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   768篇
  免费   16篇
  国内免费   35篇
林业   68篇
农学   46篇
基础科学   52篇
  75篇
综合类   139篇
农作物   93篇
水产渔业   120篇
畜牧兽医   120篇
园艺   56篇
植物保护   50篇
  2023年   3篇
  2022年   8篇
  2021年   13篇
  2020年   8篇
  2019年   9篇
  2018年   6篇
  2017年   18篇
  2016年   20篇
  2015年   23篇
  2014年   28篇
  2013年   22篇
  2012年   36篇
  2011年   57篇
  2010年   41篇
  2009年   40篇
  2008年   34篇
  2007年   41篇
  2006年   21篇
  2005年   26篇
  2004年   29篇
  2003年   26篇
  2002年   20篇
  2001年   19篇
  2000年   50篇
  1999年   28篇
  1998年   19篇
  1997年   22篇
  1996年   15篇
  1995年   24篇
  1994年   11篇
  1993年   10篇
  1992年   10篇
  1991年   12篇
  1990年   8篇
  1989年   11篇
  1988年   2篇
  1987年   8篇
  1986年   5篇
  1985年   3篇
  1984年   4篇
  1983年   2篇
  1982年   4篇
  1981年   4篇
  1980年   4篇
  1979年   6篇
  1978年   2篇
  1977年   2篇
  1955年   4篇
  1944年   1篇
排序方式: 共有819条查询结果,搜索用时 15 毫秒
81.
The structural stability of fish myosin depends upon species and temperatures of water in which fish live. Primary, secondary, and quaternary structures of myosin heavy chain (MyHC) from three species of fish living at different temperature ranges have been compared with those of rabbit MyHC in order to investigate the differences in stability. Primary structure of MyHC, although being accessible for warm-water and cold-water fish (carp and walleye pollack), was not available in previous for tropical-water fish literature; so in this study primary structure of MyHC of the tropical-water fish amberjack has been determined by cloning and sequencing its cDNA. The MyHC has 1938 amino acid residues (AA), which are almost as much as as those of carp and walleye pollack. The amberjack MyHC is 91013;95% homologous with other fish and rabbit MyHCs. There is a discernible difference between animal species with stable myosin rod (amberjack, carp, and rabbit) and walleye pollack with unstable rod. Stable rod species have a high probability of forming coiled-coil around the COOH-terminal end of the rod, while the pollack has a low coiled-coil formation probability. In addition, the average scores of the coiled-coil for myosin rod were rabbit (1.738) > amberjack (1.691) > carp (1.680) > walleye pollack (1.674) which correlated exactly with the observed stability. The results suggest that coiled-coil forming ability, particularly around the COOH-terminal end, directs structural stability of fish myosin rod.  相似文献   
82.
牙鲆仔鱼早期阶段的摄食能力   总被引:3,自引:1,他引:3  
龚小玲 《水产学报》2000,24(6):516-521
本文主要在实际观察值的基础上,利用发育反应方程C=KW^y与功能反应Holling、Ivlev方程对2~10日龄的牙鲆仔鱼的摄食能力进行了计算与预测。其结果表明:牙鲆仔鱼在2~6日龄、轮虫饵料密度不超过3000ind.L^-1的水体中,摄食强度不超过1.7μg.h^-1;随着日龄的增加,摄食强度迅速增强,在4、6、10日龄、饵料密度为3000ind.L^-1的水体中,摄食强度分别是1.35、1.6  相似文献   
83.
中籼898产量构成及高产栽培措施研究   总被引:9,自引:4,他引:9  
对中籼 898产量构成因素的分析表明 ,4个产量构成因素对产量贡献的大小依次为有效穗、每穗总粒数、结实率、千粒重。产量 9.0t/hm2 以上的合理穗粒结构为 :有效穗 (2 85± 15 )万 /hm2 ,每穗总粒数 (16 0± 10 )粒 ,结实率 (86± 2 ) % ,千粒重 2 9.5g。并提出高产栽培措施。  相似文献   
84.
IDEF0方法在智能化小区系统设计中的应用   总被引:1,自引:0,他引:1  
通过应用IDEF0系统功能模型设计方法对智能化小区系统进行设计,为完全基于“Everything Over IP”思想的网络化智能小区系统的设计提供了一个较成熟的智能化系统工程的设计思路。  相似文献   
85.
