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31.
Sven Jonasson 《Ecography》1983,6(3):295-304
Forbs and leaves of deciduous shrubs had high concentrations of Ca, Mg, K, P, and N. Deciduous dwarf shrubs had intermediate concentrations but higher than evergreen dwarf shrubs. Monocots, cryptogams, woody and belowground tissues had low concentrations.
Plant nutrient concentrations and nutrient content in soil organic matter increased from dry towards moist tundra areas.
The residence time of nutrients was considerably less than ten years in surface litter, but several decades or centuries in total organic matter. The longest residence time was found in the moist part of the tundra.
N, K, and P in Betula nana leaves were translocated to a great extent prior to leaf fall, whereas Mg and particularly Ca were only slightly translocated. As on other tundra areas shortage of nutrients probably limits plant growth. In that case short supply of N and F seems most probable due to retention in litter and soil organic matter. 相似文献
Plant nutrient concentrations and nutrient content in soil organic matter increased from dry towards moist tundra areas.
The residence time of nutrients was considerably less than ten years in surface litter, but several decades or centuries in total organic matter. The longest residence time was found in the moist part of the tundra.
N, K, and P in Betula nana leaves were translocated to a great extent prior to leaf fall, whereas Mg and particularly Ca were only slightly translocated. As on other tundra areas shortage of nutrients probably limits plant growth. In that case short supply of N and F seems most probable due to retention in litter and soil organic matter. 相似文献
32.
Vendela Parrow Eewa Nnberg Jari Heikkil Ulf Hammerling Sven Phlman 《Journal of cellular physiology》1992,152(3):536-544
SH-SY5Y human neuroblastoma cells can be induced to differentiate into a neuronal phenotype by treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA). In other cell systems, TPA treatment frequently leads to down-regulation of protein kinase C (PKC). However, we now report that TPA-treated and non-treated SH-SY5Y cells express PKC-alpha, but not PKC-beta and PKC-gamma, mRNA. Furthermore, only a slight down-regulation of the PKC-alpha protein could be seen during prolonged treatment with 16 nM TPA, the concentration giving optimal differentiation. In contrast, a higher concentration of TPA (1.6 microM) results in a poor neuronal differentiation and a complete down-regulation of PKC-alpha. PKC-alpha was rapidly translocated to the particulate fraction and remained membrane bound for at least 4 days during treatment with 16 nM TPA. In such cells a sustained increased level of the phosphorylated form of a 80,000 Dalton PKC-substrate was found. In addition to this sustained augmented phosphorylation, administration of fresh TPA at day 4 caused a small but reproducible further increased level of phosphorylated substrate. When the PKC activity was measured by the histone phosphorylation assay a substantial fraction of the initial enzyme activity could still be detected after 4 days of TPA treatment. Taken together, the data demonstrate that PKC remains functionally active during TPA induced differentiation of SH-SY5Y cells, which may suggest a continuous role for the enzyme during the differentiation process. 相似文献
33.
Sven Zea 《Helgoland Marine Research》1992,46(3):293-300
During a study of the spatial and temporal patterns of desmosponge (Porifera, Demospongiae) recruitment on rocky and coral
reef habitats of Santa Marta, Colombian Caribbean Sea, preliminary attempts were made to estimate actual settlement rates
from short-term (1 to a few days) recruitment censuses. Short-term recruitment rates on black, acrylic plastic plates attached
to open, non-cryptic substratum by anchor screws were low and variable (0–5 recruits/plate in 1–2 days, sets of n=5–10 plates),
but reflected the depth and seasonal trends found using mid-term (1 to a few months) censusing intervals. Moreover, mortality
of recruits during 1–2 day intervals was low (0–12%). Thus, short-term censusing intervals can be used to estimate actual
settlement rates. To be able to make statistical comparisons, however, it is necessary to increase the number of recruits
per census by pooling data of n plates per set, and to have more than one set per site or treatment. 相似文献
34.
