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Key message Sessile oak leaves showed a high degree of plasticity to atmospheric and pedospheric conditions. Abstract The aim of the present study was to elucidate the significance of current weather conditions for foliar traits of adult sessile oak (Quercus petraea), one of the most valuable forest tree species in Central Europe. For this purpose, structural and functional traits were analysed in fully expanded, sun exposed leaves collected in south-west Germany from five old-growth forest stands, representing the meteorological and pedospheric conditions in the growing region, but differing in aridity during the 12 days before harvest in two consecutive years. Across the forest stands, most foliar traits differed significantly between wet and dry weather conditions before harvest as indicated by partial least square discriminant analysis (PLS-DA). These traits included fresh weight/dry weight ratio, leaf hydration, leaf-C content, leaf-C/N ratio, structural N, soluble protein-N, total amino acid-N, cell wall composition, numerous specific amino acids as well as soluble sugar content. Structural biomass, δ13C signature, total N and total C as well as H2O2 contents were not affected by the weather before harvest. These results indicate a high plasticity of the foliar metabolism of drought-tolerant sessile oak to current weather conditions. They also suggest that sessile oak is characterized by a high potential to cope with the growth conditions expected as a consequence of future climate change.

Due to climate change, sessile oak (Quercus petraea) seedlings experience an increasing risk of drought during regeneration of forest stands by management practices. The present study was aimed at elucidating the potential of sessile oak seedlings originating from sites with different aridity and nitrogen (N) supply to acclimate to contrasting water availability. For this purpose, a free-air cross-exchange experiment was conducted between a dry and a humid forest stand with high and low soil N contents, respectively, during two consecutive years differing in aridity before harvest. Almost all structural and physiological foliar traits analyzed did not differ consistently between seed origins during both years, when cultivated at the same site. As an exception, the arid provenance upregulated foliar ascorbate contents under drought, whereas the humid provenance accumulated the phenolic antioxidants vescalagin and castalagin (VC) under favorable weather conditions and consumed VC upon drought. Apparently, differences in long-term aridity at the forest sites resulted in only few genetically fixed differences in foliar traits between the provenances. However, structural and physiological traits strongly responded to soil N contents and weather conditions before harvest. Foliar N contents and their partitioning were mostly determined by the differences in soil N availability at the sites, but still were modulated by weather conditions before harvest. In the first year, differences in aridity before harvest resulted in differences between most foliar traits. In the second year, when weather conditions at both sites were considerably similar and more arid compared to the first year, differences in foliar traits were almost negligible. This pattern was observed irrespective of seed origin. These results support the view that leaves of sessile oak seedlings generally possess a high plasticity to cope with extreme differences in aridity by immediate acclimation responses that are even better developed in plants of arid origin.

Phytopathogenic fungi infections induce plant defence responses that mediate changes in metabolic and signalling processes with severe consequences for plant growth and development. Sphaeropsis tip blight, induced by the endophytic fungus Sphaeropsis sapinea that spreads from stem tissues to the needles, is the most widespread disease of conifer forests causing dramatic economic losses. However, metabolic consequences of this disease on bark and wood tissues of its host are largely unexplored. Here, we show that diseased host pines experience tissue dehydration in both bark and wood. Increased cytokinin and declined indole-3-acetic acid levels were observed in both tissues and increased jasmonic acid and abscisic acid levels exclusively in the wood. Increased lignin contents at the expense of holo-cellulose with declined structural biomass of the wood reflect cell wall fortification by S. sapinea infection. These changes are consistent with H2 O2 accumulation in the wood, required for lignin polymerization. Accumulation of H2 O2 was associated with more oxidized redox states of glutathione and ascorbate pools. These findings indicate that S. sapinea affects both phytohormone signalling and the antioxidative defence system in stem tissues of its pine host during the infection process.

