全文获取类型
收费全文 | 1517篇 |
免费 | 96篇 |
国内免费 | 89篇 |
专业分类
林业 | 101篇 |
农学 | 656篇 |
基础科学 | 5篇 |
65篇 | |
综合类 | 470篇 |
农作物 | 105篇 |
水产渔业 | 88篇 |
畜牧兽医 | 94篇 |
园艺 | 51篇 |
植物保护 | 67篇 |
出版年
2024年 | 3篇 |
2023年 | 19篇 |
2022年 | 19篇 |
2021年 | 30篇 |
2020年 | 34篇 |
2019年 | 29篇 |
2018年 | 24篇 |
2017年 | 36篇 |
2016年 | 50篇 |
2015年 | 37篇 |
2014年 | 47篇 |
2013年 | 68篇 |
2012年 | 68篇 |
2011年 | 73篇 |
2010年 | 68篇 |
2009年 | 61篇 |
2008年 | 67篇 |
2007年 | 62篇 |
2006年 | 58篇 |
2005年 | 39篇 |
2004年 | 44篇 |
2003年 | 56篇 |
2002年 | 43篇 |
2001年 | 44篇 |
2000年 | 40篇 |
1999年 | 50篇 |
1998年 | 47篇 |
1997年 | 47篇 |
1996年 | 47篇 |
1995年 | 41篇 |
1994年 | 36篇 |
1993年 | 38篇 |
1992年 | 37篇 |
1991年 | 40篇 |
1990年 | 31篇 |
1989年 | 21篇 |
1988年 | 31篇 |
1987年 | 16篇 |
1986年 | 16篇 |
1985年 | 10篇 |
1984年 | 5篇 |
1983年 | 1篇 |
1982年 | 19篇 |
1981年 | 12篇 |
1980年 | 7篇 |
1979年 | 7篇 |
1978年 | 8篇 |
1977年 | 7篇 |
1976年 | 7篇 |
1955年 | 2篇 |
排序方式: 共有1702条查询结果,搜索用时 15 毫秒
51.
Summary The risk for a gene dispersal is reported for reciprocal crosses between a transgenic rapeseed variety resistant to the herbicide phosphinotricin and five related species. The first stages after pollination were cytologically observed and fertilized ovaries were established in in vitro culture for the production of interspecific hybrids. A similar classification was observed for the index of pollination compatibility and embryo yield. From the 243 embryos produced, 109 plantlets were obtained in a greenhouse. All the interspecific combinations tested were able to produce hybrid plants. A higher number of hybrids was obtained when rapeseed was used as the female parent. The hybrids had the expected triploid structure except for two amphidiploid, B. napus × B. oleracea, and one amphidiploid, B. napus × S. arvensis, plants with 56 chromosomes. The triploid hybrids were sterile or partially fertile but two of the amphidiploid plants, B. napus × B. oleracea, were fully fertile. The cytoplasm source did not seem to affect the fertility of the hybrids. 相似文献
52.
The genotypes of Avena sativa were crossed to A. magna Murph. et Terrell, from the sterile pentapliod F1 amphiploids were successfully developed after colchicine treatment. The C1 plants were observed for their morphology, cytology and fertility. These plants were characterized by longer and broader glumes, swollen pedicel attachment, pubescent lemmas, bold seeds and persistent spikelets. Other characters, viz., final plant height, rachis length, peduncle length and spikelets per panicle were intermediate. All plants hail the chromosome number 2n = 70 or close to. 7C. Due to stickiness of chromosomes, meiosis of only two decaploid plants could be studied in detail. The average chromosome Association was 1.3 I + 28.56 II + 1.4 III + 1.77 IV + 0.07 VI and 0.13 I + 28.7 II + 0.14 III + 2.09 IV + 0.59 VI; but in some cells up to 35 bivalents were observed. Seed set as well as pollen fertility and size exhibited great variation. 相似文献
53.
Summary The interspecific (C. arabica × tetraploid C. canephora) F1 hybrid showed a low but highly variable fertility. An almost complete restoration to normal fertility was observed in individual trees of BC1 and BC2 generations with C. arabica as recurrent parent. From a comparison of various tests of fertility, it was found that pollen viability is a satisfactory measure of fertility. 相似文献
54.
Eleven interspecific hybrids of cassava with the wild relatives Manihot glaziovii, M. pseudoglaziovii. M. dichotoma and M. anomala were examined to detect unreduced microspores. The frequency of dyads and triads was counted, as well as pollen viability. Of these hybrids, the second generation progeny of cassava×M. glaziovii showed a high frequency of unreduced microspores (3.7%) permitting their selection for further manipulation of this character. An association of vegetatively-reproduced genotypes and the occurrence of unreduced microspores has also been observed. 相似文献
55.
