B-absorbing compound accumulation in lemon peel. Relationships between UV-B-absorbing compounds, MDA, reactive oxygen species and pathogen protection are also discussed.

Effects of enhanced UV-B radiation on fitness of an alpine species Cerastium glomeratum Thuill

Aims Information about how species respond to extreme environments, such as high UV-B radiation, is very useful in estimating natural ecosystem structure and functions in alpine areas. Our aim is to examine the effect of enhanced UV-B radiation on the fitness of an alpine meadow annual species on Qinghai-Tibet Plateau. Methods Plants of Cerastium glomeratum Thuill. were exposed to ambient (control) or ambient plus supplemental UV-B radiation (enhanced), simulating a 9% ozone depletion over Gannan, China (10253#E, 3455#N, 2900 m in altitude), up to leaf senescence and fruit maturation. Plant height, flower phenology, biomass allocation and reproductive parameters of the species were measured. Important findings Plant height in C. glomeratum was reduced by enhanced UV-B radiation at early growth stages and compensated with ongoing development. Fruit biomass, aboveground biomass, total biomass and reproductive effort (fruit dry mass/aboveground biomass) were not affected by enhanced UV-B radiation, but a significant increase in root/shoot ratio was found. Enhanced UV-B radiation delayed onset of flowering by 1 day and shortened duration of flowering by 5 days in C. glomeratum. But because of the long period of flowering time (83–88 days), this did not make any significant effect on flower number, seed number, pollination success (number of seeds per fruit) or reproductive success (fruit to flower ratio) in C. glomeratum. Enhanced UV-B radiation had no effect on seed germination and seed mass either. And the high production and low germination rate of the seed might be the strategy of C. glomeratum to survive the extreme environments on alpine meadow. All these results showed that C. glomeratum was tolerant to enhanced UV-B radiation.

Intraspecific responses of Fagopyrum esculentum to enhanced ultraviolet B radiation

The effects of supplemental UV-B radiation on crop growth, morphology, reproduction and physiology were studied in three cultivars of Fagopyrum esculentum Moench (buckwheat) originating from high elevation (Qinghai-Tibet plateau) and lower altitudes (The Sichuan Basin). Our results showed that common buckwheat was sensitive to UV-B stress. Plant growth, development, and reproduction were inhibited by elevated UV-B radiation. Plant lipid oxidation and polyphenol oxidase (PPO) activity increased with increasing UV-B radiation, along with the concentration of phenylpropanoid compounds, superoxide dismutase (SOD) activity and ascorbic acid (Asa) concentration were also enhanced at the lowest level of supplemental UV-B radiation but decreased at the higher level of enhanced UV-B. While, a cultivar originating from elevated locations had lower dry matter accumulation and was more tolerant to UV-B radiation than cultivars originating from lower elevations. The effects on leaf thickness and increased antioxidant capacity could be linked with the improved performance of cultivar originating from high elevation when exposed to enhanced UV-B radiation. We conclude that UV-B tolerance should be considered prior to introducing or breeding common buckwheat cultivars from lowland cultivation to regions at high elevation such as the Qinghai-Tibet plateau.

Effects of supplemental ultraviolet-B and cadmium on growth, antioxidants and yield of Pisum sativum L.

The impact of supplemental UV-B irradiation (sUV-B) and cadmium (Cd, 68 μmol kg −1 soil) singly and in combination was studied on growth, pigments, enzymatic and non-enzymatic antioxidants, lipid peroxidation (LPO), uptake and translocation of Cd and yield of pea plants under field condition. Compared to the control, both the stresses individually and in combination led to in reductions in growth, photosynthetic pigments, ascorbic acid, catalase (CAT) activity and yield, whereas a reverse trend was observed for flavonoids, thiols and proline contents, superoxide dismutase (SOD) and peroxidase (POD) activities, and LPO. The uptake and translocation of Cd increased in different plant parts with duration of its treatment and also when applied with sUV-B. Combined treatment of sUV-B and Cd inhibited the growth and various metabolic processes antagonistically except the CAT activity which showed additive response. Further, CAT activity may be used as a bioindicator parameter to evaluate the individual and interactive effects of both the stresses.