European mistletoe (Viscum album L.) has largely infested Central European forests and causes high mortality probability particularly in dry years. However, little information is available about the consequences of mistletoe infestation for metabolic processes in bark and wood of its host, despite their important roles in infestation defense. We analyzed the tissue hydration, carbohydrate composition, phytohormone profile, reactive oxygen species and anti-oxidant levels in bark and wood of Scots pines (Pinus sylvestris L.), as dependent on mistletoe infestation. As a consequence of mistletoe infestation, host bark and wood showed impaired hydration and reduced total carbon content. In the bark, soluble sugar and lignin contents increased, apparently at the expense of holo-cellulose. Hydrogen peroxide accumulation was accompanied by increased glutathione and decreased reduced ascorbic acid levels. Mistletoe infestation mediated alteration of the phytohormone profile in bark and wood of its host. Cytokinins, jasmonic acid and abscisic acid levels increased in both tissues, whereas salicylic acid and indole-3-acetic acid, which were only detected in the bark, declined. The present results show that mistletoe infestation affects both the host's anti-oxidative defense system and the phytohormone profile after establishment of the xylem tapping haustorium. The significance of these processes for the development of the woody mistletoe stem and the haustorium is discussed.

The critical level for SO 2 susceptibility of Populus × canescens is approximately 1.2 μL L −1 SO 2 . Both sulfite oxidation and sulfite reduction and assimilation contribute to SO 2 detoxification. In the present study, uptake, susceptibility and metabolism of SO2 were analyzed in the deciduous tree species poplar (Populus × canescens). A particular focus was on the significance of sulfite oxidase (SO) for sulfite detoxification, as SO has been characterized as a safety valve for SO2 detoxification in herbaceous plants. For this purpose, poplar plants were exposed to different levels of SO2 (0.65, 0.8, 1.0, 1.2 μL L−1) and were characterized by visible injuries and at the physiological level. Gas exchange parameters (stomatal conductance for water vapor, CO2 assimilation, SO2 uptake) of the shoots were compared with metabolite levels (sulfate, thiols) and enzyme activities [SO, adenosine 5′-phosphosulfate reductase (APR)] in expanding leaves (80–90 % expanded). The critical dosage of SO2 that confers injury to the leaves was 1.2 μL L−1 SO2. The observed increase in sulfur containing compounds (sulfate and thiols) in the expanding leaves strongly correlated with total SO2 uptake of the plant shoot, whereas SO2 uptake rate was strongly correlated with stomatal conductance for water vapor. Furthermore, exposure to high concentration of SO2 revealed channeling of sulfite through assimilatory sulfate reduction that contributes in addition to SO-mediated sulfite oxidation to sulfite detoxification in expanding leaves of this woody plant species.

In the present study, the significance of sulfite oxidase (SO) for sulfite detoxification and sulfur assimilation was investigated. In response to sulfur dioxide (SO(2)) exposure, a remarkable expansion of sulfate and a significant increase of GSH pool were observed in wild-type and SO-overexpressing Arabidopsis. These metabolic changes were connected with a negative feedback inhibition of adenosine 5'-phosphosulfate reductase (APR), but no alterations in gas exchange parameters or visible symptoms of injury. However, Arabidopsis SO-KO mutants were consistently negatively affected upon 600 nL L(-1) SO(2) exposure for 60 h and showed phenotypical symptoms of injury with small necrotic spots on the leaves. The mean g(H2O) was reduced by about 60% over the fumigation period, accompanied by a reduction of net CO(2) assimilation and SO(2) uptake of about 50 and 35%. Moreover, sulfur metabolism was completely distorted. Whereas sulfate pool was kept constant, thiol-levels strongly increased. This demonstrates that SO should be the only protagonist for back-oxidizing and detoxification of sulfite. Based on these results, it is suggested that co-regulation of SO and APR controls sulfate assimilation pathway and stabilizes sulfite distribution into organic sulfur compounds. In conclusion, a sulfate-sulfite cycle driven by APR and SO can be postulated for fine-tuning of sulfur distribution that is additionally used for sulfite detoxification, when plants are exposed to atmospheric SO(2).

