Search

In their natural environment along coast lines, date palms are exposed to seawater inundation and, hence, combined stress by salinity and flooding. To elucidate the consequences of this combined stress on foliar gas exchange and metabolite abundances in leaves and roots, date palm seedlings were exposed to flooding with seawater and its major constituents under controlled conditions. Seawater flooding significantly reduced CO 2 assimilation, transpiration and stomatal conductance, but did not affect isoprene emission. A similar effect was observed upon NaCl exposure. By contrast, flooding with distilled water or MgSO 4 did not affect CO 2 /H 2 O gas exchange or stomatal conductance significantly, indicating that neither flooding itself, nor seawater sulfate, contributed greatly to stomatal closure. Seawater exposure increased Na and Cl contents in leaves and roots, but did not affect sulfate contents significantly. Metabolite analyses revealed reduced abundances of foliar compatible solutes, such as sugars and sugar alcohols, whereas nitrogen compounds accumulated in roots. Reduced transpiration upon seawater exposure may contribute to controlling the movement of toxic ions to leaves and, therefore, can be seen as a mechanism to cope with salinity. The present results indicate that date palm seedlings are tolerant towards seawater exposure to some extent, and highly tolerant to flooding., (© 2020 The Authors New Phytologist © 2020 New Phytologist Foundation.)

We studied acclimation of leaf gas exchange to differing seasonal climate and soil water availability in slow-growing date palm (Phoenix dactylifera) seedlings. We used an extended Arrhenius equation to describe instantaneous temperature responses of leaf net photosynthesis (A) and stomatal conductance (G), and derived physiological parameters suitable for characterization of acclimation (T opt , A opt and T equ ). Optimum temperature of A (T opt ) ranged between 20-33°C in winter and 28-45°C in summer. Growth temperature (T growth ) explained c. 50% of the variation in T opt , which additionally depended on leaf water status at the time of measurement. During water stress, light-saturated rates of A at T opt (i.e. A opt ) were reduced to 30-80% of control levels, albeit not limited by CO 2 supply per se. Equilibrium temperature (T equ ), around which A/G and substomatal [CO 2 ] are constant, remained tightly coupled with T opt . Our results suggest that acclimatory shifts in T opt and A opt reflect a balance between maximization of photosynthesis and minimization of the risk of metabolic perturbations caused by imbalances in cellular [CO 2 ]. This novel perspective on acclimation of leaf gas exchange is compatible with optimization theory, and might help to elucidate other acclimation and growth strategies in species adapted to differing climates., (© 2019 The Authors. New Phytologist © 2019 New Phytologist Trust.)

The present study was performed to elucidate the fate of carbon (C) and nitrogen (N) derived from protein of prey caught by carnivorous Dionaea muscipula. For this, traps were fed 13 C/ 15 N-glutamine (Gln). The release of 13 CO 2 was continuously monitored by isotope ratio infrared spectrometry. After 46 h, the allocation of C and N label into different organs was determined and tissues were subjected to metabolome, proteome and transcriptome analyses. Nitrogen of Gln fed was already separated from its C skeleton in the decomposing fluid secreted by the traps. Most of the Gln-C and Gln-N recovered inside plants were localized in fed traps. Among nonfed organs, traps were a stronger sink for Gln-C compared to Gln-N, and roots were a stronger sink for Gln-N compared to Gln-C. A significant amount of the Gln-C was respired as indicated by 13 C-CO 2 emission, enhanced levels of metabolites of respiratory Gln degradation and increased abundance of proteins of respiratory processes. Transcription analyses revealed constitutive expression of enzymes involved in Gln metabolism in traps. It appears that prey not only provides building blocks of cellular constituents of carnivorous Dionaea muscipula, but also is used for energy generation by respiratory amino acid degradation., (© 2017 The Authors. New Phytologist © 2017 New Phytologist Trust.)

Carnivorous Dionaea muscipula operates active snap traps for nutrient acquisition from prey; so what is the role of D. muscipula's reduced root system? We studied the capacity for nitrogen (N) acquisition via traps, and its effect on plant allometry; the capacity of roots to absorb NO₃(-), NH₄(+) and glutamine from the soil solution; and the fate and interaction of foliar- and root-acquired N. Feeding D. muscipula snap traps with insects had little effect on the root : shoot ratio, but promoted petiole relative to trap growth. Large amounts of NH₄(+) and glutamine were absorbed upon root feeding. The high capacity for root N uptake was maintained upon feeding traps with glutamine. High root acquisition of NH₄(+) was mediated by 2.5-fold higher expression of the NH₄(+) transporter DmAMT1 in the roots compared with the traps. Electrophysiological studies confirmed a high constitutive capacity for NH₄(+) uptake by roots. Glutamine feeding of traps inhibited the influx of (15)N from root-absorbed (15)N/(13)C-glutamine into these traps, but not that of (13)C. Apparently, fed traps turned into carbon sinks that even acquired organic carbon from roots. N acquisition at the whole-plant level is fundamentally different in D. muscipula compared with noncarnivorous species, where foliar N influx down-regulates N uptake by roots., (© 2014 The Authors. New Phytologist © 2014 New Phytologist Trust.)

