Effects of increased UV-B radiation and elevated levels of tropospheric ozone on physiological processes in European beech (Fagus sylvatica)

As a consequence of the ongoing reduction of the stratospheric ozone layer, the vegetation is exposed to increasing levels of UV-B radiation (280–320 nm). In addition ozone in the troposphere is a pollutant and also capable of affecting the photosynthetic machinery. In this study, 5-year-old European beech trees were exposed from 1 July to October 1993 to two levels of UV-B radiation and two levels of ozone, alone and in combination, in open-top chambers equipped with lamps. The simulated UV-B levels corresponded to either clear sky ambient level or a 14% decrease in the stratospheric ozone column over eastern Denmark, resulting in a 23% difference in biologically effective UV-B (UV-B_BE) irradiance. The maximum UV-Bbe given was 8.61 kJ m⁻² day⁻¹. The ozone levels were either the ambient (average 32 nl l⁻¹) or ambient with ozone addition (average resulting concentration 71 nl l⁻¹). Compared to the control treatment (ambient UV-B, ambient O₃) the elevated levels of UV-B and O₃ affected the trees negatively, expressed as declines in net photosynthesis (P_n), stomatal conductance (g_s), chlorophyll fluorescence (F_v/F_m) and acceleration of senescence, measured as yellowing of the leaves. The UV-B treatment induced stomatal closure before the other treatments did. The magnitude of the decreases in P_n and F_v/F_m occurred in the order: control 3 3. Compared to the control, the combination treatment with high levels accelerated the visual senescence processes by ca 27 days, while for high UV-B and O₃ alone, there was an acceleration by 14 and 21 days, respectively. UV-B and O₃ in combination enhanced the negative effects compared with UV-B and O₃ alone. The P_n and F_v/F_m results could be related to this acceleration process. The chamber effect was investigated by comparing the control plots with a plot without open-top chamber. The trees in the chambers showed a higher P_n and F_v/F_m and a 14-day delayed senescence compared to the trees outside, probably caused by higher temperatures, a more protected environment and altered conditions inside the chambers.     

Interactive effects of elevated CO2 and O3 on photosynthesis and biomass production of clonal 5-year-old Norway spruce [Picea abies (L.) Karst.] under different nitrogen nutrition and irrigation treatments

To study the single and combined effects of elevated carbon dioxide (CO₂), ozone (O₃), nitrogen nutrition, and water supply on photosynthetic gas exchange and biomass accumulation of Norway spruce, a four-factorial experiment was conducted in closed environmental chambers. Each factor was applied at two levels: (i) ambient and elevated (ambient + 200 μl 1^-1) CO₂, (ii) 20 and 80 nl 1_-1 O₃, (iii) low and high nitrogen fertilization, and (iv) a well watered and a drought treatment. Neither elevated O₃ nor CO₂ significantly changed stomatal conductances of spruce needles. Adverse effects of elevated O₃ on photosynthetic parameters such as net assimilation rate and carboxylation efficiency occurred only when the plants were well watered and in a good nutritional status. After 6 weeks enhanced atmospheric CO₂ resulted in increased net assimilation rates provided that nutrition was well balanced and plants were well watered. Acclimation processes became apparent and are interpreted as a consequence of sink regulation. While O₃-effects were apparent only in biomass of 1-year-old plant material, elevated CO₂ resulted in higher biomass of the buds expanding during the exposure and increased root biomass significantly. Above and below-ground biomass were strongly influenced by the water and nutrition treatments.     

The effects of UV-B radiation on loblolly pine. 3. Interaction with CO2 enhancement

