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   3382篇
  免费   143篇
  国内免费   441篇
林业   30篇
农学   717篇
基础科学   68篇
  442篇
综合类   1561篇
农作物   569篇
水产渔业   46篇
畜牧兽医   109篇
园艺   209篇
植物保护   215篇
  2024年   21篇
  2023年   46篇
  2022年   91篇
  2021年   98篇
  2020年   112篇
  2019年   116篇
  2018年   101篇
  2017年   134篇
  2016年   148篇
  2015年   145篇
  2014年   189篇
  2013年   219篇
  2012年   216篇
  2011年   259篇
  2010年   192篇
  2009年   186篇
  2008年   177篇
  2007年   170篇
  2006年   154篇
  2005年   151篇
  2004年   78篇
  2003年   119篇
  2002年   51篇
  2001年   62篇
  2000年   90篇
  1999年   70篇
  1998年   52篇
  1997年   64篇
  1996年   53篇
  1995年   51篇
  1994年   31篇
  1993年   43篇
  1992年   48篇
  1991年   29篇
  1990年   28篇
  1989年   36篇
  1988年   34篇
  1987年   19篇
  1986年   30篇
  1985年   10篇
  1984年   12篇
  1983年   2篇
  1982年   4篇
  1981年   3篇
  1980年   3篇
  1979年   3篇
  1978年   3篇
  1977年   8篇
  1976年   3篇
  1955年   1篇
排序方式: 共有3966条查询结果,搜索用时 31 毫秒
1.
Aspects of intensive management practices such as high-yielding cultivars, narrow-row spacings and fungicide treatments could potentially increase cereal yields in regions with a short crop-growing season and occasionally dry and hot weather. A field experiment was carried out at McGill University, Canada for three years (1987 to 1989) to test the effects of triadimefon (1-[4-chlorophenoxy]-3,3-dimethyl-1-[1H-1,2,4-triazol-1-yl]-2-butanone, or Bayleton, a trade name) fungicide (0 vs. 140 g a.i. ha−1) and row spacing (10 vs. 20 cm) on the yield components, yield and other agronomic traits (spike emergence, days to maturity, leaf disease and plant height) of spring barley ( Hordeum vulgare L.) cv. Cadette, Laurier and Leger. A cultivar by row spacing interaction resulted in a 11 to 13.5 % increase in grain yield due to narrow rows for Laurier in two out of the three years and up to 16 % for Leger in one year. Over the three years grain yields were on average increased 6 to 12 % due to use of the narrower row spacing. Fungicide application to barley at the early heading stage effectively controlled leaf diseases without influence on yield components or grain yield. Our results indicate that some components of intensive management such as narrow row spacing can be applicable in regions with a short crop-growing season.  相似文献   
2.
3种农药的微核效应研究   总被引:6,自引:0,他引:6  
运用蚕豆根尖微核技术研究3种农药(甲胺磷、甲基硫菌灵和盐酸吗啉胍)在不同浓度下的诱变效应。结果表明,3种农药诱发蚕豆根尖细胞微核率与对照组相比均有不同程度的上升(P<0.05或P<0.01),农药浓度与微核率呈剂量-效应关系,盐酸吗啉胍的诱变效应大于其他2种农药。  相似文献   
3.
The plants of field bean var. Nadwiślaνski were detopped after about 1 month since the beginning of flowering and after that all flower buds were removed from some plants. The effect of a change in the trophic balance on the setting and subsequent growth of root nodules was studied.
The removal of pods had the greater effect on the increase in the weight of the remaining organs than the detopping of plants. Detopping retarded the ageing of the plants and delayed the phase of the greatest reduction of the number of pods. The detopping of plants revealed their considerable compensative possibilities in terms of production of the pod weight and the potential of leaves for biomass production. The data obtained indicate that plant detopping may disturb the root–bacteria exchange of signals, necessary to initiate the nodule development, while the further growth of nodules depends, above all, on the amount of available photosynthates.
A very high correlation between the dry weight of the whole plant to the dry weight of leaves ratio and the weight of nodules was observed. It proves the importance of the trophic balance of the plant for the root nodule weight production.  相似文献   
4.
Field experiments were conducted to characterize the demography of Abutilon theophrasti and Setaria faberi in a conventionally managed 2‐year (maize/soya bean) rotation, and in 3‐year (maize/soya bean/triticale + red clover) and 4‐year (maize/soya bean/triticale + lucerne/lucerne) rotations managed with 72% and 79% lower herbicide inputs respectively. Rates of weed seedling recruitment, seedling survival and adult plant fecundity were determined for populations in each phase of each rotation and used to calculate annual rates of weed population change, Δ. In both years of the study, Δ for A. theophrasti populations declined or remained stable in all three rotation systems. Despite greater rates of seedling survival and fecundity in maize and soya bean in the 3‐ and 4‐year rotations, increases in Δ for A. theophrasti populations were prevented in these systems because of low fecundity in triticale and low seedling survival and fecundity in lucerne. For Setaria faberi populations, Δ remained stable in the 2‐year rotation, increased in the 3‐year rotation in both years, and increased in the 4‐year rotation in 1 year. The results of this study indicate that when herbicide use is reduced, rotations that include triticale and lucerne can facilitate the suppression of A. theophrasti. Rotations that include lucerne can contribute to restraining S. faberi population growth, given adequate levels of seedling mortality in this crop.  相似文献   
5.
