全文获取类型
收费全文 | 1188521篇 |
免费 | 94479篇 |
国内免费 | 98788篇 |
专业分类
1381788篇 |
出版年
2023年 | 8468篇 |
2022年 | 9508篇 |
2021年 | 11000篇 |
2020年 | 9976篇 |
2019年 | 11198篇 |
2018年 | 11557篇 |
2017年 | 8027篇 |
2016年 | 9063篇 |
2015年 | 10332篇 |
2014年 | 13665篇 |
2013年 | 12626篇 |
2012年 | 107805篇 |
2011年 | 121695篇 |
2010年 | 29569篇 |
2009年 | 21956篇 |
2008年 | 102113篇 |
2007年 | 106037篇 |
2006年 | 99382篇 |
2005年 | 94545篇 |
2004年 | 91639篇 |
2003年 | 86929篇 |
2002年 | 77158篇 |
2001年 | 62018篇 |
2000年 | 77371篇 |
1999年 | 32672篇 |
1998年 | 7196篇 |
1997年 | 5459篇 |
1996年 | 4655篇 |
1995年 | 4403篇 |
1994年 | 4575篇 |
1993年 | 3779篇 |
1992年 | 4359篇 |
1991年 | 3846篇 |
1990年 | 3777篇 |
1989年 | 4294篇 |
1988年 | 4270篇 |
1987年 | 3919篇 |
1986年 | 3735篇 |
1985年 | 3572篇 |
1983年 | 3390篇 |
1959年 | 3879篇 |
1958年 | 6824篇 |
1957年 | 6798篇 |
1956年 | 6073篇 |
1955年 | 5733篇 |
1954年 | 5439篇 |
1953年 | 5173篇 |
1952年 | 4656篇 |
1951年 | 4278篇 |
1950年 | 3511篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
A Starchless Mutant of Nicotiana sylvestris Containing a Modified Plastid Phosphoglucomutase 总被引:11,自引:8,他引:3 下载免费PDF全文
A mutant (NS 458) of Nicotiana sylvestris (Spegazzini and Comes) unable to synthesize leaf starch was isolated in the M2 generation following ethyl methanesulfonate mutagenesis by testing with iodine. Segregation ratios in reciprocal F2 progenies showed that the starchless phenotype resulted from a recessive mutation in a single nuclear gene. DEAE-agarose chromatography showed that the mutant is grossly deficient in plastid phosphoglucomutase (EC 2.1.5.1) activity. The structure of the enzyme is changed, as evidenced by increased Michaelis constants and by the prolonged activation period (>40 minutes) observed when the enzyme is assayed in triethanolamine buffer rather than imidazole buffer. The activity of the wild-type enzyme with saturating glucose 6-P alone was 7% of the activity when saturating glucose 1,6-P2 was also present. The results suggest that glucose 1,6-P2 is both an effector and a dissociable reaction intermediate. The growth rate of mutant and wild-type plants were not significantly different in continuous light and on an 8-hour dark, 16-hour light cycle and the mutants grew normally under greenhouse conditions. The mutant supports growth during diurnal periods of darkness by vacuolar storage of sugars instead of chloroplast storage of starch. The simplification in metabolism achieved by blocking the diversion of plastid fructose-6-P to starch facilitates the induction of oscillations in CO2 fixation. 相似文献
992.
A developmentally regulated hydroxyproline-rich glycoprotein in maize pericarp cell walls 总被引:12,自引:3,他引:9 下载免费PDF全文
We have studied the accumulation of peptidyl hydroxyproline in the pericarp of developing maize (Zea mays L., Golden cross Bantam sweet corn) kernels. Although this hydroxyproline accumulates throughout development, it is most soluble and its content per milligram dry weight greatest at midmaturation stages of development. Salt-soluble proteins containing this hydroxyproline from isolated cell walls of developing kernels were fractionated on a CsCl density gradient and on a Chromatofocusing column, resulting in the purification of an hydroxyproline-rich glycoprotein, PC-1. PC-1 is a basic protein of approximately 65 to 70 kilodaltons in molecular weight with an isoelectric point of at least 10.2 and a density of 1.38 to 1.39 in CsCl. Amino acid composition data indicate that it is rich in hydroxyproline, threonine, proline, lysine, and glycine. Its relation to dicot extensin is discussed. 相似文献
993.
