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   1858篇
  免费   97篇
  国内免费   204篇
  2023年   32篇
  2022年   28篇
  2021年   39篇
  2020年   46篇
  2019年   38篇
  2018年   45篇
  2017年   48篇
  2016年   55篇
  2015年   59篇
  2014年   54篇
  2013年   146篇
  2012年   54篇
  2011年   62篇
  2010年   62篇
  2009年   55篇
  2008年   70篇
  2007年   50篇
  2006年   67篇
  2005年   68篇
  2004年   53篇
  2003年   74篇
  2002年   58篇
  2001年   52篇
  2000年   44篇
  1999年   42篇
  1998年   38篇
  1997年   53篇
  1996年   43篇
  1995年   46篇
  1994年   46篇
  1993年   55篇
  1992年   51篇
  1991年   64篇
  1990年   42篇
  1989年   39篇
  1988年   47篇
  1987年   35篇
  1986年   34篇
  1985年   26篇
  1984年   36篇
  1983年   12篇
  1982年   20篇
  1981年   13篇
  1980年   13篇
  1979年   9篇
  1978年   10篇
  1977年   7篇
  1976年   8篇
  1975年   5篇
  1973年   2篇
排序方式: 共有2159条查询结果,搜索用时 187 毫秒
51.
In areas with short growing seasons, poor early vegetative growth of soybean (Glycine max [L.] Merr.) is often attributed to the restrictive effect of cool soil conditions on nodulation and N2-fixation by this subtropical grain legume. However, there are few studies regarding potential genetic variability of soybean and Bradyrhizobium japonicum genotypes for nodulation at cool root-zone temperatures (RZT). Experiments were conducted to (1) test for a threshold temperature for low RZT inhibition of soybean nodulation and (2) ascertain whether this threshold temperature response depends mainly on the micro- or macrosymbiont. In experiment 1 soybean seedlings (Glycine max [L.] Merr. cv. Maple Arrow) were inoculated with 1 ml of a log phase culture of B. japonicum strain 532C, H8 or H15 (the latter two strains were isolated from cold soils of Hokkaido, northern Japan) and maintained at either 16, 17.5, 19 or 25°C RZT. In experiment 2 seedlings of cv. Maple Arrow and a cold-tolerant Evans isoline were combined with strain 532C and two Hokkaido strains (H5, H30) at both 19 and 25°C RZT. Results indicated that N2-fixation at 44 days after inoculation was substantially reduced (30–40%) by RZT as high as 19°C, due to development of less nodule mass and to a delay in the onset of N2-fixation and a small decrease in the number of nodules formed. However, the number of nodules formed was sharply reduced and the time required for the first appearance of nodules was significantly delayed below an RZT of 17.5°C. Differences between cultivars for nodulation and N accumulation were apparent at 25°C, but were abolished by growth at 19°C, indicating that, in spite of differences in growth potential between the cultivars under optimum RZT, both cultivars were equally limited by low RZT. Differences between B. japonicum strains were consistent across temperatures and were largely attributable to higher rates of specific nodule activity recorded for strain 532C, which seemed well adapted to low RZT. These results suggest that the host plant mediates the sensitivity of N2-fixation under low RZT and that inoculation with B. japonicum strains from cold environments is unlikely to enhance soybean N2-fixation under cool soil conditions.  相似文献   
52.
Evaluation of enzyme activities in combination with taxonomic analyses may help define the mechanisms involved in microbial decomposition of orgaic amendments and biological control of soilborne pathogens. In this study, powdered pine bark was added to nematode-infested soil at rates of 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, and 50 g kg–1. Total fungal populations did not differ among treatments immediately after application of pine bark. After 7 days, fungal populations were positively correlated with increasing levels of pine bark. This increase was sustained through 14 and 21 days.Penicillium chrysogenum andPaecilomves variotii were the predominant fungal species isolated from soil amended with pine bark. Total bacterial populations did not change with addition of pine bark at 0, 7, and 14 days after treatment. At 21 and 63 days, total bacterial populations declined in soil receiving the highest rates of pine bark. Addition of pine bark powder to soil caused a shift in predominant bacterial genera fromBacillus spp. in nonamended soil, toPseudomonas spp. in amended soil. Soil enzyme activities were positively correlated with pine bark rate at all sampling times. Trehalase activity was positively correlated with total fungal populations and with predominant fungal species, but was not related to bacterial populations. The number of non-parasitic (non-stylet bearing) nematodes andMeloidogyne arenaria in soil and roots were not correlated with pine bark rate. However,Heterodera glycines juveniles in roots, and the number of cysts g–1 root, declined with increasing levels of pine bark.Journal Series Series No. 18-933598 Alabama Agricultural Experiment Station  相似文献   
53.