In most interspecific hybridization among diploid Brassica species (n = 8, 9, 10), few or no hybrids are obtained, and the three crosses of tetraploid (n = 17, 18, 19) by diploid species are usually only partially successful, especially in one of the reciprocal crosses. Such a cross-incompatibility complex is explained in this paper by a polar-nuclei activation (PNA) hypothesis which proposes two reversible functions, a normal reproduction and a sexual barrier, of double fertilization in higher plants. The first, i.e. the relative activating value (AV) of diploid species, was estimated as 1.0–3.5, whereas that of tetraploid species was 2.7–5.2. In addition, crosses with a polar nuclei activation index (AI) of about 15–87 % were compatible but those beyond the two limits were incompatible, 50 % of those showing normal seed development after self-pollination. 相似文献
56.
Summary Interspecific hybrids have been obtained in an otherwise incompatible cross betweenBrassica juncea × Brassica hirta through the in vitro culture of hybrid ovules and ovaries. The best response was observed from ovules and ovaries cultured 10–15 and 5–7 days after pollination respectively on a basal medium supplemented with indoleacetic acid, kinetin and casein hydrolysate. In some cases the basal cut end of the ovaries proliferated to form callus and shoots. The in vitro-derived hybrid seeds varied in their colour, size and shape, and the F1 plants in the field showed a large diversity in their morphological traits. The hybrids were sterile, and had an intermediate number of chromosomes (2n=30). 相似文献
57.
G. Kleijer 《Plant Breeding》1987,99(2):144-150
Meiotic observations of 56-chromosome plants of the C1, C2 and C3 generations of the amphidiploid Lolium multiflorum × Festuca arundinacea showed that most of the chromosomes paired as bivalents. However, uni- and multivalents were frequently observed. No difference in meiotic regularity and stability could be detected between the different generations. The average chromosome number and; the percentage of 56-chromosome plants decreased with increasing generations. The direct use of the 56-chromosome amphidiploids of L. multiflorum and F. arundinacea in a breeding programme appears to be quite difficult, but these plants can be used in a backcross programme with Lolium and Festuca. 相似文献
58.
Escape of transgenes from genetically modified oilseed rape, Brassica napus, into wild radish, Raphanus raphanistrum, depends on sexual compatibility. The variation in prezygotic barriers of two different cultivars for interspecific hybridization
with a population of wild radish was investigated by hand crossing and fluorescence microscopy of pistils. Significant differences
were observed between oil seed rape cultivars in their ability to accept wild radish pollen germinating onto their stigma
and the rate of fertilization of ovules. Some differences among the pollen donor plants were also detected. These results
suggest that the rate of interspecific hybridization in the field would depend upon the oilseed rape cultivar and the genotype
composition of the local wild populations. The implication of S-related genes, as revealed through identification by pistil
tissue prints of class I and II S-types of SLG (S-Locus Glycoprotein) and SLR1 (S-Locus Related),and immuno-IEF, was not significant.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
59.
Attempts were made to produce hybrids among various Sambucus species in order to change the plant structure of S. nigra, a medicinal plant of increasing agricultural importance. The first successful genetic recombination, involving four species, was conducted after the introduction of S. javanica from a South Pacific island. A series of genetic analyses of the involved species and some relatives was performed in order to obtain reliable data on genetic similarity among parental species and to confirm the hybrid nature of putative hybrids. The nuclear 2C DNA content revealed by flow cytometry of nine species showed limited variation, the greatest being in S. nigra (28.6 pg) and the smallest in S. caerulea (23.9 pg). Maximum parsimony analysis of seven Sambucus species was used to form two dendrograms, one based on the sequenced nuclear ribosomal DNA (nrDNA) internal transcribed spacer (ITS) region and the other based on the cpDNA trnT‐trnF region. The results showed similar clustering into distinct groups; on both dendrograms S. nigra and S. javanica ES were closely related. Analysis of hybrid origin was based on the sequenced cpDNA region, genome size differences and PCR–RFLP analysis of the nrDNA ITS region. The seed parent and maternal origin of cpDNA was confirmed for all hybrids, while the pollen parent was confirmed in 21 of 22 putative hybrid plants tested. 相似文献
60.
Three diploid perennial sunflower species are useful for variety improvement: Helianthus mollis, because of sessile leaves, H. salicifolius, because of a high oil concentration, and H. maximiliani, a potential source of resistance to Sclerotinia sclerotiorum. The crossability of these species to cultivated sunflower was examined.Hybrids were obtained from eight combinations, with 3–15 F1 plants per combination. The F1's exhibited the dominant phenotype of the wild species. Pollen viability varied between 32.1 and 69.9%. Meiosis was irregular in the F1 hybrids. At diakinesis, bivalents (62.7–97.9% of meiocytes), univalents (0–31.23%), and multivalents (3.84–7.68%) were detected. At anaphase I, chromosome bridges were detected in 6.77 to 11.44% of meiocytes. Fast chromosomes in metaphase I, and lagging chromosomes in anaphase I and telophase II were evidenced in a high percentage of meiocytes. 相似文献