首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   707篇
  免费   86篇
林业   49篇
农学   17篇
基础科学   4篇
  124篇
综合类   19篇
农作物   52篇
水产渔业   89篇
畜牧兽医   345篇
园艺   13篇
植物保护   81篇
  2023年   15篇
  2022年   22篇
  2021年   47篇
  2020年   45篇
  2019年   66篇
  2018年   42篇
  2017年   57篇
  2016年   48篇
  2015年   32篇
  2014年   33篇
  2013年   55篇
  2012年   40篇
  2011年   65篇
  2010年   23篇
  2009年   40篇
  2008年   33篇
  2007年   25篇
  2006年   28篇
  2005年   17篇
  2004年   14篇
  2003年   19篇
  2002年   7篇
  2001年   5篇
  2000年   4篇
  1999年   1篇
  1997年   1篇
  1996年   2篇
  1994年   1篇
  1993年   1篇
  1991年   1篇
  1990年   1篇
  1989年   1篇
  1988年   1篇
  1984年   1篇
排序方式: 共有793条查询结果,搜索用时 12 毫秒
121.
We characterized the pollination and fecundation times in soybean flowers and evaluated the effects of kinetin and calcium applications on physiological and productive traits of soybean plants during the reproductive stage. The anatomical study of flowers of eight soybean cultivars showed that fecundation occurred in closed flowers with visible petals, which presented embryo in the first cell divisions. These results indicate that fertilizers and growth regulation applications should be performed before flower opening, which is different from the current recommendation. Foliar applications of kinetin and calcium between the floral bud and full flowering stages did not affect carbon dioxide (CO2) assimilation, yield components and final yield. The results obtained in this research showed the lack of viability of foliar application of calcium and kinetin in order to increase pod set and number of seeds per plant in soybean crop.  相似文献   
122.
The adaptation of stevia to the growing conditions of NE Portugal is assessed, including the tolerance of this species to cold temperatures, and the potential to produce biomass when grown as an annual crop and when subjected to various nitrogen (N) rates and two harvesting regimes. Almost all the plants died during the winter of 2014 (minimum temperatures peaked at ?8.0°C), making it necessary to replant the crop the following spring. With the best cutting regime (double cut) and N rate (150 kg N ha?1), 1514.4 and 2390.0 kg ha?1 of dry leaves were produced, respectively, in 2014 and 2015. Leaf chlorophyll concentrations estimated by the SPAD (Soil and Plant Analysis Development)-502 chlorophyll meter and a NDVI (Normalized Difference Vegetation Index) carried out by the Field Scout CM 1000 spectroradiometer showed significant differences among N rates, proving to be good indicators of plant N nutritional status. Based on the leaf analysis, provisional sufficiency ranges for N are proposed, namely 25–35 g kg?1 for mid-summer and 15–25 g kg?1 for early autumn. The fluorescence of chlorophyll a and the transient fluorescence intensity performed by the OS-30p+ fluorometer failed to show any stress induced by no-N control treatments in comparison to N-treated plants.  相似文献   
123.
The aim of this study was to determine whether applying good practices of handling during the pre-weaning period have long-term effects on behavioral and physiological indicators, health status, and average daily gain (ADG) of crossbred Bos taurus × Bos indicus heifer calves. During the pre-weaning period, 98 crossbred of Holstein × Gir heifer calves were allotted into three treatments: (1) good practices of handling + brushing (GPB; n?=?25), (2) good practices of handling (GP; n?=?25), and (3) control (n?=?48). Every 2 months, four evaluation periods (EV1 to EV4) were conducted to record data. Behavioral indicators comprised time to drive (TD), flight speed (FS), flight distance (FD), and composite reactivity score (CRS). Physiological indicators of acute stress during handling comprised respiratory and heart rates. Health status comprised data regarding occurrence of most common diseases (i.e., pneumonia and anaplasmosis). Collected data were analyzed by using a linear mixed model for repeated measures, Tukey’s test, and chi-squared procedures. Treatment influenced (P?<?0.05) TD, FS, and FD but not CRS (P?=?0.78). From EV1 to EV3, the control calves had the lowest TD. The GPB group had lower FS than the control but did not differ from GP. The GPB group had lower FD means than the other two groups in EV2, EV3, and EV4. No differences (P?>?0.05) due to treatment were observed on heart and respiratory rates, ADG, or occurrence of pneumonia and anaplasmosis. It was concluded that adoption of good practices of handling during pre-weaning period may lead to long-term positive effects.  相似文献   
124.
125.
