全文获取类型
收费全文 | 112217篇 |
免费 | 1909篇 |
国内免费 | 816篇 |
出版年
2023年 | 88篇 |
2022年 | 233篇 |
2021年 | 432篇 |
2020年 | 233篇 |
2019年 | 307篇 |
2018年 | 12170篇 |
2017年 | 10946篇 |
2016年 | 8007篇 |
2015年 | 1606篇 |
2014年 | 1440篇 |
2013年 | 1690篇 |
2012年 | 5896篇 |
2011年 | 14336篇 |
2010年 | 12955篇 |
2009年 | 9085篇 |
2008年 | 10927篇 |
2007年 | 12493篇 |
2006年 | 1350篇 |
2005年 | 1554篇 |
2004年 | 1984篇 |
2003年 | 1944篇 |
2002年 | 1651篇 |
2001年 | 458篇 |
2000年 | 303篇 |
1999年 | 178篇 |
1998年 | 241篇 |
1997年 | 170篇 |
1996年 | 145篇 |
1995年 | 125篇 |
1994年 | 117篇 |
1993年 | 131篇 |
1992年 | 100篇 |
1991年 | 102篇 |
1990年 | 75篇 |
1989年 | 68篇 |
1988年 | 67篇 |
1987年 | 69篇 |
1986年 | 42篇 |
1985年 | 46篇 |
1984年 | 50篇 |
1983年 | 80篇 |
1982年 | 39篇 |
1981年 | 32篇 |
1980年 | 37篇 |
1978年 | 42篇 |
1977年 | 35篇 |
1976年 | 32篇 |
1975年 | 30篇 |
1972年 | 259篇 |
1971年 | 284篇 |
排序方式: 共有10000条查询结果,搜索用时 26 毫秒
991.
Human activities have greatly increased the input of biologically available nitrogen (N) from land-based sources to aquatic
ecosystems; yet few studies have examined how human actions influence N export in regions with a strong seasonality in water
availability. In this study, we quantified N inputs and outputs for 23 California watersheds and examined how climate, hydrology,
and land use practices influenced watershed N export. N inputs ranged from 581 to 11,234 kg N km−2 year−1 among watersheds, with 80% of total input for the region originating from agriculture (inorganic fertilizer, manure, and
legumes). Of the potential N sources examined, mean annual concentrations of dissolved organic N and dissolved inorganic N
in study rivers correlated most strongly with manure N input (r
2 = 0.54 and 0.53, respectively). Seasonal N export varied by basin and was correlated with climate, anthropogenic N inputs,
and reservoir releases. Fractional export of watershed N inputs by study rivers annually was small (median of 8%) and scaled
exponentially with runoff (r = 0.66). Collectively, our results show that anthropogenic activities have altered both the magnitude and timing of watershed
N export in California and suggest that targeted management in specific locations and times of the year could reduce N export
to downstream systems in the region. 相似文献
992.
The isolation of photosystem-I (PS-I) from spinach has been conducted using ultrafiltration with 300 kDa molecular weight
cut-off polyethersulfone membranes. The effects of ultrafiltration operating conditions on PS-I activity were optimized using
parameter scanning ultrafiltration. These conditions included solution pH, ionic strength, stirring speed, and permeate flux.
The effects of detergent (Triton X-100 and n-dodecyl-beta-D-maltoside) concentration on time dependent activity of PS-I were also studied using an O2 electrode. Under optimized conditions, the PS-I purity obtained in the retentate was about 84% and the activity recovery
was greater than 94% after ultrafiltration. To our knowledge, this is the first report of the isolation of a membrane protein
using ultrafiltration alone. 相似文献
993.
‘Green tides’ are overwhelming the coastline of our blue planet: taking the world’s largest example 总被引:8,自引:0,他引:8
Nai-hao Ye Xiao-wen Zhang Yu-ze Mao Cheng-wei Liang Dong Xu Jian Zou Zhi-meng Zhuang Qing-yin Wang 《Ecological Research》2011,26(3):477-485
A broad spectrum of events that come under the category of green tide are recognized world-wide as a response to elevated
levels of seawater nutrients in coastal areas. Green tides involve a wide diversity of sites, macroalgal species, consequences,
and possible causes. Here we review the effect of natural and man-induced environmental fluctuations on the frequency and
apparent spread of green tides. This article highlights the need for interdisciplinary research aimed at shedding light on
the basic mechanisms governing the occurrence and succession of green algae in coastal seas. This will result in more effective
management and mitigation of the effects of green tides, thus safeguarding the intrinsic and commercial value of coastal marine
ecosystems. 相似文献
994.
Thomas Proft 《Biotechnology letters》2010,32(1):1-10
Sortases are transpeptidases produced by Gram-positive bacteria to anchor cell surface proteins covalently to the cell wall.