Overexpression of bacterial γ-glutamylcysteine synthetase in the cytosol of Populus tremula × P. alba produces higher glutathione (GSH) concentrations in leaves, thereby indicating the potential for cadmium (Cd) phytoremediation. However, the net Cd(2+) influx in association with H(+) /Ca(2+) , Cd tolerance, and the underlying molecular and physiological mechanisms are uncharacterized in these poplars. We assessed net Cd(2+) influx, Cd tolerance and the transcriptional regulation of several genes involved in Cd(2+) transport and detoxification in wild-type and transgenic poplars. Poplars exhibited highest net Cd(2+) influxes into roots at pH 5.5 and 0.1 mM Ca(2+) . Transgenics had higher Cd(2+) uptake rates and elevated transcript levels of several genes involved in Cd(2+) transport and detoxification compared with wild-type poplars. Transgenics exhibited greater Cd accumulation in the aerial parts than wild-type plants in response to Cd(2+) exposure. Moreover, transgenic poplars had lower concentrations of O2 ˙(-) and H2 O2 ; higher concentrations of total thiols, GSH and oxidized GSH in roots and/or leaves; and stimulated foliar GSH reductase activity compared with wild-type plants. These results indicate that transgenics are more tolerant of 100 μM Cd(2+) than wild-type plants, probably due to the GSH-mediated induction of the transcription of genes involved in Cd(2+) transport and detoxification., (© 2014 The Authors. New Phytologist © 2014 New Phytologist Trust.)

High concentrations of sulfur dioxide (SO(2) ) as an air pollutant, and its derivative sulfite, cause abiotic stress that can lead to cell death. It is currently unknown to what extent plant fumigation triggers specific transcriptional responses. To address this question, and to test the hypothesis that sulfite oxidase (SO) is acting in SO(2) detoxification, we compared Arabidopsis wildtype (WT) and SO knockout lines (SO-KO) facing the impact of 600 nl l(-1) SO(2) , using RNAseq to quantify absolute transcript abundances. These transcriptome data were correlated to sulfur metabolism-related enzyme activities and metabolites obtained from identical samples in a previous study. SO-KO plants exhibited remarkable and broad regulative responses at the mRNA level, especially in transcripts related to sulfur metabolism enzymes, but also in those related to stress response and senescence. Focusing on SO regulation, no alterations were detectable in the WT, whereas in SO-KO plants we found up-regulation of two splice variants of the SO gene, although this gene is not functional in this line. Our data provide evidence for the highly specific coregulation between SO and sulfur-related enzymes like APS reductase, and suggest two novel candidates for involvement in SO(2) detoxification: an apoplastic peroxidase, and defensins as putative cysteine mass storages., (© 2012 The Authors. New Phytologist © 2012 New Phytologist Trust.)

• Increasing atmospheric concentrations of phytotoxic ozone (O(3) ) can constrain growth and carbon sink strength of forest trees, potentially exacerbating global radiative forcing. Despite progress in the conceptual understanding of the impact of O(3) on plants, it is still difficult to detect response patterns at the leaf level. • Here, we employed principal component analysis (PCA) to analyse a database containing physiological leaf-level parameters of 60-yr-old Fagus sylvatica (European beech) trees. Data were collected over two climatically contrasting years under ambient and twice-ambient O(3) regimes in a free-air forest environment. • The first principal component (PC1) of the PCA was consistently responsive to O(3) and crown position within the trees over both years. Only a few of the original parameters showed an O(3) effect. PC1 was related to parameters indicative of oxidative stress signalling and changes in carbohydrate metabolism. PC1 correlated with cumulative O(3) uptake over preceding days. • PC1 represents an O(3) -responsive multivariate pattern detectable in the absence of consistently measurable O(3) effects on individual leaf-level parameters. An underlying effect of O(3) on physiological processes is indicated, providing experimental confirmation of theoretical O(3) response patterns suggested previously., (© 2012 The Authors. New Phytologist © 2012 New Phytologist Trust.)

