全文获取类型
收费全文 | 9878篇 |
免费 | 643篇 |
国内免费 | 50篇 |
专业分类
林业 | 649篇 |
农学 | 478篇 |
基础科学 | 86篇 |
1803篇 | |
综合类 | 674篇 |
农作物 | 499篇 |
水产渔业 | 769篇 |
畜牧兽医 | 4547篇 |
园艺 | 150篇 |
植物保护 | 916篇 |
出版年
2023年 | 96篇 |
2022年 | 166篇 |
2021年 | 316篇 |
2020年 | 317篇 |
2019年 | 373篇 |
2018年 | 342篇 |
2017年 | 354篇 |
2016年 | 365篇 |
2015年 | 259篇 |
2014年 | 379篇 |
2013年 | 551篇 |
2012年 | 569篇 |
2011年 | 643篇 |
2010年 | 327篇 |
2009年 | 348篇 |
2008年 | 513篇 |
2007年 | 515篇 |
2006年 | 434篇 |
2005年 | 403篇 |
2004年 | 380篇 |
2003年 | 341篇 |
2002年 | 383篇 |
2001年 | 257篇 |
2000年 | 265篇 |
1999年 | 215篇 |
1998年 | 66篇 |
1997年 | 71篇 |
1996年 | 58篇 |
1995年 | 53篇 |
1994年 | 43篇 |
1993年 | 38篇 |
1992年 | 82篇 |
1991年 | 54篇 |
1990年 | 63篇 |
1989年 | 70篇 |
1988年 | 63篇 |
1987年 | 53篇 |
1986年 | 47篇 |
1985年 | 59篇 |
1984年 | 38篇 |
1983年 | 43篇 |
1982年 | 31篇 |
1980年 | 26篇 |
1979年 | 39篇 |
1978年 | 21篇 |
1977年 | 23篇 |
1974年 | 23篇 |
1973年 | 25篇 |
1972年 | 24篇 |
1970年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Isolated plant microspores, when stressed and cultured in vitro, can be diverted from their normal gametophytic pathway towards
sporophytic development, with the formation of haploid embryos and ultimately doubled-haploid plants. This process is called
androgenesis or microspore embryogenesis, and is widely used in plant breeding programmes to generate homozygous lines for
breeding purposes. Protocols for the induction of microspore embryogenesis and the subsequent regeneration of doubled haploid
(DH) plants have been successfully developed for more than 200 species. These practical advances stand in stark contrast to
our knowledge of the underlying molecular genetic mechanism controlling this process. The majority of information regarding
the genetic and molecular control of the developmental switch from gametophytic to sporophytic development has been garnered
from four intensely studied (crop) plants comprising two dicotyledonous species, rapeseed (Brassica napus) and tobacco (Nicotiana tabacum), and two monocotyledonous species, wheat (Triticum aestivum) and barley (Hordeum vulgare). In these species the efficiency of microspore embryogenesis is very high and reproducible, making them suitable models
for molecular studies. In the past, molecular studies on microspore embryogenesis have focussed mainly on the identification
of genes that are differentially expressed during this developmental transition and/or early in embryo development, and have
identified a number of genes whose expression marks or predicts the developmental fate of stressed microspores. More recently,
functional genomics approaches have been used to obtain a broad overview of the molecular processes that take place during
the establishment of microspore embryogenesis. In this review we summarise accumulated molecular data obtained in rapeseed,
tobacco, wheat and barley on embryogenic induction of microspores and define common aspects involved in the androgenic switch. 相似文献
942.
Juliano Lino Ferreira José Eustáquio de Souza Carneiro Alexsandro Lara Teixeira Fabiéli Fortunata de Lanes Paulo Roberto Cecon Aluízio Borém 《Euphytica》2007,153(1-2):165-170
This study was conducted in Brazil in order to assess the potential risk posed by gene escape from transgenic into non-transgenic plants and wild populations. A new methodology was applied to evaluate the gene flow between common bean cultivars, by means of a specially delineated experiment in two stages. The first stage consisted of the planting of one cultivar with violet flowers (BB) as pollen source (‘Diamante Negro’), and a receiver (‘Talismã’) with white flowers (bb), at different distances. The source was sown in the center of the area. The pollen receiver cultivar was sown, in concentrical squares around it. At maturity, the rows were sampled at varied distances from the source in the four cardinal directions. In the second stage, the sampled seeds of the previous stage were sown, and the percentage of outcrossing was evaluated during flowering through the presence of violet flowers (Bb). The highest frequency of natural hybrids, 0.136%, occurred at a distance of 0.5 m between the cultivars. The natural outcrossing rate was practically zero beyond a distance of 3.25 m. 相似文献
943.