Effect of Lanthanum (III) on Reactive Oxigen Metabolism of Soybean Seedlings under Supplemental UV-B Irradiation

The effect of lanthanum (III) on reactive oxygen metabolism of soybean seedling under elevated ultraviolet-B radiation (UV-B: 280 ∼320 nm) at 0.15 and 0.45 W · cm −2 levels respectively was studied through hydroponics in the laboratory. Plasmolemma permeability and contents of malonadialdehyde (MDA), hydrogen peroxide (H 2O 2), and proline gradually increased during the imposition of UV-B radiation and subsequently decreased during recovery from UV-B stress. The dynamic tendency of catalase (CAT) activity was similar to that of the above four indices. The activity of peroxidase (POD) initially increased, then remained at a high level, and finally dropped steeply when soybean seedlings were exposed to a low dosage of UV-B radiation. However, POD activity rose throughout and declined slightly on the eleventh day when soybean seedlings were stressed by a high dosage. With the addition of La(III) of 20 mg · L −1, the rising tendency of plasmolemma permeability and contents of MDA, H 2O 2, and proline were slowed down during the stress period, whereas the declining speed was accelerated during the recovery period. The activities of CAT and POD were higher than those without La(III) in all experiments. It suggested that the regulative effect of La(III) on antioxidant enzymes such as CAT and POD could strengthen their capacities to scavenge reactive oxygen species (ROS), decrease contents of MDA and proline, and maintain normal plasmolemma permeability. Further more, the protective potential of La(III) was better under low UV-B radiation than under a high one.

The interactive effects of enhanced UV-B radiation and soil drought on spring wheat

The combined effects of enhanced UV-B radiation and soil drought on the phenological development, photosynthetic pigments, ultraviolet absorbing compounds (e.g. flavonoids), physiological characteristics, biomass and yields of spring wheat ( Triticum aestivum L.) under arid and semiarid field conditions were investigated in this study. The enhanced UV-B BE radiation was 4.25 kJ m 2 day − 1 , which simulated 20% stratospheric ozone depletion on clear summer solstice (Lanzhou, 1550 m above sea level, China). Soil moisture was maintained at 20% (control groups) and 15% (drought stress groups) by water-irrigation periodically. Phenology was delayed by increased UV-B radiation and the combination of UV-B and drought. In contrast, it was prompted by drought as compared with control in all cultivars. In UV-B- or drought-treated plants, pigments contents (chlorophyll a and b), leaf relative water content and water potential were reduced, However, lipid peroxidation products (MDA) and membrane permeability (relative electronic conductance) increased, and UV-B absorbing compounds (e.g. flavonoids) were induced by UV-B or water stress. Enhanced UV-B radiation or drought could decrease the net photosynthetic capacity through different paths, and led to the reduction of root, stem and leaves biomass and yield, as well as changed biomass and the harvest index. The plant growth, photosynthetic capacities, pigment contents, biomass and yield were reduced by the combination of two stresses in comparison with single stresses. The results suggested that co-stresses of supplementary UV-B radiation and drought Synergistically functioned and one of them could alleviate the inhibitory effects of another under the condition of arid and semiarid loessial soils. In addition, the earlier-ripped cultivar was better than middle or later-matured cultivars in response to the stress.

Effect of cerium on photosynthetic characteristics of soybean seedling exposed to supplementary ultraviolet-B radiation

Effects of cerium (Ce 3+) on photosynthetic characteristics were investigated by hydroponics under laboratory conditions when soybean seedlings were exposed to two levels of supplementary UV-B radiation. UV-B radiation badly inhibited the photosynthesis in soybean seedling, leading to a reduction in net photosynthetic rate (Pn), Hill reaction activity, light saturated photosynthetic rate (Ps) and apparent quanta yield (AQY), as well as the CO 2 and light saturated photosynthetic rate (Pm) and carboxulation efficiency (CE). On the contrary, Ce obviously promoted the photosynthesis of plants by increasing Hill reaction activity, accelerating electron transport and photophosphorylation, and enhancing carboxylation efficiency. For Ce+UV-B treatments, the values of photosynthetic parameters were still lower than those of the control, but obviously higher than those of UV-B treatment. The results indicated that Ce alleviated the inhibition of UV-B radiation on the photosynthesis in soybean seedling to a certain extent. In correlating of Pn with Hill activity, AQY and CE, we found that the changes ot photosynthetic rate were mainly influenced by the regulating effect of Ce on Hill activity and AQY at low level (0.15 W/m 2) of UV-B radiation, but were dominated by the regulating effect of Ce on CE at high level (0.45 W/m 2). Thus, Ce could regulate many aspects in photosynthesis of soybean seedling under UV-B stress. The regulating mechanism was close related with the dosage of UV-B radiation.

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.

