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81.
Reflected Far-Red Light Effects on Chlorophyll and Light-Harvesting Chlorophyll Protein (LHC-II) Contents under Field Conditions 总被引:1,自引:1,他引:0
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The influence of various colors of soil cover (mulch) on the farred/red (FR/R) ratio in upwardly reflected light and on concentrations of chlorophyll (Chl) and light-harvesting Chl protein (LHC-II) were measured under field conditions. The FR/R ratios above green surfaces were higher than over white surfaces. Even though plants (Gossyplum hirsutum L. cv PD-1) were grown in full sunlight, those that received higher FR/R ratios in upwardly reflected light were taller and had thinner leaves with higher concentrations of Chl and LHC-II. A controlled environment experiment showed FR/R control of Chl and LHC-II concentrations. The results illustrate the importance of spectral distribution of reflected light on plant growth and a potential means of altering the chemistry of leaf crops under field conditions. 相似文献
82.
Antigen/MHC-specific T cells are preferentially exported from the thymus in the presence of their MHC ligand 总被引:35,自引:0,他引:35
Transgenic mice expressing a T cell receptor heterodimer specific for a fragment of pigeon cytochrome c plus an MHC class II molecule (I-Ek) have been made. We find that H-2k alpha beta transgenic mice have an overall increase in the number of T cells and express a 10-fold higher fraction of cytochrome c-reactive cells than H-2b mice. Surface staining of thymocytes indicates that in H-2b mice, T cell development is arrested at an intermediate stage of differentiation (CD4+8+, CD310). Analyses of mice carrying these T cell receptor genes and MHC class II I-E alpha constructs indicate that his developmental block can be reversed in H-2b mice by I-E expression on cortical epithelial cells of the thymus. These data suggest that a direct T cell receptor-MHC interaction occurs in the thymus in the absence of nominal antigen and results in the enhanced export of T cells, consistent with the concept of "positive selection". 相似文献
83.
F R Althaus H Naegeli C Realini G Mathis P Loetscher M Mattenberger 《Acta biologica Hungarica》1990,41(1-3):9-18
The role of poly ADP-ribosylation in DNA excision repair was studied in experimental models of various complexities. In intact cells in vivo, the unfolding of chromatin during DNA excision repair apparently requires the presence of a functional poly-ADP-ribosylation system. In vitro studies involving a reconstituted poly-ADP-ribosylation system show that the enzyme poly(ADP-ribose)polymerase has the capacity to shuttle core histones on a core DNA fragment of 146 bp. Under these conditions, the polymerase operates in a strictly processive mode. Furthermore, the polymerase adapts to different shuttling targets by producing very distinct polymer patterns. We conclude that the eukaryotic poly-ADP-ribosylation system has the capacity to regulate DNA-protein interactions and this may be an essential part of the unfolding mechanism of chromatin during excision repair in vivo. 相似文献
84.
Five species of fishes, two species of aquatic macrophytes, zooplankton, migratory goose feces, water and sediments from a eutrophic lake were analyzed for mercury, cadmium, lead and thallium. Mercury was detected in fishes and sediments only while cadmium and lead were detected in all components. Thallium was detected only in sediments. Sediments in the lake act as a ‘sink’ for the four metals. Mercury in axial musculature of largemouth bass was highly correlated with length and weight. A high degree of correlation between other metals and weight and length of other species was not evident. The feces of migratory waterfowl had high concentrations of both cadmium and lead. In view of the large quantity of waterfowl feces deposited within the drainage basin, it is suggested that this avenue is one of the major sources of contamination for the two metals and that fallout from airborne particulate matter is secondary. Fallout of airborne particulate matter may be the primary method by which mercury and thallium enter the lake although residual concentrations of the four metals in soil of the drainage basin were not determined. 相似文献
85.
Magnetic fields influence two properties of the P-870 triplet state observed in Rps.
sphaeroides reaction centers: the yield of formation and the kinetics of decay. These effects have been studied in reaction centers which were prepared in three different states: state QA
–, state QA
2– and state (– QA) (QA depleted). The triplet yields decrease with increasing magnetic fields, with B1/2's of about 140, 41 and 57 Gauss, respectively. The half-time of 3P-870 decay is not influenced by the field in state QA
–; it increases at increasing fields, in state QA
2– and state (– QA), with the same B1/2 as the triplet yield. These results are discussed in the framework of current theories of the radical-pair dynamics and of the mechanism of triplet decay.Abbreviations I
primary electron acceptor
- LDAO
lauryldimethylamine oxide
- P-870
primary electron donor
- QA
first quinone acceptor
- SDS
sodium dodecylsulfate
- YAG
Yttrium Aluminum Garnet 相似文献
86.