The poplar hybrid Populus x canescens (syn. Populus tremula x Populus alba) was subjected to salt stress by applying 75 mM NaCl for 2 weeks in hydroponic cultures. Decreasing maximum quantum yield (Fv/Fm) indicated damage of photosystem II (PS II), which was more pronounced under nitrate compared with ammonium nutrition. In vivo staining with diaminobenzidine showed no accumulation of H(2)O(2) in the leaf lamina; moreover, staining intensity even decreased. But at the leaf margins, development of necrotic tissue was associated with a strong accumulation of H(2)O(2). Glutathione (GSH) contents increased in response to NaCl stress in leaves but not in roots, the primary site of salt exposure. The increasing leaf GSH concentrations correlated with stress-induced decreases in transpiration and net CO(2) assimilation rates at light saturation. Enhanced rates of photorespiration could also be involved in preventing reactive oxygen species formation in chloroplasts and, thus, in protecting PS II from damage. Accumulation of Gly and Ser in leaves indeed indicates increasing rates of photorespiration. Since Ser and Gly are both immediate precursors of GSH that can limit GSH synthesis, it is concluded that the salt-induced accumulation of leaf GSH results from enhanced photorespiration and is thus probably restricted to the cytosol.

In a field study, the composition and concentrations of amino compounds in the xylem sap of the mistletoe, Viscum album L., and in the xylem sap of two host species, an evergreen conifer (Abies alba Mill.) and a deciduous broad-leaved tree (Populus x euramericana), were analyzed. The xylem sap of both hosts and mistletoe contained large, but similar amounts of total organic nitrogen in low molecular weight amino compounds (TONLW). Nevertheless, individual amino compounds accumulated in the xylem sap of mistletoe relative to the host xylem sap, indicating selective uptake. In the xylem sap of Populus, major amino compounds (asparagine (Asn) and glutamine (Gln)) and the bulk parameters, TONLW and proteinogenic amino acids, showed significant seasonal variation. In Abies and in mistletoe on either host, variation of amino compounds in xylem sap was largely explained by inter-annual differences, not by seasonal variation. In both hosts, TONLW in the xylem sap was dominated by Gln. There was a steady decrease in relative abundance of Gln from the host xylem sap to the mistletoe xylem sap and to the stems and leaves of mistletoe. Simultaneously, the abundance of arginine (Arg) increased. Arginine was the predominant amino compound in the stems and leaves of mistletoe, occurring at concentrations previously observed only in leaves of trees exposed to excess nitrogen. We conclude that Gln (2 mol N mol(-1)) delivered by the host xylem sap is converted, in mistletoe, to Arg (4 mol N mol(-1)) and that the organic carbon liberated from Gln contributes significantly to the parasite's heterotrophic carbon gain. Statistical analyses of the data support this conclusion. Accumulation of Arg in mistletoe is an indication of excess N supply as a result of the uptake of amino compounds from the host xylem sap and a lack of phloem uploading.

In the present field study we analysed the seasonal pattern of carbohydrate composition and contents in the xylem sap of Viscum album and the xylem sap of a deciduous (Populus x euramericana) and a coniferous (Abies alba) host tree species. The results were compared with the soluble carbohydrate composition and contents of mistletoe tissues. On both hosts significant amounts of glucose, fructose, and sucrose were found in the xylem sap of Viscum throughout the seasons. The general seasonal pattern of sugar contents, i.e. high concentrations in spring and lower concentrations in other seasons on Populus, and intermediate concentrations throughout the year on Abies, largely reflected the xylem sap carbohydrate composition and contents of the respective host. These observations provide indirect evidence for carbohydrate flux from the xylem sap of the host into the mistletoe. However, in both hosts xylem sap seems to be deviated into the mistletoe without specific control of carbohydrate flux. Differences observed between the seasonal pattern of xylem sap carbohydrate concentrations in Viscum on Populus and Abies may originate from the different time of leaf development of these species. A clear-cut seasonal pattern of soluble carbohydrates was not observed in the leaves of Viscum on both hosts. Still soluble carbohydrates seem to be reallocated from the senescing to the newly developed leaves of Viscum indicating that the seasonal requirement of carbohydrate for growth and development can only completely be met by carbohydrate acquisition from the host and their own photosynthesis.

