全文获取类型
收费全文 | 4662篇 |
免费 | 269篇 |
国内免费 | 991篇 |
专业分类
5922篇 |
出版年
2024年 | 7篇 |
2023年 | 78篇 |
2022年 | 96篇 |
2021年 | 122篇 |
2020年 | 143篇 |
2019年 | 146篇 |
2018年 | 121篇 |
2017年 | 128篇 |
2016年 | 176篇 |
2015年 | 167篇 |
2014年 | 181篇 |
2013年 | 359篇 |
2012年 | 212篇 |
2011年 | 229篇 |
2010年 | 177篇 |
2009年 | 220篇 |
2008年 | 196篇 |
2007年 | 251篇 |
2006年 | 271篇 |
2005年 | 268篇 |
2004年 | 256篇 |
2003年 | 279篇 |
2002年 | 233篇 |
2001年 | 195篇 |
2000年 | 183篇 |
1999年 | 139篇 |
1998年 | 107篇 |
1997年 | 147篇 |
1996年 | 125篇 |
1995年 | 92篇 |
1994年 | 86篇 |
1993年 | 76篇 |
1992年 | 65篇 |
1991年 | 60篇 |
1990年 | 64篇 |
1989年 | 41篇 |
1988年 | 43篇 |
1987年 | 32篇 |
1986年 | 13篇 |
1985年 | 26篇 |
1984年 | 24篇 |
1983年 | 14篇 |
1982年 | 21篇 |
1981年 | 14篇 |
1980年 | 8篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1976年 | 6篇 |
1973年 | 4篇 |
排序方式: 共有5922条查询结果,搜索用时 0 毫秒
41.
Evolution of HCN from both rice ( Oryza sativa ) and cocklebur ( Xanthium pennsylvanicum ) seeds increased during a pre-germination period and preceded the evolution of (C2 H4 ). These two species were adopted as the representatives of starchy and fatty seeds, respectively. Ethylene promotes seed germination of many species. However, HCN evolution declined abruptly when the radicles emerged and before the peak in C2 H4 evolution. More-over, both rice and soybean ( Glycine max ) seeds showed some activity of β-cyanoalanine synthase (CAS, EC 4.4.1.9) even in the unimbibed dry state. The activities of CAS in the lower seed of cocklebur and in soybean seeds increased rapidly after emergence of the radicle. However, the CAS of rice seeds, with high activity in the dry state, exhibited a bimodal change, gradually decreasing until radicle emergence had occurred, but then increaing. It is thus likly that HCN evolution during initial imbibition may be derived from cyanogenic reserves and controlled by both pre-existing and subsequently-developing CAS. The exogenous application of C2 H4 stimulated the activities of CAS in both rice and upper cocklebur seeds and reduced their cyanogen contents. Therefore, the decline of HCN evolution after germination seems to be due to the increased activities of CAS by endogenously produced C2 H4 . 相似文献
42.
Iron toxicity and other chemical soil constraints to rice in highland swamps of Burundi 总被引:1,自引:0,他引:1
Iron toxicity is suspected to be a major nutritional disorder in rice cropping systems established on flooded organic soils that contain reductible iron. A pot trial was carried out to assess Fe toxicity to rice in flooded Burundi highland swamp soils with a wide range of organic carbon contents. Soil and leaf analyses were performed and total grain weight was determined. Clear Fe toxicity was diagnosed, based on leaf Fe content at panicle differentiation. Leaf Fe contents higher than 250 g g–1 dry matter induced lower Mg (and probably Mn) uptake, and a 50% total grain weight reduction. These features were associated with exchangeable Fe equivalent fractions higher than 86%. Besides, several non-Fe toxic soils exhibited an Mg-Mn imbalance. 相似文献
43.
44.
In a two-choice test, moreS. furcifera females settled more often on exposed plants than on parafilm-masked ones, regardless of the susceptibility of rice varieties. This indicates that rice volatiles play an important role in the insect's short-range orientation to its host. The fact that more insects settled on exposed resistant Rathu Heenati (RHT) than to masked susceptible Taichung Native 1 (TN1) suggests that there must be certain common volatiles released by both varieties. Few females landed on masked plants of either RHT or TN1. This implies that the insect could not recognize at a distance that a plant was resistant or susceptible without olfactory stimuli.S. furcifera excreted less honeydew on masked plants than on exposed ones for both varieties and more on masked TN1 than on exposed RHT. The electronic monitoring of feeding behavior demonstrates that the insect made more frequent probes and had shorter phloem ingestion durations on exposed RHT than on exposed TN1 and on masked RHT than on masked TN1. Moreover, the insect had longer phloem ingestion durations on masked TN1 than on exposed RHT. These results suggest that volatile chemicals given off by resistant RHT plants have a negative effect on feeding. 相似文献
45.