To investigate whether PCB 126 exposure duringembryonic development induces an endocrine stressresponse in larval carp, eggs were exposed,containing 0.01% ethanol (vehicle-control), 10-11,immediately after fertilization, for 48 h to water10-10 or 10-9 mol l-1 PCB in 0.01% ethanol. Eggsincubated in water served as controls. After transferto PCB-free water, mortality, the incidence ofyolk-sac and pe-ricardial oedema, wet and dry weight,rate of skin pigmentation, and whole-body contents ofthe stress hormones ACTH, 0">-MSH and cortisol weredetermined at 48, 96, 144, 168, 192 and 216 hpost-fertilization. Except for the dry weight, allparameters of animals exposed to 10-10 and 10-9 moll-1 PCB increased in a concentration-related manner.However, these changes became evident only at 144 hpost-fertilization, i.e. after resorption of theyolk-sac. Swelling of the yolk sac and pericardiumoccurred, and whole-body ACTH, 0">-MSH and cortisollevels increased. Although animals exposed to 10-10and 10-9 mol l-1 PCB displayed stable but elevatedwhole-body ACTH and 0">-MSH levels until 216 h,whole-body cortisol concentrations gradually decreasedfrom 168 h post-fertilization, and were significantlybelow control values at 216 h post-fertilization.Exposure of the carp embryos to 10-11 mol l-1 PCB only increased whole-body 0">-MSH levels. Increased whole-body ACTH and cortisol levels indicate that PCBinduces a stress response in carp larvae, possiblymediated by a disturbed hydromineral balance (oedema).We further suggest that the PCB-stimulated bodypigmentation is mediated by a stimulation of 0">-MSHsecretion.  相似文献   
86.
Thein vitro secretion of 170">,200">-dihydroxy-4-pregnen-3-one 20-sulphate (170">,200">-P-sulphate) and the free steroid 170">,200">-dihydroxy-4-pregnen-3-one (170">,200">-P), by rainbow trout (Oncorhynchus mykiss) gonads, in response to gonadotropin (GTH) I and GTH II, were studied during the final stages of sexual maturation. Substantial amounts of 170">,200">-P-sulphate were produced, by both mature ovaries and testes, indicating considerable 200">-hydroxysteroid sulphotransferase (200">-HST) activity within these tissues. In the post-ovulatory ovary the level of 170">,200">-P-sulphate (36.6 ng ml013;1) greatly exceeded that of 170">,200">-P (8.59 ng ml013;1). The amount of 170">,200">-P-sulphate produced in incubations of both mature ovary and testes was unaffected by either GTH I or GTH II treatment at physiological concentrations up to 100 ng ml013;1. Similarly, incubations of maturing ovary and testes, treated with GTH I or GTH II, in the presence of added 170">,200">-P at 100 ng ml013;1 of medium, produced levels of 170">,200">-P-sulphate that were similar to those of the controls. In incubations of mature ovarian follicles at the stages of germinal vesicle breakdown and preovulation, both GTHs significantly stimulated secretion of 170">,200">-P, although GTH II was always more potent than GTH I. GTH II significantly elevated the levels of 170">,200">-P in testicular incubations from mature males more than 4-fold relative to GTH I and controls, which did not differ from one another.In conclusion, 200">-HST, the enzyme responsible for the sulphate conjugation of 170">,200">-P, was found to be active in the ovaries and testes of rainbow troutin vitro. However, the levels of this enzyme do not appear to be regulated by either GTH I or GTH II.  相似文献   
87.
Mediators and mechanisms responsible for the inhibitory modulation of trout intestinal smooth muscle were examined using a series of putative mediators and substances known to modulate neurotransmission in mammalian systems. Frequency response relationships to transmural stimulation and concentration response relationships to 5-hydroxytryptamine, carbachol, and substance P were established on paired segments of rainbow trout intestinein vitro in the presence and absence of putative modulatory agents. Modulation of neurally-mediated contractions of trout intestine was achieved with dibutyryl cyclic AMP and forskolin, agents that increase intracellular levels of cyclic AMP. The effect appears to be at the level of the smooth muscle, since the adenylate cyclase activator, forskolin, inhibited muscarinic and serotoninergic contractions as well as transmurally stimulated contractions. Substance P-induced contractions were unaffected by forskolin. The endogenous agonists/neurotransmitters which would increase cyclic AMP levels in rainbow trout intestinal smooth muscle are as yet unknown. The effects do not appear to be modulated by vasoactive intestinal peptide (VIP), calcitonin, calcitonin gene-related peptide (CGRP), or agents that activate 0">-adrenoceptors. Prostaglandin E2 (PGE2) and 0">2-adrenergenic agonists are possible agents which will decrease contractility of the smooth muscle. They were only active in the proximal intestine and on transmurally stimulated contractions. The effects of both PGE2 and 0">2-agonists appear to be prejunctional, decreasing release of contractile neurotransmitters in the enteric nervous system.  相似文献   
88.