• Salinity causes osmotic stress and limits biomass production of plants. The goal of this study was to investigate mechanisms underlying hydraulic adaptation to salinity. • Anatomical, ecophysiological and transcriptional responses to salinity were investigated in the xylem of a salt-sensitive (Populus × canescens) and a salt-tolerant species (Populus euphratica). • Moderate salt stress, which suppressed but did not abolish photosynthesis and radial growth in P. × canescens, resulted in hydraulic adaptation by increased vessel frequencies and decreased vessel lumina. Transcript abundances of a suite of genes (FLA, COB-like, BAM, XET, etc.) previously shown to be activated during tension wood formation, were collectively suppressed in developing xylem, whereas those for stress and defense-related genes increased. A subset of cell wall-related genes was also suppressed in salt-exposed P. euphratica, although this species largely excluded sodium and showed no anatomical alterations. Salt exposure influenced cell wall composition involving increases in the lignin : carbohydrate ratio in both species. • In conclusion, hydraulic stress adaptation involves cell wall modifications reciprocal to tension wood formation that result in the formation of a novel type of reaction wood in upright stems named 'pressure wood'. Our data suggest that transcriptional co-regulation of a core set of genes determines reaction wood composition., (© 2011 The Authors. New Phytologist © 2011 New Phytologist Trust.)

Temperature crucially affects the speed of metabolic processes in poikilotherm organisms, including plants. The instantaneous temperature responses of O(2)-reduction and CO(2)-release can be approximated by Arrhenius kinetics, even though respiratory gas exchange of plants is the net effect of many constituent biochemical processes. Nonetheless, the classical Arrhenius equation must be modified to account for a dynamic response to measurement temperatures. We show that this dynamic response is readily explained by combining Arrhenius and Michaelis-Menten kinetics, as part of a fresh appraisal of metabolic interpretations of instantaneous temperature responses. In combination with recent experimental findings, we argue that control of mitochondrial electron flow is shared among cytochrome oxidase and alternative oxidase under in vivo conditions, and is continuously coordinated. In this way, upstream carbohydrate metabolism and downstream electron transport appear to be optimized according to the demand of ATP, TCA-cycle intermediates and anabolic reducing power under differing metabolic states. We provide a link to the 'Growth and Maintenance Paradigm' of respiration and argue that respiratory temperature responses can be used as a tool to probe metabolic states of plant tissue, such that we can learn more about the mechanisms that govern longer-term acclimatization responses of plant metabolism., (© 2011 The Authors. New Phytologist © 2011 New Phytologist Trust.)

Based on computational prediction of RNA secondary structures, a long antisense RNA (asRNA) was found in chloroplasts of Arabidopsis, Nicotiana tabacum and poplar, which occurs in two to three major transcripts. Mapping of primary 5' ends, northern hybridizations and quantitative real-time reverse transcription polymerase chain reaction (qPCR) experiments demonstrated that these transcripts originate from a promoter that is typical for the plastid-encoded RNA polymerase and are over their full length in antisense orientation to the gene ndhB and therefore were designated asRNA_ndhB. The asRNA_ndhB transcripts predominantly accumulate in young leaves and at physiological growth temperatures. Two nucleotide positions in the mRNA that are subject to C-to-U RNA editing and which were previously found to be sensitive to elevated temperatures are covered by asRNA_ndhB. Nevertheless, the correlation between the accumulation of asRNA_ndhB and RNA editing appeared weak in a temperature shift experiment. With asRNA_ndhB, we describe the first asRNA of plant chloroplasts that covers RNA editing sites, as well as a group II intron splice acceptor site, and that is under developmental control, raising the possibility that long asRNAs could be involved in RNA maturation or the control of RNA stability.

Amino acid concentration and composition in xylem and phloem sap and in plant tissues are good markers of plant performance and general plant nitrogen (N)-supply. Here, we tested if amino acid pools in Eucalyptus regnans, growing in southeastern Australia were increased by understory acacias in 70-yr-old stands, and if xylem N-transport of temperate Acacia spp. differs from their tropical counterparts. We analysed amino-N concentrations and composition in foliage, xylem and phloem. In a novel approach we coupled amino-N concentrations of xylem with long-term sap flow measurements to calculate total stand N-transport. Xylem N-transport of E. regnans is largely based on amino compounds of the glutamate group (more than 90%). By contrast, Acacia spp. transport mainly aspartate group amino acids in xylem (up to 80%). Amino compound diversity and concentration in tissues and xylem and phloem sap were universally greater in acacias compared to eucalypts. Acacias investigated here can be classified as 'amide transporters'. We conclude that N-status and growth potential of aging E. regnans forest is not enhanced by a contribution of N from understory acacias, and that xylem N-transport in temperate Acacia spp. differs from acacias located in the tropics.