John A. Buswell Karl-Erik Eriksson Jugal K. Gupta Sven G. Hamp Inger Nordh 《Archives of microbiology》1982,131(4):366-374
Metabolism of vanillic acid, a product of lignin degradation, has been studied in selected representatives of soft-rot, brown-rot and white-rot fungi. All of the brown-and white-rot species examined decarboxylated vanillate to methoxyhydroquinone oxidatively. Mycelium extracts of all these fungi, except Pleurotus ostreatus contained high levels of an NAD(P)H-dependent vanillate hydroxylase. P. ostreatus also released 14CO2 from 14COOH-vanillate but by a different mechanism possibly involving phenoloxidases. Most of these fungi also contained a dioxygenase which catalysed the intra-diol cleavage of hydroxyquinol (1,2,4-trihydroxybenzene) to form maleylacetate. No 3-O-demethylase activity was detected, and data indicate that in some of the fungi examined cleavage of the aromatic ring occurs without prior removal of the methoxyl group. None of the soft-rot fungi tested contained vanillate hydroxylase or hydroxyquinol 1,2-dioxygenase, but very low levels of protocatechuate 3,4-dioxygenase were detected in mycelium extracts. Vanillate catabolism among members of this group occurs via a different route which may involve ring demethylation although no 3-O-demethylase activity was detected in this study. The enzyme NAD(P)H-quinone oxidoreductase was demonstrated to exist in all the studied groups of fungi. 相似文献
35.
Summary Hagfish hemoglobin has three main components, one of which is Hb III. It is monomeric and consists of 148 amino acid residues (M = 17 350). Its complete primary structure, previously published, is discussed here. The proximal amino acid (F8) of the heme linkage is histidine as always in the hemoglobins, but the regularly expected distal histidine E7 is substituted by glutamine. This substitution, leading to a new kind of heme linkage, has hitherto only been demonstrated in opossum hemoglobin. It is suggested that E7, Gln, is directed out of the heme pocket, and that the adjacent Ell, Ile, is directed toward the inside of the pocket, giving the distal heme contact instead of histidine.Myxine Hb III has an additional tail of 9 amino acid residues at its N-terminal end, as has the hemoglobin ofLampetra fluviatilis. The genetic codes ofMyxine andLampetra hemoglobins show 117 differences, in spite of many morphological resemblances between hagfish and lamprey. Their primary hemoglobin structures show differences substantial enough to bo compatible with the divergence of the two families some 400–500 million years ago. 相似文献
36.
Prof. Sven O. E. Ebbesson 《Cell and tissue research》1980,213(2):179-212
Summary Recently discovered neocortical equivalents in anamniotes and certain patterns of interspecific variability in brain organization provide new insights into evolutionary and ontogenetic mechanisms of development. The new data suggest that nervous systems become more complex, not by one system invading another, but by a process of parcellation that involves the selective loss of connections of the newly formed daughter aggregates and subsystems. The parcellation process is reflected in the normal ontogenetic development of the CNS in a given species and can be manipulated, to a certain extent, by deprivation or surgically induced sprouting.The parcellation theory allows certain predictions about the range of variation of a given system at all levels of analysis including the cellular and aggregate levels. For example, the interspecific variability in organization of cortical columns, thalamic nuclei, cortical areas and tectal layers can be explained. The findings, summarized here, suggest that diffuse, undifferentiated systems existed in the beginning of vertebrate evolution and that during the evolution of complex behaviors, and analytical capacities related to these behaviors, a range of patterns of neural systems evolved that relate to these functions. One principle underlying the growth, differentiation and multiplication of neural systems appears to be the process of parcellation as defined by the theory.Presented in part at the meeting of the British Society for Experimental Biology, Belfast, Ireland, July 1979 相似文献
37.
Partially purified prostacyclin synthase from pig aorta converted the prostaglandin (PG) endoperoxide PGH2 to prostacyclin (PGI2), and PGH1 to 12-hydroxy-8,10-heptadecadienoic acid (HHD). Both reactions were inhibited by 15-hydroperoxy-5,8,11,13-eicosatetraenoic acid (15-HP) in a dose-dependent fashion. However, the reactions PGH2 → PGI2 and PGH1 → HHD appeared to differ: substrate availability was rate limiting in the latter reaction, while the enzyme became rapidly saturated with PGH2 and a steady rate of prostacyclin formation was observed at higher substrate levels. 相似文献
38.
By priming female C57BL/6 mice with syngeneic male spleen cells and enriching inguinal and paraaortic lymph node cells in long-term culture (LTC) by repeated restimulations, H-Y-specific T helper cells can be produced. In response to male spleen cells carrying I-Ab antigens these cells activate antigenexpressing B cells to secrete polyclonal antibody. Before the end of the second week in LTC it was impossible to detect any helper activity. Induction of plaque-forming cells (PFC) also requires simultaneous recognition of antigen and I-A-encoded determinants in the stimulator-responder spleen-cell population. The testing of spleen cells fromH-2 recombinant strains as stimulator-responders to anti-H-Y helper T cells of C57BL/6 origin also revealed that other genes, telomeric toI-A, control the magnitude of both specific T-cell proliferation and helper-dependent B-cell activation. 相似文献
39.
40.