全文获取类型
收费全文 | 20589篇 |
免费 | 233篇 |
国内免费 | 2篇 |
专业分类
林业 | 3789篇 |
农学 | 1478篇 |
基础科学 | 149篇 |
3511篇 | |
综合类 | 1060篇 |
农作物 | 2615篇 |
水产渔业 | 2139篇 |
畜牧兽医 | 2823篇 |
园艺 | 1172篇 |
植物保护 | 2088篇 |
出版年
2023年 | 29篇 |
2022年 | 88篇 |
2021年 | 163篇 |
2020年 | 97篇 |
2019年 | 121篇 |
2018年 | 2832篇 |
2017年 | 2773篇 |
2016年 | 1293篇 |
2015年 | 212篇 |
2014年 | 219篇 |
2013年 | 274篇 |
2012年 | 1093篇 |
2011年 | 2447篇 |
2010年 | 2275篇 |
2009年 | 1438篇 |
2008年 | 1571篇 |
2007年 | 1839篇 |
2006年 | 291篇 |
2005年 | 312篇 |
2004年 | 314篇 |
2003年 | 336篇 |
2002年 | 198篇 |
2001年 | 135篇 |
2000年 | 132篇 |
1999年 | 57篇 |
1998年 | 28篇 |
1997年 | 17篇 |
1996年 | 9篇 |
1995年 | 10篇 |
1993年 | 20篇 |
1992年 | 23篇 |
1991年 | 14篇 |
1990年 | 23篇 |
1989年 | 22篇 |
1988年 | 18篇 |
1987年 | 10篇 |
1986年 | 7篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 5篇 |
1980年 | 5篇 |
1978年 | 11篇 |
1977年 | 6篇 |
1974年 | 5篇 |
1972年 | 6篇 |
1971年 | 3篇 |
1970年 | 3篇 |
1969年 | 4篇 |
1968年 | 8篇 |
1967年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Yang Yang Yuezhi Pan Xun Gong Moutian Fan 《Genetic Resources and Crop Evolution》2010,57(8):1239-1248
Citrus hongheensis is a critically endangered species endemic to the Honghe river region in southeastern Yunnan, China. Its genetic diversity
and differentiation were investigated using Inter-Simple Sequence Repeat (ISSR) markers. One hundred primers were screened,
and a total of 245 loci were amplified from seven natural populations by 13 informative and reliable primers. Of these 245
ISSR loci, 233 were polymorphic and the detected variations revealed a relatively high level of intraspecific genetic diversity.
At the population level, the mean percentage of polymorphic loci (PPB) was 36.50%, while the average expected heterozygosity (He) and Shannon diversity index (Ho) were 0.1327 and 0.1972, respectively. At the species level (across all populations), PPB was 95.10%, while He and Ho were 0.3520 and 0.5195, respectively. A high Gst value (0.6247) indicated that there is significant differentiation among populations, which was confirmed by AMOVA analysis
(Φst = 0.6420). Pairwise genetic identity (I) values among populations ranged from 0.6341 to 0.7675, with a mean of 0.7008. We propose that the high level of genetic
differentiation may be the result of habitat fragmentation and limited gene flow (Nm = 0.1502). For effective in situ conservation and population restoration of C. hongheensis it will be important to maintain historical processes, including high outbreeding rates, sufficient gene flow, and large
effective population sizes. 相似文献
972.
In this study, fully vitellogenic oocytes of the longchin goby (Chasmichthys dolichognathus) were exposed to in vitro xenoestrogens such as diethylstilbestrol (DES), bisphenol A (BPA) and nonylphenol (NP) at concentrations of 100 ng/ml in the presence of [3H]17α-hydroxyprogesterone as precursor. The major metabolites produced in vitro are progestogens [17α-hydroxy,20α-dihydroprogesterone (17α20αOHP) and 17α-hydroxy,20β-dihydroprogesterone (17α20βOHP)], androgens [androstenedione (A4) and testosterone (T)] and estrogens [estrone (E1) and estradiol-17β (E2)]. Comparative activities of these chemicals in oocyte steroid biosynthesis showed as follows: NP treatment resulted in clear stimulation of estrogen production, while the opposite occurred in DES treated incubation. Treatment with DES resulted in a higher synthesis of 17α20αOHP. BPA treatment increased the androgen, while estrogen synthesis was slightly inhibited. These results suggest that NP exhibited clearly estrogenic activity on the three chemicals. 相似文献
973.
