全文获取类型
收费全文 | 39015篇 |
免费 | 4468篇 |
国内免费 | 1835篇 |
出版年
2023年 | 303篇 |
2022年 | 411篇 |
2021年 | 1227篇 |
2020年 | 932篇 |
2019年 | 1174篇 |
2018年 | 1218篇 |
2017年 | 891篇 |
2016年 | 1316篇 |
2015年 | 1828篇 |
2014年 | 2113篇 |
2013年 | 2262篇 |
2012年 | 2697篇 |
2011年 | 2576篇 |
2010年 | 1502篇 |
2009年 | 1428篇 |
2008年 | 1736篇 |
2007年 | 1522篇 |
2006年 | 1432篇 |
2005年 | 1182篇 |
2004年 | 996篇 |
2003年 | 925篇 |
2002年 | 830篇 |
2001年 | 2555篇 |
2000年 | 2383篇 |
1999年 | 1766篇 |
1998年 | 614篇 |
1997年 | 671篇 |
1996年 | 557篇 |
1995年 | 532篇 |
1994年 | 422篇 |
1993年 | 350篇 |
1992年 | 886篇 |
1991年 | 726篇 |
1990年 | 636篇 |
1989年 | 483篇 |
1988年 | 394篇 |
1987年 | 324篇 |
1986年 | 239篇 |
1985年 | 196篇 |
1984年 | 128篇 |
1983年 | 112篇 |
1982年 | 64篇 |
1981年 | 57篇 |
1980年 | 41篇 |
1979年 | 42篇 |
1978年 | 37篇 |
1977年 | 31篇 |
1976年 | 38篇 |
1974年 | 30篇 |
1973年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Photosynthetic pathways (e.g. C3, C4) and morphological functional types (e.g. trees, shrubs, high perennial grasses, perennial forbs) were identified for the native species from the Saskatchewan mixed
prairie, using the data from references published between 1950 and 2003. Of the total 219 identified species in 145 genera
and 45 families, 208 species in 137 genera and 44 families were found with C3 photosynthesis, and most of these species are dominants (e.g. Agropyron dasystachyum Hook. and Stipa spartea var. curtiseta Hitchc.). 11 species in 10 genera and 3 families were identified with C4 photosynthesis (e.g. Atriplex argentea Nutt., Andropogon scoparius Michx., Boutelou gracilis Lag., Calamovilfa longifolia Hook.). The amount of total identified C4 species in the region is much less than that from the South Dakota mixed prairie (27 species). Gramineae is the leading family with C4 photosynthesis (8 species), Chenopodiaceae ranks the second (2 species). Relatively less forb types [50 % perennial forbs (PEF) and 12 % annual forbs (ANF)] and more
graminoid types (25 %) composition suggested that the rangelands in the region are relatively stable. Lacking of the knowledge
on the optimal traits for PFTs classification in the region, further studies (e.g. C3 and C4 plant identification and optimal trait selection) are needed to explore the relationships between PFTs and vegetation variations,
as well as land-use and climate changes. 相似文献
992.
Y.-P. Guo M.-J. Hu H.-F. Zhou L.-C. Zhang J.-H. Su H.-W. Wang Y.-G. Shen 《Photosynthetica》2006,44(3):333-337
Spraying low concentrated (0.5–5.0 mM) solutions of NaHSO3 on Satsuma mandarin (Citrus unshiu Marc.) leaves resulted in enhancement (maximal about 15 % at 1 mM NaHSO3) of net photosynthetic rate (P
N) for 6 d. The potential photochemical efficiency of photosystem 2 (PS2, Fv/Fm) and the quantum yield of PS2 electron transport (ΦPS2) were increased under strong photon flux density (PFD). The slow phase of millisecond delayed light emission (ms-DLE) was
increased, showing that the transmembrane proton motive force related to photophosphorylation was enhanced. We also observed
that low concentrations of NaHSO3 promoted the production of ATP in irradiated leaves. We suggest that the increase in P
N in Satsuma mandarin leaves caused by low concentrations of NaHSO3 solution may have been due to the stimulation of photophosphorylation and, hence, the increase in photochemical efficiency
through speeding-up of PS2 electron transport. Photoinhibition of photosynthesis in leaves was modified by NaHSO3 treatment under high PFD. Hence the increase in leaf dry mass seems to be associated with the mitigation of photoinhibition
caused by strong PFD. 相似文献
993.
Examining the Role of Collective Action in an Informal Seed System: A Case Study from the Central Valleys of Oaxaca, Mexico 总被引:1,自引:0,他引:1
Lone B. Badstue Mauricio R. Bellon Julien Berthaud Xóchitl Juárez Irma Manuel Rosas Ana María Solano Alejandro Ramírez 《Human ecology: an interdisciplinary journal》2006,34(2):249-273
This paper explores social arrangements associated with seed transactions among small-scale maize farmers in the Central Valleys of Oaxaca, Mexico, where no formal seed supply system exists. We test the hypothesis that individual farmers have strong incentives to participate in some form of collective action to ensure their access to seed. Six communities were studied, three of them in detail, using in-depth, semistructured interviews with key informants; focus group discussions; and a tracer study that followed seed flows among farmers. Farmers mostly saved seed and only occasionally acquired seed from outside sources. We found no evidence of a specialized social organization based on collective action to mediate seed flows. Seed transactions are infrequent, bilateral, and ad hoc, although trust is an important component, as it ensures reliable information about the seed is provided. Implications of these findings are discussed, especially for genetic diversity if the current supply system breaks down. 相似文献
994.