Projected depletions in the stratospheric ozone layer will result in increases in solar ultraviolet-B radiation (290–320 nm) reaching the earth's surface, These increases will likely occur in concert with other environmental changes such as increases in atmospheric carbon dioxide concentrations. Currently very little information is available on the effectiveness of UV-B radiation within a CO₂-enriched atmosphere, and this is especially true for trees. Loblolly pine (Pinus taeda L.) seedlings were grown in a factorial experiment at the Duke University Phytotron with either 0, 8.8 or 13.8 kJ m⁻² of biologically effective UV-B radiation (UV-B_BE). The CO₂ concentrations used were 350 and 650 μmol mol⁻¹. Measurements of chlorophyll fluorescence were made at 5-week intervals and photosynthetic oxygen evolution and leaf pigments were measured after 22 weeks, prior to harvest. The results of this study demonstrated a clear growth response to CO₂ enrichment but neither photosynthetic capacity nor quantum efficiency were altered by CO₂. The higher UV-B irradiance reduced total biomass by about 12% at both CO₂ levels but biomass partitioning was altered by the interaction of CO₂ and UV-B radiation. Dry matter was preferentially allocated to shoot components by UV-B radiation at 350 μmol mol⁻¹ CO₂ and towards root components at 650 μmol mol⁻¹ CO₂. These subtle effects on biomass allocation could be important in the future to seedling establishment and competitive interactions in natural as well as agricultural communities.     

Effects of increasing UV-B radiation and atmospheric CO2 on photosynthesis and growth: implications for terrestrial ecosystems

Increases in UV-B radiation reaching the earth as a result of stratospheric ozone depletion will most likely accompany increases in atmospheric CO₂ concentrations. Many studies have examined the effects of each factor independently, but few have evaluated the combined effects of both UV-B radiation and elevated CO₂. In general the results of such studies have shown independent effects on growth or seed yield. Although interspecific variation is large, high levels of UV-B radiation tends to reduce plant growth in sensitive species, while CO₂ enrichment tends to promote growth in most C₃ species. However, most previous studies have not looked at temporal effects or at the relationship between photosynthetic acclimation to CO₂ and possible photosynthetic limitations imposed by UV-B radiation. Elevated CO₂ may provide some protection against UV-B for some species. In contrast, UV-B radiation may limit the ability to exploit elevated CO₂ in other species. Interactions between the effects of CO₂ enrichment and UV-B radiation exposure have also been shown for biomass allocation. Effects on both biomass allocation and photosynthetic acclimation may be important to ecosystem structure in terms of seedling establishment, competition and reproductive output. Few studies have evaluated ecosystem processes such as decomposition or nutrient cycling. Interactive effects may be subtle and species specific but should not be ignored in the assessment of the potential impacts of increases in CO₂ and UV-B radiation on plants.     

CO(2) Enhancement of Growth and Photosynthesis in Rice (Oryza sativa) : Modification by Increased Ultraviolet-B Radiation

Two cultivars of rice (Oryza sativa L.) IR-36 and Fujiyama-5 were grown at ambient (360 microbars) and elevated CO₂ (660 microbars) from germination through reproduction in unshaded greenhouses at the Duke University Phytotron. Growth at elevated CO₂ resulted in significant decreases in nighttime respiration and increases in photosynthesis, total biomass, and yield for both cultivars. However, in plants exposed to simultaneous increases in CO₂ and ultraviolet-B (UV-B) radiation, CO₂ enhancement effects on respiration, photosynthesis, and biomass were eliminated in IR-36 and significantly reduced in Fujiyama-5. UV-B radiation simulated a 25% depletion in stratospheric ozone at Durham, North Carolina. Analysis of the response of CO₂ uptake to internal CO₂ concentration at light saturation suggested that, for IR-36, the predominant limitation to photosynthesis with increased UV-B radiation was the capacity for regeneration of ribulose bisphosphate (RuBP), whereas for Fujiyama-5 the primary photosynthetic decrease appeared to be related to a decline in apparent carboxylation efficiency. Changes in the RuBP regeneration limitation in IR-36 were consistent with damage to the photochemical efficiency of photosystem II as estimated from the ratio of variable to maximum chlorophyll fluorescence. Little change in RuBP regeneration and photochemistry was evident in cultivar Fujiyama-5, however. The degree of sensitivity of photochemical reactions with increased UV-B radiation appeared to be related to leaf production of UV-B-absorbing compounds. Fujiyama-5 had a higher concentration of these compounds than IR-36 in all environments, and the production of these compounds in Fujiyama-5 was stimulated by UV-B fluence. Results from this study suggest that in rice alterations in growth or photosynthesis as a result of enhanced CO₂ may be eliminated or reduced if UV-B radiation continues to increase.     