In the present field study with adult trees inhabited by Viscum album, the question was addressed as to whether European mistletoes are able to remove reduced sulphur from the xylem sap of its hosts. For this purpose the reduced sulphur composition and content of the xylem sap of Viscum album and the corresponding hosts Populus x euramericana and Abies alba were analysed. The xylem sap of Viscum was enriched in reduced sulphur compared to the hosts but still reflected the higher reduced sulphur content of Populus compared to Abies. Despite similar xylem sap composition of the hosts with glutathione as the dominating thiol, Viscum on Populus contained predominantly cysteine, Viscum on Abies predominantly glutathione in its xylem sap. These findings suggest selective and different removal of reduced sulphur from these hosts. Still the amount of reduced sulphur removed was too small to result in changes of the concentration of thiols in the xylem sap of the hosts that are statistically significant, probably due to the high variability encountered under field conditions. Despite the differences in the reduced sulphur composition and contents of the xylem sap between Viscum on Populus and Viscum on Abies, total thiol content as well as thiol composition of Viscum leaves on the two hosts were similar throughout the seasons. The seasonal pattern in the thiol composition and contents of Viscum leaves showed high levels in spring and autumn and low levels in summer. The significance of these seasonal changes is discussed.

Wild-type hybrid poplar (Populus tremula × P. alba) plants and transgenic lines over-expressing glutathione reductase (GR) either in the cytosol (ca. 5-fold) or in the chloroplast (150- to 200-fold) or glutathione synthetase (GSS) in the cytosol (ca. 200-fold) were exposed for 3 days to ambient air or air containing 300 nl l–1 ozone for 7 h day–1. The contents and oxidation state of antioxidants (glutathione; ascorbate), the in vitro activities of target enzymes (GR, GSS), as well as the in vitro activities of ascorbate consuming (ascorbate peroxidase, APX) and regenerating (monodehydroascorbate reductase, MDAR; dehydroascorbate reductase, DHAR) enzymes were determined in the developing 2nd and the mature 5th leaves from the apex of the plants. The changes in the activities of the target enzymes GR and GSS also affected other components of the leaves' antioxidative system. Over-expression of GR in the chloroplast enhanced total glutathione contents of the leaves 2- to 3-fold. Foliar GSSG levels were enhanced in all transgenic poplar lines compared to wild-type plants. In developing leaves of transgenic plants, over-expressing GR in the cytosol or chloroplasts, APX and DHAR activities were higher, in mature leaves similar or lower than in wild-type plants. These differences between poplar lines did not mediate different sensitivities of the leaves to acute ozone exposure (Strohm et al., J Exp Bot 50:365–374). Still, components of the antioxidative system of the leaves showed both similar or specific reactions to acute ozone exposure in different poplar lines. Irrespective of leaf age total glutathione contents increased 2- to 3-fold in ozone-exposed leaves of all poplar lines. Also GSSG contents of the leaves increased, with the exception of plants over-expressing GR in the chloroplasts. Still a highly reduced state of glutathione was maintained in wild-type (>90%) and transgenic lines (ca. 80%) irrespective of ozone exposure. Apparently, GR activity was sufficient in all lines to compensate for ozone-mediated glutathione oxidation. Foliar GR activity increased up to 100% as a consequence of ozone treatment in wild-type and transgenic plants over-expressing GR in the cytosol, but not in plants over-expressing GR in the chloroplasts. The combined increase in total glutathione contents and GR activity in response to acute ozone exposure did not prevent injury and, therefore, cannot be considered a useful mechanism of defence, but rather an indicator of ozone-mediated oxidative stress. Total ascorbate contents and the activities of ascorbate consuming (APX) or regenerating enzymes (MDAR, DHAR) were not significantly affected by acute ozone exposure in all poplar lines. Ozone exposure strongly oxidised the foliar ascorbate pool except in leaves over-expressing GR in the chloroplast. Apparently, these leaves possess an enhanced capacity for ascorbate regeneration as a consequence of the strongly enhanced chloroplast GR activity and the simultaneously increased APX activity and glutathione levels. The enhanced antioxidative capacity did not prevent ozone-mediated injury to the leaves, most likely due to different sites of primary ozone reactions. Despite higher total ascorbate contents and higher APX activities in developing than in mature leaves, these and other components of the antioxidative system analysed in this study were not responsible for the higher sensitivity to acute ozone exposure of mature compared to developing poplar leaves.