Temporal changes in growth, plasma thyroid hormone, cortisol, growth hormone (GH) and non-esterified fatty acid (NEFA) concentrations, hepatic T3 content and hepatic 5001/xxlarge8242.gif" alt="prime" align="BASELINE" BORDER="0">-monodeiodinase activity were measured in rainbow trout (Oncorhynchus mykiss) subjected to a sustained fast for up to eight weeks, and during a four-week re-feeding period. The purpose of the study was to examine aspects of the endocrine control of energy partitioning processes characteristic of short-term (acute; fasting) and long-term (chronic; starvation) food-deprivation states in fish, and to explore the role of the thyroid hormones, cortisol and GH in the energy repartitioning that takes place during an acute anabolic (re-feeding) state following chronic food deprivation.Differences in growth rate between fed and fasted groups were evident after two weeks, but significant weight loss by the fasted groups was not evident until between four and six weeks into the fast. Hepatosomatic indices (HSIs) were significantly reduced in the fasted fish within seven days, and as early as two days in one study; recovery of the HSI in fasted fish was evident within three days of re-feeding. Liver protein content (expressed as % wet weight) was consistently depressed in the fasted fish in only one of the three studies. Liver total lipid content (expressed as % wet weight) was depressed in the fasted fish within two days of food deprivation. Because of the rapid and sustained decrease in the HSI of fasted fish, the hepatic total protein and lipid reserves, when considered on a body weight basis, were markedly lowered within the first few days of the fast. Plasma GH concentrations exhibited a bi-modal pattern of change, with a transient fall in levels, followed by a sustained increase in fasted fish. The indicators of interrenal activity were suggestive of a depressed pituitary-interrenal axis in fasted animals; plasma cortisol levels were elevated to levels of fed animals within one day of re-feeding. The indicators of thyroid hormone economy (plasma thyroid hormone levels, liver triiodothyronine content, hepatic 5001/xxlarge8242.gif" alt="prime" align="BASELINE" BORDER="0">-monodeiodinase (MD) activity, thyroid epithelial cell height) were similarly indicative of a depressed pituitary-thyroid axis in fasted animals, with recovery to levels of the fed animals within one week. Despite the compensatory changes in accumulation of reserves (as indicated by a compensatory increase in HSI), there were no apparent compensatory changes in any of the endocrine parameters evident during the re-feeding period.  相似文献   
89.
Rainbow trout, Oncorhynchus mykiss, were exposed to 48h of environmental hypoxia (water partial pressure of oxygen = 8.0 kPa) at either 5 or 15°C. Blood was sampled during hypoxia via a dorsal aorta cannula to measure arterial blood partial pressure of oxygen and plasma catecholamine concentrations. After 48h, the number (Bmax) and dissociation constant (Kd) of red blood cell surface 03r7873r437/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-adrenoceptors were determined using a radioligand-displacement binding assay. At 5°C, plasma catecholamine levels were elevated at 24h whereas at 15°C, levels were elevated at 48h. At either temperature, following 48h of hypoxia, there was no change in Bmax or Kd of red blood cell surface 03r7873r437/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-adrenoceptors, compared to normoxic control fish. This study demonstrates that chronic exposure to moderate hypoxia does not affect the number or affinity of cell surface 03r7873r437/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-adrenoceptors on trout red blood cells.  相似文献   
90.
Changes in immunoreactivity of Na+/K+-ATPase 06x173u11w528r/xxlarge945.gif" alt="agr" align="BASELINE" BORDER="0">-subunit in gill sections of wild masu salmon (Oncorhynchus masou) during the parr-smolt transformation (smoltification) were compared with changes in gill Na+/K+-ATPase specific activity. Gill Na+/K+-ATPase specific activity increased from April and peaked in May. Immunohistochemical analysis, using an antiserum against a synthetic oligopeptide based on the conserved region of the Na+/K+-ATPase 06x173u11w528r/xxlarge945.gif" alt="agr" align="BASELINE" BORDER="0">-subunit, revealed that immunoreactivity was confined to chloride cells in the surface layer of primary lamellae and the proximal end of secondary lamellae. The size and number of these cells increased gradually from February to May; however, the number of chloride cells of the secondary lamellae decreased in May. These data suggest that the synthesis of Na+/K+-ATPase and the proliferation of chloride cells occur prior to the elevation of enzyme activity. Moreover, it is likely the proliferation and hypertrophy of chloride cells on primary lamellae prepare smolts for entry into seawater and migration in the ocean.  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