Iwasaki Yasunaga Mae Tadahiko Fukazawa Chikafusa Makino Amane Ohira Koji Ojima Kunihiko 《Plant and Soil》1993,(1):211-214
Glutelin accumulation in the apical spikelet of the top primary branch (superior spikelet) and the second spikelet of the lowest secondary branch (inferior spikelet) of the ear of the rice plant (Oryza sativa L.) was characterized during grain filling.In the superior spikelet, the accumulation of dry matter and nitrogen started immediately after flowering and rapidly reached the maturation level by 20 days after heading (DAH). At 7 DAH, total RNA content had already reached its maximum level and glutelin mRNA content 70% of its maximum. The increase in glutelin mRNA was followed by a rapid increase in glutelin between 7 and 16 DAH.In the inferior spikelet dry matter, nitrogen and glutelin accumulation were low immediately after flowering and increased only after grain filling of the superior spikelet was almost complete. Total RNA and glutelin mRNA increased much later at slower rates than in the superior spikelet.It is very likely that the retardation of dry matter, total nitrogen and glutelin accumulation in the inferior spikelet is due to retardation of differentiation and development of endosperm tissue, and to glutelin gene expression in endosperm cells. It is suggested that the delayed development resulted from limited partitioning of nutrients to the inferior spikelet at the early stage of ripening. 相似文献
46.
At two sites, one with a 4-year-old (4-Y) secondary vegetation and the other with a 20-year-old (20-Y) vegetation, the influence of burning slashed vegetation on crop performance was studied during three seasons. In the first season after clearing, also the influence on weed growth was studied. At both sites, burning significantly decreased the number of weed seedlings. The lowest number of seedlings was found on the burnt plots of the 20-Y site. Burning increased yield and nutrient uptake significantly in the first and second season after clearing. In the third season after burning, only at the 4-Y site a significantly higher yield and nutrient uptake were found. At the 20-Y site the effect had disappeared. Calculations of efficiency of utilization of absorbed N, P and K indicated that P was the least available nutrient, also after burning. At both sites three consecutive crops absorbed approximately 40% of P applied in ash, while the cumulative recovery of K was at least 36% at the 4-Y site and at least 59% at the 20-Y site. On non-burnt plots, yields were not lower in the third season than in the first season after clearing, thus indicating that the inherent soil fertility did not decrease. Hence, yield decline on the burnt plots could be ascribed to ash depletion. It was concluded that in the local shifting cultivation system, the combination of ash depletion and infestation of weeds are the main reasons for abandoning the fields. 相似文献
47.
Ming-Tsair Chan Hsin-Hsiung Chang Shin-Lon Ho Wu-Fu Tong Su-May Yu 《Plant molecular biology》1993,22(3):491-506
We have successfully transferred and expressed a reporter gene driven by an -amylase promoter in a japonica type of rice (Oryza sativa L. cv. Tainung 62) using the Agrobacterium-mediated gene transfer system. Immature rice embryos (10–12 days after anthesis) were infected with an Agrobacterium strain carrying a plasmid containing chimeric genes of -glucuronidase (uidA) and neomycin phosphotransferase (nptII). Co-incubation of potato suspension culture (PSC) with the Agrobacterium inoculum significantly improved the transformation efficiency of rice. The uidA and nptII genes, which are under the control of promoters of a rice -amylase gene (Amy8) and Agrobacterium nopaline synthase gene (nos), respectively, were both expressed in G418-resistant calli and transgenic plants. Integration of foreign genes into the genomes of transgenic plants was confirmed by Southern blot analysis. Histochemical localization of GUS activity in one transgenic plant (R0) revealed that the rice -amylase promoter functions in all cell types of the mature leaves, stems, sheaths and roots, but not in the very young leaves. This transgenic plant grew more slowly and produced less seeds than the wild-type plant, but its R1 and R2 progenies grew normally and produced as much seeds as the wild-type plant. Inheritance of foreign genes to the progenies was also confirmed by Southern blot analysis. These data demonstrate successful gene transfer and sexual inheritance of the chimeric genes. 相似文献
48.
49.
Ar^+,远红外激光,γ射线单一及复合处理水稻的诱变效果 总被引:1,自引:0,他引:1
用Ar~+、远红外两种激光和~(60)Co—r射线单一或复合处理两个籼稻品种的干种子,分析和比较了不同处理对水稻的当代生物学效应及处理二代的变异频率。结果表明,两种激光对当代的几个性状均表现为刺激效应,并有减轻,射线辐射损伤的作用,复合处理二代的变异频率和变异类型数明显高于相应的单一处理。说明复合处理是提高激光育种效果的有效方法。 相似文献
50.