External salinization can affect different steps of nitrogen (N) metabolism (ion uptake, N assimilation, and amino acid and protein synthesis) depending on the inorganic N source. Here, we assessed the net uptake of N supplied as nitrate or ammonium and N assimilation (combining metabolite analyses with molecular biological approaches) in grey poplar (Populus x canescens) plants grown under saline (75 mM NaCl) and control conditions. The specific (micromol N g(-1) dry weight fine roots h(-1)) and total plant (micromol N per plant h(-1)) N net uptake rates, total plant N content, total plant biomass and total leaf protein concentration were reduced under saline conditions when plants were supplied with ammonium. In both nutritional groups, salt treatment caused pronounced accumulation of soluble N compounds in the leaves. The mRNAs of genes coding for enzymes catalyzing rate-limiting steps of both proline synthesis and degradation (delta-1-pyrroline-5-carboxylate synthase and proline dehydrogenase) as well as for NADH-dependent glutamate synthase were accumulated under saline conditions. Whereas under control conditions the plant N status seemed to be superior when ammonium was supplied, the N balance of ammonium-fed plants was more severely affected by salt stress than that of plants supplied with nitrate. Possible metabolic implications of stress-related accumulation of particular amino acids are discussed.

To test the effects of calcium on wood formation, Populus tremula x Populus tremuloides clones were supplied with Hoagland solution modified in its calcium contents. Energy-dispersive X-ray analysis (EDXA) revealed an increase in calcium in the phloem, the cambium and the xylem elongation zone with increasing Ca(2+) supply in the nutrient solution. Using light and electron microscopy, a strong impact was shown on the cambial and the elongation zones under calcium starvation. Using Fourier transform infrared (FTIR) spectroscopy on wood and bark cells formed under calcium starvation, we detected a reduction of some absorptions, such as carbonyl and methoxy groups from S-lignin. Also, a significant reduction in fiber length was detected with decreasing calcium supply in the nutrient solution. High-performance liquid chromatography (HPLC) analysis revealed a large increase in sugar concentrations in the leaves, but reduced concentrations in the bark under Ca(2+) deficiency. In conclusion, our results show a significant influence of calcium on the structure, chemistry and physiology of wood formation. Thus, efficient Ca(2+) supply has to be considered a decisive factor in wood formation.

Transition metals such as zinc (Zn) are essential micronutrients for many physiological processes, but they become toxic at elevated levels. Zinc is one of the most abundant trace heavy metals present in agro-ecosystems. Populus spp. have been suggested as good candidates for using to study the removal and/or immobilization of environmental organic and inorganic pollutants. In order to understand the physiological and biochemical bases of this assumption for Zn, plants of Populus deltoides x P. nigra (P. x euramericana) were grown in hydroponics with different concentrations of Zn [1 microm (control), and 1, 5 and 10 mm] in the nutrient solution.Shoot biomass decreased at 5 and 10 mm Zn, while the Zn content of young leaves increased progressively with increasing Zn concentration (1-10 mm). Total glutathione (GSH+GSSG) content was reduced with increasing Zn concentration, while the contribution of oxidized to total glutathione increased. Despite these observations, semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) revealed that the gene expression of GSH reductase (GR, chloroplastic and cytosolic isoform) and gamma-glutamylcysteine synthetase (gamma-ECS) increased in young leaves of poplars treated with excess Zn. We conclude that GSH synthesis, consumption and redox status play a central role in the response of poplars to high concentrations of Zn.

The potential negative influence of competition from early successional species like blackberry (Rubus fruticosus) may be decisive for the natural regeneration success of drought-sensitive beech (Fagus sylvatica), especially in the light of climate change. With a split plot glasshouse experiment, we investigated the influence of two air temperature and irradiance levels on the competitive interference of blackberry on the water, nitrogen (N) and carbon (C) balance of beech seedlings under moderate drought. When increased temperature was accompanied by low irradiance the biomass, root-to-shoot ratio, N uptake and assimilation rates of blackberry were lower compared with beech, either grown alone or with blackberry. By contrast, when elevated temperature and high irradiance were combined, the root-to-shoot ratio and specific N uptake rate of blackberry were substantially increased, while the N acquisition of beech was impaired. Under lower temperature, with either full light or shade, the presence of blackberry had no significant effects on beech, for almost all tested parameters. Under elevated air temperature beech was impaired by the presence of blackberry at high irradiance. These findings emphasize the interacting effects between environmental factors and competition on the establishment of beech regeneration, which should be considered for future forest management in the frame of climate change.

•  The spectrum and diurnal course of carbonyl exchange of mature Norway spruce (Picea abies) was analysed in a temperate forest and under controlled conditions. In parallel, plant physiological and meteorological parameters were determined. •  Spruce emitted acetaldehyde, formaldehyde and acetone under field and laboratory conditions. •  Carbonyl emissions were highest at midday, for acetaldehyde amounting up to 100 nmol m -2  min -1 . During darkness uptake was also observed. Fumigation of spruce seedlings with acetaldehyde indicated a compensation point of c. 6 ppb. The exchange rates were strongly correlated with temperature and mass flow of ethanol in the xylem sap. The studies further indicated that the height of a twig on the tree affects its carbonyl emission rates. •  The present findings support the view that acetaldehyde emission by spruce is related to mass flow of ethanol in the xylem sap, as previously shown for tree seedlings under controlled conditions. The basis of formaldehyde and acetone emissions by spruce is still not clear and remains to be studied in further experiments.

