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   341篇
  免费   17篇
林业   32篇
农学   27篇
基础科学   5篇
  99篇
综合类   19篇
农作物   51篇
水产渔业   40篇
畜牧兽医   51篇
园艺   6篇
植物保护   28篇
  2023年   2篇
  2022年   8篇
  2021年   14篇
  2020年   15篇
  2019年   16篇
  2018年   17篇
  2017年   22篇
  2016年   25篇
  2015年   12篇
  2014年   21篇
  2013年   39篇
  2012年   23篇
  2011年   19篇
  2010年   12篇
  2009年   11篇
  2008年   25篇
  2007年   13篇
  2006年   9篇
  2005年   4篇
  2004年   5篇
  2003年   5篇
  2002年   4篇
  2000年   2篇
  1998年   2篇
  1997年   2篇
  1996年   2篇
  1995年   2篇
  1994年   1篇
  1993年   2篇
  1992年   2篇
  1991年   2篇
  1990年   1篇
  1989年   3篇
  1988年   1篇
  1987年   1篇
  1986年   1篇
  1985年   2篇
  1984年   1篇
  1982年   1篇
  1981年   2篇
  1979年   3篇
  1975年   1篇
  1971年   1篇
  1966年   1篇
  1961年   1篇
排序方式: 共有358条查询结果,搜索用时 0 毫秒
321.
Wheat is one of the most important crops in the world. Its study has involved anatomy, ecology, physiology, molecular biology, biotechnology, and cultivation. Knowledge of the relationship of diploid, tetraploid and hexaploid wheat with stomatal characteristics and water-use efficiency is spare and is subject to argument. The present study was aimed to examine different characteristics of stomata in 28 Iranian landraces belonging to the species Triticum monococcum (2n = 2x = 14), Triticum durum (2n = 4x = 28) and Triticum aestivum (2n = 6x = 42), respectively and investigated a possible relationship between these characteristics and water use efficiency by pot cultivation experiments. The results revealed large variation among landraces as well as between the different species for stomatal frequency and size. The diploid wheat species had the highest stomatal frequency and the lowest stomatal length and width. The hexaploid species had a lower stomatal frequency than the tetraploid species. A highly significant relationship was noted between the value of those traits on the adaxial and abaxial surfaces of the leaf. The ratio of adaxial to abaxial values appeared to be constant for a given species. Both stomatal frequency and size were associated with ploidy level, but also with the nature of the constituting genomes of the species. A highly significant negative association was found among accessions between stomatal frequency and size. As a consequence, the variation for stomatal area per unit leaf area was less than for the other traits. Diploid and hexaploid wheat have significantly higher water use efficiency, compared to tetraploid wheat. No clear association was found, however, between water use efficiency and stomatal characteristics.  相似文献   
322.
In this study, the efficiency of a novel droplet vitrification technique along with different doses of fish antifreeze protein (AFP) type III on Persian sturgeon thawed spermatozoa quality (motility duration and motility percentage) was investigated. Semen of seven male individuals was pooled in equal volumes and diluted with 4°C Tris‐Hcl (100 mM), pH = 8 extenders containing 0, 5, 10, 15 μM of AFP type III in a ratio of 1:1 (semen/extenders). Treated semen was dropped into liquid nitrogen. Solidified droplets were stored for 2, 60 and 120 days and thawed by plunging them into a tube containing 5 mL Tris‐Hcl (100 mM), pH=8 with 1% BSA at 37°C. Motility duration in all treatments had no significant difference comparing to fresh sperm (P > 0.05), but their motility percentage was significantly lower. Treatment with 10 μM of AFP had significantly higher motility percentage (16.11 ± 0.5%) comparing to other treatments (P < 0.05). There was no significant difference between 0, 5, 15 μM of antifreeze protein treatments (P > 0.05), suggesting that antifreeze protein effectiveness are highly dose dependent, and dose of 10 μM is appropriate in Persian sturgeon spermatozoa droplet vitrification. Besides, the present technique obtained higher quality of spermatozoa comparing to its analogue techniques.  相似文献   
323.

