Effects of ozone on organic acids in needles of Norway spruce and Scots pine

Summary The effect of ozone, needle age, and season on the pH of homogenate and acid contents of Scots pine and Norway spruce needles is presented. In addition enzyme activities of cytochrome C-oxidase (cyt. C-ox), phosphoenolpyruvate-carboxylase (PEPC), shikimic acid-dehydrogenase (SHDH) and malate-dehydrogenase (MDH) were measured in Scots pine needles. In freshly sprouted spruce needles the level of quinic acid is high and the pH of the needle homogenate is low. Shikimic acid starts at low levels, increases with increasing needle age and becomes dominant, whereas the quinic acid content decreases. Malic acid has a marked seasonal trend; no trend was found in citric acid. Ozone (200 g/m³) decreased shikimic acid and quinic acid, whereas pH, malic acid and citric acid increased. Ozone (100 g/m³) had a similar effect, except in the current-year spruce needles. In Scots pine needles ozone led to increased enzymatic activities of cyt. C-ox, PEPC and SHDH, and a decrease in the activity of MDH. This effect was more pronounced in summer than in autumn, but the visible damage was greater in autumn. These effects can be found with other stresses and are not specific for ozone.     

Fatty acids,starch and biomass of Scots pine needles and roots in open-air ozone exposure

Scots pine (Pinus sylvestris L.) seedlings were fumigated with 1.2–1.5 x ambient ozone (cumulative exposure) over 2 seasons in an open-air experiment. Starch and fatty acid concentrations were analyzed in needle and root tissue in the summer, autumn and early winter. Seedling growth was determined by measuring the height of the stem and the total shoot and root biomass. Significant decreases in growth were found in exposed seedlings, even though visible symptoms were lacking. Almost significant reductions in needle and root starch concentrations were found. In the ozone treated foliage, significant increases in myristic acid (140) were detected, but the major fatty acids remained unchanged. Fatty acid ratios showed that the degree of unsaturation decreased in treated needles in the summer. In the roots of ozone treated seedlings, changes in fatty acids were different from those in the foliage. Decreases of the main root fatty acids (160, 180, 181, 18:2, 183) were detected in the summer. These results show that Scots pine is susceptible to enhanced levels of ozone. If the tropospheric ozone levels continue to increase it may have deleterious effects on Scots pine forests in Finland.     

Nitrogen metabolism of Douglas fir and Scots pine as affected by optimal nutrition and water supply under conditions of relatively high atmospheric nitrogen deposition

Nitrogen metabolism of the needles of 40-year-old Douglas fir and Scots pine trees, growing in two forest stands on cation-poor and acidic sandy soil with a relatively high atmospheric nitrogen deposition was studied. The composition of the free amino acid (FAA) pool, the concentrations of total nitrogen and soluble protein and the activities of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) were determined in the needles. An excessive nitrogen supply by a high atmospheric nitrogen deposition in both forest stands was indicated by the high concentrations of total nitrogen and the amino acids arginine, glutamic acid, glutamine and aspartic acid in control trees. In addition the effect of optimal nutrition and water supply (fertigation) on the needle nitrogen metabolism was evaluated. The total concentration of the FAA pool in needles of both tree species was lower in the fertigated than in the non-fertigated (control) trees, except for 1-year-old needles of Scots pine, in which the concentration after fertigation did not differ from the control. The lower total FAA concentration in the fertigated trees could be attributed to arginine, the concentration of which was on average 60% lower than in the control. Neither the concentration of soluble protein nor the activity of GS were influenced by fertigation. The activity of GDH in fertigated trees only differed significantly from the control in October. Scots pine needles had higher concentrations of protein (50%) and higher activities of GS (44%) and GDH (25%) than Douglas fir needles. Possible explanations for the lower vitality of Douglas fir compared to Scots pine are given.     