A carbonaceous sorbent was prepared from the wood of Japanese larch (Larix leptolepis) by dehydration with concentrated sulfuric acid in a 69% yield. The abilities of the sorbent to remove Cr(VI) from aqueous
solutions were investigated. Research parameters included the initial solution pH, temperature, and initial concentration
of Cr(VI) in solution. The removal of Cr(VI) was highly solution pH dependent and was mainly governed by physicochemical sorption
under weak acidic conditions. The equilibrium data fit well in the Langmuir isotherm model. The Langmuir constants were calculated
at different temperatures, and the sorption capacity increased with rising temperature, indicating the endothermic nature
of the Cr(VI) sorption onto the sorbent. The desorption experiments suggest that the Cr(VI) sorption is generally irreversible,
owing to strong interaction of HCrO
4
−
with the active sites of the sorbent. 相似文献
974.
Mélanie Saulnier Jean-Louis Edouard Christophe Corona Frédéric Guibal 《Annals of Forest Science》2011,68(1):189-200
•Introduction
In the context of climate change, assessing climate–growth relationships is of high importance in order to understand how forest ecosystems evolve and to test climate models at regional scale. 相似文献975.
Paśko P Zagrodzki P Bartoń H Chłopicka J Gorinstein S 《Plant foods for human nutrition (Dordrecht, Netherlands)》2010,65(4):333-338
The effect of Chenopodium quinoa seeds on lipid profile, glucose level, protein metabolism and selected essential elements (Na, K, Ca, Mg) level was determined
in high—fructose fed male Wistar rats. Fructose decreased significantly LDL [42%, p < 0.01] and activity of alkaline phosphatase [20%, p < 0.05], and increased triglycerides level [86%, p < 0.01]. The analysis of blood of rats fed quinoa indicated, that these seeds effectively reduced serum total cholesterol
[26%, p < 0.05], LDL [57%, p < 0.008] and triglycerides [11%, p < 0.05] when compared to the control group. Quinoa seeds also significantly reduced the level of glucose [10%, p < 0.01] and plasma total protein level [16%, p < 0.001]. Fructose significantly decreased HDL [15%, p < 0.05] level in control group but when the quinoa seeds were added into the diet the decrease of HDL level was inhibited.
Quinoa seeds did not prevent any adverse effect of increasing triglyceride level caused by fructose. It was shown in this
study that quinoa seeds can reduce most of the adverse effects exerted by fructose on lipid profile and glucose level. 相似文献
976.
To standardize conditions during the final maturation and ovulation of ovarian follicles from Japanese eel, we have developed
a culture system for the production of fertilizable eggs from post-vitellogenic ovarian follicles in vitro. Post-vitellogenic
ovarian follicles were incubated in culture medium supplemented with 17α,20β-dihydroxy-4-pregnen-3-one (DHP) with or without
bovine serum albumin (BSA) to assess the effects of protein concentration. Eggs that ovulated during incubation were fertilized,
and the remaining follicles were incubated in prostaglandin F2α (PGF2α) for a further 3 or 6 h before fertilization. Male eels were injected repeatedly with human chorionic gonadotropin. The quality
of eggs obtained under the different culture conditions was evaluated after artificial fertilization in terms of hatching
success. Hatching rates tended to decrease with increasing concentrations of BSA in the incubation medium in a dose-dependent
manner. The addition of PGF2α drastically increased the number of eggs that ovulated, but the rate of hatching was greatly decreased compared with eggs
obtained earlier by DHP incubation alone. The larvae obtained from artificially fertilized eggs produced in vitro survived
for 14 days without feeding. We conclude that in vitro culture systems thus have a great potential for the acquisition of
good quality eggs under tightly controlled artificial conditions, culminating in the production of eel larvae. 相似文献
977.
Quantifying landscape pattern and ecosystem service value changes in four rapidly urbanizing hill stations of Southeast Asia 总被引:1,自引:0,他引:1
Context
Hill stations are known for their favorable cool climate and natural environments which generate valuable ecosystem services that benefit the local population, tourists and visitors. However, rapid urbanization threatens the sustainability of these highly valued fragile landscapes.Objectives
We aim to characterize and quantify the changes in the landscape patterns and ecosystem service values (ESVs) of Baguio (Philippines), Bogor (Indonesia), Dalat (Vietnam), and Pyin Oo Lwin (Myanmar), and discuss their implications to landscape sustainability.Methods
We used remote sensing imagery to map land-use/cover (2001 and 2014), and spatial metrics and gradient analysis to characterize the changes in landscape pattern. We employed a benefit transfer method to estimate the changes in ESV and human-to-ESV ratio. A land-change model was used to simulate different scenarios of future built-up expansions (2014–2030).Results
The landscapes of Dalat and Pyin Oo Lwin are becoming more fragmented, while those of Baguio and Bogor are getting more aggregated. Dalat had the highest decrease (absolute change) in ESV and H-ESV ratio, while Bogor had the highest percentage decrease (2001–2014).Conclusions
Rapid urbanization has been a major factor in the landscape transformation of Baguio, Bogor, Dalat and Pyin Oo Lwin. If the current built-up expansion rate will speed up, the decline in future ESV and H-ESV ratio (2014–2030) will be higher than if the rate will continue or slow down. Unless the concept of landscape sustainability is taken seriously in landscape and urban planning, the respective ‘values’ of these precious hill stations will become less and less.978.