The effect of soluble silicon (Si) on photosynthetic parameters and soluble sugar concentrations was determined in leaves of rice cv. Oochikara and mutant plants of Oochikara defective in active Si uptake [low silicon 1 (lsi1)]. Plants were grown in hydroponic culture amended with 0 (?Si) or 2 mm Si (+Si), under either low or high photon flux density (PFD) and with or without inoculation with Bipolaris oryzae, the causal agent of brown spot of rice. Leaf Si concentration increased by 141 and 435% in +Si cv. Oochikara and by 119 and 251% in +Si lsi1 mutant plants under high and low PFD, respectively, compared with ?Si plants. Plant biomass accumulation was improved by Si regardless of PFD, especially plants for cv. Oochikara. Brown spot severity was highest in ?Si plants for cv. Oochikara and lsi1 mutant plants under low PFD. In the presence of Si, disease severity in plants grown under both low and high PFD was reduced, except for lsi1 mutant plants under high PFD. Plant inoculation reduced the photosynthetic parameters measured regardless of plant material or Si supply. A decrease of net carbon assimilation rate (A) of inoculated plants under low PFD compared with non‐inoculated plants was associated with damage in the photosynthetic apparatus, except for +Si cv. Oochikara in which stomatal restriction [low water vapour conductance (gs)] contributed to A reduction. Under high PFD, damage to the photosynthetic apparatus of inoculated plants was the main reason for the reduction in A for +Si and ?Si lsi1 mutant plants. In addition, for ?Si cv. Oochikara, a reduction in gs contributed to reduced A. However, for +Si cv. Oochikara, gs was the limiting factor for A. Inoculated plants of +Si cv. Oochikara had higher A values than +Si lsi1 mutant plants, regardless of environmental conditions. Soluble sugars were not detected in leaf tissues of plants under low PFD. For high PFD, Si improved the hexose concentration in non‐inoculated plants at 144 h after inoculation (hai) for lsi1 mutant plants and from 96 hai onwards for cv. Oochikara compared with ?Si plants. However, plant inoculation reduced hexose concentration compared with non‐inoculated plants, mainly in +Si plants, regardless of plant material. Sucrose concentration increased in leaves of cv. Oochikara in the presence of Si whether inoculated or not. For +Si lsi1 mutant plants, sucrose concentration increased only at 48 hai compared with ?Si plants, whether inoculated or not. The results of this study show that a minimum Si concentration is needed in leaf tissues of rice plants to avoid the negative impact of B. oryzae infection on photosynthesis and sugar concentration. High leaf Si concentration resulted in an increased soluble sugar concentration and together, but in independent ways, soluble sugar and Si reduced brown spot severity of rice.  相似文献   
126.
With the development of the textile industry, there has been a demand for dye removal from contaminated effluents. In recent years, attention has been directed toward various natural solid materials that are capable of removing pollutants from contaminated water at low cost. One such material is sugarcane bagasse. The aim of the present study was to evaluate adsorption of the dye Acid Violet Alizarin N with different concentrations of sugarcane bagasse and granulometry in agitated systems at different pH. The most promising data (achieved with pH 2.5) was analyzed with both Freundlich and Langmuir isotherms equations. The model that better fits dye adsorption interaction into sugarcane bagasse is Freundlich equation, and thus the multilayer model. Moreover, a smaller bagasse granulometry led to greater dye adsorption. The best treatment was achieved with a granulometry value lower than 0.21 mm at pH 2.50, in which the total removal was estimated at a concentration of 16.25 mg mL−1. Hence, sugarcane bagasse proves to be very attractive for dye removal from textile effluents.  相似文献   
127.
The petroleum industry activities provide potential risks to the environment because they can contaminate ecosystems with different organic compounds in the production chain. Several accidents with transport and handling of petroleum and related products occurred in urban areas with harmful effects to the quality of life and economy. In the 1990s, bioremediation and phytoremediation technologies as economically feasible alternatives to repair the environmental damage were developed. In this study, the potential of the willows Salix rubens and Salix triandra were evaluated with regard to the phytoremediation of soils contaminated with petroleum-derived hydrocarbons (total hydrocarbons and polycyclic aromatic hydrocarbons (PAHs)). The PAHs were quantified by extraction from soils and plants using dichloromethane under ultrasonication. The HPLC analysis was performed with GC/MSD equipment. The total hydrocarbons present in uncontaminated soil were quantified by the sum of animal/vegetable oils and greases and mineral oils and greases according to Standard Methods 5520 (1997). The two willows species S. rubens and S. triandra were resistant during the project development. In the contaminated soil, in which both species were planted, the total hydrocarbons concentration was reduced near 98?%. The PAHs content was remarkably reduced as well. Pyrene showed an initial concentration of 23.06???g?kg?1, decreasing in most cases to 0.1???g?kg?1 or to undetectable levels. Chrysene decreased from 126.27???g?kg?1 to undetectable levels. Benzo[k]fluoranthene and benzo[a]pyrene concentrations had also showed a decrease from 28.44 and 3.82???g?kg?1, respectively, to undetectable levels.  相似文献   
128.