A thirty percent increase in UV-B has no impact on photosynthesis in well-watered and droughted pea plants in the field

It has been suggested that field experiments which increase UV-B irradiation by a fixed amount irrespective of ambient light conditions (‘square-wave’), may overestimate the response of photosynthesis to UV-B irradiation. In this study, pea (Pisum sativum L.) plants were grown in the field and subjected to a modulated 30% increase in ambient UK summer UV-B radiation (weighted with an erythemal action spectrum) and a mild drought treatment. UV-A and ambient UV control treatments were also studied. There were no significant effects of the UV-B treatment on the in situ CO₂ assimilation rate throughout the day or on the light-saturated steady-state photosynthesis. This was confirmed by an absence of UV-B effects on the major components contributing to CO₂ assimilation; photosystem II electron transport, ribulose 1,5-bisphosphate regeneration, ribulose 1,5-bisphosphate carboxylase/oxygenase carboxylation, and stomatal conductance. In addition to the absence of an effect on photosynthetic activities, UV-B had no significant impact on plant biomass, leaf area or partitioning. UV-B exposure increased leaf flavonoid content. The UV-A treatment had no observable effect on photosynthesis or productivity. Mild drought resulted in reduced biomass, a change in partitioning away from shoots to roots whilst maintaining leaf area, but had no observable effect on photosynthetic competence. No UV-B and drought treatment interactions were observed on photosynthesis or plant biomass. In conclusion, a 30% increase in UV-B had no effects on photosynthetic performance or productivity in well-watered or droughted pea plants in the field.     

Growth and maintenance respiration in leaves of northern red oak seedlings and mature trees after 3 years of ozone exposure

A two-component model of growth and maintenance respiration is used to study the response of northern red oak (Quercus rubra L.) seedlings and 32-year-old trees to sub-ambient (10 μmol h; cumulative dose based on 7 h daily mean), ambient (43 μmol h), and twice-ambient (85 μmolh) ozone. The relative growth rates (RGR) of leaves sampled from seedlings and trees were similar across treatments, as were specific leaf respiration rates (SRR). Growth coefficients estimated from the SRR versus RGR relationship averaged 25-3 mol CO2 kg^?1 leaf dry mass produced for seedlings and 21-5 mol kg^?1 for trees. Maintenance coefficients ranged from 0-89 to 1-07 mol CO₂ kg^?1 leaf dry mass d^?1 for seedlings and from 0-64 to 0-84 mol kg-1 d^?1 for trees. Neither coefficient was affected by ozone. Leaves sampled throughout the growing season also showed little response of respiration to ozone. This occurred despite a 30% reduction in net photosynthesis for trees grown at twice-ambient ozone. These results suggest that growth and maintenance respiration in young northern red oak leaves are not affected by ozone and that in older leaves injury can occur without a parallel increase in so-called ‘maintenance’ respiration.     

Effects of solar ultraviolet-B radiation,temperature and CO2 on growth and physiology of sunflower and maize seedlings

The effects of solar UV-B radiation, in combination with elevated temperature (4 °C ) and CO₂ (680 L L^-1 concentration, on sunflower and maize seedlings were studied from May to August in 1991 at the research station Quinta de São Pedro in Portugal (38.7°N). The ambient solar radiation of Portugal was reduced to levels of Central European latitudes by using the ozone filter technique. This radiation served as control, while the ambient solar radiation of Portugal was to simulate intense UV-B treatment (+30%). All plants were grown up to 18 days in 4 climate controlled growth chambers simulating a daily course of temperature with T_max=28 °C or 32 °C , resp., and ambient CO₂ concentrations (340 L L^-1); in one chamber the CO₂ concentration was twice as high (680 L L^-1). Under intense UV-B and at 28 °C (T_max) all growth parameters (height, leaf area, fresh and dry weight, stem elongation rate, relative growth rate) of sunflower and maize seedlings were reduced down to 35% as compared to controls. An increase in growing temperature by 4 °C , alone or in combination with doubled CO₂, compensated or even overcompensated the UV-B effect so that the treated plants were comparable to controls. Chlorophyll content, on a leaf area basis, increased under intense UV-B radiation. This increase was compensated by lower leaf areas, resulting in comparable chlorophyll contents. Similar to growth, also the net photosynthetic rates of sunflower and maize seedlings were reduced down to 29% by intense UV-B calculated on a chlorophyll basis. This reduction was compensated by an increased temperature. Doubling of CO₂ concentration had effects only on sunflower seedlings in which the photosynthetic rates were higher than in the controls. Dark respiration rates of the seedlings were not influenced by any experimental condition. Transpiration and water use efficiency (wue) were not influenced by intense UV-B. Higher temperatures led to higher transpiration rates and lower water use efficiencies, resp.. Doubling of CO₂ reduced the transpiration rate drastically while for wue maximum values were recorded.     

