全文获取类型
收费全文 | 21811篇 |
免费 | 683篇 |
国内免费 | 507篇 |
出版年
2023年 | 211篇 |
2022年 | 157篇 |
2021年 | 293篇 |
2020年 | 346篇 |
2019年 | 405篇 |
2018年 | 390篇 |
2017年 | 302篇 |
2016年 | 345篇 |
2015年 | 586篇 |
2014年 | 1424篇 |
2013年 | 1476篇 |
2012年 | 1212篇 |
2011年 | 1518篇 |
2010年 | 1218篇 |
2009年 | 898篇 |
2008年 | 986篇 |
2007年 | 987篇 |
2006年 | 903篇 |
2005年 | 762篇 |
2004年 | 787篇 |
2003年 | 618篇 |
2002年 | 491篇 |
2001年 | 327篇 |
2000年 | 338篇 |
1999年 | 402篇 |
1998年 | 375篇 |
1997年 | 334篇 |
1996年 | 332篇 |
1995年 | 351篇 |
1994年 | 373篇 |
1993年 | 325篇 |
1992年 | 337篇 |
1991年 | 317篇 |
1990年 | 260篇 |
1989年 | 283篇 |
1988年 | 242篇 |
1987年 | 244篇 |
1986年 | 188篇 |
1985年 | 199篇 |
1984年 | 225篇 |
1983年 | 152篇 |
1982年 | 231篇 |
1981年 | 152篇 |
1980年 | 162篇 |
1979年 | 159篇 |
1978年 | 92篇 |
1977年 | 95篇 |
1976年 | 59篇 |
1972年 | 25篇 |
1971年 | 24篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Toshiyuki Nagata Kazuya Okada Tetsu Kawazu Itaru Takebe 《Molecular & general genetics : MGG》1987,207(2-3):242-244
Summary An experimental system to study cell cycle specific gene expression in plant cells was developed using protoplasts from tobacco cells synchronized by aphidicolin treatment. Chimeric plasmids consisting either of the chloramphenicol acetyltransferase (CAT) gene downstream of the cauliflower mosaic virus (CaMV) 35 S promoter or the nopaline synthase (nos) promoter were introduced into synchronized protoplasts of four cell cycle stages by electroporation. In the case of the CaMV 35 S promoter cyclic oscillation of CAT activity was observed which paralleled the cell cycle of the recipient cells. The peak of CAT activity was found in the S phase, while no such cyclic change was observed in the case of the nos promoter. This system clearly shows that it is feasible to search for a cell cycle specific promoter. The significance of these observations is discussed in relation to the study of plant cells. 相似文献
82.
Summary Investigations into iron deficiency have been hindered by the lack of a satisfactory diagnostic tissue test, which in turn
results from the total iron content of plant tissue commonly being an unreliable index of the iron status. Our measurements
of chlorotic and normal leaves of field grown groundnut (Arachis hypogaea L.) showed that total iron was unsatisfactory as the measure of iron status of plant tissue. It was found that iron status
was better assessed from an estimate of the ferrous iron content of fresh leaf materials obtained by extraction with o-phenanthroline.
Extractable iron content increased with leaf age. Chlorotic buds or the first fully opened leaf always contained less than
6μg extractable-Fe/g fresh tissue.
Approved for publication as ICRISAT Journal Article No. 307. 相似文献
83.
Advances in salt tolerance 总被引:6,自引:0,他引:6
Summary Advances in and prospects for the development of salt tolerant crops are discussed. The genetic approach to the salinity problem
is fairly new, but research has become quite active in a short span of time. Difficulties and opportunities are outlined.
Salinity varies spatially, temporally, qualitatively, and quantitatively. In addition, the responses of plants to salt stress
vary during their life cycle. Selection and breeding, including the use of wide crosses, are considered the best short-term
approaches to the development of salt tolerant crops, but the new biotechnological and molecular biological techniques will
make increasingly important contributions. Cooperation is called for among soil and water scientists, agronomists, plant physiologists
and biochemists, cytologists, and plant geneticists, breeders, and biotechnologists. Given such cooperation and adequate support
for these endeavors, the potential for increasing productivity in salt-affected areas can be realized. 相似文献
84.
