Preferential use of root litter compared to leaf litter by beech seedlings and soil microorganisms

Background and aims

Litter decomposition is regulated by e.g. substrate quality and environmental factors, particularly water availability. The partitioning of nutrients released from litter between vegetation and soil microorganisms may, therefore, be affected by changing climate. This study aimed to elucidate the impact of litter type and drought on the fate of litter-derived N in beech seedlings and soil microbes.

Methods

We quantified ¹⁵N recovery rates in plant and soil N pools by adding ¹⁵N-labelled leaf and/or root litter under controlled conditions.

Results

Root litter was favoured over leaf litter for N acquisition by beech seedlings and soil microorganisms. Drought reduced ¹⁵N recovery from litter in seedlings thereby affecting root N nutrition. ¹⁵N accumulated in seedlings in different sinks depending on litter type.

Conclusions

Root turnover appears to influence (a) N availability in the soil for plants and soil microbes and (b) N acquisition and retention despite a presumably extremely dynamic turnover of microbial biomass. Compared to soil microorganisms, beech seedlings represent a very minor short-term N sink, despite a potentially high N residence time. Furthermore, soil microbes constitute a significant N pool that can be released in the long term and, thus, may become available for N nutrition of plants.     

Synopsis of the CASIROZ case study: carbon sink strength of Fagus sylvatica L. in a changing environment--experimental risk assessment of mitigation by chronic ozone impact

Databases are needed for the ozone (O(3)) risk assessment on adult forest trees under stand conditions, as mostly juvenile trees have been studied in chamber experiments. A synopsis is presented here from an integrated case study which was conducted on adult FAGUS SYLVATICA trees at a Central-European forest site. Employed was a novel free-air canopy O(3) fumigation methodology which ensured a whole-plant assessment of O(3) sensitivity of the about 30 m tall and 60 years old trees, comparing responses to an experimental 2 x ambient O(3) regime (2 x O(3), max. 150 nl O(3) l (-1)) with those to the unchanged 1 x ambient O(3) regime (1 x O(3)=control) prevailing at the site. Additional experimentation on individual branches and juvenile beech trees exposed within the forest canopy allowed for evaluating the representativeness of young-tree and branch-bag approaches relative to the O(3) sensitivity of the adult trees. The 2 x O(3) regime did not substantially weaken the carbon sink strength of the adult beech trees, given the absence of a statistically significant decline in annual stem growth; a 3 % reduction across five years was demonstrated, however, through modelling upon parameterization with the elaborated database. 2 x O(3) did induce a number of statistically significant tree responses at the cell and leaf level, although the O(3) responsiveness varied between years. Shade leaves displayed an O(3) sensitivity similar to that of sun leaves, while indirect belowground O(3) effects, apparently mediated through hormonal relationships, were reflected by stimulated fine-root and ectomycorrhizal development. Juvenile trees were not reliable surrogates of adult ones in view of O(3) risk assessment. Branch sections enclosed in (climatized) cuvettes, however, turned out to represent the O(3) sensitivity of entire tree crowns. Drought-induced stomatal closure decoupled O(3) intake from O(3) exposure, as in addition, also the "physiologically effective O(3) dose" was subject to change. No evidence emerged for a need to lower the "Critical Level for Ozone" in risk assessment of forest trees, although sensitive tree parameters did not necessarily reflect a linear relationship to O(3) stress. Exposure-based concepts tended to overestimate O(3) risk under drought, which is in support of current efforts to establish flux-related concepts of O(3) intake in risk assessment.     

Interactions of NH4+ and L-glutamate with NO3- transport processes of non-mycorrhizal Fagus sylvatica roots

The processes of NO₃^– uptake and transport and the effectsof NH₄⁺ or L-glutamate on these processes were investigatedwith excised non-mycorrhizal beech (Fagus sylvatica L.) roots.NO₃^– net uptake followed uniphasic Michaelis-Menten kineticsin a concentration range of 10µM to 1 mM with an apparentK_m of 9.2 µM and a V_max of 366 nmol g^–1 FW h^–1.NH₄⁺, when present in excess to NO₃^–, or 10 mM L-glutamateinhibited the net uptake of NO₃^– Apparently, part of NO₃^–taken up was loaded into the xylem. Relative xylem loading ofNO₃^– ranged from 3.21.6 to 6.45.1% of NO₃^– netuptake. It was not affected by treatment with NH₄⁺ or L-glutamate.¹⁶N/¹³N double labelling experiments showed that NO₃^–efflux from roots increased with increasing influx of NO₃^–and, therefore, declined if influx was reduced by NH₄⁺ or L-glutamateexposure. From these results it is concluded that NO₃^–net uptake by non-mycorrhizal beech roots is reduced by NH₄⁺or L-glutamate at the level of influx and not at the level ofefflux. Key words: Nitrate transport, net uptake, influx, efflux, ammonium, Fagus, Fagaceae     