•  Here, the effects were assessed of local climate and canopy thinning on the carbon isotope composition (δ 13 C) signatures of different plant groups in the understorey of a beech (Fagus sylvatica) forest in southern Germany. The relationship between δ 13 C and environmental parameters, within different time integrals, was also studied. •  δ 13 C was analysed in different tissues of beech regeneration, and herbaceous and woody understorey vegetation, in thinned and untreated control stands differing in aspect and, hence, local climate, on three dates during the growing season. •  Generally, tissues were 13 C-depleted on the north-east, compared with the south-west aspect. Thinning had variable effects on δ 13 C, depending on plant group and sampling date. δ 13 C in beech leaves and roots in control stands was mostly influenced by mean soil water potential in 4- and 8-wk integrals before sampling, respectively, and in leaves, additionally, by mean radiation in a 4-wk time integral. Shoot water potential and transpiration influenced foliar δ 13 C of beech whereas δ 13 C in the beech wood was modulated by soil temperature integrated over a 4-wk period before sampling. Above-ground tissues of woody and herbaceous plants were 13 C-enriched in mid-summer; their δ 13 C was poorly related to environmental factors. •  δ 13 C of various tissues of beech regeneration appears to be indicative of recent environmental conditions within the forest understorey and, consequently, this easy-to-determine physiological parameter could be used widely to assess effects of silvicultural treatments.

•  The effects of N-availability and elevated atmospheric CO 2 partial pressure ( pCO 2 ) on growth, allometry and N-metabolism of poplar plants are reported here. •  Poplar plants were grown hydroponically at deficient and sufficient N-supply under ambient and elevated pCO 2 . The N-fluxes within the plants were estimated by comparing the fate of newly acquired 15 N-NO 3 - in plants either severely N-limited or with sufficient N-supply. •  At deficient N-supply, plants accumulated less biomass and exhibited an increased root : shoot ratio compared with sufficient N-supply; a larger fraction of newly acquired 15 N was allocated to the youngest leaves immediately after exchange of the nutrient solution. Increasing the external N-supply from deficient to sufficient shifted the site of nitrate reduction from roots to leaves. •  Elevated pCO 2 increased total biomass and the root : shoot ratio at deficient N-supply, but had no effect at sufficient N-supply. Elevated pCO 2 decreased rates of N-uptake in both treatments. Increased root : shoot ratio at deficient N-supply coincided with enhanced nitrate reduction in the root and elevated pCO 2 also enhanced the allocation of newly acquired 15 N to the youngest leaves. Root nitrate reduction as a possible factor controlling the root : shoot ratio and N-allocation is discussed.

•  In the present study NH 3 and NO 2 exchange between the atmosphere and needles of adult spruce ( Picea abies ) trees at a field site ('Höglwald') exposed to high loads of atmospheric nitrogen was assessed. •  Twigs were fumigated with different NH 3 (C NH3 ) or NO 2 (C NO2 ) concentrations using the dynamic chamber technique. Beside fluxes of NH 3 (J NH3 ) and NO 2 (J NO2 ), transpiration (J H2O ), leaf conductance for water vapour (g H2O ), photosynthetic activity (J co2 ), photosynthetic photon fluence rate (PPFR), air temperature (T) and relative air humidity (RH) were determined. •  Both fluxes, J NH3 and J NO2 , depended linearly on C NH3 and C NO2 in concentration ranges representative for the field site and g H2O as a measure of stomatal aperture. For both trace gases compensation points could be determined amounting to 2.5 nmol mol -1 for NH 3 and to 1.7 nmol mol -1 for NO 2 . •  The fluxes of NH 3 and NO 2 could not be explained exclusively by exchange through the stomata. In both NH 3 and NO 2 fumigation experiments additional deposition onto the needle surface was observed and increased with increasing C NH3 and C NO2 . 15 N[NH 3 ] fumigation experiments with adult spruce trees confirmed the results of gas exchange measurements and revealed that NH 3 -N deposited to spruce needles is subjected to long distance transport within the plant, supplying the plant with additional nitrogen from the atmosphere.