Global climate change causes an increase in ambient air temperature, a major environmental factor influencing plant physiol-ogy and growth that already has been perceived at the regional scale and is expected to become even more severe in the future. In the present study, we investigated the effect of elevated ambient air temperature on the nitrogen metabolism of two interior provenances of Douglas fir (Pseudotsuga menziesii var. glauca) originating from contrasting habitats, namely the provenances Monte Creek (MC) from a drier environment and Pend Oreille (PO) from a more humid environment. Three- to four-year-old seedlings of the two provenances were grown for 3 months in controlled environments under either control temperature (day 20 °C, night 15 °C) or high temperature (HT, 30/25 °C) conditions. Total nitrogen (N), soluble protein, chlo-rophyll and total amino acid (TAA) contents as well as individual amino acid concentrations were determined in both current-year and previous-year needles. Our results show that the foliar total N contents of the two provenances were unaffected by HT. Arginine, lysine, proline, glutamate and glutamine were the most abundant amino acids, which together contributed 88% to the TAA pool of current- and previous-year needles. High temperature decreased the contents of most amino acids of the glutamate family (i.e., arginine, proline, ornithine and glutamine) in current-year needles. However, HT did not affect the concentrations of metabolites related to the photorespiratory pathway, such as NH4+, glycine and serine. In general, current-year needles were considerably more sensitive to HT than previous-year needles. Moreover, provenance PO originating from a mesic environment showed stronger responses to HT than provenance MC. Our results indicate provenance-specific plasticity in the response of Douglas fir to growth temperature. Provenance-specific effects of elevated temperature on N-use efficiency suggest that origin might determine the sensitivity and growth potential of Douglas fir trees in a future warmer climate. Global climate change causes an increase in ambient air temperature, a major environmental factor influencing plant physiology and growth that already has been perceived at the regional scale and is expected to become even more severe in the future. In the present study, we investigated the effect of elevated ambient air temperature on the nitrogen metabolism of two interior provenances of Douglas fir (Pseudotsuga menziesii var. glauca) originating from contrasting habitats, namely the provenances Monte Creek (MC) from a drier environment and Pend Oreille (PO) from a more humid environment. Three- to four-year-old seedlings of the two provenances were grown for 3 months in controlled environments under either control temperature (day 20 °C, night 15 °C) or high temperature (HT, 30/25 °C) conditions. Total nitrogen (N), soluble protein, chlorophyll and total amino acid (TAA) contents as well as individual amino acid concentrations were determined in both current-year and previous-year needles. Our results show that the foliar total N contents of the two provenances were unaffected by HT. Arginine, lysine, proline, glutamate and glutamine were the most abundant amino acids, which together contributed ∼88% to the TAA pool of current- and previous-year needles. High temperature decreased the contents of most amino acids of the glutamate family (i.e., arginine, proline, ornithine and glutamine) in current-year needles. However, HT did not affect the concentrations of metabolites related to the photorespiratory pathway, such as NH+4, glycine and serine. In general, current-year needles were considerably more sensitive to HT than previous-year needles. Moreover, provenance PO originating from a mesic environment showed stronger responses to HT than provenance MC. Our results indicate provenance-specific plasticity in the response of Douglas fir to growth temperature. Provenance-specific effects of elevated temperature on N-use efficiency suggest that origin might determine the sensitivity and growth potential of Douglas fir trees in a future warmer climate.

