全文获取类型
收费全文 | 21249篇 |
免费 | 241篇 |
国内免费 | 4篇 |
专业分类
林业 | 3987篇 |
农学 | 1901篇 |
基础科学 | 241篇 |
3934篇 | |
综合类 | 895篇 |
农作物 | 2480篇 |
水产渔业 | 2104篇 |
畜牧兽医 | 2424篇 |
园艺 | 1259篇 |
植物保护 | 2269篇 |
出版年
2023年 | 44篇 |
2022年 | 94篇 |
2021年 | 115篇 |
2020年 | 164篇 |
2019年 | 188篇 |
2018年 | 2899篇 |
2017年 | 2871篇 |
2016年 | 1349篇 |
2015年 | 212篇 |
2014年 | 206篇 |
2013年 | 477篇 |
2012年 | 1034篇 |
2011年 | 2357篇 |
2010年 | 2274篇 |
2009年 | 1411篇 |
2008年 | 1482篇 |
2007年 | 1757篇 |
2006年 | 236篇 |
2005年 | 251篇 |
2004年 | 209篇 |
2003年 | 262篇 |
2002年 | 156篇 |
2001年 | 94篇 |
2000年 | 124篇 |
1999年 | 75篇 |
1998年 | 57篇 |
1997年 | 53篇 |
1996年 | 42篇 |
1995年 | 41篇 |
1994年 | 36篇 |
1993年 | 47篇 |
1992年 | 47篇 |
1991年 | 51篇 |
1990年 | 45篇 |
1989年 | 52篇 |
1988年 | 67篇 |
1987年 | 45篇 |
1986年 | 35篇 |
1985年 | 29篇 |
1984年 | 32篇 |
1983年 | 31篇 |
1982年 | 26篇 |
1981年 | 27篇 |
1980年 | 38篇 |
1979年 | 48篇 |
1978年 | 24篇 |
1971年 | 24篇 |
1970年 | 38篇 |
1969年 | 29篇 |
1968年 | 30篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
4.
Differences in gas exchange parameters i.e. carbon exchange rate (CER), transpiration (Tr), water vapor conductance (g) were investigated, under a controlled environment, in a semi-dwarf (SD) Triticum aestivum line, its tall (TL) near isoline and the Payne (PA) variety characterized by small leaves. The plants were maintained in: (a) optimal substrate moisture condition (CTR); (b) salinized by watering with a 0.12 m NaCl solution (SLT); (c) water stressed by withholding waterings during a period of six days (STR); (d) stressed and salinized during the same period and with the same saline solution (S + S). CER and Tr were negatively affected by the stresses; SLT and S + S treatments had a higher WUE compared to their respective controls CTR and STR.
SD line had a better performance in terms of CER and WUE, particularly when water and salt stresses interacted. Leaf anatomy and assimilation/internal CO2 concentration curves contributed to explaining the SD performance. 相似文献
SD line had a better performance in terms of CER and WUE, particularly when water and salt stresses interacted. Leaf anatomy and assimilation/internal CO
5.
Praveen C. Verma Debasis Chakrabarty Satya Narayan Jena Devesh K. Mishra Pradhyumna K. Singh Samir V. Sawant Rakesh Tuli 《Industrial Crops and Products》2009,29(2-3):581-589
Vanilla is a large genus of about 110 species in the orchid family (Orchidaceae), including the species Vanilla planifolia from which commercial vanilla flavoring is derived. Since most species of vanilla are considered rare and endangered there is an urgent need to conserve them through genetic analysis and propagation/conservation studies on this crop.The present study investigated the genetic diversity among nine leafy- and leaf-less Vanilla species employing 30 decamer RAPD primers and 10 ISSR primers. The species under study were diverse and displayed a range of variability (0–66% and 0–81% for RAPD and ISSR, respectively). A total of 154 RAPD polymorphic markers (83.24%, h = 0.378) and 93 ISSR polymorphic markers (86.11%, h = 0.363) were used to generate a genetic similarity matrix followed by the cluster analysis. Specific groupings were revealed by each cluster analysis with slight variation between two different markers. Among the nine species studied, V. planifolia, Vanilla aphylla and Vanilla tahitensis revealed very low level of variation within their collections, thus indicating a narrow genetic base. The large genetic distance of Vanilla andamanica from other species suggests its different origin. A close genetic affinity was observed between the pairs V. planifolia, V. tahitensis and Vanilla albida, V. aphylla. These are the first comparative results for RAPD and ISSR reporting inter-relationship among nine cultivated, wild and hybrid Vanilla species. 相似文献
6.