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 CO₂ 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 CO₂ concentrations and transpiration rate in eight cultivars. UV-B radiation might change stomatal conductance, intercellular CO₂ concentrations and transpiration rate, thus resulting in changes in net photosynthetic rate.     

The Regulation of Photosynthesis in Leaves of Field-Grown Spring Wheat (Triticum aestivum L., cv Albis) at Different Levels of Ozone in Ambient Air

Wheat (Triticum aestivum L. cv Albis) was grown in open-top chambers in the field and fumigated daily with charcoal-filtered air (0.015 microliters per liter O₃), nonfiltered air (0.03 microliters per liter O₃), and air enriched with either 0.07 or 0.10 microliters per liter ozone (seasonal 8 hour/day [9 am-5 pm] mean ozone concentration from June 1 until July 10, 1987). Photosynthetic ¹⁴CO₂ uptake was measured in situ. Net photosynthesis, dark respiration, and CO₂ compensation concentration at 2 and 21% O₂ were measured in the laboratory. Leaf segments were freeze-clamped in situ for the determination of the steady state levels of ribulose 1,5-bisphosphate, 3-phosphoglycerate, triose-phosphate, ATP, ADP, AMP, and activity of ribulose, 1,5-bisphosphate carboxylase/oxygenase. Photosynthesis of flag leaves was highest in filtered air and decreased in response to increasing mean ozone concentration. CO₂ compensation concentration and the ratio of dark respiration to net photosynthesis increased with ozone concentration. The decrease in photosynthesis was associated with a decrease in chlorophyll, soluble protein, ribulose bisphosphate carboxylase/oxygenase activity, ribulose bisphosphate, and adenylates. No decrease was found for triose-phosphate and 3-phosphoglycerate. The ratio of ATP to ADP and of triosephosphate to 3-phosphoglycerate were increased suggesting that photosynthesis was limited by pentose phosphate reductive cycle activity. No limitation occurred due to decreased access of CO₂ to photosynthetic cells since the decrease in stomatal conductance with increasing ozone concentration did not account for the decrease in photosynthesis. Ozonestressed leaves showed an increased degree of activation of ribulose bisphosphate carboxylase/oxygenase and a decreased ratio of ribulose bisphosphate to initial activity of ribulose bisphosphate carboxylase/oxygenase. Nevertheless, it is suggested that photosynthesis in ozone stressed leaves is limited by ribulose bisphosphate carboxylation possibly due to an effect of ozone on the catalysis by ribulose bisphosphate carboxylase/oxygenase.     

The photosynthetic irradiance-response of Norway spruce exposed to a long-term elevation of CO2 concentration

During an open-top chamber experiment performed in a mountain stand of young (12-year-old) Norway spruce (Picea abies [L.] Karst.), the trees were exposed to one of two CO² concentrations (ambient CO², AC, or AC + 350 μmol mol-1 = elevated CO₂, EC) continuously over three growing seasons. To evaluate the EC influence, measurements of the relations between the rate of net CO₂ uptake (P _N ) and incidental photosynthetically active photon flux density (PPFD), as well as the content of photosynthetic pigments and chlorophyll (Chl) a fluorescence were taken in the third growing season. The short-term response to EC was evident mainly on ribulose-1,5-bisphosphate carboxylase/oxygenase kinetics without any significant change to the utilization of radiant energy. The long-term effect of EC was responsible for a decrease in P _N , content of Chl a + b, Fv/Fm ratio, quantum yield of fluorescence, and photochemical quenching. Changes of stoichiometry between the electron transport, Calvin cycle and the end-product synthesis were confirmed for responses to the long-term import of EC and led to a definition of the photosynthetic acclimation to EC in Norway spruce. This revised version was published online in August 2006 with corrections to the Cover Date.     