summary Elevated [CO2] has been shown to protect photosynthesis and growth of wheat against moderately elevated [O3]. To investigate the role of ozone exclusion and detoxification in this protection, spring wheat (Triticum aestivum L. ev. Wembley) was grown from seed, in controlled-environment chambers, under reciprocal combinations of [CO2] at 350 or 700 μmol mol−1and [O3] peaking at < 5 or 60 nmol mol−1, respectively. Cumulative ozone dose to the mesophyll and antioxidant status were determined throughout flag leaf development. Catalase activity correlated with rates of photorespiration and declined in response to elevated [CO2] and/or [O3]. Superoxide dismutase activity was not significantly affected by either condition. Neither ascorbate nor glutathione content was enhanced by elevated [CO2]. In wheat, at moderately elevated [O3], our results show that stomatal exclusion plays a major role in the protective effect of elevated [CO2] against O3 damage.

summary Needles from young and mature Scots pine trees (Pinus sylvestirs) were collected at three field sites in eastern Germany, i.e. Menz, Taura, and Gossa, which differed in SO2 pollution. Current and previous year's needles from the polluted sites Taura and Gossa (yearly mean af SO2: 26 ppb) contained approximately 100-120 μmol sulphate g −1 d. wt, whereas needles tram the unpolluted site, Menz (yearly mean of SO2: 3 ppb) contained 30-65 μmol sulphate g−1 d. wt. The apoplastic sulphate content was in the order of o.3-1% of total sulphate and corresponded to concentrations of 0.6-1.2 mM in needles from Mtnz and to 1.5–2 mn in needles from Gossa. In needles from the polluted sites the contents of glutathione, cysteine, γ glutamyleysteine, and soluble protein were higher than in needles from Menz. Despite the clear increases in sulphur-containing compounds, pigment content and activities of superoxide dismutase, total peroxidase and apoplastic peroxidase (on the basis of d. wt) were not significantly different. When enzymatic activities were related to protein, lowest activities were found in needles from the site with the highest SO2-pollution. The results show that it is not possible to use the activities of peroxidases or superoxide dismutases as biomarkers for SO2-mediated stress in pine needles. It remains open whether the needles from the polluted sites suffered from greater oxidative stress than the needles from the less polluted site.

Drought is a major environmental stress affecting growth and vitality of forest ecosystems. In the present study, foliar nitrogen (N) and carbon (C) metabolism of two Douglas fir (Pseudotsuga menziesii) provenances with assumed different drought tolerance were investigated. We worked with 1-year-old seedlings of the interior provenance Fehr Lake (FEHR) originating from a dry environment and the coastal provenance Snoqualmie (SNO) from a more humid origin. Total C and N, structural N and the concentrations of soluble protein, total amino acids (TAAs) and individual amino acids as well as the relative abundance of polar, low-molecular-weight metabolites including antioxidants were determined in current-year needles exposed either to 42 days of drought or to 42 days drought plus 14 days of rewatering. The seedlings reacted in a provenance-specific manner to drought stress. Coastal provenance SNO showed considerably increased contents of TAAs, which were caused by increased abundance of the quantitatively most important amino acids arginine, ornithine and lysine. Additionally, the polyamine putrescine accumulated exclusively in drought-stressed trees of this provenance. In contrast, the interior provenance FEHR showed the opposite response, i.e., drastically reduced concentrations of these amino acids. However, FEHR showed considerably increased contents of pyruvate-derived and aromatic amino acids, and also higher drought-induced levels of the antioxidants ascorbate and α-tocopherol. In response to drought, both provenances produced large amounts of carbohydrates, such as glucose and fructose, most likely as osmolytes that can readily be metabolized for protection against osmotic stress. We conclude that FEHR and SNO cope with drought stress in a provenance-specific manner: the coastal provenance SNO was mainly synthesizing N-based osmolytes, a reaction not observed in the interior provenance FEHR; instead, the latter increased the levels of scavengers of reactive oxygen species. Our results underline the importance of provenance-specific reactions to abiotic stress.

