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101.
DIMITRIOS SARRIS DIMITRIOS CHRISTODOULAKIS CHRISTIAN KÖRNER† 《Global Change Biology》2007,13(6):1187-1200
We present evidence of a recent drying in the eastern Mediterranean, based on weather and tree‐ring data for Samos, an island of the eastern Aegean Sea. Rainfall declined rapidly after the late 1970s following trends for the entire Mediterranean and was associated with reduced tree‐ring width in Pinus brutia. The most recent decline led to the lowest annual radial stem increment after the last 100 years (as far as records reach). As moisture availability decreased best correlations of tree growth with rainfall were obtained for progressively longer integration periods (1–2 years in moister periods, 5–6 years during the severe dryness of 20th century's last decades), suggesting increasing dependency in deep soil water. Such long‐term integration periods of tree‐growth responses to precipitation have not been reported before. They may reflect a tree‐rooting pattern adapted to cope with even several successive dry years. In late summer 2000, moisture reserves became exhausted, however, and a substantial fraction of low altitude pines died, including some 80‐year‐old trees, which underlines the exceptional extent this trend had reached. Our findings provide empirical support for Intergovernmental Panel on Climate Change projections derived from global circulation models that the Mediterranean, its eastern basin in particular, should become drier as temperature rises, as was the case in the recent past. 相似文献
102.
Electron transfer of dissolved organic matter and its potential significance for anaerobic respiration in a northern bog 总被引:6,自引:0,他引:6
TOBIAS HEITMANN TOBIAS GOLDHAMMER JULIA BEER CHRISTIAN BLODAU 《Global Change Biology》2007,13(8):1771-1785
We investigated electron transfer processes of dissolved organic matter (DOM) and their potential importance for anaerobic heterotrophic respiration in a northern peatland. Electron accepting and donating capacities (EAC, EDC) of DOM were quantified using dissolved H2S and ferric iron as reactants. Carbon turnover rates were obtained from porewater profiles (CO2, CH4) and inverse modeling. Carbon dioxide was released at rates of 0.2–5.9 mmol m−2 day−1 below the water table. Methane (CH4) formation contributed <10%, and oxygen consumption 2% to 40%, leaving a major fraction of CO2 production unexplained. DOM oxidized H2S to thiosulfate and was reduced by dissolved ferric iron. Reduction with H2S increased the subsequently determined EDC compared to untreated controls, indicating a reversibility of the electron transfer. In situ redox capacities of DOM ranged from 0.2 to 6.1 mEq g−1 C (EAC) and from 0.0 to 1.4 mEq g−1 C (EDC), respectively. EAC generally decreased with depth and changed after a water table drawdown and rebound by 20 and −45 mEq m−2, respectively. The change in EAC during the water table fluctuation was similar to CH4 formation rates. In peatlands, electron transfer of DOM may thus significantly contribute to the oxidation of reduced organic substrates by anaerobic heterotrophic respiration, or by maintaining the respiratory activity of sulfate reducers via provision of thiosulfate. Part of the anaerobic electron flow in peat soils is thus potentially diverted from methanogenesis, decreasing its contribution to the total carbon emitted to the atmosphere. 相似文献
103.
104.
Field and laboratory experiments on net uptake of nitrate and ammonium by the roots of spruce (Picea abies) and beech (Fagus sylvatica) trees 总被引:4,自引:0,他引:4
105.
CHRISTIAN T. CHIMIMBA NICO J. DIPPENAAR TERENCE J. ROBINSON 《Biological journal of the Linnean Society. Linnean Society of London》1999,67(4):501-527
Five species of African rock rats, Aethomys namaquensis, A. granti, A. silindensis, A. nyikae and A. chrysophilus, have to date been recognized in southern Africa. While the first three species are craniometrically well-delineated, morphometric analysis of cytogenetically known and other specimens of A. chrysophilus revealed that it comprises two sympatric, morphologically similar species, the nominate species and the newly recognized A. ineptus. This is in agreement with observations on cranial morphology and earlier investigations involving cytogenetics, protein electrophoresis, and gross sperm and bacular morphology. Contrary to published reports, the Central African A. nyikae does not extend into southern Africa, and the single previous record from eastern Zimbabwe is probably based on a misidentification. 相似文献
106.
107.