Effects of ultraviolet-B radiation on crop growth, development, yield and leaf pigment concentration of tartary buckwheat ( Fagopyrum tataricum) under field conditions

Tartary buckwheat ( Fagopyrum tataricum Gaertn.) crops were grown in sheltered field plots for two consecutive seasons under reduced, near-ambient and two supplemental levels of ultraviolet-B radiation (UV-B, 280–315 nm). The effects of different UV-B treatments on crop growth, development, yield and leaf pigment levels were determined. All four treatments were replicated three times in a randomized complete block design. Near-ambient solar UV-B (approximately 84–88% of solar UV-B) was provided by using a Teflon film, attenuated solar UV-B radiation (43–49% reduction in solar UV-B) by using a cellulose acetate film, and supplemental UV-B radiation by the use of UV fluorescent lamps (two levels: 5.30 and 8.50 kJ m −2 day −1). The prevailing ambient solar radiation caused a negative effect on plant growth and yield. The crop growth, total biomass, final seed yield, thousand-grain weight and the amount of photosynthetic pigments decreased under near-ambient and enhanced UV-B radiation, while crop development was promoted by enhanced UV-B radiation. Leaf rutin concentration and the amount of UV-B absorbing compounds were increased by UV-B with the exception of the 8.50 kJ m −2 day −1 supplemental levels. Our results showed that tartary buckwheat is a potentially UV-B sensitive species, and the crop response to UV-B radiation is associated with UV-B intensity, environmental factors and growing season.

Interactive effects between supplemental ultraviolet-B radiation and heavy metals on the growth and biochemical characteristics of Spinacia oleracea L.

The impact of supplemental UV-B (sUV-B) radiation and heavy metals: Cd (68 µmol.kg-1 soil) and Ni (68 µmol.kg-1 soil) singly and in combination was studied on photosynthetic and non-photosynthetic pigments, metabolites, enzyme activities, biomass accumulation and the net primary productivity of spinach (Spinacia oleracea L. var. All Green), a common leafy vegetable plant. Both the stresses individually and in combination resulted in the reduction of photosynthetic pigments, ascorbic acid, and catalase activity, whereas a reverse trend was observed for anthocyanin, flavonoids and proline content, lipid peroxidation and peroxidase activity. The interactive effects of both the stresses were, however, less than additive. Combined treatment of sUV-B + Cd was more deleterious as compared to sUV-B + Ni and other individual treatments.

Responses of Strawberry (Fragaria×ananassa) to Supplemental UV-B Radiation and Selenium Under Field Conditions

In greenhouse experiments, selenium (Se) has been shown to defend plants against detrimental effects of heavy UV-B radiation stress. The aim of this study was to investigate whether this positive effect can be found in open-field conditions with enhancement of UV-B radiation. In the experiment, conducted with strawberry (Fragaria×ananassa, cultivars “Jonsok” and “Polka”) over two growing seasons, plants were exposed to UV-B radiation (including UV-A) and cultivated without Se or supplied with Se added at two levels (0.1 and 1.0 mg kg−1). The plants were monitored for growth, flavonoids, chlorophyll fluorescence, net photosynthesis as well as tissue and cell structure. Photosystem II was observed to be sensitive to UV-B stress under field conditions. In the leaves, a decrease in Fv/Fm was seen at the end of the growing season, implying a cumulative effect of UV-B stress. Several parameters, especially cell and tissue structures, were affected by UV-B and UV-A treatments, which proves the need for UV-A control in outdoor UV-B supplementation studies. Addition of Se did not ameliorate the harmful effects of UV-B but the lower Se-increment level increased leaf growth. The effects of UV-B and Se differed during the two experimental years, indicating the need to repeat experiments during several growing seasons.

Long-term exposure to enhanced UV-B radiation has no significant effects on growth or secondary compounds of outdoor-grown Scots pine and Norway spruce seedlings

The effects of long-term enhanced UV-B radiation on growth and secondary compounds of two conifer species were studied in an outdoor experiment. Scots pine (Pinus sylvestris) seedlings were exposed for two growing seasons and Norway spruce (Picea abies) seedlings for three growing seasons to supplemental UV-B radiation, corresponding to a 30% increase in ambient UV-B radiation. The experiment also included appropriate controls for ambient and increased UV-A radiation. Enhanced UV-B did not affect the growth of the conifer seedlings. In addition, neither the concentrations of terpenes and phenolics in the needles nor the concentrations of terpenes in the wood were affected. However, in the UV-A control treatment the concentrations of diterpenes in the wood of Scots pine decreased significantly compared to the ambient control. Apparently, a small increase in UV-B radiation has no significant effects on the secondary compounds and growth of Scots pine and Norway spruce seedlings.