Background

There is evidence in the scientific literature of the adverse physiological and psychological effects of shift work. The work of nurses in hospitals is connected with shift and night work. Several publications have described gastrointestinal disturbances in shift workers. The aim of this study was to compare the frequency of gastrointestinal (GI) complaints of nurses on a rotating shift with that of nurses on a regular day shift.

Methods

The study involved 160 nurses (133 working in shifts and at night and 27 working on day shifts) in the Shahid Beheshti Hospital in Kashan, Iran. These nurses answered a Gastrointestinal Symptom Questionnaire regarding the presence of gastrointestinal symptoms (including heartburn, regurgitation, constipation, diarrhea and bloating). Positive responses required frequent symptom occurrence in the past 4 weeks. Significance of group differences was assessed by chi-square and Fisher-exact tests.

Results

Prevalence of GI symptoms was significantly higher (p = 0.009) in rotating-shift nurses (81.9%) than in day-shift nurses (59.2%). Irregular meal consumption (p = 0.01) and GI medications (p = 0.002) were all significantly higher among the rotating shift nurses. In both groups, regurgitation was the most common symptom.

Conclusion

Nurses on rotating shifts in Iran experience more GI disturbances than do nurses on day shifts.  相似文献   
324.
在于田绿洲弃耕地典型地段布设两条基本平行的地下水采样带,用GPS定位,共设置13个样点,现场采样并送实验室分析,以研究其化学特征。运用统计分析发现,地下水离子平均含量,阳离子K++Na+Mg2+Ca2+,阴离子Cl-HCO3-SO42-CO32-,就变异系数而言,CO32-和SO42-较大,其余离子均介于30%~65%;地下水埋深、溶解性总固体和电导率的变异系数较大,均大于50%,pH值的变异系数最小,为3.06%,且pH值的变化范围在8.35~9.28之间,呈弱碱性。通过主成分分析发现,影响研究区浅层地下水化学特征的主成分共有3个,第一主成分F1与K++Na+、Cl-和CO32-具有显著的相关性,第二主成分F2与埋深和SO42-具有较显著的相关关系,第三主成分F3与Mg2+相关关系显著,且这3个主成分已涵盖所有8项指标信息的83%以上。线性回归分析显示,溶解性总固体与K++Na+和Cl-的相关关系显著,且拟合方程具有显著的统计学意义。  相似文献   
325.
326.
To determine the most important traits influencing seed yield of sesame, 18 lines were evaluated in a randomized block design with three replicates in two years during the 2013 and 2014 growing seasons. Following a combined analysis of variance, multivariate statistical analyses such as simple correlation coefficients, factor analysis, stepwise analysis, path analysis, and cluster analysis were administered on combined means. The correlation coefficient analysis showed a significant negative correlation between seed yield and number of capsules per plant and number of seeds per capsule. Correlation between germination period and seed yield was positive and significant at 5% probability level. Factor analysis divided the 14 measured variables into five factors. In the stepwise regression analysis for seed yield as the independent variable, four traits including number of capsules per plant, first capsule height, number of seeds per capsule, and stem length to first capsule were entered into the regression model in four steps with r2 =0.72. The regression coefficient of number of capsules per plant, number of seeds per capsule, and stem length to first capsule were negative and the regression coefficient of the first capsule height was positive and significant at the 5% probability level. The path coefficient analysis based on seed yield as a dependent variable implicated that the number of seeds per capsule had the highest negative direct effect on seed yield. The highest negative indirect effect on seed yield was related to the number of capsules per plant through the number of seeds per capsule. Cluster analysis using Ward’s method divided 18 investigated lines into five clusters.  相似文献   
327.
Flower abscission and pod set behaviour of mungbean in relation to the degree of defoliation at the reproductive stage was studied under field conditions. In the undefoliated and partially defoliated plants flowering occurred in two successive flushes, although the second flush of flowering was less distinct in partially defoliated plants. Most of the pods a plant attained came from the second flush of flowering. Undefoliated and partially defoliated plant had 23 and 8 pods, respectively. Magnitude of flower abscission and pod set in undefoliated plants was much higher than it was in partially defoliated plants. In the completely defoliated plants none of the pods survived to mature and death of plants occurred within a week following defoliation.  相似文献   