UV-B增强下施硅对稻田CH4和N2O排放及其增温潜势的影响

大气平流层臭氧损耗导致的地表紫外辐射增强作为全球变化重要问题之一,受到广泛关注。硅是水稻生长有益元素,但施硅是否影响稻田CH_4和 N_2O排放,迄今相关报道尚不多见。通过大田试验,研究UV-B增强下施硅对水稻生长、稻田甲烷(CH_4)和氧化亚氮( N_2O)排放及其增温潜势的影响。UV-B辐照设2水平,即对照(A,自然光)和增强20%(E);施硅量设2水平,即对照(Si0,0 kg SiO_2/hm2)和施硅(Si1,200 kg SiO_2/hm2)。结果表明,UV-B增强降低了成熟期水稻地上部和地下部生物量,而施硅能缓解UV-B增强对水稻生长的抑制作用,使水稻地上部和地下部生物量增加。UV-B增强可显著提高稻田CH_4和 N_2O排放通量和累积排放量,增加稻田CH_4和 N_2O排放的综合增温潜势。施硅能明显降低稻田CH_4排放,促进 N_2O排放,降低稻田CH_4和 N_2O排放的综合增温潜势。研究表明,施硅显著降低稻田CH_4和 N_2O的全球增温潜势,缓解UV-B增强对稻田CH_4和 N_2O的全球增温潜势的促进作用。     

Extraordinary drought of 2003 overrules ozone impact on adult beech trees (Fagus sylvatica)

The extraordinary drought during the summer of 2003 in Central Europe allowed to examine responses of adult beech trees (Fagus sylvatica) to co-occurring stress by soil moisture deficit and elevated O₃ levels under forest conditions in southern Germany. The study comprised tree exposure to the ambient O₃ regime at the site and to a twice-ambient O₃ regime as released into the canopy through a free-air O₃ fumigation system. Annual courses of photosynthesis (A _max), stomatal conductance (g _s), electron transport rate (ETR) and chlorophyll levels were compared between 2003 and 2004, the latter year representing the humid long-term climate at the site. ETR, A _max and g _s were lowered during 2003 by drought rather than ozone, whereas chlorophyll levels did not differ between the years. Radial stem increment was reduced in 2003 by drought but fully recovered during the subsequent, humid year. Comparison of AOT40, an O₃ exposure-based risk index of O₃ stress, and cumulative ozone uptake (COU) yielded a linear relationship throughout humid growth conditions, but a changing slope during 2003. Our findings support the hypothesis that drought protects plants from O₃ injury by stomatal closure, which restricts O₃ influx into leaves and decouples COU from high external ozone levels. High AOT40 erroneously suggested high O₃ risk under drought. Enhanced ozone levels did not aggravate drought effects in leaves and stem.     

The effect of enhanced ultraviolet-B radiation on growth,photosynthesis and stable carbon isotope composition (δ13C) of two soybean cultivars (Glycine max) under field conditions

Two Chinese cultivars of Glycine max, namely Heidou and Jindou, were exposed to ambient and supplemental levels of ultraviolet-B (UV-B) radiation simulating a 24% depletion in stratospheric ozone over a 9-week growing period at an outdoor experimental site. Enhanced UV-B irradiation significantly reduced leaf, stem and root biomass, and plant height in the Heidou cultivar. These changes were associated with a diminished photosynthetic (net CO₂) rate, stomatal conductance, transpiration rate and water use efficiency, and accompanied by decreased foliar chlorophyll a and b, and total carotenoid concentrations and elevated foliar flavonoid levels. In contrast, the Jindou cultivar displayed only a significantly reduced stem mass and stomatal conductance, but no changes in pigment composition under elevated UV-B. The greater tolerance of elevated UV-B exposures by the Jindou cultivar was attributed partly to its higher foliar flavonoid content, smaller leaf size, thicker leaf cuticle and scabrous (hairy) lamina. Nevertheless both the Heidou cultivar and the less UV-B sensitive Jindou cultivar displayed an altered carbon isotope composition (δ¹³C) in their tissues following exposure to elevated UV-B. Such carbon isotope composition changes in plant tissues suggested a means of early detection of photosynthetic disruption in plants with anticipated increase in UV-B due to stratospheric ozone depletion.     

