Putative role of γ -aminobutyric acid (GABA) as a long-distance signal in up-regulation of nitrate uptake in Brassica napus L.

The relationship between nitrate influx, BnNrt2 nitrate transporter gene expression and amino acid composition of phloem exudate was investigated during N‐deprivation (short‐term experiment) and over a growth cycle (long‐term experiment) in Brassica napus L. The data showed a positive correlation between γ‐aminobutyric acid (GABA) in phloem exudate and nitrate uptake in the short‐ and the long‐term experiments. The hypothesis that this non‐protein amino acid could up‐regulate nitrate uptake via a long‐distance signalling pathway was tested by providing an exogenous GABA supply to the roots. The effect of GABA was compared with the effects of Gln, Glu and Asn, each known to be inhibitors of nitrate uptake. The results showed that GABA treatment induced a significant increase of BnNrt2 mRNA expression, but had less effect on nitrate influx. By contrast, Gln, Glu and Asn significantly reduced nitrate influx and BnNrt2 mRNA expression compared with the control plants. This study provides the first evidence that GABA may act as a putative long‐distance inter‐organ signal molecule in plants in conjunction with negative control exerted by Gln. The up‐regulation effect of GABA on nitrate uptake is discussed in the context of its role in N metabolism, nutritional stress and the recent discovery of a putative role of GABA as a signal molecule in plant development.     

Long-term exposure to enhanced ultraviolet-B radiation in the sub-arctic does not cause oxidative stress in Vaccinium myrtillus

The aim of this work was to assess whether or not oxidative stress had developed in a dwarf shrub bilberry ( Vaccinium myrtillus L.) under long-term exposure to enhanced levels of ultraviolet-B (u.v.-B) radiation. The bilberry plants were exposed to increased u.v.-B representing a 15% stratospheric ozone depletion for seven full growing seasons (1991–1997) at Abisko, Swedish Lapland (68°N). The oxidative stress was assessed on leaves and stems by analysing ascorbate and glutathione concentrations, and activities of the closely related enzymes ascorbate peroxidase (EC 1.11.1.11) and glutathione reductase (EC 1.6.4.2). The affects of autumnal leaf senescence and stem cold hardening on these variables were also considered. The results showed that the treatment caused scarcely any response in the studied variables, indicating that u.v.-B flux representing a 15% ozone depletion under clear sky conditions is not sufficient to cause oxidative stress in the bilberry. It is suggested that no strain was evoked since adaptation was possible under such u.v.-B increases. The studied variables did, however, respond significantly to leaf senescence and especially to stem cold hardening.     

Near-zero recent carbon accumulation in a bog with high nitrogen deposition in SW Sweden

We present data on the accumulation of carbon and nitrogen into an open oceanic ombrotrophic bog, SW Sweden, with high levels of anthropogenic nitrogen deposition. The aim was to investigate if this peatland currently acts as a sink for atmospheric carbon. Peat cores were sampled from the top peat layer in five different vegetation types. Small pines were used to date the cores. The cores bulk density and carbon and nitrogen content were determined. A vegetation-classified satellite image was used to estimate the areal extent of the vegetation types and to scale up these results to bog level. The rate of current carbon input into the upper oxic acrotelm was 290 g m⁻² yr⁻¹, and there were no significant differences in accumulation rates among the vegetation types. This organic matter input to the acrotelm was almost completely decomposed before it was deposited for storage in the deeper peat layers (the catotelm) and only a small fraction (≪1%) or 0.012 g m⁻² yr⁻¹ of the carbon would be left, assuming a residence time of 100 years in the acrotelm. Nitrogen accumulation rates differed between the vegetation classes, and the average input via primary production varied from 5.33 to 16.8 g m⁻² yr⁻¹. Current nitrogen input rates into the catotelm are much lower, 0–0.059 g m⁻² yr⁻¹, with the highest accumulation rates in lawn-dominated communities. We suggest that one of the main causes of the low carbon input rates is the high level of nitrogen deposition, which enhances decomposition and changes the vegetation from peat-forming Sphagnum -dominance to dominance by dwarf shrubs and graminoids.     

SCALE-DEPENDENT SUMMER RESOURCE SELECTION BY REINTRODUCED ELK IN WISCONSIN,USA

Abstract: Identifying how habitat use is influenced by environmental heterogeneity at different scales is central to understanding ungulate population dynamics on complex landscapes. We used resource selection functions (RSF) to study summer habitat use in a reintroduced and expanding elk (Cervus elaphus nelsoni) population in the Chequamegon National Forest, Wisconsin, USA. Factors were examined that influenced where elk established home ranges and that influenced habitat use within established home ranges. We also determined grain sizes over which elk responded to environmental heterogeneity and the number of categories of habitat selection from low to high that the elk distinguished. At a large spatial extent, elk home-range establishment was largely explained by the spatial distribution of wolf (Canis lupus) territories. Forage abundance was also influential but was relatively more important at a small spatial extent when elk moved within established home ranges. Areas near roads were avoided when establishing a home-range, but areas near roads were selected for use within the established home range. Elk distinguished among 4 different categories of habitat selection when establishing and moving within home ranges. Spatial and temporal cross validation demonstrated that to improve the predictive strength of habitat models in areas of low inter-annual variability in the environment, it is better to follow more individuals across diverse environmental conditions than to follow the same individuals over a longer time period. Last, our results show that the effects of environmental variables on habitat use were scale-dependent and reemphasize the necessity of analyzing habitat use at multiple scales that are fit to address specific research questions.     

CH4 production and oxidation processes in a boreal fen ecosystem after long‐term water table drawdown

Fens, which extend over vast areas in the Northern hemisphere, are sources of the greenhouse gas CH₄. Climate change scenarios predict a lowering water table (WT) in mires. To study the effect of WT drawdown on CH₄ dynamics in a fen ecosystem, we took advantage of a WT drawdown gradient near a ground water extraction plant. Methane fluxes and CH₄ production and oxidation potentials were related to microbial communities responsible for the processes in four mire locations (wet, semiwet, semidry, and dry). Principal component analyses performed on the vegetation, pH, CH₄, and WT results clearly separated the four sampling locations in the gradient. Long‐term lowering of WT was associated with decreased coverage of Sphagnum and aerenchymatic plants, decreased CH₄ field emissions and CH₄ production potential. Based on mcrA terminal restriction fragment length polymorphism the methanogen community structure correlated best with the methane production and coverage of aerenchymatic plants along the gradient. Methanosarcinaceae and Methanocellales were found at the pristine wet end of the gradient, whereas the Fen cluster characterized the dry end. The methane‐oxidizing bacterial community consisted exclusively of Methylocystis bacteria, but interestingly of five different alleles (T, S, R, M, and O) of the particulate methane monooxygenase marker gene pmoA. The M allele was dominant in the wet locations, and the occurrence of alleles O, S, and T increased with drainage. The occurrence of the R allele that characterized the upper peat layer correlated with CH₄ oxidation potential. These results advance our understanding of mire dynamics after long‐term WT drawdown and of the microbiological bases of methane emissions from mires.