328.
Name and address for reprints: A. B. Forbes, Technical Services Manager, MSD-AGVET, Princess Mary House, 4 Bluecoats Avenue, Hertford, Herts. SG14 1PB, UK.  相似文献   
329.
The pharmacokinetics of tripelennamine (T) was compared in horses (n = 6) and camels (n = 5) following intravenous (i.v.) administration of a dose of 0.5 mg/kg body weight. Furthermore, the metabolism and urinary detection time was studied in camels. The data obtained (median and range in brackets) in camels and horses, respectively, were as follows: the terminal elimination half-lives were 2.39 (1.91-6.54) and 2.08 (1.31-5.65) h, total body clearances were 0.97 (0.82-1.42) and 0.84 (0.64-1.17)L/h/kg. The volumes of distribution at steady state were 2.87 (1.59-6.67) and 1.69 (1.18-3.50) L/kg, the volumes of the central compartment of the two compartment pharmacokinetic model were 1.75 (0.68-2.27) and 1.06 (0.91-2.20) L/kg. There was no significant difference (Mann-Whitney) in any parameter between camels and horses. The extent of protein binding (mean +/- SEM) 73.6 + 8.5 and 83.4 +/- 3.6% for horses and camels, respectively, was not significantly statistically different (t-test). Three metabolites of T were identified in urine samples of camels. The first one resulted from N-depyridination of T, with a molecular ion of m/z 178, and was exclusively eliminated in conjugate form. This metabolite was not detected after 6 h of T administration. The second metabolite, resulted from pyridine ring hydroxylation, had a molecular ion of m/z 271, and was also exclusively eliminated in conjugate form. This metabolite could be detected in urine sample for up to 12 h after T administration. The third metabolite has a suspected molecular ion of m/z 285, was eliminated exclusively in conjugate form and could be detected for up to 24 h following T administration. T itself could be detected for up to 27 h after i.v. administration, with about 90% of eliminated T being in the conjugated form.  相似文献   
330.
Growth, photosynthesis and mineral ions accumulation of aromatic small grain, local coarse grain and HYV types of rice were evaluated under saline conditions. Three cultivars from each type were subjected to 0, 50, 100, 150 and 200 mvi NaCl concentrations. After three weeks of 200 mvi NaCl application only Pokkali and Kalobail, both belonging to the local coarse grain type and IPK 37011 of the HYV type were able to survive. Plant height, green leaf area, leaf weight, shoot and root growth were seriously decreased by salinity. However, leaf area was decreased more than other growth parameters. In general aromatic small grain type showed higher salt-sensitiveness in relation to growth than other types of rice. Photosynthesis (Pn) was decreased by salinity and apparently stomatal resistance was partially responsible for the decreased Pn. Kalijira, an aromatic small grain eultivar, showed serious reduction in Pn, especially at the 150mM NaCl level. Na accumulation was increased while K accumulation was decreased by salinity in all types. Salt tolerant eultivars in all types of rice accumulated less Na and more K than susceptible ones. The K/Na ratio was also higher in tolerant eulti-vars. Ca and Mg eoncentrations were decreased by salinity although not all the eultivars showed clear accumulation patterns as observed for Na and K. As HYV and local coarse grain types of rice in general showed high salt tolerance, a comprehensive breeding programme should be of value for the improvement of salt tolerance of aromatic small grain rice which has a high demand in rice importing countries.  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