The Staphylococcus aureus sortase A (SrtA) cleaves a short C-terminal recognition motif (LPXTG) on the target protein followed by the formation of an amide bond with the pentaglycine
cross-bridge in the cell wall. Over recent years, several researchers have exploited this specific reaction for a range of
biotechnology applications, including the incorporation of non-native peptides and non-peptidic molecules into proteins, the
generation of nucleic acid–peptide conjugates and neoglycoconjugates, protein circularisation, and labelling of cell surface
proteins on living cells. 相似文献
995.
Katsunori Motosaka Masayuki Koganezawa Satoko Narikawa Akira Furuyama Kenji Shinozaki Kunio Isono Ichiro Shimada 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》2007,193(2):279-283
Acute choice behavior in ingesting two different concentrations of sucrose in Drosophila is presumed to include learning and memory. Effects on this behavior were examined for four mutations that block associative
learning (dunce, rutabaga, amnesiac, and radish). Three of these mutations cause cyclic AMP signaling defects and significantly reduced taste discrimination. The exception
was radish, which affects neither. Electrophysiological recordings confirmed that the sensitivity of taste receptors is almost indistinguishable
in all flies, whether wild type or mutant. These results suggest that food choice behavior in Drosophila involves central nervous learning and memory operating via cyclic AMP signaling pathways. 相似文献
996.
Mariëtte van Amstel Claar de Brauw Peter Driessen Pieter Glasbergen 《Biodiversity and Conservation》2007,16(14):4109-4129
This paper seeks to understand why multinationals prefer to launch a label specific to their own product and examines how
reliable these product-specific eco-labels are. A new methodology is applied to assess the extent to which eco-labels live
up to claims about their contribution to conservation and the sustainable use of agricultural biodiversity. Product-specific
eco-labels are considered as industry self-regulation and all three regulatory stages are studied: the planning, implementation
and outcome stage. There are major differences between the product specific eco-labels in the degree in which agrobiodiversity
management is part of the normative labeling schemes. Although there are some problems of reliability, such as transparency
in the implementation stage and the monitoring in the outcome stage, the degree of reliability of product-specific labels
is comparable with eco-labels of international labeling families. The conclusion is that only one of the product-specific
eco-labels examined here is reliable when examined in the light of all three stages. The main reason why multinationals establish
a product-specific eco-label instead of adopting one from an existing labeling family is that they want to profile themselves
as distinct from other companies. The unique character of a product-specific label creates a market opportunity for them. 相似文献
997.
Fushi Wen Ho Hyung Woo Elizabeth A. Pierson Toril D. Eldhuset C. G. Fossdal N. E. Nagy Martha C. Hawes 《Plant Molecular Biology Reporter》2009,27(1):58-68
Root cap development in cereals and legumes is self-regulated by a repressor that accumulates in the extracellular environment,
and immersing the root tip into water results in renewed cap development. By exploiting this phenomenon, root cap mitosis
and differentiation can be synchronously induced among populations. In Pisum sativum L., messenger RNA (mRNA) differential display revealed changes in expression of approximately 1% of the sample mRNA population
within minutes of induced cap turnover. This profile changes sequentially over a period of 30 min, then stabilizes. Microarray
analysis of Medicago truncatula root caps confirmed changes in expression of approximately 1% of the target population, within minutes. A cell specific marker
for cap turnover exhibited the same temporal and spatial expression profile in the gymnosperm species Norway spruce (Picea abies) as in pea. Induced cap development provides a means to profile cell-specific gene expression among phylogenetically diverse
species from the early moments of mitosis and cellular differentiation. 相似文献
998.
Seedlings of two barley genotypes (‘Maresi’ and wild form of Hordeum spontaneum) were treated with jasmonic acid (JA 5 μM and 15 μM) for 24 h, and then subjected to water stress (PEG 6000 solution of −
1.5 MPa). JA caused an increase in the content of ABA but not in that of proline and spermidine in the two studied genotypes.
The effect of the treatment did not depend on the applied JA concentration. The pre-stress treatment with JA changed plant
response to water deficit with regard to membrane injury. Treatment with a lower JA concentration (5 μM) caused a substantial
reduction of the stress-induced membrane damage in the both genotypes. A higher JA concentration (15 μM) caused the reduction
of membrane injury only in H. spontaneum and was ineffective in ‘Maresi’. JA had no influence on the leaf water status in water-stressed plants. A possible role of
JA in leaf ABA accumulation and alleviation of cell membrane injury under water deficit is discussed.
The work was partly supported by the Polish Committee For Scientific Research, grant No 5 PO6A 036 18 相似文献
999.
1000.