全文获取类型
收费全文 | 3983篇 |
免费 | 249篇 |
国内免费 | 301篇 |
出版年
2024年 | 4篇 |
2023年 | 73篇 |
2022年 | 63篇 |
2021年 | 97篇 |
2020年 | 147篇 |
2019年 | 140篇 |
2018年 | 109篇 |
2017年 | 98篇 |
2016年 | 126篇 |
2015年 | 137篇 |
2014年 | 140篇 |
2013年 | 279篇 |
2012年 | 116篇 |
2011年 | 142篇 |
2010年 | 122篇 |
2009年 | 177篇 |
2008年 | 153篇 |
2007年 | 191篇 |
2006年 | 178篇 |
2005年 | 147篇 |
2004年 | 142篇 |
2003年 | 155篇 |
2002年 | 131篇 |
2001年 | 92篇 |
2000年 | 79篇 |
1999年 | 95篇 |
1998年 | 95篇 |
1997年 | 91篇 |
1996年 | 85篇 |
1995年 | 101篇 |
1994年 | 83篇 |
1993年 | 88篇 |
1992年 | 83篇 |
1991年 | 71篇 |
1990年 | 56篇 |
1989年 | 87篇 |
1988年 | 44篇 |
1987年 | 48篇 |
1986年 | 41篇 |
1985年 | 44篇 |
1984年 | 35篇 |
1983年 | 19篇 |
1982年 | 33篇 |
1981年 | 27篇 |
1980年 | 13篇 |
1979年 | 14篇 |
1978年 | 18篇 |
1977年 | 12篇 |
1976年 | 4篇 |
1973年 | 5篇 |
排序方式: 共有4533条查询结果,搜索用时 78 毫秒
131.
Avoidance thresholds and 96-h LC50 values were determined for golden shiners, Noiemigonus crysoleucas , for five individual elements: chromium, Cr; copper, Cu; cadmium, Cd; arsenic, As; selenium, Se. The avoidance concentrations were 73, 26 and 28 μgl-1 for Cr-VI, Cu and As-III, respectively. Cadmium and Se were not avoided at experimental concentrations up to 68 and 3489 μg1-1 , respectively. Acute flow-through 96-h LC50 values were Cr-VI 55·0, Cu 84·6, and As-III 12·5 mg 1-1 , which were more than two orders of magnitude above avoidance concentrations. The acute flow-through 96-h LC50 values for Cd and Se were 2·8 and 11·2 mg 1-1 , respectively. These concentrations are 31 and 2·2 times the highest concentration employed in the avoidance tests, neither of which were avoided by the test organisms. Thus, simple toxicity tests do not identify the environmental hazard of some elements, and the most toxic elements may not elicit a behavioural response. When used in concert with tests of organism function, more realistic indicators of environmental hazard or safety may be determined. 相似文献
132.
SIBANANDA PANIGRAHI SACHIDANANDA PADHY RABINDRA N PADHY 《The Annals of applied biology》2003,143(2):195-202
The toxicity of a commercial formulation of the insecticide parathion‐methyl to the N2‐fixing filamentous cyanobacterium (blue‐green alga) Cylindrospermum, sp. was studied. A concentration of parathion‐methyl of 0.5 ppm caused growth increase in liquid growth media. The minimum inhibitory concentration of parathion‐methyl for both types (N2, fixing and nitrate supplemented) of liquid and solid media was 1.0 ppm. LC50 values were: 4.4 ppm (liquid, N2, fixing), 5.5 ppm (liquid, nitrate supplemented), 3.3 ppm (agar, N2‐fixing) and 4.0 ppm (agar, nitrate supplemented). LC100 values for N2‐fixing liquid and both types of agar media were 10.0 ppm, while for the liquid nitrate supplemented medium the LC100 was 12.0 ppm. Both akinete (spore) formation and germination were inhibited below the highest permissive concentration of 8.0 ppm, with the insecticide incorporated in the agar media. In soil, the LC50 and LC100 values for parathion‐methyl were 13.6 and 30 ppm, respectively. Both the dehydrogenase activity of heterocysts (monitored by 2,3,5‐triphenyl tetrazolium chloride reduction) and the nitrogen concentration of cultures (estimated by the micro‐Kjeldahl method) were affected by the insecticide, but the latter (N2‐fixation) was more sensitive. The Kruskal‐Wallis H test on the numbers of vegetative cells in the filaments revealed that the insecticide significantly affected the division of vegetative cells. The cyanobacterium could detoxify the growth medium containing high levels (30 and 40 ppm) of the insecticide in short‐term exposures at the expense of cell viability. 相似文献
133.