Summary Fluorescein or rhodamine conjugates of seventeen different lectins were tested for their ability to label the plasma membrane of live plant protoplasts. During the investigation, a strong effect of calcium was observed on the binding of several lectins to protoplasts derived from suspension cultured rose cells (Rosa sp. Paul's Scarlet). The binding of these lectins was increased by elevating the calcium concentration from 1 to 10 mM in the buffer. Other divalent cations had variable, but similar, effects on lectin binding. The mechanism of this effect appeared to involve the protoplast surface rather than the lectins. Although the cell wall-degrading enzymes used to isolate protoplasts had generally no effect on lectin binding, one clear exception was observed. Binding ofArachis hypogaea agglutinin was markedly reduced on protoplasts isolated with Driselase as compared to protoplasts isolated with a combination of Cellulysin and Pectolyase Y-23. Although most of the lectins that labeled protoplasts derived from cultured rose cells or from corn root cortex (Zea mays L. WF9 × Mo17) had specificities for galactose or N-acetylgalactosamine, some differences in protoplast labeling between lectins of the same saccharide specificity were observed. Two different analyses of the interaction betweenRicinus communis agglutinin and rose protoplasts showed that binding was cooperative with an apparent association constant of 7.2 × 105M–1 or 9.8 × 105M–1 with a maximum of approximately 108 lectin molecules bound per protoplast. Treatment of protoplasts with glycosidases which hydrolyze either N- or O-glycosidic linkages of glycoproteins slightly enhanced labeling of protoplasts byRicinus communis agglutinin. Interpretation of these results are discussed.Abbreviations MPR
medium, minimal organic medium (Nothnagel andLyon 1986)
- APA
Abrus precatorius agglutinin
- CSA
Cytisus sessilifolius agglutinin
- ECA
Erythrina cristagalli agglutinin
- GS-I
Griffonia simplicifolia agglutinin
- LcH
Lens culinarus agglutinin
- PNA
Arachis hypogaea agglutinin
- SBA
Glycine max agglutinin
- VAA
Viscum album agglutinin
- VFA
Vicia faba agglutinin
- WGA
Triticum vulgaris agglutinin
- Con A
Canavalia ensiformis agglutinin
- HPA
Helix pomatia agglutinin
- TPA
Tetragonolobus purpureas agglutinin
- RCA
Ricinus communis agglutinin
- DBA
Dolichos biflorus agglutinin
- SJA
Sophora japonica agglutinin
- BPA
Bauhinia purpurea agglutinin
- FITC
fluorescein isothiocyanate
- Ga1NAc
N-acetylgalactosamine
- FDA
fluorescein diacetate
- 2-O-Me-D-Fuc
2-O-methyl-D-fucose
Parts of the work presented here are also submitted in partial fulfillment of requirements for the Ph.D. degree. 相似文献
85.
Summary The mechanism of water movement across roots is, as yet, not well understood. Some workable black box theories have already been proposed. They, however, assumed unrealistic cell membranes with low values of , or were based on a poor anatomical knowledge of roots. The role of root stele in solute and water transport seems to be especially uncertain. An attempted explanation of the nature of root exudation and root pressure by applying the apoplast canal theory (Katou andFurumoto 1986 a, b) to transport in the root stele is given. The canal equations are solved for boundary conditions based on anatomical and physiological knowledge of the root stele. It is found that the symplast cell membrane, cell wall and net solute transport into the wall apoplast are the essential constituents of the canal system. Numerical analysis shows that the canal system enables the coupled transport of solutes and water into a xylem vessel, and the development of root pressure beyond the level predicted by the osmotic potential difference between the ambient medium and the exudate. Observations on root exudation and root pressure previously reported seem to be explained quite well. It is concluded that the movement of water in the root stele although apparently active is essentially osmotic.Abbreviations J
v
ex
volume exudation per root surface
- J0
non-osmotic exudation
- Lr
overall radial hydraulic conductivity of an excised root
-
reflection coefficient
- Cs
difference in the osmotic concentration between the bathing medium and the exudate
- R
gas constant
- T
absolute temperature
- CK
molar concentration of K+
- CCl
molar concentration of Cl–
- Cj
molar concentration of ion species j
- Pj
membrane permeability of ion j
- zj
valence of ion j
- F
Faraday constant
- Vix
intracellular electric potential with reference to the canal 相似文献
86.
A. Grębecki 《Protoplasma》1987,141(2-3):126-134
Summary The transverse velocity profiles of the anterograde flow of particles on the cell surface and around it are approximately parabolic. The peak velocity is recorded close to the membrane and the descendent arm of the profile is viscosity-dependent. It indicates that the extracellular forward flow is probably generated by a forward movement of the fluid fraction of the membrane itself. The retrograde component of extracellular movements is manifested by particles kept on the cell surface by adhesion, which behave exactly as the ectoplasmic layer on the opposite side of the membrane,i.e., they probably reflect the movement of that fraction of the surface material which is attached to the cortical microfilaments. In the longitudinal profile, the velocity of anterograde flow rises from the tail to the front of amoeba, but is generally related to the effective cell locomotion rate and not to the movements of any intracellular layer. Around the cells deprived of any attachment to the substratum, which cannot locomote but manifest vigorous intracellular movements, the anterograde flow ceases at least along 2/3 of their lenght. It persists, however, around the frontal fountain zone, where other particles still move backwards together with the retracted ectoplasmic layer. This indicates that the role of the forward flow of and on the cell surface is to compensate for: (1) the increase of the surface area in the frontal regions due to locomotion, (2) the withdrawal of a part of material which is hauled back by the retracting cortical layer. A comprehensive scheme of the velocity distribution within the different layers of a moving amoeba and around it has been constructed on the basis of present and earlier data.Study supported by the Research Project CPBP 04.01 of the Polish Academy of Science.I dedicate this paper to Professor K. E. Wohlfarth-Bottermann with the best wishes for his 65th birthday. 相似文献
87.