•  In order to test whether xylem-transported carbohydrates are a potential source for isoprene biosynthesis, [U- 13 C]-labelled α- d -glucose was fed via cut ends of stems into the xylem of Quercus robur seedlings and the incorporation of 13 C into isoprene emitted was studied. Emission of 13 C-labelled isoprene was monitored in real time by proton-transfer-reaction mass spectrometry (PTR-MS). •  A rapid incorporation of 13 C from xylem-fed glucose into single (mass 70) and double (mass 71) 13 C-labelled isoprene molecules was observed after a lag phase of approx. 5-10 min. This incorporation was temperature dependent and was highest (up to 13% 13 C of total carbon emitted as isoprene) at the temperature optimum of isoprene emission (40-42°C), when net assimilation was strongly reduced. •  Fast dark-to-light transitions led to a strong single or double 13 C-labelling of isoprene from xylem-fed [U-13C]glucose. During a period of 10-15 min up to 86% of all isoprene molecules became single or double 13 C-labelled, resulting in a 13 C-portion of up to 27% of total carbon emitted as isoprene. •  The results provide evidence that xylem-transported glucose or its degradation products can potentially be used as additional precursors for isoprene biosynthesis and that this carbon source becomes more important under conditions of limited photosynthesis.

•  The effects of drought on European beech (Fagus sylvatica) were assessed in a pot experiment under controlled conditions. •  Plants from 11 autochthonous provenances originating from regions in Germany, which differed in annual precipitation, were exposed to a 3-wk drought period in a glasshouse after the first stage of shoot growth had been completed. •  Drought reduced the water content to 97% of control in leaves and axes and to 92% in the roots. A strong reduction of predawn water potential in roots and shoots, as well as on transpiration rate, was found. In the roots, the effect on water potential was the same for all provenances, but differences were observed in the shoot water potential. Leaf concentrations of abscisic acid (ABA), proline and sucrose increased in the drought-treated plants compared with the controls. •  Two extreme clusters from opposite climatic sites were identified by cluster analysis. A drought-sensitive cluster, originating from regions with high annual precipitation, had low water potential and transpiration rates, as well as high concentrations of fructose, ABA and proline after drought. Water potential and transpiration rates were less affected by drought in the other cluster, which comprised two provenances of relatively dry habitats, and concentrations of hexose, ABA and proline were low.

•  The effect of elevated atmospheric CO 2 on the carbonyl emissions of leaves from two Mediterranean oak species (Quercus ilex and Q. pubescens) was analyzed under field conditions. •  Physiological and meteorological parameters were determined in parallel with measurements of carbonyl emissions. Gas exchange was quantified in dynamic cuvettes combined with an infrared gas analyzer. •  Acetaldehyde and acetone emissions from leaves of Q. ilex were enhanced by elevated CO 2 in the autumn (from 14-40 nmol m -2  min -1 and from 2-8 nmol m -2  min -1 , respectively), but not in the summer. No significant effects were found for leaves of Q. pubescens. The effects of CO 2 on Q. ilex were mainly a result of decreased emissions by control trees under ambient CO 2 concentrations in the autumn; emissions from trees exposed to elevated CO 2 remained at a high level. •  Elevated atmospheric CO 2 causes autumnal changes in carbonyl emissions from Quercus ilex. These effects suggest that the production of acetaldehyde and acetone depend on developmental factors. It is not yet clear whether the altered carbonyl emissions are a unique feature of Q. ilex.

The dynamic-chamber technique was used to investigate the correlation between NH 3 and NO 2 fluxes and different climatic and physiological parameters: air temperature; relative air humidity; photosynthetic photon fluence rate; NH 3 and NO 2 concentrations; transpiration rate; leaf conductance for water vapour; and photosynthetic activity. The experiments were performed with twigs from the sun crown of mature beech trees (Fagus sylvatica) at a field site (Höglwald, Germany), and with 12-wk-old beech seedlings under controlled conditions. Both sets of experiments showed that NO 2 and NH 3 fluxes depended linearly on NO 2 and NH 3 concentration, respectively, in the concentration ranges representative for the field site studied, and on water-vapour conductance as a measure for stomatal aperture. The NO 2 compensation point determined in the field studies (the atmospheric NO 2 concentration with no net NO 2 flux) was 1.8-1.9 nmol mol -1 . The NH 3 compensation point varied between 3.3 and 3.5 nmol mol -1 in the field experiments, and was 3.0 nmol mol -1 in the experiments under controlled conditions. The climatic factors T and PPFR were found to influence both NO 2 and NH 3 fluxes indirectly, by changing stomatal conductance. Whilst NO 2 flux showed a response to changing relative humidity that could be explained by altered stomatal conductance, increased NH 3 flux with increasing relative humidity (>50%) depended on other factors. The exchange of NO 2 between above-ground parts of beech trees and the atmosphere could be explained exclusively by uptake or emission of NO 2 through the stomata, as indicated by the quotient between measured and predicted NO 2 conductance of approx. 1 under all environmental conditions examined. Neither internal mesophyll resistances nor additional sinks could be observed for adult trees or for beech seedlings. By contrast, the patterns of NH 3 flux could not be explained by an exclusive exchange of NH 3 through the stomata. Deposition into additional sinks on the leaf surface, as indicated by an increase in the quotient between measured and predicted NH 3 conductance, gained importance in high air humidity, when the stomata were closed or nearly closed and/or when atmospheric NH 3 concentrations were high. Although patterns of NH 3 gas exchange did not differ between different months or years at high NH 3 concentrations (c. 140 nmol mol -1 ), it must be assumed that emission or deposition fluxes at low ambient NH 3 concentration (0.8 and 4.5 nmol mol -1 ) might vary significantly with time because of variation in the NH 3 compensation point.

