全文获取类型
收费全文 | 9545篇 |
免费 | 881篇 |
国内免费 | 1816篇 |
出版年
2023年 | 177篇 |
2022年 | 174篇 |
2021年 | 252篇 |
2020年 | 324篇 |
2019年 | 329篇 |
2018年 | 331篇 |
2017年 | 339篇 |
2016年 | 372篇 |
2015年 | 392篇 |
2014年 | 419篇 |
2013年 | 568篇 |
2012年 | 388篇 |
2011年 | 500篇 |
2010年 | 351篇 |
2009年 | 452篇 |
2008年 | 524篇 |
2007年 | 535篇 |
2006年 | 525篇 |
2005年 | 495篇 |
2004年 | 431篇 |
2003年 | 422篇 |
2002年 | 360篇 |
2001年 | 292篇 |
2000年 | 263篇 |
1999年 | 290篇 |
1998年 | 266篇 |
1997年 | 221篇 |
1996年 | 241篇 |
1995年 | 242篇 |
1994年 | 206篇 |
1993年 | 196篇 |
1992年 | 177篇 |
1991年 | 148篇 |
1990年 | 156篇 |
1989年 | 159篇 |
1988年 | 155篇 |
1987年 | 96篇 |
1986年 | 83篇 |
1985年 | 80篇 |
1984年 | 75篇 |
1983年 | 31篇 |
1982年 | 47篇 |
1981年 | 30篇 |
1980年 | 26篇 |
1979年 | 23篇 |
1978年 | 22篇 |
1977年 | 9篇 |
1976年 | 11篇 |
1974年 | 8篇 |
1958年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Two new sesquiterpenoids (1 and 2) and a new ent-pimarane type diterpenoid (3), together with eighteen known compounds (4–21), were isolated from the whole plants of Siegesbeckia pubescens. The structures of the new compounds were determined on the basis of 1D-, 2D NMR and HRESIMS data. All compounds were evaluated for their inhibitory effects on LPS-induced nitric oxide production in RAW 264.7 macrophages. Of these, highly oxygenated germacrane type sesquiterpenoids (1–2 and 13–14) showed significant inhibitory effects with IC50 values ranging from 3.9 to 16.8 μM. 相似文献
2.
A. Rigotto S.R. Cotta A.C.F. Dias J.L.N. Carvalho F.D. Andreote 《Letters in applied microbiology》2020,71(5):444-450
Sugarcane cultivation supports Brazil as one of the largest world sugar and ethanol producer. In order to understand the impact of changing sugarcane harvest from manual to mechanized harvest, we studied the effect of machinery traffic on soil and consequently soil compaction upon soil microbial communities involved in nitrogen cycling. The impact of sugarcane harvest was dependent on soil depth and texture. At deeper soil layers, mechanized harvesting increases the abundance of nitrogen fixers and denitrifying communities (specifically nosZ clade I and II) while manual harvesting increases the abundance of ammonia oxidizers (specifically AOA) and increases denitrifying communities (nosZ clade I and II) on top and at intermediate depth. The effect of change on the harvest system is more evident on sandy soil than on clay soil, where soil indicators of compaction (bulk density and penetration resistance) were negatively correlated with soil microorganisms associated with the nitrogen cycle. Our results point to connections between soil compaction and N transformations in sugarcane fields, besides naming biological variables to be used as proxies for alterations in soil structure. 相似文献
3.
The water relations of shoots of young jack pine (Pinus banksiana Lamb.) seedlings were examined 6 and 15 weeks after the initiation of four different dynamic nitrogen (N) treatments using a pressure-volume analysis. The N treatments produced a wide range of needle N concentrations from 12 to 32 mg g?1 dry mass and a 10-fold difference in total dry mass at 15 weeks. Osmotic potential at full turgor did not change over the range of needle N concentrations observed. Osmotic potential at turgor-loss point, however, declined as N concentrations decreased, indicating an increased ability of N-deficient jack pine plants to maintain turgor. The increase could be attributed largely to an increase in cell wall elasticity, suggesting that elasticity changes may be a common, significant adaptation of plants to environmental stresses. Dry mass per unit saturated water almost doubled as needle N level dropped from 32 to 12 mg g?1 and was inversely correlated to the bulk modulus of elasticity. This suggests that cell wall elasticity is determined more by the nature of its cross-linking matrix than by the total amount of cell wall material present. Developmental change was evident in the response of some water relation variables to N limitation. 相似文献
4.