Monitoring of ozone pollution and the physiological activity of <Emphasis Type="Italic">Pinus halepensis</Emphasis> (Mill.) by electron paramagnetic resonance and other parameters

An investigation of selected Aleppo pines in the forests of Mt Hymettus and Mt Parnis near Athens (Greece) was undertaken at three different sites in the period 1999–2003, because a considerable proportion of pine trees showed visible signs of chlorotic mottle. This condition is characteristic of high and prolonged levels of ozone exposure. Needles from Aleppo pine trees (Pinus halepensis Mill.) were analyzed for their manganese content in combination with Electron Paramagnetic Resonance (EPR) spectra of Mn²⁺, involved in photosystem II. Manganese is considered as an important bioindicator for the vitality of trees. Also, we investigated the EPR spectrum of the needles in the region of g=2.0045 for healthy and diseased trees. The antioxidant capacity of the needles extract was measured from trees by the DPPH method. Finally, seasonal changes in chlorophyll concentration in the needles were measured to evaluate the effects of ozone. Measurements of ozone concentrations at the three sites showed that there were elevated levels during the summer months. Our experimental results suggest that the concentration of manganese in the needles was lower in the area with higher ozone concentrations, supported by EPR measurements. Higher ozone concentrations also affected the antioxidant potential of the needles and their chlorophyll content during summer months. Our findings also confirmed the resilience of Aleppo pines under stressful conditions and recovery in winter months. Despite the experimental problems, EPR spectra of Mn²⁺ in combination with other methods can be used as a sensitive bioindicator for ozone pollution, and is the result of oxidative stress affecting the growth cycle of the pine trees and their photosynthetic mechanisms.     

Peroxisome proliferation in Norway spruce induced by ozone

Summary Catalase (EC 1.11.1.6) activity (both total and specific activity) of particulate fractions of needles of Norway spruce [Picea abies (L.) Karst.] was elevated approximately 2-fold following exposure of trees to 60–70 g/m³ of ozone during the growing season compared to trees receiving charcoal filtered air (about 15 g/m³ ozone). Measurements were from homogenates fractionated into particulate and soluble (supernatent) activities. In contrast, the catalase activity of the supernatant was unchanged in response to ozone treatment. Catalase activity declined as the needles aged comparing current, 1-, and 2-year needles but the ozone-induced increment remained constant. Electron microscope cytochemistry using peroxidatic coupling with 3,3-diaminobenzidine carried out in parallel, revealed catalase-containing peroxisomes both in situ and in the particulate fractions analyzed for catalase activity. The tissue volume occupied by peroxisomes in response to needle age and ozone appeared to vary approximately in proportion to the measured catalase activity. Overall cytochemical reactivity for catalase declined with needle age, but, for all years, was greater in needles of trees receiving air supplemented with ozone compared to those of trees receiving charcoal filtered air.Abbreviations DAB 3,3-diaminobenzidine tetrahydrochloride     

Molecular cloning and functional expression of a stress-induced multifunctional O-methyltransferase with pinosylvin methyltransferase activity from Scots pine (Pinus sylvestris L.)

Formation of pinosylvin (PS) and pinosylvin 3-O-monomethyl ether (PSM), as well as the activities of stilbene synthase (STS) and S-adenosyl-l-methionine (SAM):pinosylvin O-methyltransferase (PMT), were induced strongly in needles of Scots pine seedlings upon ozone treatment, as well as in cell suspension cultures of Scots pine upon fungal elicitation. A SAM-dependent PMT protein was purified and partially characterised. A cDNA encoding PMT was isolated from an ozone-induced Scots pine cDNA library. Southern blot analysis of the genomic DNA suggested the presence of a gene family. The deduced protein sequence showed the typical highly conserved regions of O-methyltransferases (OMTs), and average identities of 20–56% to known OMTs. PMT expressed in Escherichia coli corresponded to that of purified PMT (40 kDa) from pine cell cultures. The recombinant enzyme catalysed the methylation of PS, caffeic acid, caffeoyl-CoA and quercetin. Several other substances, such as astringenin, resveratrol, 5-OH-ferulic acid, catechol and luteolin, were also methylated. Recombinant PMT thus had a relatively broad substrate specificity. Treatment of 7-year old Scots pine trees with ozone markedly increased the PMT mRNA level. Our results show that PMT represents a new SAM-dependent OMT for the methylation of stress-induced pinosylvin in Scots pine needles.     