E. Argese C. Bettiol P. Miana L. Iuzzolino G. Giurin 《Journal of Aquatic Ecosystem Stress and Recovery (Formerly Journal of Aquatic Ecosystem Health)》1996,5(2):125-134
The effects on mitochondrial respiratory parameters of heavy metals, such as Cu, Ni, Pb, Cd, Zn, Ag, Hg, were recorded by using thein vitro response of submitochondrial particles (SMP) from beef heart mitochondria.The toxicity of these elements was estimated by determining their effects on the energy-coupled reverse electron transfer (RET), which is induced by ATP and succinate at first site level of the respiratory chain in SMP.The RET rate was easily monitored by recording spectrophotometrically at 340 nm the production of NADH, arising from the reduction of exogenous NAD+ by RET.The toxicity values were expressed as the toxicant molar concentration which decreases the rate of reduction of NAD+ to an extent of 50 percent (EC50). The toxicity increased in the following order: Ni2+2+2+< Cd2+2+2++.The SMP data were compared with the toxicity values obtained from a variety of biological systems currently used for toxicity testing. The results obtained demonstrate that the SMP test generally provides a good estimate of metal toxicity for several fish and invertebrate species. This is demonstrated by the statistical parameters obtained in the regression analysis. The broadened 95% confidence intervals and, in particular, the poor correlations obtained for some aquatic organisms can be ascribed to the more complex metabolic interactions and competing toxic pathways in aquatic organisms, when compared to SMP. 相似文献
134.
Copper-Putrescine-Pyridine (Cu-PuPy) is a membrane-permeable complex which efficiently dismutates superoxide. In excess of 0.1 mM it is highly cytotoxic and oxidizes cellular GSH with concomitant production of H2O2. Here we show that treatment of CHO cells with 0.2 mM Cu-PuPy (0-200 min) leads to an accumulation of H2O2. Organic hydroperoxides which are also formed at low levels in the presence of Cu-PuPy, increase significantly after removal of the copper complex. We conclude that Cu-PuPy acts as an oxidant until cellular GSH is depleted. However, by interfering with radical chain propagation reactions, it suppresses lipid peroxidation and thus substitutes for consumed physiological antioxidants in a later stage of treatment. This consistently explains our previous, seemingly paradox, finding that longer Cu-PuPy treatments may be significantly less toxic than shorter ones. 相似文献
135.
Summary Iron tolerance of rice (Oryza sativa L.) was investigated using an oxygen depleted hydroculture system. Treatment with high concentrations of Fe2+ induced yellowing and bronzing symptoms as well as iron coatings at the root surface. Root and shoot growth were inhibited by increasing iron concentration in the medium. All symptoms were more pronounced in an iron sensitive cultivar (IR 64) compared to an iron tolerant one (IR 9764-45-2). Superoxide dismutase and peroxidase activity of root extracts of IR 97 were about twice that of IR 64 in untreated control plants. No significant increase of peroxidase activity was detected with increasing iron concentration in the medium. Catalase activity of IR 64 was slightly higher than that of IR 97, independent of iron concentration.Abbreviations SOD
Superoxide dismutase (EC 1.15.1.1)
- POD
peroxidase (EC 1.11.1.7)
- EDTA
ethylenediamintetraacetic acid
- fwt
fresh weight
- Hepes
(N-[2-hydroxyethyl]piperazine-N-[2-ethanesulfonic acid])
- BSA
bovine serum albumin
- IR 97 IR 9764-45-2
an iron tolerant rice cultivar
- IR 64
iron sensitive rice cultivar
- PM
plasma membrane 相似文献
136.