Symbiotic Effectiveness and Host-Strain Interactions of Rhizobium fredii USDA 191 on Different Soybean Cultivars 总被引:1,自引:0,他引:1
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Daniel W. Israel James N. Mathis W. Mark Barbour Gerald H. Elkan 《Applied microbiology》1986,51(5):898-903
Nodulation, acetylene reduction activity, dry matter accumulation, and total nitrogen accumulation by nodulated plants growing in a nitrogen-free culture system were used to compare the symbiotic effectiveness of the fast-growing Rhizobium fredii USDA 191 with that of the slow-growing Bradyrhizobium japonicum USDA 110 in symbiosis with five soybean (Glycine max (L.) Merr.) cultivars. Measurement of the amount of nitrogen accumulated during a 20-day period of vegetative growth (28 to 48 days after transplanting) showed that USDA 110 fixed 3.7, 39.1, 4.6, and 57.3 times more N2 than did USDA 191 with cultivars Pickett 71, Harosoy 63, Lee, and Ransom as host plants, respectively. With the unimproved Peking cultivar as the host plant, USDA 191 fixed 3.3 times more N2 than did the USDA 110 during the 20-day period. The superior N2 fixation capability of USDA 110 with the four North American cultivars as hosts resulted primarily from higher nitrogenase activity per unit nodule mass (specific acetylene reduction activity) and higher nodule mass per plant. The higher N2-fixation capability of USDA 191 with the Peking cultivar as host resulted primarily from higher nodule mass per plant, which was associated with higher nodule numbers. There was significant variation in the N2-fixation capabilities of the four North American cultivar-USDA 191 symbioses. Pickett 71 and Lee cultivars fixed significantly more N2 in symbiosis with USDA 191 than did the Harosoy 63 and Ransom cultivars. This quantitative variation in N2-fixation capability suggests that the total incompatibility (effectiveness of nodulation and efficiency of N2 fixation) of host soybean plants and R. fredii strains is regulated by more than one host plant gene. These results indicate that it would not be prudent to introduce R. fredii strains into North American agricultural systems until more efficient N2-fixing symbioses between North American cultivars and these fast-growing strains can be developed. When inoculum containing equal numbers of USDA 191 and of strain USDA 110 was applied to the unimproved Peking cultivar in Perlite pot culture, 85% of the 160 nodules tested were occupied by USDA 191. With Lee and Ransom cultivars, 99 and 85% of 140 and 96 nodules tested, respectively, were occupied by USDA 110. 相似文献
87.
The primary donor of Photosystem II (PS II), P-680, was photo-oxidized by a short flash and its rate of reduction was measured at different pH values by following the recovery of the absorption change at 820 nm in chloroplasts pretreated with a high concentration of Tris. The re-reduction is biphasic with a fast phase (dominant after the first flash) attributed to the donation by a donor, D1, and a slow phase (usually dominant after the second flash) attributed to a back-reaction with the primary acceptor.
It is found that pH has a strong influence on the donation from D1 (τ = 2 μs at pH 9, 44 μs at pH 4), but no influence on the back reaction (τ ≈ 200 μs). pH also influences the stability of the charge separation since the contribution of donation from D1 at the second flash increases at lower pH, getting close to 100% at pH 4. 相似文献
88.
HMG proteins (1 + 2) form beaded structures when complexed with closed circular DNA. 总被引:4,自引:2,他引:2
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Structures bearing a resemblance to nucleosomes can be assembled by incubating calf thymus High Mobility Group proteins (1 + 2) with closed circular DNA. These HMG proteins are capable of forming beads and inducing superhelicity when bound to DNA. However, they do not protect from nuclease digestion the discrete DNA fragments characteristic of nucleosomes. The relationship between HMGs (1 + 2) and the "primitive" histone-like DNA-packaging proteins from prokaryotes and mitochondria is discussed. 相似文献
89.
The influence of temperature on the rate of reduction of P-680+, the primary donor of Photosystem II, has been studied in the range 5–294 K, in chloroplasts and subchloroplasts particles. P-680 was oxidized by a short laser flash. Its oxidation state was followed by the absorption level at 820 nm, and its reduction attributed to two mechanisms: electron donation from electron donor D1 and electron return from the primary plastoquinone (back-reaction).Between 294 and approx. 200 K, the rate of the back-reaction, on a logarithmic scale, is a linear function of the reciprocal of the absolute temperature, corresponding to an activation energy between 3.3 and 3.7 kcal · mol?1, in all of the materials examined (chloroplasts treated at low pH or with Tris; particles prepared with digitonin). Between approx. 200 K and 5 K the rate of the back-reaction is temperature independent, with . In untreated chloroplasts we measured a of 1.7 ms for the back-reaction at 77 and 5 K.The rate of electron donation from the donor D1 has been measured in darkadapted Tris-treated chloroplasts, in the range 294–260 K. This rate is strongly affected by temperature. An activation energy of 11 kcal · mol?1 was determined for this reaction.In subchloroplast particles prepared with Triton X-100 the signals due to P-680 were contaminated by absorption changes due to the triplet state of chlorophyll a. This triplet state has been examined with pure chlorophyll a in Triton X-100. An Arrhenius plot of its rate of decay shows a temperature-dependent region (292–220 K) with an activation energy of 9 kcal · mol?1, and a temperature-independent region (below 200 K) with . 相似文献
90.