The use of doubled haploids improves the efficiency of cultivar development in many crops and can be helpful in genetic and molecular studies. The major problem with this approach is the low efficiency of green plant regeneration. We describe here an efficient method for inducing embryos and regenerating green plants directly from isolated microspores of durum wheat cv. ‘Jennah Khetifa’. Tillers from donor plants were pretreated in 0.3 m mannitol and were stored at 4°C at various times: 3, 5, 6, 7, 8, 10 and 12 days. Our results showed clearly that the novel pretreatment combined mannitol 0.3 m and cold for 7 days had a strong effect on the number of embryos produced and regenerated green plants. Under this condition 13 475 mature embryos were produced from 2 693 500 microspores. Moreover, 85 green plants were obtained. High green plants regeneration frequency was recorded. As an average 11.55 green plants were produced per 100 000 microspores (about the equivalent of six plants per spike). Therefore, this study showed clearly that our results are the best ones published until now in durum wheat. 相似文献
944.
Among the most important Mediterranean annual crops durum wheat is widely grown in drought-prone areas. Therefore improving water-use efficiency 《分子植物育种》2007,5(2):223-223
Among the most important Mediterranean annual crops, durum wheat is widely grown in drought-prone areas. Therefore, improving water-use efficiency (WUE) of durum wheat represents a major breeding goal. IDu-WUE (Improving Durum wheat for Water Use Efficiency and yield stability through physiological and molecular approaches) is a collaborative project among public and private research centres in Italy, Spain and WANA (West Asia and North Africa) countries (Morocco, Tunisia, Syria and Lebanon) funded by the European Union aimed at investigating the genetic variation for WUE and yield stability in durum wheat grown in Mediterranean droughtprone areas. During the first year of the project, a number of morpho-physiological traits (e.g. early vigour, flowering time, leaf rolling, number of fertile tillers, etc.), WUE, WUE-related traits (e.g. carbon isotope discrimination, canopy temperature, chlorophyll fluorescence, etc.), yield and its components have been investigated in a RIL population (249 lines) and a collection of ca. 190 durum wheat accessions characterized by a high level of linkage disequilibrium (Maccaferri et al., 2005), 相似文献
945.
Giovani Greigh de Brito Eveline Teixeira Caixeta Ana Paula Gallina Eunize Maciel Zambolim Laércio Zambolim Valdir Diola Marcelo Elhers Loureiro 《Euphytica》2010,173(2):255-264
The most important disease of Coffea arabica is coffee leaf rust caused by the fungus Hemileia vastatrix. The purpose of this study was to characterize the inheritance of coffee resistance gene(s) to race II of this pathogen and
to identify and map molecular markers linked to this trait. Different populations were used: F2 (160 plants), BCr (20), and BCs (135), derived from a cross between the resistant genotype Híbrido de Timor UFV 427-15 and
the susceptible cultivar Catuaí Amarelo UFV 2143-236 (IAC 30). The segregation analysis showed that the resistance of Híbrido
de Timor to race II of the H. vastatrix is conferred by a single dominant gene. The amplification of 176 AFLP (Amplified fragment length polymorphism) primer combinations
using bulked segregant analysis (BSA) allowed the identification of three molecular markers linked to the resistance gene.
Genetic mapping of these three markers in the F2 population indicated that they are distributed on both sides, flanking the resistance gene. The markers E.CTC/M.TTT405 and
E.CGT/M.TGT300 were found linked to the resistance gene at 8.69 cM (LOD 18.91) and 25.10 cM (LOD 5.37), respectively, while
E.CCT/M.TTC230 was localized on the other side of the gene, at 20.50 cM (LOD 6.15). These markers are the first rust resistance
markers identified in Híbrido de Timor and can be useful for marker assisted selection in coffee breeding programs. 相似文献
946.