Relationship of aluminium and calcium to net CO2 exchange among diverse Scots pine provenances under pollution stress in Poland

Light-saturated net photosynthesis (A_sat), dark respiration (RD), and foliar nutrient content of eight European Scots pine (Pinus sylvestris L.) provenances were measured at experimental sites in western Poland. Two-year-old seedlings were planted in 1984 at two sites with similar soils in areas of contrasting air pollution. One site was near a point source of SO₂ and other pollutants, and another 12 km to the southeast in an area free of acute air pollution was treated as a control. The eight provenances were from a large north-tosouth latitudinal range (60 to 43° N). At the heavily polluted site Scots pine trees exhibited lower growth rates and crown dieback and deformation. Soil pH, Ca and Mg were at least 10 times lower, and Al 10 times higher at the polluted than the control site. In 1991, concentrations of Al, P, Ca, S, Mn, Fe, and Zn in oneyear old Scots pine foliage were higher and Mg lower at the polluted than control site. At both sites foliar Mg levels were within the range considered deficient (0.6 mg g^-1), and at the polluted site, Al concentrations were very high (670 to 880 g g^-1). In all provenances, RD of one-year-old needles was higher (by 22% on average) and A_sat was lower (by 37% on average) at the polluted than the control site. The ratio of A_sat: RD was half as great in all provenances at the polluted (4 to 6) than control site (8 to 11). Provenances of southern origin had greater increases in RD and water-use efficiency at the polluted site than other provenances. Within the polluted site alone, or across both sites, A_sat in Scots pine was negatively correlated to the Al: Ca ratio (p<0.001, r=–0.93). Across sites RD increased with needle N and Al (multiple regression, p<0.001). The data suggest that at the polluted site there is excessive soil Al and deficient Mg availability, low needle Mg and high Al concentrations and high Al: Ca ratios, and that these have resulted in reduced photosynthetic capacity and increased respiration.     

Effect of ozone exposure on polyamines in Scots pine trees

Effects of ozone exposure on polyamines in Pinus sylvestris L. were studied in a long-term experiment. Ten- to 15-year-old Scots pines were exposed to target ozone levels which began at ambient + 40 ppb in May, decreasing to ambient air only by September for 3 growing seasons. The amount of ozone applied followed the natural pattern of variation in ozone concentrations in Northern Finland. The free, soluble conjugated and insoluble conjugated polyamines were analyzed during the experiment and shortly after termination of exposure as well as at the beginning of the following growing season. A carry-over effect was observed as ozone-induced reduction of free spermidine in the oldest needle year class, which developed during the first exposure season of the experiment. This reduction was observed both after the second and the third ozone exposure season. Conversely, after termination of the experiment, levels of free polyamines increased in the following growing season, and soluble conjugated polyamines decreased in the developing needles. The post-treatment changes in polyamine concentrations are hypothesized to be caused by stress-induced injuries or delayed recovery of metabolic processes rather than protective responses. It is noteworthy that some responses in polyamines were found in the developing needles nine months after terminating the ozone exposure. This suggests that stress-induced injuries to older needles affected metabolism of new developing needles.     

Some effects of long-term ozone fumigation on Norway spruce

Summary Potted cuttings of a 12-year-old Picea abies tree were fumigated with ozone, 100 or 300 g O₃· m^–3 (50 or 150 ppb O₃) being added to charcoal-filtered air during the 1985 growing season for a total of 1215 h. The wax structure of ozone-fumigated needles was no different from that of controls. Because flattened wax structures and fused wax fibrils also occurred in controls, these phenomena could not serve as bioindications for the ozone concentrations applied. A smooth layer was found beneath the soluble wax layer and covered needle surface and stomatal openings of ozone-fumigated needles to a greater extent than in controls. Wax quantity was considerably reduced by fumigation with 300 g O₃ · m^–3. Leaf pigments (as extracted with the wax) were less abundant in needles treated with 300 gO₃; the smooth layer probably contributed to the impeded extraction of pigments.     