E. S. Jensen 《Plant and Soil》1991,133(1):83-92
Simultaneous determination of 15N and total N using an automated nitrogen analyser interfaced to a continuous-flow isotope ratio mass spectrometer (ANA-MS method) was evaluated. The coefficient of variation (CV) of repeated analyses of homogeneous standards and samples at natural abundance was lower than 0.1%. The CV of repeated analyses of 15N-labelled plant material and soil samples varied between 0.3% and 1.1%. The reproductibility of repeated total N analyses using the automated method was comparable to results obtained with a semi-micro Kjeldahl procedure. However, the automated method gave results which were 3% to 5% higher than those obtained with the Kjeldahl procedure. Since only small samples can be analysed, careful sample homogenization and fine grinding are very important. Evaluation of a diffusion method for preparing nitrate and ammonium in solution for automated 15N analysis showed that the recovery of inorganic N in the NH3 trap was lower when the N was diffused from water than from 2 M KCl. The results also indicated that different proportions of the NO3
- and the NH4
+ in aqueous solution were recovered in the trap after combined diffusion. The method is most suited for diffusing either NO3
- or NH4
+ alone, but can be used for combined diffusion of the two ions. 相似文献
5.
J. P. Pimentel F. Olivares R. M. Pitard S. Urquiaga F. Akiba J. Döbereiner 《Plant and Soil》1991,137(1):61-65
Bacteria causing mottled stripe disease in sugar cane, known asPseudomonas rubrisubalbicans, were shown to be able to fix molecular N2 and to grow on it. The root associated diazotroph known asHerbaspirillum seropedicae, after artificial inoculation caused mottled stripe disease symptoms on sorghum and Napier grass but not on sugar cane. Both bacteria could be reisolated from leaves even 60 days after. Sugar cane leaves contained large numbers of these bacteria even in the uninoculated controls. Additional physiological characteristics of six strains ofP. rubrisubalbicans were compared with those of twoH. seropedicae strains and were shown to be very similar. 相似文献
6.
7.
8.
A novel group of 1,3,4-oxadaiazoles, a group known for their anti-inflammatory activity, is hybridized with nitric oxide (NO) releasing group, oxime, for its gastro-protective action and potential synergistic effect. The synthesized hybrids were evaluated for their anti-inflammatory, analgesic, antioxidant and ulcerogenic activities. Most of the tested compounds showed excellent anti-inflammatory activity with compound 8e being more active than indomethacin. They also showed moderate analgesic activity but no antioxidant one. The ability of the synthesized compounds to inhibit COX-1 and COX-2 is studied and the prepared compounds were able to inhibit both COXs non-selectively with IC50s of 0.75–70.50 μM. Docking studies revealed the mode of interaction of the tested compounds into the empty pocket of the isozymes. All of the synthesized compounds interact with COXs active site with energy scores comparable to that of ibuprofen. All compounds showed a safer profile on the stomach tissue integrity compared to conventional NSAIDs. The designed strategy was applied to ibuprofen to introduce ibuprofen/oxadiazole/NO hybrid. The synthesized ibuprofen hybrid is a promising alternative to ibuprofen having similar anti-inflammatory activity but with safer GIT profile. 相似文献
9.
10.
Effects of different N/P ratios on several root parameters and on net P uptake were studied in winter wheat, Triticum aestivum cv. Starke II, grown in water culture. In the First experiment N/P ratios of (0/4, 2/3, 4/2, 6/1 and 8/0) were used, and plants were harvested at age 3, 5, 8, 11 and 14 days. In the second experiment N/P ratios of 6/1, 10/1, 15/1, 17/1, 20/1 and 25/1 were applied at two different N,P levels. Root length and number were determined using a digitizer connected to a computer. In the first experiment. the 6/1 N/P ratio gave the largest plants at day 14, and growth decreased with decreasing N/P ratio, The same pattern was found fur lateral root length and root number (seminal and lateral). In the second experiment the root weights decreased with increasing N/P ratio within each level. Lateral root number and overall length decreased with increasing N/P ratio at both levels as did the average lateral root length at the high N,P level. At the low N.P level, average lateral root length was about the same at all N/P ratios. Increasing the N/P ratio increased net uptake of P at the low N,P level, but decreased net P uptake at the high N,P level. Net P uptake increased with increasing P concentration in the roots and then decreased with further increase in P concentration. Net P uptake based on calculated root length [m (g root)−1 ] showed no significant deviation from weight-based uptake plots. The effect of N and P on root structure is discussed as well as the interaction of N and P in P uptake. The relevance of a proper basis for expressing root activity is stressed. 相似文献