A field experiment was conducted to find out the critical physiological stages of irrigation schedules inducing better growth, physiological efficiency and seed yield potential of berseem ( Trifolium alexandrinum L., Var. S-99-1). For this purpose eight irrigation treatments were made comprised of four treatments of three irrigation (W1 , W2 , W3 and W4 ), three treatments of four irrigation (W5 , W7 and W8 ) and one treatment of five irrigation (W6 ) at various physiological stages i. e. regeneration, flower initiation, full bloom, seed initiation and advance seed development stage.
Thus based on the experimental results the physiological role of watering in berseem seed production could be discussed as:
With-holding of irrigation either at regeneration or at full bloom stage developed potential water stress in plants as indicated by high proline content of irrigation treatments — W4 , W1 and W7 ; and further brought out disturbance on the formation of carotene, synthesis of water soluble sugar and translocation of sugar towards reproductive organs during grain development stage. These stresses adversely affected the plant growth and flowering behaviour. The irrigation at seed initiation stage increased the seed yield. Continuous irrigation did not appear to be useful. Thus it can be concluded that irrigation at three critical physiological stages i. e. regeneration, full bloom and seed initiation was found to be essential for obtaining potential seed yield of berseem. 相似文献
Thus based on the experimental results the physiological role of watering in berseem seed production could be discussed as:
With-holding of irrigation either at regeneration or at full bloom stage developed potential water stress in plants as indicated by high proline content of irrigation treatments — W
7.
D. Sharifi J. Singh P.K. Peshin A.P. Singh 《Veterinary anaesthesia and analgesia》1991,18(Z1):309-314
Cardio pulmonary embarrassment was induced with thiopentone sodium in 35 healthy adult donkeys divided in to seven groups of five animals. “To effect” anaesthetic dose was (12.72 ± 0.69 mg/kg) which caused transient hypotension, tachycardia and hypoxemia. A stable fall in MAP 1. 6.77 ± 0.287 Kpa (50.78 ± 2.15 mmHg) and a flat EEG were considered to indicate acute circulatory insufficiency due to thiopentone overdose. 相似文献
8.
The relative amounts of primary and secondary sulfates in atmospheric aerosols and precipitation can be estimated from measurements of the stable oxygen isotope ratios. The oxygen-18 content of sulfates formed in power plant stack gases before emission into the atmosphere is significantly higher than that of sulfates formed from sulfur dioxide after emission. Results show that 20 to 30 percent of the sulfates in rain and snow at Argonne, Illinois, are of primary origin. 相似文献
9.
Variation of Parthenium hysterophorus in response to soil quality: implications for invasiveness 总被引:1,自引:0,他引:1
Summary The performance of Parthenium hysterophorus , native to the tropical Americas and invasive to several countries, was evaluated for response to soil quality. Phenological (six stages) and quantitative growth variables [relative growth rates in height (Rh ) and diameter (Rd )] were measured every fortnight. Based on harvest data, the variables, root:shoot (R:S) ratio, specific leaf area (SLA), relative growth rate in biomass (Rw ), net assimilation rate (NAR) and dry matter allocation to plant components, were determined. High clay content in soils prolonged the rosette stage, enhanced Rh and Rd and hampered root growth, but promoted biomass allocation to shoots. The extreme degree of plant mortality was observed, with only 33.3% individuals surviving to reproduce in the soils with highest clay contents. This appears to be one of the most important findings that certainly has a major bearing on the range of adaptation of the weed. Seed mass declined whereas seed production increased in relatively coarser soils with the exception of nearly pure sand. In soils rich in clay, plants produced a smaller number of larger seeds. These data suggest that variation in fundamental functional traits would enable P. hysterophorus to adjust to a variety of habitat conditions. 相似文献
10.