Growth and reproduction of aphids and levels of free amino acids in Scots pine and Norway spruce in an open-air fumigation with ozone

To estimate the susceptibility of conifer seedlings to aphids under future tropospheric ozone levels, Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) seedlings were exposed to ambient and elevated ozone levels in an open-air exposure system. Growth and reproduction of the aphids Schizolachnus pineti and Cinara pinea on Scots pine and Cinara pilicornis on Norway spruce were monitored. Levels of free amino acids in foliage and young shoots were used as indicators of host plant quality. In elevated treatment plots the ozone doses were between 1.2 and 1.7 times the dose in ambient plots in 1990–93. Half of the seedling material in 1992–93 was subjected to nitrogen fertilization treatment to evaluate the effects of increased N deposition. In 1990, population density of S. pineti on pine did not differ between ambient and elevated ozone treatments during growing season, but remained higher in the elevated ozone plot than in the ambient plot at the end of the growing season. This was associated with elevated levels of glutamic acid in foliage. In August 1992, the numbers of S. pineti were consistent between the two ambient ozone plots, but deviated highly between the two ozone-fumigated plots. Glycine concentration in pine foliage was elevated by ozone, but free amino acid concentrations were not related to aphid performance. In 1993, ozone and nitrogen did not significantly affect the relative growth rate (RGR) of S. pineti or C. pinea nymphs on Scots pine, but glutamic acid concentration in foliage was increased by nitrogen fertilization. On Norway spruce, fecundity of C. pilicornis females was higher in elevated ozone treatment, but RGR of nymphs was not affected in 1992. In 1993, RGR of C. pilicornis nymphs was increased by nitrogen fertilization in June, but not affected by ozone. Nitrogen fertilization increased the levels of total free amino acids, aspartic acid, glutamic acid and proline in elongating shoots of Norway spruce, and ozone reduced the concentrations of valine and γ-butyric acid. Our results suggest that availability of nitrogen from soil has a stronger impact on the concentrations of free amino acids in conifer seedlings than ozone. Some episodes of high ozone concentration may increase free amino acids in foliage. Aphid response to ozone was extremely variable, in agreement with previous laboratory experiments. The expected 20–70% increase in ambient concentrations of tropospheric ozone may in some occasions enhance aphid performance on Scots pine and Norway spruce seedlings, but in most cases the ozone effect on the susceptibility of conifer seedlings to sucking insect pests is not important.     

Effects of Fertilization and Irrigation on the Seasonal Changes of Carbohydrate Reserves in Different Age-Classes of Needle on 20-Year-Old Scots Pine Trees (Pinus silvestris)

The effects of fertilization, irrigation or both on the seasonal changes of starch and soluble carbohydrates (glucose, fructose, myo-inositol, pinitol and sucrose) in needles of 20-year-old Scots pine trees (Pinus silvestris L.) were studied during three consecutive years. The starch content of the mature needles increased during spring and early summer to about 25% of dry weight. Neither fertilization nor irrigation affected the general pattern of starch accumulation during the spring. The starch reserves were mobilized when the shoot started to grow. Starch content decreased more rapidly in needles from fertilized than in those from unfertilized trees. The current needles from the control trees accumulated starch while they were still growing. The current needles of the fertilized trees did so to a lesser extent. The amount of starch was closely correlated to the air temperature and to the growth rate. Large amounts were found at low temperatures and low growth rates. The concentrations of soluble carbohydrates showed the well-known seasonal variation, with the highest value during the winter. The levels of sugars were nearly similar, irrespective of fertilization. An exception was sucrose, which was found in small quantities in needles from fertilized plots. Small amounts of sucrose were also found in growing current needles. The results are discussed in relation to growth limitation by assimilate availability and indicate that the ‘sink demand’ is the limiting factor.     