The flux of inorganic and organic nitrogen into the mistletoe Viscum album L. from the xylem sap of a deciduous (Populus x euamericana) and a coniferous host (Abies alba Mill.) was analyzed. For this purpose, a perfusion system was developed in which the xylem sap of the host was replaced by an artificial perfusion solution. With this system flux rates into the mistletoe were determined in feeding experiments either with the organic nitrogen source [1,2-13C2]glutamine at high and the inorganic nitrogen source 15NO3- at low concentration or vice versa. Glutamine influx was already saturated at the low concentration in the xylem sap and was--different from nitrate--not enhanced, when a 250-fold higher concentration was applied. Nitrate influx matched glutamine influx only at high inorganic/organic nitrogen ratios in the perfusion solution. This result indicates a preferential influx of glutamine over nitrate from the host xylem into the mistletoe at the concentrations found in the xylem sap of trees. Surprisingly, a high percentage of both N sources were accumulated in the mistletoe stem, indicating excessive N nutrition of the mistletoe leaves. Since 13C isotope signature was significantly reduced in the outflowing perfusion solution, either an upload of organic compounds from the phloem into the xylem, or an efflux of organic compounds from haustorium of mistletoe into the xylem has to be assumed. 15N isotope signatures enriched in the outflowing perfusion solution support the idea of a nitrate uptake system at the host xylem-haustorium interface, which favors the light N isotope of nitrate.

Mixed spruce-beech plantations grown in large open-top chambers (OTC) were used to study consequences of elevated CO2, nitrogen-deposition and soil type on plant internal nitrogen and sulphur cycling of juvenile beech (Fagus sylvatica L.) and spruce (Picea abies Karst.) in a competitive situation. Processes of re-cycling as a consequence of protein turnover during leaf senescence in autumn were of further interest. For this purpose, phloem sap was collected in September 1998 and analysed for the composition and concentrations of organic and inorganic nitrogen and sulphur compounds. The phloem exudate of spruce showed higher total soluble non-protein nitrogen (TSNN) concentration on calcareous soil than on acidic soil, independent of the treatment. N-fertilization increased the N-concentration of phloem exudate significantly on both soil types, mainly by an increase of Arg and Gln concentrations. Elevated CO2 slightly increased TSNN on calcareous, but not on acidic soil. The combination of elevated CO2 and augmented N-deposition induced a further increase of TSNN on calcareous soil, but caused a lower N-effect on TSNN on acidic soil. Arg, the main TSNN component in phloem exudate, mediated this effect. Since Arg is considered to be a major nitrogen storage compound, it is concluded that in autumn elevated CO2 and augmented N-deposition, influence storage of N rather than N-supply of spruce. An effect of elevated CO2 and augmented N-deposition on GSH and sulphate concentrations in phloem exudate of spruce was not observed on acidic soil. On calcareous soil augmented N-deposition enhanced, elevated CO2 decreased phloem exudate GSH contents. In combination, elevated CO2 compensated the positive effect of N-deposition. The effects of elevated CO2 and augmented N-deposition on phloem sap N- and S-contents described above were not observed for beech trees. Apparently, elevated CO2 and augmented N-deposition did not affect plants internal S and N cycling of beech grown in spruce-beech plantations.

To study the role of low UV-B radiation in modulating the response of antioxidants to ozone, 4-year-old pine (Pinus sylvestris L.) and spruce (Picea abies L.) seedlings potted in natural soil, were exposed in phytochambers to fluctuating ozone concentrations between 9 and 113 nl I -1 according to field data recorded at Mt Wank (1175 m above sea level, Bavaria, Germany) and two-times ambient O 3 levels. UV-B radiation was either added at a biologically effective level of ca 1.2 kJ m -2 day -1 , which is close to that found in March at Mt Wank, or was excluded by filters (