Diurnal pattern of acetaldehyde emission by flooded poplar trees

The emission of the tropospheric trace gas acetaldehyde was determined in leaves of 4-month-old poplar trees ( Populus tremula × P. alba ) grown under controlled environmental conditions in a greenhouse. Using a dynamic cuvette system together with a high sensitivity laser-based photoacoustic detection unit, rates of acetaldehyde emission were measured with the high time resolution of about 15 min. Submergence of the roots resulted in the emission of acetaldehyde by the leaves. The emission increased linearly before reaching more or less steady-state values (ca 350 nmol m⁻² min⁻¹; ca 470 ng g⁻¹ dry weight min⁻¹) after approximately 6 h. Prolonged flooding of poplar trees resulted in a clear diurnal rhythm of acetaldehyde emission. The emission rates decreased when the light was switched off in the evening and peaked in the morning after the light was turned on again. This pattern significantly correlated with diurnal rhythms of stomatal conductance, photosynthesis, transpiration and with the concentrations of ethanol, the assumed precursor of acetaldehyde, in the xylem sap of flooded poplar trees. It may be concluded that under conditions of diminished stomatal conductance, acetaldehyde emission declines because its diffusive flux is reduced. Alternatively, reduced transpiration may decrease ethanol transport from the roots to the shoots and appreciable amounts of the acetaldehyde precursor ethanol are lacking in the leaves. The present results support the view that acetaldehyde emitted by the leaves of plants is derived from ethanol produced by alcoholic fermentation in submerged roots and transported to the leaves with the transpiration stream.     

Oxidation and reduction of sulfite contribute to susceptibility and detoxification of SO2 in Populus × canescens leaves

Key Message

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.

Abstract

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.     

Isoprene emission protects photosynthesis in sunfleck exposed Grey poplar

In the present study, we combined transient temperature and light stress (sunfleck) and comparably analyzed photosynthetic gas exchange in Grey poplar which has been genetically modified in isoprene emission capacity. Overall, we demonstrate that for poplar leaves the ability to emit isoprene is crucial to maintain photosynthesis when exposed to sunflecks. Net CO₂ assimilation and electron transport rates were strongly impaired in sunfleck-treated non-isoprene emitting poplars. Similar impairment was not detected when the leaves were exposed to high light (lightflecks) only. Within 10 h non-isoprene emitting poplars recovered from sunfleck-related impairment as indicated by chlorophyll fluorescence and microarray analysis. Unstressed leaves of non-isoprene emitting poplars had higher ascorbate contents, but also higher contents of malondialdehyde than wild-type. Microarray analyses revealed lipid and chlorophyll degradation processes in the non-isoprene emitting poplars. Thus, there is evidence for an adjustment of the antioxidative system in the non-isoprene emitting poplars even under normal growth conditions.     

Developmental changes in the potential for h(2)s emission in cucurbit plants

Based on results obtained with leaf discs exposed to sulfate, leaves on cucurbit plants (Cucurbita pepo L. cv Small Sugar Pumpkin and Cucumis sativus cv Chipper) 1 to 2.5 weeks old have a low potential for H₂S emission (less than 10 picomoles per min per cm² leaf area) in response to sulfate, whereas discs from most of the leaves on plants 3 to 4 weeks old emit H₂S at a higher rate (50 to 150 picomoles per min per cm² leaf area). This difference is determined by the age of the plant, and is independent of the leaves' age or developmental stage. In response to l-cysteine, however, discs from leaves on cucurbit plants 1 to 2.5 weeks old emit H₂S at higher rates (15 to 50 picomoles per min per cm² leaf area) than in response to sulfate. Furthermore, the potential for H₂S emission in response to l-cysteine decreases with increasing age of the individual leaf. Thus, most of the potential for H₂S emission in response to l-cysteine is developed during germination and the early growth of cucurbit plants, but most of the potential for H₂S emission in response to sulfate arises later in the development of the plants.