全文获取类型
收费全文 | 20626篇 |
免费 | 1751篇 |
国内免费 | 2725篇 |
出版年
2024年 | 74篇 |
2023年 | 437篇 |
2022年 | 679篇 |
2021年 | 899篇 |
2020年 | 853篇 |
2019年 | 973篇 |
2018年 | 861篇 |
2017年 | 770篇 |
2016年 | 835篇 |
2015年 | 847篇 |
2014年 | 1183篇 |
2013年 | 1420篇 |
2012年 | 901篇 |
2011年 | 974篇 |
2010年 | 708篇 |
2009年 | 998篇 |
2008年 | 993篇 |
2007年 | 1119篇 |
2006年 | 980篇 |
2005年 | 875篇 |
2004年 | 770篇 |
2003年 | 726篇 |
2002年 | 625篇 |
2001年 | 519篇 |
2000年 | 503篇 |
1999年 | 440篇 |
1998年 | 426篇 |
1997年 | 450篇 |
1996年 | 349篇 |
1995年 | 307篇 |
1994年 | 274篇 |
1993年 | 273篇 |
1992年 | 254篇 |
1991年 | 204篇 |
1990年 | 198篇 |
1989年 | 179篇 |
1988年 | 152篇 |
1987年 | 155篇 |
1986年 | 148篇 |
1985年 | 156篇 |
1984年 | 118篇 |
1983年 | 75篇 |
1982年 | 104篇 |
1981年 | 81篇 |
1980年 | 44篇 |
1979年 | 56篇 |
1978年 | 36篇 |
1977年 | 28篇 |
1976年 | 18篇 |
1975年 | 28篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Differential thermal analysis indicated that the frost resistance of winter rape leaves ( Brassica napus L. var. oleifera L. cv. Gòrczanski), collected from plants grown in the cold (5/2°C), relies mainly on their ability to supercool to −9 to −11°C, i.e. consists in freezing avoidance. Initiation of ice formation in the cold-acclimated leaves resulted in the death of more than 50% of the cells as determined with a conductivity method. The development of freezing tolerance appeared to be an attribute of the second stage of plant hardening and was induced by the exposure of plants to a slightly subzero temperature (−5°C) for 18 h. Such a treatment brought about a sudden and persistent water potential decrease in the leaves, despite the fact that they had reabsorbed water from the medium prior to water potential measurements. Water potential changes were associated with a higher growth capability of the leaves as checked by determinations of disk area increments. It is suggested that the increased frost tolerance of the cold-grown winter rape leaves, subjected to subfreezing temperature, is related to the decreased water potential of the tissue caused by changes in turgor and/or in osmotic pressures of the cells. 相似文献
82.
Stanislaw Ulaszewski Elisabetta Balzi André Goffeau 《Molecular & general genetics : MGG》1987,207(1):38-46
Summary In the yeast Saccharomyces cerevisiae, the pma1 mutations confers vanadate-resistance to H+-ATPase activity when measured in isolated plasma membranes. In vivo, the growth of pma1 mutants is resistant to Dio-9, ethidium bromide and guanidine derivatives. This phenotype was used to man the pma1 mutation adjacent to LEU1 gene on chromosome VII. From a cosmid library of a wild-type Saccharomyces cerevisiae genome, a large 30 kb DNA fragment was isolated by complementation of a leu1-pma1 double mutant. A 5 kb HindIII fragment was subcloned and it restored both Leu+ and Pma+ phenotypes after integrative transformation. The restriction map of the 5 kb HindIII fragment and Southern blot analysis reveal that the cloned fragment contains the entire structural gene for the plasma membrane ATPase and the 5 end of the adjacent LEU1 gene. The pma1 mutation conferring vanadate-resistance is thus located in the structural gene for the plasma membrane ATPase.Publication no 2456 from the Biology Directorate of the Commission of European Communities 相似文献
83.
The fate of15N labeled nitrogen applied to mature citrus trees 总被引:1,自引:0,他引:1
Summary The efficiency and balance of nitrogen from one year's application was studied in a long-term fertigation experiment. Enriched
nitrogen fertilizer, K15NO3, was applied to a 22-year-old Shamouti orange tree with a history of high N applications (N3) and to an N-starved tree (N1). The distribution of N in the different parts of the trees and in the soil was determined after the experimental trees were
excavated.
Similar total recovery of the labeled fertilizer N was found in the trees and soil in both treatments (N1−61.7% N3−56%). However, the distribution between tree and soil was different. The amount of recovered residual fertilizer in the soil
was much larger in the N3 treatment than in N1. The highest percentage of fertilizer N was found in the new organs,i.e. fruits, twigs and leaves. The roots and branches took up only 6–14% from the labeled fertilizer.
Only 20.9% of the leaf N and 23.4% of the fruit N in the N3 tree originated in the labeled fertilizer, indicating translocation of N from older parts of the tree to new growth.
Evidence was found of storage of N in the wooded branches, while the roots contained a surprisingly small part of labeled
fertilizer.
Contribution 1599E. 相似文献
84.
R. B. Sharma 《Plant and Soil》1987,98(3):429-432
Summary A greenhouse study in which 24, 54 and 71 per cent roots of wheat (Triticum aestivum L.) were pruned on the 73rd day from the date of planting (anthesis stage) showed that during a 7-day period following root
pruning, total transpiration and leaf water potential were significantly lower (P=0.05) and the stomatal resistance was significantly higher (P=0.05) where 54 and 71 per cent roots were pruned, as compared to no root pruning or 24 per cent root pruning. The leaf relative
water content, however, showed no significant differences. Thus about one-fourth root sytem could be reduced without adversely
affecting the plant-water status. 相似文献
85.