The free and N-acetyl glucosamine contents, serving as a measure of the amounts of chitosan and chitin respectively, were
determined in the chitinase hydrolysates of the cell wall of a wild strain ofNeurospora crassa. Chitinase, obtained from cultures ofSerratia marcescens, could hydrolyse the cell wall completely apart from being capable of hydrolysing preparations of chitin and chitosan. The
free and N-acetyl glucosamines, released by chitinase hydrolysis, were determined by a modified Morgan-Elson reaction carried
out in the presence and absence of acetic anhydride. The method is capable of estimating chitin and chitosan contents in as
little as 100 μg of cell wall material. 相似文献
88.
Arie H. Mulder Francois Hogenboom George Wardeh Anton N. M. Schoffelmeer 《Journal of neurochemistry》1987,48(4):1043-1047
Synaptosomes prepared from rat cerebral cortex and labeled with [3H]noradrenaline (NA) were superfused with calcium-free Krebs-Ringer-bicarbonate medium and exposed to 10 mM K+ plus 0.1 mM Ca2+ so that [3H]NA release was induced. 6,7-Dihydroxy-N,N-dimethyl-2-aminotetralin (TL-99) strongly inhibited synaptosomal K+-induced [3H]NA release (EC50 = 5-10 nM) by activating alpha 2-adrenoceptors. Release was also inhibited (maximally by 40-50%) by morphine (EC50 = 5-10 nM), [Leu5]enkephalin (EC50 = approximately 300 nM), [D-Ala2,D-Leu5]enkephalin (DADLE), and Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol (DAGO) (EC50 values = approximately 30 nM). In contrast to the mu-selective opioid receptor agonists morphine and DAGO, the highly delta-selective agonist [D-Pen2,D-Pen5]enkephalin (1 microM) did not affect [3H]-NA release. Furthermore, the inhibitory effect of DADLE, an agonist with affinity for both delta- and mu-opioid receptors, was antagonized by low concentrations of naloxone. The findings strongly support the view that, like alpha 2-adrenoceptors, mu-opioid receptors mediating inhibition of NA release in the rat cerebral cortex are localized on noradrenergic nerve terminals. 相似文献
89.
A "fatigue" of acetylcholine (ACh) release is described in cholinergic synaptosomes stimulated with the calcium ionophore A23187 or gramicidin. A small conditioning calcium entry, which did not trigger a large ACh release, led to a decrease of transmitter release elicited by a second large calcium influx. This fatigue was half-maximal at approximately 30 microM external calcium and developed in a few minutes. In contrast, activation of release by calcium was very rapid and was half-maximal at approximately 0.5 mM external calcium. Activation and desensitization of release could be attributed to the recently identified presynaptic membrane protein, the "mediatophore." Proteoliposomes equipped with purified mediatophore showed a calcium-dependent activation and "fatigue" of ACh release similar to that of synaptosomes. It was found that the ionophore A23187 rapidly equilibrated internal and external calcium concentrations in proteoliposomes. Thus, the external calcium concentration gave the internal concentration required for activation or desensitization of proteoliposomal ACh release. The mediatophore showed remarkable calcium binding properties (20 sites/molecule) with a KD of 25 microM. The physiological implications of desensitization on the organization of release sites are discussed. 相似文献
90.
Enhancement of Stimulation-Evoked Catecholamine Release from Cultured Bovine Adrenal Chromaffin Cells by Forskolin 总被引:16,自引:9,他引:7
Katsuya Morita Toshihiro Dohi Shigeo Kitayama Yutaka Koyama Akira Tsujimoto 《Journal of neurochemistry》1987,48(1):243-247
Acetylcholine (ACh) increased cyclic AMP levels in cultured bovine chromaffin cells with a peak effect at 1 min after the addition. Pretreatment with forskolin (0.3 microM) enhanced the ACh-evoked cyclic AMP increase. The catecholamine (CA) release induced by ACh was enhanced by forskolin, but forskolin alone did not enhance the CA release. The effect of forskolin increased dose-dependently up to 1 microM, but decreased at higher concentrations. Dibutyryl cyclic AMP (DBcAMP) also enhanced ACh-evoked CA release, but the effect was less potent than that of forskolin. Forskolin enhanced both [3H]norepinephrine ([3H]NE) and endogenous CA release evoked by 30 mM K+ from cells that were preloaded with [3H]NE. The effects of forskolin were substantial when CA release was evoked with low concentrations of ACh or excess K+, but decreased with higher concentrations of the stimulants. Forskolin also enhanced the CA release induced by ionomycin and veratrine, or by caffeine in Ca2+-free medium. The potentiation by forskolin of the ACh-evoked CA release was manifest in low Ca2+ concentrations in the medium, but decreased when Ca2+ concentration was increased. These results suggest that cyclic AMP may play a role in the modulation of CA release from chromaffin cells. 相似文献