Although the role of habitat fragmentation in species declines is well recognised, the effect of habitat quality on species
distributions is often studied using presence–absence models that ignore metapopulation dynamics. We compared three approaches
to model the presence–absence of North Island robins in 400 sites among 74 fragments of native forest in a 15,000-ha agricultural
landscape in New Zealand. The first approach only considered local habitat characteristics, the second approach only considered
metapopulation factors (patch size and isolation), and the third approach combined these two types of factors. The distribution
of North Island robins was best predicted by patch isolation, as their probability of occurrence was negatively correlated
with isolation from neighbouring patches and from the closest major forests, which probably acted as a source of immigrants.
The inclusion of habitat factors gave only a slight increase in predictive power and indicated that robins were more likely
to occur in areas with tall canopy, tall understory and low density of young trees. We modelled the effect of isolation using
an index of functional patch connectivity based on dispersal behaviour of radio-tracked juveniles, and this functional index
greatly improved the models in comparison to classical indices relying on Euclidean distances. This study highlights the need
to incorporate functional indices of isolation in presence–absence models in fragmented landscapes, as species occurrence
can otherwise be a misleading predictor of habitat quality and lead to wrong interpretations and management recommendations. 相似文献
979.
A total of 24 candidate plus trees (CPTs) of Pongamia pinnata (L.) Pierre. were selected to elucidate their variation and diversity based on thirteen quantitative traits (4 pod traits,
6 seed traits of parent trees and 3 progeny traits) at Forest Research Centre, Institute of Forest Productivity — Mandar,
Ranchi district during 2005–2007. The results show that, CPT-19 had maximum for seven traits viz, pod length (65.6 mm), 100-pod
weight (542.4 g), seed 2D (two dimension) area (351.2 mm2), seed length (27.9 mm), seed breadth (17.4 mm), 100-seed weight (217.9 g) and plant height (164.3 cm). The traits, 100-pod
weight and 100-seed weight had a high heritability (98.4%, 96.9%) accompanied with high genetic advance (46.0%, 34.9%). There
is a positive significant correlation between 100-pod weight and 100-seed weight traits at both genotypic and phenotypic levels
with plant height, collar diameter and volume index at 30 MAS (months after sowing). Volume index expressed a moderate heritability
(47.4%) accompanied with high genetic advance (48.4%), indicating that the character is governed by additive gene effects.
In divergence study, 24 accessions were grouped into 6 clusters on the basis of non-hierarchical euclidian cluster analysis.
The genotypes in cluster IV (CPT-5, CPT-6, CPT-7, CPT-12, CPT-16, CPT-18, CPT-22) and cluster III (CPT-4, CPT-8, CPT-9, CPT-20,
CPT-21) were most heterogeneous and can be best used within group hybridization. The wide diversity exists between the cluster
V and II, followed by cluster II and I and crosses between CPTs of these clusters may result in substantial segregates. It
is revealed that the existence of substantial variation and diversity can be utilized for genetic resource conservation and
further tree improvement programmers of the species. 相似文献
980.
Motoaki Tojo Ken Watanabe Kenichi Kida Yingjie Li Shuhei Numata 《Journal of General Plant Pathology》2007,73(2):121-124
Severe mottle necrosis (Shirogusare-byo in Japanese) was found on mature tubers of sweet potato (Ipomoea batatas) in Ibaraki Prefecture in October 2004. The causal organism was identified as Pythium scleroteichum hitherto unknown in Japan. Sweet potato cultivar Purple Sweet Lord was more susceptible than cultivars Beniazuma, Benimasari,
Koukei-14, and Tamayutaka to the pathogen at 25°C, while this difference in the susceptibility was not clear at 15°C. 相似文献