ABSTRACT: Changes in cerebrospinal fluid (CSF) and anatomical and histopathological central nervous system (CNS) lesions were evaluated, and the presence of Trypanosoma vivax in CNS tissues was investigated through PCR. Twelve adult male goats were divided into three groups (G): G1, infected with T. vivax and evaluated during the acute phase; G2, infected goats evaluated during the chronic phase; and G3, consisting of non-infected goats. Each goat from G1 and G2 was infected with 1.25 × 105 trypomastigotes. Cerebrospinal fluid (CSF) analysis and investigation of T. vivax was performed at the 15th day post-infection (dpi) in G1 goats and on the fifth day after the manifestation of nervous system infection signs in G2 goats. All goats were necropsied, and CNS fragments from G1 and G2 goats were evaluated by PCR for the determination of T. vivax. Hyperthermia, anemia and parasitemia were observed from the fifth dpi for G1 and G2, with the highest parasitemia peak between the seventh and 21st dpi. Nervous system infection signs were observed in three G2 goats between the 30th and 35th dpi. CSF analysis revealed the presence of T. vivax for G2. Meningitis and meningoencephalitis were diagnosed in G2. PCR were positive for T. vivax in all the samples tested. In conclusion, T. vivax may reach the nervous tissue resulting in immune response from the host, which is the cause of progressive clinical and pathological manifestations of the CNS in experimentally infected goats.  相似文献   
129.
The aim of the present study was to evaluate the effects of different medium replacement intervals on the viability, antral cavity formation, growth and in vitro maturation (IVM) of oocytes from caprine and ovine pre‐antral follicles. Pre‐antral ovarian follicles (≥150 μm) were isolated from the ovarian cortex of goats and sheep and were individually cultured for 24 days using two different medium replacement intervals [2 days (T1) or 6 days (T2)]. Follicle development was evaluated on the basis of antral cavity formation, increases in follicular diameter and the presence of healthy cumulus oocyte complexes and fully grown oocytes. For caprine species, results showed a higher percentage (p < 0.05) of viable follicles in T1 than T2 from day 6 until the end of the culture. In addition, when comparing both treatments after the same culture duration, the rate of antrum formation was significantly higher in T1 than in T2 from day 12 onwards. Yet, in ovines, when both treatments were compared on day 24 of the culture, there were more viable follicles in T2 than in T1 (p < 0.05). In the caprine species, percentages of fully grown oocytes (≥110 μm) acceptable for IVM after 24 days of culture were significantly higher in normal follicles cultured in T1 (30.0%) than in T2 (6.7%; p < 0.05). On the other hand, in ovines, at the end of the culture, the percentage of oocytes destined for IVM was higher in T2 than in T1 (23.5% vs 2.9%; p < 0.05). In conclusion, under the same conditions, the frequency of medium replacement significantly affected the in vitro development of caprine and ovine pre‐antral follicles. To improve the efficiency of the culture system, the medium must be replaced every 2 and 6 days for goat and sheep pre‐antral follicles, respectively.  相似文献   
130.
The objective of this study was to demonstrate how different factors can simultaneously influence the oxidative stability of an oil-in-water emulsion, and how these factors can be used to enlarge the variation range of oxidation markers, expressed as peroxide value (PV) and TBARS. Initially, a Plackett-Burman design was used to screen seven factors (temperature, pH, and iron, copper, ascorbyl palmitate, ascorbic acid, and sodium chloride concentrations). A temperature elevation of 30 to 60 °C reduced PV and TBARS, a pH change from 3.0 to 7.0 increased PV and reduced TBARS, and the presence of ascorbic acid (1 mmol/L) had no significant effect on PV but increased TBARS (p < 0.05). Thus, the temperature was fixed at 30 °C, and an emulsion was formulated with different combinations of ascorbic acid, iron, and pH according to a central composite rotatable design. Regression models were fitted to PV and TBARs responses and optimized to get the higher values of both markers of oxidation. The optimized emulsion contained 1.70 mmol/L AH (ascorbic acid) and 0.885 mmol/L FeSO(4) · 7H(2)O (1.0 mmol/L Fe(2+)) at pH 5.51 and 30 °C. The range of variation observed for oxidation markers in the optimized emulsion model (PV, 0-4.27 mequiv/L; TBARS, 0-13.55 mmol/L) was larger than the variation observed in the nonoptimized model (PV, 0-1.05 mequiv/L; TBARS, 0-1.00 mmol/L). The antioxidant activity of six compounds (Trolox, α-tocopherol, caffeic acid, gallic acid, catechin, and TBHQ) was evaluated using the optimized emulsion conditions. After application of the Tukey HSD post hoc statistical test, the samples that were not different (p < 0.05) in the nonoptimized emulsions showed a significant difference in the optimized emulsions. Considering the importance of the interactions on oxidation studies, our model represents a significant improvement in a direct methodology that can be applied to evaluate natural compounds under different combination of factors.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号