From laboratory experiments with seedlings and young trees of Norway spruce (Picea abies L. Karst.), a cycling pool of soluble non-protein N compounds is thought to be indicative of the N-nutritional status of trees. In order to test whether this assumption can be transferred to mature trees grown in the field, xylem sap and phloem exudate were collected from spruce trees in two remote forest stands: (1) a N-limited stand (Villingen site), and (2) a stand where trees are sufficiently supplied with N from the soil (Schluchsee site). Trees at these sites were c. 80-100 (Villingen site) and c. 40-60 (Schluchsee site) yr old. In addition to untreated control areas, one entire watershed area at both sites was subjected to (NH 2 ) 2 SO 4 fertilization to the soil. In the xylem sap of the spruce roots at both sites Gln, Asp and Arg were the dominant total soluble non-protein nitrogen (TSNN) compounds. In the xylem sap of the trunk and the twigs Arg was virtually absent and Gln plus Asp dominated TSNN. On average, TSNN in the xylem sap of trees at the Schluchsee site was 1·5-2-fold higher than those at Villengen. Highest TSNN contents in the xylem sap were found during growth and development of current year tissues, while the lowest TSNN contents were found in summer. At the Villingen site (NH 4 ) 2 SO 4 fertilization caused an increase in the Gln content in the xylem sap of all tree sections analysed as well as an increase in the Arg content in the xylem sap of the roots. At the Schluchsee site only a small increase in TSNN contents of the xylem was observed, mostly in the xylem sap of the roots. In phloem exudates TSNN contents were much higher in trees on the Schluchsee than on the Villingen site. The seasonal pattern of TSNN in phloem exudates was similar to the seasonal pattern found in the xylem sap. During spring and early summer Gln was the predominant TSNN compound in phloem exudates, but during late summer and autumn Arg became predominant. At the Villingen site (NH 4 ) 2 SO 4 fertilization caused a significant increase in TSNN contents in phloem exudates of twigs and roots, but at Schluchsee an increase in TSNN was found only in phloem exudates of the roots. At both field sites Arg that was not transported to the shoot by xylem transport, was allocated from the leaves to the roots by phloem transport and was cycled within the root system by both xylem and phloem transport. From these results it is calculated that shoot-to-root signalling by long-distance transport of amino compounds can also contribute to the regulation of N-nutrition of mature spruce trees. Apparently, the internal cycling of individual N compounds within spruce trees differs considerably.

Beech nuts (Fagus sylvatica L.) were germinated and grown in soil inoculated with the ectomycorrhizal fungus Laccaria laccata or Paxillus involutus for 18-20 wk. The success of mycorrhizal infection was monitored by measuring the ergosterol contents of the mycorrhizas. Ergosterol levels ranged from 122±23 μg g -1 d. wt (Laccaria mycorrhizas) to 94±36 μg g -1 d. wt (Paxillus mycorrhizas), indicating that ectomycorrhizal symbiosis was established. In root incubation chambers, rates of sulphate uptake and the xylem loading of sulphate of excised mycorrhizas were investigated. Both types of mycorrhizas showed saturation kinetics in external sulphate concentrations from 2·5-1000 μmol l -1 . Linearization of these kinetics revealed two phases with low apparent K m (Laccaria mycorrhizas: 15±3 μmol l -1 ; Paxillus mycorrhizas: 13±3 μmol l -1 ) and V max (Laccaria mycorrhizas: 19±3 nmol h -1 g -1 f. wt; Paxillus mycorrhizas: 25±4 nmol h -1 g -1 f. wt) at low external sulphate concentrations and significantly higher kinetic constants at higher sulphate supplies. Relative xylem loading, i.e. the portion of sulphate loaded into the xylem that was taken up, remained constant over the entire concentration range investigated (c. 4-7% of the sulphate taken up). If trees were supplied for 72 h with different N and sulphur concentrations, both uptake of sulphate and relative xylem loading were unaffected by sulphur availability, but modulated by N supply. Nitrogen depletion diminished the rates of sulphate uptake in Laccaria and Paxillus mycorrhizas. In response to higher N availability combined with sulphur depletion, sulphate uptake of Laccaria mycorrhizas, but not of Paxillus mycorrhizas, increased. Organic compounds considered to be possible signals for the regulation of sulphate uptake were fed to excised mycorrhizas. l-Cysteine but not l-methionine and glutathione (γ-Glu-Cys-Gly) inhibited sulphate uptake of the two mycorrhizas and xylem loading of sulphate was stimulated rather than inhibited by l-Cys in both types. In Paxillus mycorrhizas glutathione had a similar effect. O-Acetyl-l-serine (OAS), a precursor of l-cysteine, stimulated sulphate uptake, but did not affect xylem loading. Apparently, OAS, generated in N metabolism, and l-cysteine, a product of assimilatory sulphate reduction, act as antagonists, together mediating regulation of sulphate uptake.