Photosynthetic parameters of birch (Betula pendula Roth) leaves growing in normal and in CO2- and O3- enriched atmospheres

Two silver birch (Betula pendula Roth) clones K1659 and V5952 were grown in open‐top chambers over 3 years (age 7–9 years). The treatments were increased CO₂ concentration (+CO₂, 72 Pa), increased O₃ concentration (+O₃, 2 × ambient O₃ with seasonal AOT40 up to 28 p.p.m. h) and in combination (+CO₂ + O₃). Thirty‐seven photosynthetic parameters were measured in the laboratory immediately after excising leaves using a computer‐operated routine of gas exchange and optical measurements. In control leaves the photosynthetic parameters were close to the values widely used in a model (Farquhar, von Caemmerer and Berry, Planta 149, 78–90, 1980). The distribution of chlorophyll between photosystem II and photosystem I, intrinsic quantum yield of electron transport, uncoupled turnover rate of Cyt b₆f, Rubisco specificity and K_m (CO₂) were not influenced by treatments. Net photosynthetic rate responded to +CO₂ with a mean increase of 17% in both clones. Dry weight of leaves increased, whereas protein, especially Rubisco content and the related photosynthetic parameters decreased. Averaged over 3 years, eight and 17 mechanistically independent parameters were significantly influenced by the elevated CO₂ in clones K1659 and V5952, respectively. The elevated O₃ caused a significant decrease in the average photosynthetic rate of clone V5952, but not of clone K1659. The treatment caused changes in one parameter of clone K1659 and in 11 parameters of clone V5952. Results of the combined treatment indicated that +O₃ had less effect in the presence of +CO₂ than alone. Interestingly, changes in the same photosynthetic parameters were observed in chamberless grown trees of clone V5952 as under +O₃ treatment in chambers, but this was not observed for clone K1659. These results suggest that during chronic fumigation, at concentrations below the threshold of visible leaf injuries, ozone influenced the photosynthetic parameters as a general stress factor, in a similar manner to weather conditions that were more stressful outside the chambers. According to this hypothesis, the sensitivity of a species or a clone to ozone is expected to depend on the growth conditions: the plant is less sensitive to ozone if the conditions are close to optimal and it is more sensitive to ozone under conditions of stress.     

The effects of enhanced UV-B radiation on physiological activity and growth of Norway spruce planted outdoors over 5 years

The responses of Norway spruce [Picea abies (L.) Karst.] to enhanced UV-B radiation during the 5-year treatment performed outdoors have been subjected to ecophysiological and growth analysis. The plants were exposed to UV-B radiation, simulating 17% ozone depletion. Ecophysiological parameters were monitored three times a year on three needle age classes, while growth was analysed at the end of each growth season. Spruce exhibited great variability in the amounts of photosynthetic pigments and methanol-soluble UV-B absorbing compounds, light use efficiency, photosynthesis and respiratory potential. The needle, branch and plant biomass production was not significantly affected during the 5-year treatment. The repeated-measures procedure comparing growth parameters through subsequent seasons, revealed a decrease of branch diameter under enhanced UV-B, which could be interpreted as a cumulative UV-B effect. The effects of UV-B radiation depended on needle development stage, interaction with environmental conditions and stresses. A reduced negative effect of UV-B radiation was observed during the prolonged drought in 2003, which was hypothesised as an alleviating effect. The tolerance of Norway spruce to elevated UV-B was to a large extent due to the high content of methanol-soluble UV-B absorbing compounds that was related neither to environmental conditions, including UV-B dose, nor to the developmental stage of the needles. The current year needles exhibited a tendency to increased production of UV-B absorbing compounds under elevated UV-B radiation. The outdoor study performed under variable environmental conditions showed great complexity of spruce response to enhanced UV-B.     