The Effect of Calcium Starvation on Assimilate Partitioning and Mineral Distribution of the Phloem 总被引:1,自引:0,他引:1
Populus plants were grown in a medium lacking calcium and exposedto 14CO2. In contrast to plants in the complete nutrient medium,the percentage amount of 14C-assimilates increased in the leavesof calcium-deficient plants and decreased in the stem and theroots. When plants were grown without potassium or magnesiumno differences in the amount of 14C-label occurred in comparisonwith plants in the complete nutrient medium. Translocation wasrecorded by microautoradiography. It was observed that considerableamounts of labelled photoassimilates were unloaded from thephloem in the middle part of the stem in plants of the completenutrient medium. In contrast, during calcium starvation 14C-labelwas restricted to the phloem of the stem. In addition, the concentrationsof magnesium and phosphorus showed a remarkable increase instem sieve tubes of calcium-deficient plants. When sieve tubesof source leaves from Populus, barley and maize were comparedwith those of sink leaves, the latter showed higher calciumconcentrations. The results suggest that calcium is a necessaryfactor in the regulation of phloem translocation. Key words: Calcium deficiency, phloem translocation, sieve element loading and unloading, X-ray microanalysis 相似文献
108.
Dendrite formation and extension, which comprise a characteristic morphology of human normal melanocytes in the skin, represent one of the functional activities of melanocytes, the ability to transfer melanosomes into neighboring keratinocytes. However, the morphology of the melanocyte in vitro is usually quite different from that observed in vivo. it is probably due to the hyperproliferative condition of the melanocytes in culture. No studies have ever compared the effects of a single factor on both dendricity and proliferation at the same time. Therefore, we have compared the effects of six growth-promoting agents commonly used for melanocyte cultures on dendrite formation and proliferation. The addition of agents that increase the intracellular levels of cyclic adenosine monophosphate (cAMP)—dibutyryl cyclic adenosine monophosphate (db cAMP; 1 mM) or isobutylmethyl xanthine (IBMX; 0.1 mM)—had a strong effect on dendrite formation and a negative effect on proliferation. This was especially true with db cAMP. In the presence of 2% or 5% of heat-inactivated fetal bovine serum (FBS), dendrite formation was significantly increased as was proliferation. The number of dendrites was decreased in the culture with 12-o-tetradecanoylphorbol-13-acetate (TPA), but cell growth was slightly increased. With human recombinant basic fibroblast growth factor (bFGF) (0.5, 1.0 ng/ml) in the presence of bovine pituitary extract (BPE) (60 μg/ml), cell growth was increased. With 2 ng/ml of bFGF, however, a strong inhibitory effect on proliferation was observed. However, dendrite formation was constant at all concentrations of bFGF tested (0.5, 1.0 or 2.0 ng/ml) with BPE (30 or 60 μg/ml). In this study, we have demonstrated that dendrite formation was suppressed by the reagents that stimulate melanocyte proliferation, and vice versa, with the only exception being heat-inactivated FBS. Both dendrite formation and proliferation were induced by the heat-inactivated FBS. This approach is crucial to the development of an adequate culture system for proliferation and/or dendrite formation of normal human melanocytes. It is necessary to keep these aspects in mind as we further investigate the biology of melanocytes, especially the cell-to-cell interactions between melanocytes and keratinocytes, involved in melanogenesis and melanin pigmentation in vivo. This study also provides practical and important information for a future reconstitutive skin system composed of melanocytes, keratinocytes, and fibroblasts in a single culture medium. 相似文献
109.
Sulphate uptake in the blue-green alga Anacystis nidulans appears based upon an active mechanism with a Km of 0.75 μM and Vmax of 0.7 pmol/min × 106 cells. Sulphate uptake is competitively inhibited by thiosulphate and sulphite. The sulphate uptake has a pH optimum at 8 and a temperature optimum at 40°C. By increasing the extracellular sulphate concentration from 0.1 to 10 μM the sulphate pool in Anacystis was altered from 8.3, 10?5M to 5.9, 10?4M. 相似文献
110.
Abstract: Exceptionally preserved specimens of the genera Myophoria, Neoschizodus and Trigonodus (Bivalvia, Palaeoheterodonta, Trigonioida) from carbonate sediments of the Muschelkalk (Anisian, Ladinian) contain phosphatized soft‐tissues. This is the oldest record of soft‐tissue preservation in bivalves and the first from the German Muschelkalk. The phosphatized remains are here interpreted as relics of the originally chitinous gill supports, the gill axis, the labial palps (?), the adductor muscles, the pedal retractor muscles, the mantle margin including the radial mantle musculature, and the ‘siphons’. According to microprobe analysis, the mineral replacing the soft‐tissue is mainly apatite and, more rarely, francolite. Additionally, quartz filled voids within the gill supports, and in one sample it occurs in minute crystals in the phosphatized remains of the adductor muscle of Neoschizodus. Myophoria, Neoschizodus and Trigonodus were soft‐bottom dwellers and five of the specimens were discovered in life position. This is indicated by geopetal structures in three specimens. 相似文献