Eggplants (Solanum melongena L. cv. Bonica) were grown in a glasshouse during summer under natural light with one unbranched shoot or one shoot with 3
to 4 branches and with or without fruit in quartz sand buffered and not buffered with 0.5% CaCO3 (w : v), respectively. Nutrient solutions supplied contained nitrate or ammonium as the sole nitrogen source.
Compared with nutrient solutions containing nitrate (10 mM), solutions containing ammonium (10 mM) caused a decrease in net photosynthesis of eggplants during early stages of vegetative growth when grown in quartz sand
not buffered with CaCO3. The decrease was not observed before leaves showed interveinal chlorosis. In contrast, net photosynthesis after bloom at
first increased more rapidly in eggplants supplied with ammonium than with nitrate nitrogen. However, even in this case, net
photosynthesis decreased four weeks later when ammonium nutrition was continued. The decrease was accompanied by epinasty
and interveinal chlorosis on the lower leaves and later by severe wilting, leaf drop, stem lesions, and hampered growth of
stems, roots, and fruits. These symptoms appeared later on plants not bearing fruits than on plants bearing fruits. If nutrient
solutions containing increasing concentrations of ammonium (0.5–30 mM) were supplied after the time of first fruit ripening, shoot growth and set of later flowers and fruits were promoted. In
contrast, vegetative growth and reproduction was only slightly affected by increasing the concentration of nitrate in the
nutrient solutions.
In quartz sand buffered with CaCO3 ammonium nutrition caused deleterious effects only under low light conditions (shade) and on young plants during rapid fruit
growth. If eggplants were supplied with ammonium nitrogen before bloom, vegetative growth was promoted, and set of flowers
and fruit occurred earlier than on plants supplied with nitrate. Furthermore, the number of flowers and fruit yield increased.
These effects of ammonium nutrition were more pronounced when plants were grown with branched shoots than with unbranched
shoots.
The results indicate that vegetative and reproductive growth of eggplants may be manipulated without causing injury to the
plants by supplying ammonium nitrogen as long as the age of the plants, carbohydrate reserves of the roots, quantity of ammonium
nitrogen supplied, and pH of the growth medium are favourable. T W Rufty Section editor 相似文献
137.
Comparative toxic and sublethal effects of fluvalinate on two-spotted spider mite and European red mite 总被引:1,自引:0,他引:1
Four fluvalinate formulations differed in their residual toxicity to female two-spotted spider mite (TSM), Tetranychus urticae adults; the emulsifiable concentrate (EC) was the most toxic. In contrast, there was little difference in toxicity between the formulations with the European red mite (ERM) Panonychus ulmi with the exception of the EC formulation which was the least toxic. Fluvalinate 2F caused minimal (<10%) TSM and ERM egg mortality. Fluvalinate 2F was more toxic and caused greater larval dispersal for the TSM compared to the ERM at the field concentration and below. The toxicity of fluvalinate 2F to TSM and ERM protonymphs, deutonymphs and adults was low, approximately <20% at field concentration. Dispersal was the main response to fluvalinate and this was positively correlated with increasing concentration. The combined mortality and dispersal LC50 was five times lower for ERM protonymphs and adults, but 11 times higher for ERM deutonymphs compared to equivalent TSM life stages. Fluvalinate 2F reduced TSM development from the protonymph and deutonymph stages to a greater extent compared to the ERM. The mortality response to fluvalinate 2F was unaffected by host type (peach or apple) for the TSM whereas ERM mortality was higher on apple compared to peach. TSM dispersal was higher from apple compared to peach whereas ERM dispersal was similar on both host types. Oviposition by both mite species was lower on apple than peach leaves. A 1 h exposure to fluvalinate 2F reduced ERM oviposition for 12 days. 相似文献
138.