Ammonia Production and Assimilation in Glutamate Synthase Mutants of Arabidopsis thaliana 总被引:6,自引:6,他引:0 下载免费PDF全文
Ammonia production and assimilation1 were examined in photorespiratory mutants of Arabidopsis thaliana L. lacking ferredoxin-dependent glutamate synthase (Fd-GluS) activity. Although photosynthesis was rapidly inhibited in these mutants in normal air, NH4+ continued to accumulate. The accumulation of NH4+ was also seen after an initial lag of 30 minutes in 2% O2, 350 microliters per liter of CO2 and after 90 minutes in 2% O2, 900 microliters per liter of CO2. The accumulation of NH4+ in normal air and low O2 was also associated with an increase in the total pool of amino acid-N and glutamine, and a decrease in the pools of glutamate, aspartate, alanine, and serine. Upon return to dark conditions, or to 21% O2, 1% CO2 in the light, the NH4+ which had accumulated in the leaves was reassimilated into amino acids. The addition of methionine sulfoximine (MSO) resulted in higher accumulations of NH4+ in glutamate synthase mutants and prevented the reassimilation of NH4+ upon return to the dark. The addition of MSO also resulted in the accumulation of NH4+ in glutamate synthase mutants in the light and in 21% O2, 1% CO2. These results indicate that glutamine synthetase is essential for the reassimilation of photorespiratory NH4+ and for primary N assimilation in the leaves and strongly suggest that glutamate dehydrogenase plays only a minimal role in the assimilation of ammonia. Levels of NADH-dependent glutamate synthase (NADH-GluS) appear to be sufficient to account for the assimilation of NH4+ by a GS/NADH-GluS cycle. 相似文献
994.
Comparison of Triazine-Resistant and -Susceptible Biotypes of Senecio vulgaris and Their F(1) Hybrids 总被引:3,自引:3,他引:0 下载免费PDF全文
The relationship of triazine resistance to decreased plant productivity was investigated in Senecio vulgaris L. F1 reciprocal hybrids were developed from pure-breeding susceptible (S) and resistant (R) lines. The four biotypes (S, S × R, R, R × S) were compared in terms of atrazine response, electron transport, carbon fixation, and biomass production. Atrazine response, carbon fixation rate, and PSII and whole-chain electron transport rates of hybrids were nearly identical to those of their respective maternal parents. Significant differences occurred between the two susceptible (S, S × R) and two resistant (R, R × S) biotypes in atrazine response (I50), carbon fixation rate, and PSII and whole-chain electron transport rates; PSI rates were identical in all four biotypes. Coupled and uncoupled, whole-chain electron transport rates of thylakoids of the two susceptible biotypes were approximately 50% greater than those of the two resistant biotypes at photon flux densities greater than 215 micromoles per square meter per second. Carbon exchange rates of the two susceptible biotypes were 23% greater than those of the two resistant biotypes. Hybrid biotypes (S × R, R × S) were not identical to their maternal parents in biomass production. The S, S × R, and R × S plants all achieved greater biomass than R plants. These results suggest that while the resistance mutation influences thylakoid performance, reduced productivity of triazine-resistant plants cannot be ascribed solely to decreases in electron transport or carbon assimilation rates brought about by the altered binding protein. Since the F1 hybrids differed from their maternal parents only in nuclear genes, it appears that the detrimental effects of the triazine resistance mutation on plant growth may be attenuated by interactions of the plastid and nuclear genomes. 相似文献
995.
Localization of Nitrogen-Assimilating Enzymes in the Chloroplast of Chlamydomonas reinhardtii 总被引:7,自引:3,他引:4 下载免费PDF全文
The specific activities of nitrate reductase, nitrite reductase, glutamine synthetase, glutamate synthase, and glutamate dehydrogenase were determined in intact protoplasts and intact chloroplasts from Chlamydomonas reinhardtii. After correction for contamination, the data were used to calculate the portion of each enzyme in the algal chloroplast. The chloroplast of C. reinhardtii contained all enzyme activities for nitrogen assimilation, except nitrate reductase, which could not be detected in this organelle. Glutamate synthase (NADH- and ferredoxin-dependent) and glutamate dehydrogenase were located exclusively in the chloroplast, while for nitrite reductase and glutamine synthetase an extraplastidic activity of about 20 and 60%, respectively, was measured. Cells grown on ammonium, instead of nitrate as nitrogen source, had a higher total cellular activity of the NADH-dependent glutamate synthase (+95%) and glutamate dehydrogenase (+33%) but less activity of glutamine synthetase (−10%). No activity of nitrate reductase could be detected in ammonium-grown cells. The distribution of nitrogen-assimilating enzymes among the chloroplast and the rest of the cell did not differ significantly between nitrate-grown and ammonium-grown cells. Only the plastidic portion of the glutamine synthetase increased to about 80% in cells grown on ammonium (compared to about 40% in cells grown on nitrate). 相似文献
996.
During indoleacetic acid (IAA) oxidation by horseradish peroxidase the water soluble model polyene, crocin, is bleached. IAA-oxidation and crocin bleaching are stimulated at acidic pH as well as by the monophenol p-hydroxyacetophenone. IAA oxidation and crocin bleaching are neither influenced by catalase or superoxide dismutase nor by different OH-radical scavengers, whereas both ascorbate and propylgallate are inhibitory. 相似文献
997.