The effects of Rhizobium strain and its interaction with plant cultivar were examined in glasshouse-grownPhaseolus vulgaris in two experiments where the physiological attributes defining the symbiotic efficiency were determined.
Strains of Rhizobium significantly affected nodulation, rates of N accumulation, partitioning of N within the mature shoot
and remobilizaton of the N stored in the vegetative organs to the seeds.
The most efficient symbiosis (strain CO5 with Negro Argel), in comparison with the least efficient symbiosis (strain 127 K-17
with Venezuela-350) showed higher rates of C2H2 reduction from flowering to mid pod fill stage, evolved less hydrogen from nodules and showed higher rates of N transport
as well as higher percentages of ureide-N in the xylem sap. At maturity, the best cultivar/strain association exceeded the
total N accumulated in the seed and the harvest index of the poorest symbiosis in 88% and 20%, respectively. The other symbiotic
combinations were intermediate in all characteristics.
Nitrogen accumulation in plant shoot showed highly significant correlation with acetylene reduction rates, nodule relative
efficiency, total N transport in the xylem sap and percentage of N transported as ureides. 相似文献
86.
M. Buiatti C. Simeti S. Vannini G. Marcheschi A. Scala P. Bettini P. Bogani M. G. Pellegrini 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1987,75(1):37-40
Summary To obtain Tomato cell lines with an altered capacity to respond to heat-released cell wall components (elicitor) of a tomato pathogen (Fusarium oxysporum f. sp. lycopersici), positive and negative selection experiments, using BUdR enrichment techniques, were carried out on suspension cultures of the susceptible, low phytoalexin producer cultivar Red River. Both high and low phytoalexin producing clones were isolated. Further tests demonstrated that not all phytoalexin-producing clones were more susceptible to the elicitor toxic effect, and that they were altered also in the speed of response to fungal cell wall components. Cells selected with Fusarium elicitor showed the same behaviour when challenged by Phytophthora infestans elicitor, thus suggesting in this case lack of specificity. The results are finally discussed with a view to using the technique both as a tool to study the genetics and physiology of hostparasite interactions and as a possible new method for the selection of pathogen resistant genotypes.Paper no. 1224 IPRA-CNR; research supported by an EEC-BAP contract 相似文献
87.
M. M. Kyle R. Provvidenti 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1987,74(5):595-600
Summary Resistance to watermelon mosaic virus-2 in Phaseolus vulgaris L. is conferred by two distinct dominant alleles at independent loci. Based on segregation data one locus is designated Wmv, the other, Hsw. The dominant allele Wmv from cv. Great Northern 1140 prevents systemic spread of the virus but viral replication occurs in inoculated tissue. In contrast, Hsw confers both local and systemic resistance to WMV-2 below 30C. At higher temperatures, plants that carry this allele in the absence of modifying or epistatic factors develop systemic veinal necrosis upon inoculation with the virus that results in rapid death. Patho-type specificity has not been demonstrated for either allele; both factors confer resistance to every isolate tested. A temperature-sensitive shift in epistasis is apparent between dominant alleles at these loci. Because Hsw is very tightly linked if not identical to the following genes for hypersensitivity to potyviruses I, (bean common mosaic virus), Bcm, (blackeye cowpea mosaic virus), Cam, (cowpea aphid-borne mosaic virus) and Hss (soybean mosaic virus), parental, reciprocal dihybrid F1 populations, and selected F3 families were inoculated with each of these viruses and held at 35 C. F1 populations developed vascular necrosis completely or primarily limited to inoculated tissue, while F3 families from WMV-2-susceptible segregates were uniformly susceptible to these viruses. The relationship between Hsw, Wmv and other genes for potyvirus resistance suggest patterns in the evolution of resistance and viral pathogenicity. Characterization of the resistance spectrum associated with each factor provides an additional criterion to distinguish genes for plant virus resistance. 相似文献
88.
Relation between electrokinetic potentials and growth in callus cultures ofTrigonella foenum-graecum
Callus cultures ofTrigonella foenum-graecum were initiated from radicle or cotyledon portions of seedlings and young leaves and maintained on modified 1-B5 medium. The
callus mass was disaggregated by mechanical agitation and the discrete cells thus obtained were used to measure their electrokinetic
potential. Studies pertaining to the effects of ageing on electrokinetic potential and growth index revealed a relationship
between these two parameters. Thus, the rate of change of electrokinotie potential with age could be employed as a parameter
to study the growth kinetics of cells in callus cultures. 相似文献
89.
The organic content of the surficial sediment: a method for the study of ecosystems development in abandoned river channels 总被引:1,自引:0,他引:1
To characterise different abandoned river channels, a simple index of the surface sediment was developed i.e. the content of organic matter (expressed as C × N) plotted in relation to that of CaCO3. The age of the channels studied ranges from 20 to 300 years. Some of them still contain water; others are silted up. Two types are distinguished. The ecosystems of the first one are closed and show a slow rate of development governed by autogenous processes. Those of the second type are more open and show a fast rate of development mainly controlled by allogenous processes. These distinctions are used in a diagrammatic model of the dynamics of Rhône River alluvial plain to be used in fundamental or applied future research. 相似文献
90.
利用 XAD-4憎水性吸附树脂采集墨红头香,以毛细管气相色谱双柱保留指数和 GC/MS/DS 联用方法鉴定头香的化学成份。共分离鉴定或初步鉴定了45种组份,其中含量较大的有乙酸芳樟酯(14.98%),柠檬烯(12.07%),甲基苯甲醚(9.88%),香茅醇(4.82%),乙酸巳酯(3.98%),β-石竹烯(4.55%),芳樟醇(3.18%),正巳醇(3.17%)等. 相似文献