Two potyvirus isolates, one from germplasm of yard-long bean (Vigna unguiculata ssp.sesquipedalis) introduced into the Netherlands, and another one from soybean plants (Glycine max) in Indonesia, were compared with two virus isolates of blackeye cowpea mosaic virus (BICMV) from the USA and a Moroccan isolate of cowpea aphid-borne mosaic virus (CAMV). It is proposed that all five isolates be now considered BICMV on the basis of host ranges, symptoms and serology. From our results, and a reassessment of the literature it is suggested to drop the name CAMV in favour of BICMV.Samenvatting Twee potyvirussen, de een in Nederland ingevoerd met genenmateriaal vanVigna unguiculata ssp.sesquipedalis en de ander uit planten van sojaboon (Glycine max) in Indonesië, werden vergeleken met twee isolaten van blackeye cowpea mosiac virus (BICMV) en een Marokkaans isolaat van cowpea aphid-borne mosaic virus (CAMV). Op grond van waardplantenreeksen, symptomen en serologie stellen de auteurs voor om alle vijf isolaten te beschouwen als BICMV. Gebaseerd op de verkregen resultaten en een kritische beschouwing van de literatuur wordt de aanbeveling gedaan om de naam CAMV te laten vallen ten gunste van BICMV.  相似文献   
6.
不同含水量菜豆种子老化过程中生理特性的研究   总被引:2,自引:0,他引:2  
 对不同含水量的菜豆种子( 2.1%、3.4%、3.8%、6.9%和9.0% ) 老化过程中的生理特性进行了研究。结果表明, 菜豆种子在含水量为318%时, 发芽率, 保护酶系统( SOD、POD和CAT) 活性和脱氢酶活性明显高于其它含水量的种子, 而脂氧合酶(LOX) 活性和丙二醛(MDA) 含量显著低于其它含水量的种子, 表现出较高的生活力。此结果表明适度降低菜豆种子的含水量可以提高其抗老化劣变能力;不同生活力的菜豆种子POD同工酶酶谱表现不同, 而SDS2PAGE蛋白质谱带则基本相同。  相似文献   
7.
宁夏春小麦磷素利用效率的基因型差异研究   总被引:4,自引:0,他引:4  
在不同施磷土壤条件下,采用肥料田间试验和测试分析方法,研究了宁夏春小麦磷素利用效率及其有关性状的基因型差异。结果表明,不论施磷与否,小麦籽粒产量、生物学产量、磷肥增产率、地上部茎叶和籽粒中含磷量以及磷素吸收累积量、磷素利用效率等均存在明显基因型差异,且这种差异在低磷条件下更为明显。在低磷条件下,籽粒生产中的磷素利用效率变化在160.1~448.3kg/kg之间,平均为223.1kg/kg,变异系数为23.05%;干物质生产中的磷素利用效率变化在332.6~898.5kg/kg之间,平均为458.4kg/kg,变异系数为22.57%。从中筛选出低磷高产高效品种8个。在较高供磷条件下,籽粒生产中的磷素利用效率变化在150.9~350.9kg/kg之间,平均为229.8kg/kg,变异系数为20.63%;于物质生产中的成素利用效率变化在358.3~746.5 kg/kg之间,平均为482.Ikg地,变异系数为 18.76%。从中筛选出高磷高产高效品种7个。通过相关分析,研究了不同供成条件下磷素利用效率与其它营养性状之间的关系,提出了培育磷高效品种的有用性状指标。  相似文献   
8.
优质高产菜豆新品种‘无筋2号''   总被引:2,自引:0,他引:2  
‘无筋2号’为优质高产菜豆新品种,生长势旺盛,耐肥抗倒,播种至采鲜荚生育期45d左右,结荚性好,豆荚饱满,商品性好,适宜速冻加工出口,鲜荚产量17—18t/hm^2,高产地块可达20t/hm^2以上。  相似文献   
9.
In March 1999, we found prairie gentian (Eustoma grandiflorum) infected with powdery mildew in a greenhouse in Oita Prefecture, Japan. Morphological observation revealed that the causal fungus belongs to the mitosporic genus Oidium subgenus Pseudoidium [teleomorph: Erysiphe sensu Braun and Takamatsu (2000)]. Precise taxonomic position of the fungus, however, is uncertain due to lack of the perfect stage. We determined the nucleotide sequence of the rDNA ITS region of the fungus. Comparison of the sequence with those obtained from DNA databases of this fungal group revealed that the sequence is identical to those of powdery mildews from garden four-o'clock (Mirabilis jalapa) and broad bean (Vicia faba). Inoculation of an isolate from garden four-o'clock caused mildew on prairie gentian and broad bean, suggesting that the prairie gentian mildew originates from garden four-o'clock or broad bean. Molecular phylogenetic analysis indicated a close relationship of this fungus to Erysiphe baeumleri on Vicia spp. and E. trifolii on Trifolium pratense. From these results, we propose that prairie gentian mildew diverged from a Fabaceae-parasitic ancestor. Received 14 March 2002/ Accepted in revised form 28 May 2002  相似文献   
10.
The genetic and virulence variability of 112 isolates of Phaeoisariopsis griseola , collected from various locations in Central America, were studied using seven random amplified polymorphic DNA (RAPD) primers and 12 common-bean differential genotypes. Broad molecular diversity ( H  = 0·92) among isolates was found using RAPD markers. Fifty pathotypes were identified on 12 differential bean genotypes, 29 of which were represented by only one isolate. Only 18 pathotypes were found in two or more countries. Pathotype 63-63 was the most virulent and caused leaf spots on all 12 common-bean differential genotypes. Comparison of virulence phenotypes and RAPD profiles to known Andean P. griseola isolates confirmed that all isolates belonged to the Mesoamerican group. Pairwise comparison between individual RAPD loci showed that the majority were in gametic phase linkage disequilibrium, revealing that P. griseola maintains a genetic structure that is consistent with asexual reproduction. The molecular and virulence diversities of P. griseola isolates from Central America imply that using single resistance genes to manage angular leaf spot is inadequate and stacking resistance genes may be necessary to manage the disease effectively.  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