Advances in the positional cloning of nodulation genes in soybean

The effect of liming on the decomposition of Norway spruce needle litter was studied in 40–60-year-old Norway spruce stands. Finely-ground limestone had been spread about 30 years ago at a dose of 2 t ha^–1 and reliming was carried out about 20 yr later at a dose of 4 t ha^–1. Needle litter was collected from both control and limed plots, and it was placed in litter bags in the middle of the humus layer of the plot from which they originated, and similarly to the other plot in May. Litter bags were sampled after 4, 12 and 16 months. The site of origin of the needle litter, whether from control plot or from limed plot, affected mainly the early stages of decomposition. Initially the effect of liming was seen as decreased concentration of water soluble material and then, during decomposition, as decreased mass loss and decreased degradation of lignin, and increased C/N ratio. The incubation site, whether the control or the limed plot, did not affect decomposition significantly.Decomposition of Scots pine needles in a young Scots pine plantation was also studied. The treatments were: 2 t ha^–1 of finely-ground limestone and 2.5 t ha^–1 of bark ash spread 8 months before this study. The treatments did not affect decomposition much, but some stimulation of the treatments on decomposition was observed. Compared to spruce needles, the C/N ratio of pine seedles was lower, they contained less lignin and more water soluble material, and decomposed faster in the first summer.     

Effects of drought and waterlogging on ultrastructure of Scots pine and Norway spruce needles

Effects of water stress on needle ultrastructure of 2-year-old Scots pine (Pinus sylvestris L.) and 5-year-old Norway spruce [Picea abies (L.) Karst.] seedlings were studied in greenhouse experiments. Drought stress was induced by leaving seedlings without watering, and waterlogging stress was produced by submerging the seedling containers in water. Needle samples for ultrastructural analyses were collected several times during the experiments, and samples for nutrient analyses at the end of the experiments. In drought stress, plasmolysis of mesophyll and transfusion parenchyma tissues, aggregation of chloroplast stroma and its separation from thylakoids and decreased size and abundance of starch grains in needles of both species were observed. The concentration of lipid bodies around the chloroplasts were detected in pine needles. Calcium and water concentrations in spruce needles were lower by the end of the experiments compared to controls. In waterlogging treatment, swelling of phloem cells in pine needles and large starch grains, slight swelling of thylakoids and increased translucency of plastoglobuli in chloroplasts of both species studied were observed. The phosphorus concentration in pine needles was higher while phosphorus, calcium and magnesium concentrations in spruce needles were lower in the waterlogging treatments compared to controls. Typical symptoms induced by drought stress, e. g. aggregation of chloroplast stroma and its separation from thylakoids, were detected, but, in waterlogging stress, ultrastructural symptoms appeared to be related to the developing nutrient imbalance of needles.     

Free Radical Scavengers and Photosynthetic Pigments in Pinus Cembra L. Needles as Affected by Ozone Exposure

The goal of this study was the characterization of the antioxidative protection system of current and 1-year-old needles of a cembran pine (Pinus cembra L.) and its possible responses to elevated concentrations of atmospheric O₃. Twigs of a mature cembran pine at the alpine timberline (1950 m a.s.l.) were exposed in climate-controlled twig chambers for 91 d to charcoal-filtered air (CF), ambient air O₃ concentration (A), and two-fold ambient air O₃ concentration (2A). Additionally, a chamberless control group (AA) was used to examine chamber effects. At the end of the fumigation period the contents of free radical scavengers and photosynthetic pigments were measured in the needles. Independent from O₃ exposure, total ascorbate and -tocopherol contents were higher in 1-year-old needles compared to the current flush while the opposite was found for glutathione. The amounts of pigments and antioxidants in P. cembra needles were comparable to those in other conifers growing at high-elevation sites. The only hint toward O₃ induced changes in the composition of antioxidants was an increase in the glutathione redox state toward more oxidation in 1-year-old needles upon exposure to A or AA conditions, but not upon 2A exposure. Chlorophyll and carotenoid contents were not affected by O₃ neither in current- nor in previous-year needles. The de-epoxidation state of the xanthophyll cycle pigments, however, was significantly increased in 1-year-old needles under A and AA compared to the CF control, but not under 2A. Hence, Pinus cembra, which is well adapted to the extreme environment of the timberline ecotone, exhibited only marginal biochemical changes in response to elevated O₃.     