野生大豆种子蛋白含量差异的生理及结构基础的探讨   总被引:1,自引:0,他引:1  
利用SDS-聚丙烯酰胺凝胶电泳、电子显微镜、蛋白及酰脲含量测定等技术,对高蛋白含量(50.7% )的50359 和低蛋白含量(40.8% )的50305 两个野生大豆在种子发育过程中贮藏蛋白积累的速率、蛋白组分合成的起始时间、蛋白体发育的进程以及幼茎的酰脲含量进行了比较研究。结果表明:野生大豆50359的高蛋白含量是与其种子发育过程中较高的植株酰脲含量、较早较快的贮藏蛋白合成及积累速率,液泡中高效的蛋白贮藏方式以及蛋白体在子叶细胞中占有较大体积相联系的  相似文献   
54.
Stomatal numbers of soybean and response to water stress   总被引:2,自引:0,他引:2  
The relationship among stomatal density, photosynthetic rate, leaf conductance, plant growth, bean yield and kaempferol triglucoside (K9) in the leaves of soybean (Glycine max (L.) Merr.) was examined in two field tests. K9 in the leaves was associated with reduced stomatal density, reduced photosynthetic rate, reduced stomatal conductance, reduced plant weight and lower bean yield. Plants with high stomatal frequency (lacking K9) were better able to take advantage of increased water supply by increasing stomatal conductance (upper surface), transpiration and bean yield. Plants with low stomatal frequency (with K9) were unresponsive to irrigation and in this sense were more tolerant of water stress, but their overall yield was low.  相似文献   
55.
The present study aims at characterizing plant water status under field conditions on a daily basis, in order to improve operational predictions of plant water stress. Ohm's law analog serves as a basis for establishing daily soil-plant relationships, using experimental data from a water-limited soybean crop: 227-1. The daily transpiration flux, T, is estimated from experimental evapotranspiration data and simulated soil evaporation values. The difference, 227-2, named the effective potential gradient, is derived from i) the midday leaf potential of the uppermost expanded leaves and ii) an effective soil potential accounting for soil potential profile and an effectiveness factor of roots competing for water uptake. This factor is experimentally estimated from field observation of roots. G is an apparent hydraulic conductance of water flow from the soil to the leaves. The value of the lower potential limit for water extraction, required to assess the effective soil potential, is calculated with respect to the plant using the predawn leaf potential. It is found to be equal to –1.2 MPa. It appears that over the range of soil and climatic conditions experienced, the daily effective potential gradient remains constant (1.2 MPa), implying that, on a daily basis, transpiration only depends on the hydraulic conductance. The authors explain this behaviour by diurnal variation of osmotic potential, relying on Morgan's theory (1984). Possible generalization of the results to other crop species is suggested, providing a framework for reasoning plant water behaviour at a daily time step.  相似文献   
56.
Adams  M. L.  Norvell  W. A.  Peverly  J. H.  Philpot  W. D. 《Plant and Soil》1993,155(1):235-238
Leaf reflectance and fluorescence characteristics of soybean (Glycine max cv Bragg) are influenced strongly by Mn availability. This report evaluates the effects of leaflet choice, leaf age, and leaf nodal position on several spectral characteristics. Leaves were obtained from soybeans grown hydroponically under controlled environmental conditions with wide differences in Mn supply. The ratio of constant yield fluorescence (Fo) to variable yield fluorescence (Fv), the ratios of reflectance at 750 nm to 550 nm and that at 650 nm to 550 nm, the position of the "red edge" near 700 nm, and an index of leaf "yellowness" were measured periodically. Increasing leaf age caused increases in the "red edge" and in both reflectance ratios. Leaf "yellowness" and the fluorescence ratio Fo/Fv decreased with leaf age and increased with leaf nodal position, primarily in Mn deficient leaves. Effects arising from leaf choice were smaller than those caused by Mn deficiency.  相似文献   
57.