稀有鮈鲫──一种新的鱼类毒性试验材料 总被引:7,自引:0,他引:7
本文研究了稀有鲫(Gobiocyprisrarus)作为毒性试验材料的可行性。采用换水式试验,在硬度为200mg/L(以CaCO3计)、pH7.8±0.2、温度24-25℃条件下研究了铬、铜、锌和五氯酚(PCP)对稀有鲫的急性毒性。重铬酸钾对2日龄稀有鲫的24h和96h和LC50控制范围分别263.6-334.7和1153-178.5mg/L(n=8)。铬、铜、锌和五氯酚对2日龄稀有鲫的急性毒性值(96hLC50)范围,从铜的52.2μg/L到铬的52000μg/L,毒性大小的顺序是铜>五氯酚>锌>铬。研究结果表明,稀有鲫有可能发展成为一种较为理想的毒性试验材料。 相似文献
139.
Geochemical partitioning and bioavailability of copper to aquatic plants in an artificial oxide-organic sediment 总被引:2,自引:0,他引:2
This study investigated the effects of competition between binding substrates (organic matter and iron oxide) and between
metals (cadmium and copper), on the partitioning of sedimentary copper and its subsequent bioavailability to an aquatic plant.
Organic matter and a synthesized iron oxide, ferrihydrite, were added singly and in combination to a series of sand sediments,
which were then dosed with environmentally realistic concentrations of cadmium and copper and planted with rice,Oryza sativa.
Organic matter controlled copper partitioning and bioavailability, whereas the synthetic ferrihydrite bound negligible amounts
of either metal, even in the absence of organic matter. As organic matter concentrations increased, operationally-defined
leachable copper decreased, organic-associated copper increased and the survival of rice plants improved in an approximately
linear fashion. At a nominal starting copper concentration of 5.8 μg g dry wt−1, plant survival after four weeks averaged 0–8% in sediments without organic matter, 25% in a sediment containing 0.18% organic
matter and 58% in a sediment containing 0.36% organic matter. These results suggest that organic-associated forms of copper
are unavailable to plants, and that the operational definition of ‘leachable’ copper (extracted with dilute ammonium acetate)
adequately represents the species of copper that is (are) available to plants. Our study using a well-characterized artificial
sediment supports the copper fractionation patterns and correlations between copper partitioning and bioavailability reported
from the heterogeneous, poorly characterized sediments of natural lake and river sediments. 相似文献
140.
Brain Quinolinic Acid in Chronic Experimental Hepatic Encephalopathy: Effects of an Exogenous Ammonium Acetate Challenge 总被引:1,自引:1,他引:0
Peter B. F. Bergqvist Melvyn P. Heyes †Mogens Bugge Finn Bengtsson 《Journal of neurochemistry》1995,65(5):2235-2240
Abstract: Elevated brain concentrations of the neurotoxin and NMDA receptor agonist quinolinic acid (QUIN) have been demonstrated in portacaval-shunted (PCS) rats, a chronic hepatic encephalopathy (HE) model. Increased brain QUIN levels have also been shown in acute hyperammonemic rats. In the present study, the plasma and brain (neocortical) QUIN levels in chronic PCS rats were investigated. The study also included a single exogenous ammonium acetate (NH4 Ac; 5.2 mmol/kg, i.p.) challenge to precipitate a reversible hepatic coma. Compared with sham-operated controls, chronic PCS rats exhibited decreased rather than increased plasma and brain QUIN levels. The plasma-to-brain QUIN ratio was not found to be altered. The NH4 Ac administration induced coma in all of the PCS rats 20–25 min after the challenge, and this coma was resolved within 60–75 min. No relevant temporal relationship between changes in brain QUIN levels and the neurological status in the PCS rats was observed. Therefore, our results do not support the contention that increased brain QUIN levels per se are involved in the pathogenesis of HE. 相似文献