The effects of UV-B radiation on epidermal anatomy in loblolly pine (Pinus taeda L.) and Scots pine (Pinus sylvestris L.)

The effects of enhanced UV‐B radiation on the needle anatomy of loblolly pine (Pinus taeda L.) and Scots pine (Pinus sylvestris L.) were studied in the field under supplemental UV‐B radiation supplied by a modulated irradiation system. The supplemental UV‐B levels were designed to simulate either a 16 or 25% loss of stratospheric ozone over College Park, Maryland. Enhanced UV‐B radiation caused different responses in these two species. The needles of loblolly pine had larger amounts of tannin in the lumen of epidermal cells and more wall‐bound phenolics in the outer epidermal walls of UV‐B‐treated needles, whereas the most pronounced effect on Scots pine needles was increased cutinization. In both species, the outer epidermal cell walls thickened and the needle cross‐sectional and mesophyll areas decreased (statistically significantly only in Scots pine). This suggests that more carbon may have been allocated to the protection mechanisms at the expense of photosynthetic area. The difference in response between these species suggests that the response to UV‐B radiation is not mediated by a single mechanism and that no generalization with regard to the effects of UV‐B on conifers can be made.     

The role of low-level ozone exposure and mycorrhizas in chemical quality and insect herbivore performance on Scots pine seedlings

The effects of low‐level ozone exposure and suppression of natural mycorrhizas on the above‐ground chemical quality of Scots pine (Pinus sylvestris L.) needles and insect herbivore performance were studied in a two‐year field experiment. Seedlings were fumigated with the ozone doses 1.5–1.7 times the ambient, and natural mycorrhizal infection level was about 35% reduced in roots with fungicide propiconazole. On ozone‐exposed seedlings the mean relative growth rate (MRGR) of Lygus rugulipennis Popp. nymphs was lower than on ambient ozone seedlings, but Gilpinia pallida Klug sawfly larvae grew better on elevated ozone seedlings than on ambient ozone seedlings. MRGR of Schizolachnus pineti Fabr. and Cinara pinea L. aphid nymphs or Neodiprion sertifer Geoffr. sawfly larvae or the oviposition of L. rugulipennis and N. sertifer were not affected by ozone exposure. Although ozone exposure did not affect total phenolics, total terpene, total or individual resin acid, total free amino acid, nutrient or sugar concentrations in needles, MRGR of L. rugulipennis positively correlated with total terpenes and MRGR of G. pallida positively with total amino acids. In addition, ozone exposure increased serine and proline concentration and marginally also starch concentration in needles. When mycorrhizas were reduced with fungicide, only MRGR of L. rugulipennis nymphs increased, but performance of other insect herbivores studied was not changed. However, number of L. rugulipennis eggs correlated positively with mycorrhizal infection level and also with total sugars. Reduction of mycorrhizas did not strongly affect the concentrations of analysed compounds in needles, because only phosphorus and potassium and some individual resin acids were reduced by fungicide treatment. These results suggest that low‐level ozone exposure and moderately declined mycorrhizal infection do not drastically affect either the above‐ground chemical quality of Scots pine seedlings or performance of studied insect herbivores.     