Effects of enhanced ultraviolet radiation and carbon dioxide concentration on the moss Hylocomium splendens

In a laboratory experiment interaction effects of UV-B and CO₂ on photosynthesis and growth of the moss Hylocomium splendens were studied. The plants were exposed to two CO₂ levels (350 ppm and 600 ppm) and three UV-B levels (no UV-B, ambient UV-B and that corresponding to 20% ozone depletion) for 5 months. The effects were recorded by measuring the photosynthetic response and growth of the plants. There was a statistically significant change in photosynthetic efficiency and maximum photosynthetic rates due to time and to enhanced CO₂ concentration, whereas there was no effect due to UV-B. There was a decreased growth due to both UV-B and CO₂ and an interaction effect on growth (in length). The UV-B dose corresponding to the ambient level had a larger reducing effect on growth than the highest UV-B dose. This was a counter-intuitive result and the following tentative interpretation was made: differences in the measured UV-A/UV-B/PAR ratios between the treatments could explain the result provided there was a non-linear response to UV over the range of irradiance levels used.     

Influence of ultraviolet-B (UV-B) radiation on photosynthetic and growth characteristics in field-grown cassava (Manihot esculentum Crantz)

The effects of ultraviolet-B (UV-B between 290 and 320 nm) on photosynthesis and growth characteristics were investigated in field grown cassava (Manihot esculentum Crantz). Plants were grown at ambient and ambient plus a 5.5kJ m^?2 d^?1 supplementation of UV-B radiation for 95 d. The supplemental UV-B fluence used in this experiment simulated a 15% depletion in stratospheric ozone at the equator (0°N). Carbon dioxide exchange, oxygen evolution, and the ratio of variable to maximum fluorescence (F_v/F_m) were determined for fully expanded leaves after 64–76 d of UV-B exposure. AH plants were harvested after 95 d of UV-B exposure, assayed for chlorophyll and UV-B absorbing compounds, and separated into leaves, petioles, stems and roots. Exposure to UV-B radiation had no effect on in situ rates of photosynthesis or dark respiration. No difference in the concentration of UV-B absorbing compounds was observed between treatments. A 2-d daytime diurnal comparison of F_v to F_m ratios indicated a significant decline in F_v/F_m ratios and a subsequent increase in photoinhibition under enhanced UV-B radiation if temperature or PPF exceeded 35°C or 1800μmol m^?2 s^?1, respectively. However, UV-B effects on fluorescence kinetics appeared to be temporal since maximal photosynthetic rates as determined by oxygen evolution at saturated CO₂ and PPF remained unchanged. Although total biomass was unaltered with UV-B exposure, alterations in the growth characteristics of cassava grown with supplemental UV-B radiation are consistent with auxin destruction and reduced apical dominance. Changes in growth included an alteration of biomass partitioning with a significant increase in shoot/root ratio noted for plants receiving supplemental UV-B radiation. The increase in shoot/root ratio was due primarily to a significant decrease in root weight (–32%) with UV-B exposure. Because root production determines the harvest-able portion of cassava, UV-B radiation may still influence the yield of an important tropical agronomic species, even though photosynthesis and total dry biomass may not be directly affected.     

The influence of ultraviolet-B radiation on the growth, pigment production and chlorophyll fluorescence of Norway spruce seedlings

Norway spruce (Picea abies (L.)Karst.) from seven seed sources was grown in a greenhouse with 8.3 and 14.7 kJ·m⁻²·d⁻¹ m UV-B_BE (biologically effective UV-B: 280–320 nm) irradiation, and with no supplemental irradiation as control. The seedlings total biomass (dry weight) and shoot growth decreased with high UV-B treatment but spruce from low elevation seed sources were more affected. The seedlings grown at the highest UV-B irradiance (14.7 kJ·m⁻²·d⁻¹) showed from 5 to 38% inhibition of total biomass and 15 to 70 % shoot growth inhibition. Norway spruce populations from higher altitude seed sources manifested greater tolerance to UV-B radiation compared to plants from low altitudes. Changes in phospholipids and protective pigments were also determined. The plants grown at the lower UV-B irradiance (8.3 kJ·m⁻²·d⁻¹) showed greater ability to concentrations UV-B-absorbing pigments then control plants. Chlorophyll a fluorescence parameter R_fd, (R_fd=(F_m-F_s)/F_s) showed a significant decrease in needles of UV-B treated plants and this correlated with the altitude of seed source. Exposure to UV-B affect levels of the ratio of variable to maximum fluorescence (F_v/F_m). Results from this study suggest that the response to increased levels of UV-B radiation is depended upon the ecotypic differentiation of Norway spruce and involved changes in metabolites in plant tissues.