Summary Lepidopteran insects are major defoliating pests of soybean in the southeastern United States. Soybean plants transgenic for a nativecryIA(b) gene fromBacillus thuringiensis var.kurstaki HD-1 were obtained. Embryogenic cultures were induced by plating cotyledons on a Murashige and Skoog-based medium supplemented with 40 mg/liter of 2,4-dichlorophenoxyacetic acid (2,4-D). The embryogenic cultures were maintained in liquid medium containing 5 mg/liter 2,4-D. These cultures were subjected to microprojectile bombardment, followed by selection on 50 mg/liter hygromycin. Resistant embryogenic cell lines were transferred to growth regulator-free medium to permit recovery of mature somatic embryos. After a desiccation period, the somatic embryos were returned to growth regulator-free medium for conversion into plants. Southern hybridization analysis verified transformation. Feeding assays of T1 plants from one cell line deterred feeding, development, and survival of velvetbean caterpillar at a level comparable to that of GatIR81-296, a soybean breeding line with a high level of insect resistance. Reduced feeding on T1 plants correlated with the presence of the transgene.  相似文献   
58.
Flowering time is one of important agronomic traits determining the crop yield and affected by high temperature. When facing high ambient temperature, plants often initiate early flowering as an adaptive strategy to escape the stress and ensure successful reproduction. However, here we find opposing ways in the short-day crop soybean to respond to different levels of high temperatures, in which flowering accelerates when temperature changes from 25 to 30 °C, but delays when temperature reaches 35 °C under short day. phyA-E1, possibly photoperiodic pathway, is crucial for 35 °C-mediated late flowering, however, does not contribute to promoting flowering at 30 °C. 30 °C-induced up-regulation of FT2a and FT5a leads to early flowering, independent of E1. Therefore, distinct responsive mechanisms are adopted by soybean when facing different levels of high temperatures for successful flowering and reproduction.  相似文献   
59.
大豆是事关人民生活和经济社会发展的重要农产品之一,提高大豆生产水平和增加自给能力,是中国农业生产必须解决的重大问题。由于中国耕地资源不足的限制,科技创新是提升大豆生产能力的唯一出路。转基因育种是推动大豆生产发展的颠覆性技术,对美国、巴西和阿根廷等世界主产国大豆产业的发展发挥了重要作用。经过20多年的科技创新,中国转基因耐除草剂和抗虫育种技术已经成熟,这些产品的产业化种植可显著降低大豆生产成本和提升单产水平。基于中国转基因大豆技术发展进度和大豆生产的国情特点,我们提出了采用如下策略科学有序推进产业化工作。一是,在产品应用时间上,按照单一耐草甘膦除草剂、多个基因耐草甘膦和草铵膦等多种除草剂,以及耐除草剂与抗虫等复合性状等产品,依次推进相关种子的产业化;二是,在产品区域布局上,按照靶标杂草和害虫的地理分布特点顶层设计各种耐除草剂和抗虫大豆产品的种植区域;三是,在生物安全管理上,研发应用抗性杂草和害虫种群监测与治理技术,延长转基因产品的使用寿命。同时,还要加强野生大豆资源的保护工作,降低转基因大豆基因漂移对野生大豆生物多样性的影响。  相似文献   
60.
The short-term stimulation of the net rate of carbon dioxide exchange of leaves by elevated concentrations of CO2 usually observed in C3 plants sometimes does not persist. Experiments were conducted to test whether the patterns of response to the environment during growth were consistent with the hypotheses that photosynthetic adjustment to elevated CO2 concentration is due to (1) feedback inhibition or (2) nutrient stress. Soybean [Glycine max (L.) Merr. cv. Williams] and sugar beet (Best vulgaris L. cv. Mono Hye-4) were grown from seed at 350 and 700 μl? CO2, at 20 and 25°C, at a photon flux density of 0.5 and 1.0 mmol m?2 S?1 and with three nutrient regimes until the third trifoliolate leaf of soybean or the sixth leaf of sugar beet had finished expanding. Net rates of CO2 exchange of the most recently expanded leaves were then measured at both 350 and 700 μl 1?1 CO2. Plants grown at the elevated CO2 concentration had net rates of leaf CO2 exchange which were reduced by 33% in sugar beet and 23% in soybean when measured at 350 μl 1?1 CO2 and when averaged over all treatments. Negative photosynthetic adjustment to elevated CO2 concentration was not greater at 20 than at 25°C, was not greater at a photon flux density of 1.0 than at 0.5 mmol m?2 S?1 and was not greater with limiting nutrients. Furthermore, in soybean, negative photosynthetic adjustment could be induced by a single night at elevated CO2 concentration, with net rates of CO2 exchange the next day equal to those of leaves of plants grown from seed at the elevated concentration of CO2. These patterns do not support either the feedback-inhibition or the nutrient-stress hypothesis of photosynthetic adjustment to elevated concentrations of CO2.  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