The effects of ozone and acid mist on Scots pine saplings

Summary It has been suggested that the forest decline (Neuartige Waldschäden) seen recently in parts of West Germany is due to the direct effects of ozone combined with acid mists, rather than soil-mediated effects of acid deposition. It has been proposed that ozone (a) makes the needles of affected conifers more susceptible to leaching by acid mist and (b) damages the photosynthetic apparatus, giving rise to diminished carbohydrate reserves which reduce the ability of affected trees to replace the leached nutrients. This nutrient deficiency (especially of Ca and Mg) is a characteristic symptom of the Waldschäden, which progresses through growth decline, needle loss, and eventually death. Parts of this hypothesis were tested in a preliminary experiment in which 3-year old Pinus sylvestris (Scots pine) saplings were exposed to 4 different O₃ levels, with and without acid mist (pH 3) treatment, for 56 days between July and September, 1983 in outdoor solardome fumigation chambers. The visual symptoms observed at >100 g m^-3 were more characteristic of the chlorotic mottle seen on O₃-affected trees in the USA than the general chlorosis of affected stands in Germany. O₃ at mean concentrations of >200 g m^-3 for 56 days reduced the fine root biomass and accelerated the senescence of older needles, in keeping with field effects observed in Germany. However, these O₃ levels increased, rather than decreased, the concentrations of most elements in the needles. Acid mist had no effect on needle concentrations, and there was no O₃-acid mist interaction. O₃ up to 300 g m^-3 also had no effect on the amount of ions leached from the needles, whereas acid mist increased the leaching of some ions, and again there was no interaction. The only nutritional effect of O₃ was to reduce the foliar uptake of NO _- ³ from the acid mist solution. An aphid infection part way through the experiment caused a large increase in leaching, particularly of K, and affected the intermediate O₃ and watersprayed plants most. Caution is needed in extrapolating these results to the field, as the experiments were of short duration on young trees with fully-formed needles, growing in a soil better supplied with nutrients than field soils. Nevertheless, these preliminary results do not support the hypothesis of an O₃-mediated increase in foliar leaching as the major cause of forest decline nor were the symptoms of O₃-injury on Scots pine comparable with those reported in the field.     

Ameliorating effect of UV-B radiation on the response of Norway spruce and Scots pine to ambient ozone concentrations

Elevated levels of both ozone and UV-B radiation are typical for high-altitude sites. Few studies have investigated their possible interaction on plants. This study reports interactive effects of O₃ and UV-B radiation in four-year-old Norway spruce and Scots pine trees. The trees were cultivated in controlled environmental facilities under simulated climatic conditions recorded on Mt Wank, an Alpine mountain in Bavaria, and were exposed for one growing season to simulated ambient or twice-ambient ozone regimes at either near ambient or near zero UV-B radiation levels. Chlorotic mottling and yellowing of current year needles became obvious under twice-ambient O₃ in both species at the onset of a high ozone episode in July. Development of chlorotic mottling in relation to accumulated ozone concentrations over a threshold of 40 nL L^–1 was more pronounced with near zero rather than ambient UV-B radiation levels. In Norway spruce, photosynthetic parameters at ambient CO₂ concentration, measured at the end of the experiment, were reduced in trees cultivated under twice-ambient O₃, irrespective of the UV-B treatment. Effects on photosynthetic capacity and carboxylation efficiency were restricted to trees exposed to near zero levels of UV-B radiation, and twice-ambient O₃. The data indicate that UV-B radiation, applied together with O₃, ameliorates the detrimental effects of O₃. The data also demonstrate that foliar symptoms develop more rapidly in Scots pine than in Norway spruce at higher accumulated ozone concentrations. Symbols and abbreviations: LSD, least significant difference; PAS300, UV-B irradiance weighted according to the plant action spectrum of Green et al. (1974) normalized at 300 (nm); AOT40, (AOT = accumulated over threshold) reflects the sum of hourly ozone concentrations above 40 nL L^–1 during daylight hours (> 50 Wm^–2) ( Kärenlampi & Skärby 1996 ); A₃₅₀, net photosynthesis at ambient CO₂; G₃₅₀, stomatal conductance for water vapour at ambient CO₂; A₂₅₀₀, net photosynthesis at saturating CO₂ (maximal potential photosynthetic activity); CE, carboxylation efficiency; ROS, reactive oxygen species; RuBP, ribulose 1,5-bisphosphate; Rubisco, ribulose 1,5-bisphosphate carboxylase/oxygenase; GLM, general linear model.