During the vegetation periods 1994 and 1995, net uptake of nitrate and ammonium by roots of adult spruce (Picea abies (L.) Karst) and beech (Fagus sylvatica L.) trees was studied at a field site exposed to high loads of N ('Höglwald', Germany). In addition, uptake experiments were carried out under controlled conditions with young spruce and beech trees grown at normal N supply. In the field, nitrate was not taken up by the roots of spruce trees in appreciable amounts. This was also true for beech except during September 1995. Apparently, beech trees was capable of taking up nitrate, but the environmental condition prevailing at the field site usually prevented net uptake. Net uptake of ammonium in both tree species showed a seasonal course, with maximum rates in mid summer. Rates of ammonium uptake by both species correlated with soil temperature at the field site. Laboratory experiments on the influence of root temperature on uptake of nitrate indicated that uptake rates at temperatures found in the field were low compared with the uptake capacity at optimum temperature. At temperatures of 10 and 15°C, frequently found in the soil at the field site, net uptake of nitrate by spruce and beech amounted to c. 16% and 11%, respectively, of maximum uptake at 25°C. By contrast, net uptake of ammonium at 10°C reached 73% and 31% of the maximum uptake for spruce and beech trees, respectively. Independent of temperature, rates of nitrate uptake were considerably lower than those of ammonium. In young spruce and beech trees, net uptake of nitrate was significantly inhibited by ammonium at nitrate∶ammonium ratios found in the soil solution at the forest site. Preincubation of roots of both species, with amino acids present in the phloem of adult trees at the field site, led to an increase in the amino acid pool in the roots. For spruce trees a correlation between inhibition of uptake of nitrate and enrichment of the roots with the amino compounds Glu, γ-amino butyric acid (Gaba), Gln, and Asn was observed. In beech trees, enrichment of Asp and Gln in the roots correlated with a decrease in net uptake of nitrate. The results of laboratory experiments on the effects of temperature, the nitrate to ammonium ratio in the nutrient solution, and amino acid enrichment in the roots are discussed with special emphasis on the patterns of net uptake of ammonium and nitrate observed in the field.

Short-term exposure of maize, pumpkin, spinach and spruce to various atmospheric H 2 S concentrations resulted in high H 2 S fluxes to the shoots. In all species an almost linear relation was observed between flux and H 2 S concentration up to around 0.3 μl -1 . At higher H 2 S concentrations the flux reached a maximum, which was about 0.08, 0.04, 0.1 and 0.03 μmol g f. wt -1 h -1 for maize, pumpkin, spinach and spruce, respectively. The transpiration rate was not affected during H 2 S exposure. Up to a level of 0.3 μl 1 -1 , the shoot conductance for a influx of the various species varied between 61 and 92% of that predicted from shoot conductance for aqueous vapour efflux. There was no relation between the H 2 S flux to the shoots and the sensitivity of the species towards H 2 S. A 12 days' exposure of maize, pumpkin and spinach to 0.75 μl 1 -1 H 2 S resulted in a reduction of shoot yield of 1.36 and 69%, respectively. H 2 S flux to the shoots of pumpkin, spinach and spruce remained rather constant during exposure for three or four days to about 0.22μl l -l H 2 S. Even at a concentration of about 0.8 μl 1 -1 H 2 , which reduced the growth of pumpkin and spinach, the fluxes remained nearly constant. Fluxes of H 2 S to the shoots did, however, vary diurnally, being high during the light and low during the dark period. The H 2 S flux to transpiration ratio was constant during both light and dark periods, indicating that uptake was predominantly via the stomata. Only part of the total flux of H 2 S to the shoots of pumpkin and spinach could be recovered in the water-soluble non-protein sulphydryl fraction of the shoots (maximum 34%). It is proposed that cysteine synthase is directly involved in the fixation and metabolism of atmospheric H 2 S by the plant.