The mechanism that controls the proportion of cannabichromene (CBC), a potential pharmaceutical, in the cannabinoid fraction
of Cannabis sativa L. is explored. As with tetrahydrocannabinol (THC) and cannabidiol (CBD), CBC is an enzymatic conversion product of the precursor
cannabigerol (CBG). CBC is reported to dominate the cannabinoid fraction of juveniles and to decline with maturation. This
ontogeny was confirmed in inbred lines with different mature chemotypes. A consistent CBC presence was found in early leaves
from a diverse clone collection, suggesting that CBC synthase is encoded by a fixed locus. Morphological variants possessing
a ‘prolonged juvenile chemotype’ (PJC), a substantial proportion of CBC persisting up to maturity, are presented. PJC is associated
with a reduced presence of floral bracts, bracteoles, and capitate-stalked trichomes. Genetic factors causing these features
were independent of the allelic chemotype locus B that was previously postulated and regulates THC and CBD synthesis and CBG accumulation. In contrast to previously described
Cannabis chemotypes, the cannabinoid composition of PJCs showed plasticity in that reduced light levels increased the CBC proportion.
The ability of PJC plants to enable the production of pharmaceutical raw material with high CBC purity is demonstrated. 相似文献
947.
FEMlab3.3(Finite Element Model Lab) is used to develop an energy delivery model for implanted devices. We designed three different shape electrodes made of Ag to enhance current transmitting efficiency. Several voltages were used for pursuing perfect goal. With the body safety current, we can get the results with a current transmitting efficiency of 20% and the energy delivery efficiency of 4%~9%. It is clear that the high energy delivery efficiency can be obtained and the results provide references for further study of energy delivery through body conduction. 相似文献
948.
A novel resonator for accelerometer with sensing and actuating combs was designed and fabricated based on micro electromechanical system (MEMS) process. Driving force and demand voltage of the resonator were analyzed by analytical method and Finite Element Method (FEM). Sensing capacitance on resonance state and Q factor under atmosphere was calculated by these two methods. The micro fabrication process was planned and realized using MEMS (Micro electromechanical System) method. The testing results show the performance of the resonator is high; the natural frequency of the resonator is 54 523.5 Hz, which has the error of 13.6% with the theorical value. 相似文献
949.
氮硫交互作用对小麦苗期生长和养分吸收的影响 总被引:15,自引:1,他引:15
通过基质盆栽冬小麦,研究了氮硫交互作用对小麦苗期生长及养分吸收的影响。研究结果表明,氮、硫对苗期小麦的生长都有一定影响,氮的增产效果远大于硫,当氮肥用量为200mg/盆时,小麦生物量的最大增产幅度89%;在低氮水平下,硫肥的增产效果最佳,最高可增产125。硫肥的施用可以提高苗期小麦植株体内硫的累积量,在氮素胁迫的情况下最为显著,可提高1倍以上;同时降低植株体内氮的累积,但降低幅度不大。氮素充足时,地上部与地下部生物量之比为2.4-3.9,氮素胁迫条件下,小麦的地上部和地下部的生物量相当,说明氮素养分不足时,小麦合成的有机物质更倾向于分配给根系,促进根系的生长,这可能是小麦抵抗胁迫环境条件的生理机制之一。 相似文献
950.
In a pear orchard, when leaf senescence occurs, nitrogen (N) is added to the soil by the fallen leaves and can be re-used by the tree after undergoing decomposition and mineralization processes. Studies on leaf decomposition and N mineralization in orchards are scarce but essential to understand the N balance in the tree–soil ecosystem in a sustainable or precision agriculture. This study aimed to quantify the contribution of pear tree senescent leaves to N cycling in the orchard and its re-cycling by the crop. ‘Rocha’ pear unlabelled leaves were incubated in situ using the litter-bag technique and 15N-enriched leaves were placed at the soil surface in undisturbed confined cores.One- to six-year-old pear trees returned to the soil between 1 kg N ha?1 year?1 and 6 kg N ha?1 year?1 from senescent leaves that decomposed at rates varying from 0.0025 day?1 (d?1) to 0.0047 d?1 (estimated by both techniques, respectively). In the litter-bags, after 506–641 days, only 18–35% of initial DW was recovered in the soil, whereas in the soil cores the weight loss was higher, resulting in only 30–6% of initial DW after 398–406 d. After this period, between 36% and 110% of the initial N of the senescent leaves was recovered as organic 15N in the surface soil layer (0–7.5 cm), depending on climatic conditions, and being more prone to be absorbed by weeds. 相似文献