In order to investigate the effect of large isoform of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) activase (RuBPCO-A)
on photosynthesis, cDNA of the enzyme (rca) was transferred to rice cultivars (Oryza sativa f. japonica cv. Nipponbare) under the control of RuBPCO small subunit gene promoter (rbcS) via Agrobacterium tumefaciens-mediated transformation. Transgenic rice plants were identified by polymerase chain reaction (PCR) and Southern and Western
blot analyses. Net photosynthetic rate (P
N) values of the T1 transgenic lines 34 (T34) and 40 (T40) were 45.26 and 46.32 % higher than that of the control plants, respectively. At the
same time, their carboxylation efficiency and RuBPCO initial activity, quantum yield of electron transport in photosystem
2 (ΦPS2), and steady state photochemical fluorescence quenching (qP) increased. In addition, heading time of the transgenic rice was advanced. Thus increasing the amount of large isoform of
RuBPCO-A in the transgenic rice might have a stimulatory effect on both photosynthesis and plant growth. 相似文献
995.
996.
为了筛选出最佳的小鼠原核期受精卵的体外发育培养系统,分别进行了四个试验。试验I:在体外分别用自配的M16、mM16、KSOM、mKSOM、CZB进行体外发育培养,进而筛选出一种最佳的体外培养系统;试验Ⅱ、Ⅲ、Ⅳ分别:探讨了血清、PVA、rhLIF对小鼠胚胎发育的影响。结果,试验I中胚胎发育到2-细胞的比率差异不明显,但是在mM16和mKSOM中,发育到4-细胞的比率94.7%,90.7%(91/96;78/86)和发育到桑椹胚/囊胚的比率分别为78.1%,67.4%(75/96;58/86)均明显高于其他三种培养液;试验II用10?S代替M16中BSA时,胚胎的发育率均下降,即使在mM16中桑椹胚/囊胚率仅为4.8%(12/35)与对照组M16(40.5%)差异显著(p<0.05);试验Ⅲ用PVA取代mM16和mKSOM中的BSA其体外发育率显著下降,胚胎均无一例发育到桑椹胚/囊胚;试验Ⅳ:rhLIF能提高胚胎在体外的发育率可使mM16培养的胚胎囊胚率、囊胚脱出率分别达到84%(47/56)、39.2%(22/56)。结论:在不添减其他成分前提下,只在M16中添加0.1mMolEDTA、0.5mMol牛磺酸、1000IU/mlrhLIF便可获得84%的囊胚率,同时证明在M16或mM16添加血清都会降低其体外发育率;PVA还不能有效的取代mM16、mKSOM中的血清。 相似文献
997.
998.
铜胁迫对玉米幼苗生长、叶绿素荧光参数和抗氧化酶活性的影响 总被引:32,自引:1,他引:31
本文研究了铜(Cu)胁迫下玉米(Zeamays)幼苗生长、叶绿素含量、叶绿素荧光参数和抗氧化酶活性的变化。研究结果表明,5 ̄20μmol.L-1Cu处理10天明显抑制玉米幼苗根系生长,并减少玉米幼苗的干物重,以及增加玉米幼苗地上部和根系含Cu量;玉米幼苗吸收的Cu大部分积累在根系,在地上部分布较少。Cu处理还降低玉米叶片的叶绿素含量和Fv/Fm、ETR、qP和qy值。在10天的Cu处理期间,根系中SOD、POD、CAT和GR活性呈现先上升后下降的趋势。而叶片中的SOD、POD、CAT和GR活性在处理前期不受Cu胁迫的显著影响,处理后期则因Cu胁迫而增强。实验表明抗氧化酶在抵御过量Cu引起的氧化胁迫中发挥了一定的作用。 相似文献
999.
通过分析37个SSR座位在琼海与三亚两普通野生稻(Oryza rufipogon)居群中的遗传变异,结果表明,SSR座位在三亚普通野生稻居群中的变异高于其在琼海普通野生稻居群中的变异。根据遗传相似性和遗传距离公式得到琼海与三亚普通野生稻居群间的遗传相似性为0.6385,遗传距离为0.4486cM。Wright的啪验结果表明,这37个SSR座位在两居群之间存在着中等程度的遗传分化,FST=0.3909。此分化结果主要是由两居群间弱的基因漂移导致的(Nm=0.3895)。 相似文献
1000.
马槟榔甜蛋白基因(MBLⅡ)的拼接与表达载体的构建 总被引:1,自引:0,他引:1
从云南植物Capparismasaikai中提取总DNA,设计引物克隆马槟榔甜蛋白基因MBLⅡ,序列测定后经同源性分析表明与GeneBank中登陆的马槟榔甜蛋白基因MBLⅡ(cDNA)序列一致,表明MBLⅡ不存在内含子序列。利用重叠区扩增基因拼接法(genesplicingbyoverlapextension,geneSOEing)进行体外基因剪接,剪接片段与植物表达载体pVKH相连,构建pVKH-35S-MBLⅡ-Pa、pVKH-35S-S-A B-pA、pVKH-35S-A B-pA、pVKH-35S-S-A-pA、pVKH-35S-S-B-pA,为寻找mabinlin的活性表达形式和翻译后剪接机制打下基础。 相似文献