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排序方式: 共有277条查询结果,搜索用时 296 毫秒
271.
272.
Matamala R Gonzàlez-Meler MA Jastrow JD Norby RJ Schlesinger WH 《Science (New York, N.Y.)》2003,302(5649):1385-1387
Estimates of forest net primary production (NPP) demand accurate estimates of root production and turnover. We assessed root turnover with the use of an isotope tracer in two forest free-air carbon dioxide enrichment experiments. Growth at elevated carbon dioxide did not accelerate root turnover in either the pine or the hardwood forest. Turnover of fine root carbon varied from 1.2 to 9 years, depending on root diameter and dominant tree species. These long turnover times suggest that root production and turnover in forests have been overestimated and that sequestration of anthropogenic atmospheric carbon in forest soils may be lower than currently estimated. 相似文献
273.
Cerebral tryptophan 5-hydroxylase and tyrosine hydroxylase are stimulated by a solution of lyophilized, protein-free 30,000g supernatant of brain homogenates (concentrate 1). This preparation can be further purified by passage through Sephadex G-75 and phosphocellulose columns to yield concentrate 2. Unlike concentrate 1, concentrate 2 is unstable after several days of storage even at -27 degrees C. Neither preparation is active without the presence of a reduced pteridine cofactor. The stimulating factor appears to be thermostable, alkali-labile, dialyzable, and light-sensitive. 相似文献
274.
275.
Moyà-Solà S Köhler M Alba DM Casanovas-Vilar I Galindo J 《Science (New York, N.Y.)》2004,306(5700):1339-1344
We describe a partial skeleton with facial cranium of Pierolapithecus catalaunicus gen. et sp. nov., a new Middle Miocene (12.5 to 13 million years ago) ape from Barranc de Can Vila 1 (Barcelona, Spain). It is the first known individual of this age that combines well-preserved cranial, dental, and postcranial material. The thorax, lumbar region, and wrist provide evidence of modern ape-like orthograde body design, and the facial morphology includes the basic derived great ape features. The new skeleton reveals that early great apes retained primitive monkeylike characters associated with a derived body structure that permits upright postures of the trunk. Pierolapithecus, hence, does not fit the theoretical model that predicts that all characters shared by extant great apes were present in their last common ancestor, but instead points to a large amount of homoplasy in ape evolution. The overall pattern suggests that Pierolapithecus is probably close to the last common ancestor of great apes and humans. 相似文献
276.
L. Potenza M. A. Cafiero A. Camarda G. La Salandra L. Cucchiarini M. Dachà 《Veterinary research communications》2009,33(7):611-618
In the present work mites previously identified as Dermanyssus gallinae De Geer (Acari, Mesostigmata) using morphological keys were investigated by molecular tools. The complete internal transcribed
spacer 1 (ITS1), 5.8S ribosomal DNA, and ITS2 region of the ribosomal DNA from mites were amplified and sequenced to examine
the level of sequence variations and to explore the feasibility of using this region in the identification of this mite. Conserved
primers located at the 3’end of 18S and at the 5’start of 28S rRNA genes were used first, and amplified fragments were sequenced.
Sequence analyses showed no variation in 5.8S and ITS2 region while slight intraspecific variations involving substitutions
as well as deletions concentrated in the ITS1 region. Based on the sequence analyses a nested PCR of the ITS2 region followed
by RFLP analyses has been set up in the attempt to provide a rapid molecular diagnostic tool of D. gallinae. 相似文献
277.