A Mutant of Nicotiana sylvestris Lacking Serine:Glyoxylate Aminotransferase: Substrate Specificity of the Enzyme and Fate of [2-C]Glycolate in Plants with Genetically Altered Enzyme Levels 下载免费PDF全文
The photorespiratory mutant of Nicotiana sylvestris, NS 349, lacking serine:glyoxylate aminotransferase (SGAT) grows in 1% CO2 but not in normal air (NA McHale, EA Havir, I Zelitch 1988 Theor Appl Genet. In press). Alanine:hydroxypyruvate and asparagine:hydroxypyruvate aminotransferase activities were also lacking in the mutant, and plants heterozygous with respect to SGAT which grow in normal air had 50% of the activities present in homozygous plants. Therefore, all these activities are associated with the same enzyme. On feeding [2-14C]glycolate to leaf discs in the light, NS 349 showed reduced incorporation of radioactivity into the neutral and organic acid fractions and increased incorporation into the amino acid fraction, principally into serine. The effect of reducing SGAT by 50% in heterozygous plants produced little change in the metabolism of [2-14C]glycolate, showing there is a large excess of this enzyme in wild-type plants. 相似文献
998.
Elicitor-active preparations from the fungal pathogen of bean Colletotrichum lindemuthianum stimulated the accumulation of products characteristic of lipid peroxidation in treated bean tissues. Bean suspension cells treated with crude and purified elicitors accumulated `lipofuscin-like pigment' (LEP) and malondialdehyde. The accumulation of LFP after about 6 h of treatment coincided with the onset of visible browning and production of the bean phytoalexins kievitone, phaseollin, and phaseollinisoflavan. The induction of phytoalexins and accumulation of LFP were also triggered by treatments with generators of activated oxygen species, xanthine:xanthine oxidase and Fe:ethylenediaminedi-o-hydroxyphenylacetic acid. These data suggest that generation of active oxygen species may be involved in lipid peroxidation triggered by elicitors. 相似文献
999.
Biochemical Studies of Paraquat-Tolerant Mutants of the Fern Ceratopteris richardii 总被引:3,自引:1,他引:2 下载免费PDF全文
Enzymes and metabolites associated with mitigation of paraquat toxicity were compared in two paraquat-tolerant mutants and a sensitive wild-type strain of the fern Ceratopteris richardii Brongn. In 21-day-old gametophytes, the specific activities of superoxide dismutase, catalase, peroxidase, glutathione reductase, dehydroascorbate reductase, and ascorbate peroxidase showed no differences that would explain mutant tolerance. Constitutive levels of ascorbate and glutathione also did not differ significantly in the three strains. An experiment testing the inducibility of paraquat tolerance revealed no change in the dose response of mutant or wild type gametophytes after exposure to sublethal concentrations of the herbicide. Uptake of paraquat by whole gametophytes was also equivalent in mutants and wild type. These data suggest that the physiological basis for tolerance in these mutants, unlike several other tolerant biotypes reported, does not lie in the oxygen radical scavenging system, in an inducible stress response, or in a block to whole-plant uptake. 相似文献
1000.
Evaluation of selectable markers for obtaining stable transformants in the gramineae 总被引:13,自引:5,他引:8 下载免费PDF全文
Hauptmann RM Vasil V Ozias-Akins P Tabaeizadeh Z Rogers SG Fraley RT Horsch RB Vasil IK 《Plant physiology》1988,86(2):602-606
Cell suspension cultures of Triticum monococcum, Panicum maximum, Saccharum officinarum, Pennisetum americanum, and a double cross trispecific hybrid between Pennisetum americanum, P. purpureum, and P. squamulatum were tested for resistance to kanamycin, hygromycin, and methotrexate for use in transformation studies. All cultures showed high natural levels of resistance to kanamycin, in excess of 800 milligrams per liter, and variable levels of resistance to hygromycin. Methotrexate was a potent growth inhibitor at low concentrations with all species. Kanamycin and hygromycin were growth inhibitory only if added early (within 5 days after protoplast isolation and culture). Protoplasts of T. monococcum, P. maximum, S. officinarum, and the tri-specific hybrid were electroporated with plasmid DNA containing hygromycin (pMON410), kanamycin (pMON273), or methotrexate (pMON806) resistance genes. Resistant colonies were obtained at low frequencies (1 × 10−5 to 2 × 10−6) when selected under conditions which were growth inhibitory to protoplasts electroporated without DNA. Southern blot hybridization confirmed stable integration of plasmid DNA into T. monococcum using hygromycin vectors and P. maximum using the methotrexate vector with 1 to 10 copies integrated per haploid genome. 相似文献