全文获取类型
收费全文 | 1214篇 |
免费 | 100篇 |
国内免费 | 122篇 |
出版年
2023年 | 6篇 |
2022年 | 15篇 |
2021年 | 25篇 |
2020年 | 25篇 |
2019年 | 16篇 |
2018年 | 27篇 |
2017年 | 28篇 |
2016年 | 18篇 |
2015年 | 26篇 |
2014年 | 24篇 |
2013年 | 36篇 |
2012年 | 28篇 |
2011年 | 40篇 |
2010年 | 25篇 |
2009年 | 32篇 |
2008年 | 46篇 |
2007年 | 51篇 |
2006年 | 56篇 |
2005年 | 54篇 |
2004年 | 46篇 |
2003年 | 43篇 |
2002年 | 37篇 |
2001年 | 38篇 |
2000年 | 41篇 |
1999年 | 55篇 |
1998年 | 49篇 |
1997年 | 55篇 |
1996年 | 54篇 |
1995年 | 72篇 |
1994年 | 51篇 |
1993年 | 52篇 |
1992年 | 34篇 |
1991年 | 46篇 |
1990年 | 35篇 |
1989年 | 39篇 |
1988年 | 40篇 |
1987年 | 23篇 |
1986年 | 13篇 |
1985年 | 13篇 |
1984年 | 4篇 |
1983年 | 6篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1975年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有1436条查询结果,搜索用时 31 毫秒
141.
Jacqueline A. Aitkenhead-Peterson Jess E. Alexander Jana Albrechtová Pavel Krám Barrett Rock Pavel Cudlín Jakub Hruška Zuzana Lhotaková Ryan Huntley Filip Oulehle Tomáš Polák William H. McDowell 《Plant and Soil》2006,283(1-2):187-201
Dissolved organic carbon and nitrogen (DOC and DON) produced in the forest floor are important for ecosystem functions such
as microbial metabolism, pedogenesis and pollutant transport. Past work has shown that both DOC and DON production are related
to litterfall and standing stocks of C and N in the forest floor. This study, conducted in spring, 2003, investigated variation
in forest floor water extractable DOC (WEDOC) and DON (WEDON) and forest floor C and N as a function of lignin, cellulose
and N contained in live canopy foliage across eight Picea abies [L.] Karst stands in northern Bohemia. Based on Near Infrared Spectroscopy (NIR) analysis of foliar materials, lignin:N and
cellulose:N content of the youngest needles (those produced in 2002) were positively and significantly related to WEDOC (R2 = 0.82–0.97; P<0.01) and to forest floor C:N ratio (R = 0.72–0.78; P<0.01). Foliar N was strongly and negatively related to WEDOC and C:N ratio (R = −0.91 and 0.72; P<0.05) among our study sites. WEDON was positively correlated to foliar lignin:N (R = 0.48; P<0.05; n=40). Forest floor C pools were not positively correlated with foliar lignin and cellulose and forest floor N pools were not
positively correlated with foliar N. Instead, a significant negative correlation was found between forest floor N pools and
foliar cellulose (R=−0.41; P<0.05), and between forest floor C pools and foliar N (R = −0.44; P<0.05). From a remote sensing standpoint, our results are important because canopy reflectance properties are primarily influenced
by the most recent foliage, and it was the chemistry of the most recently produced needles that showed a stronger relationship
with forest floor WEDOC and C:N ratio suggesting forest floor production of WEDOC can be calculated regionally with remote
sensing. 相似文献
142.
Background and Aims
Reconstructions have identified the 20th century as being uniquely warm in the last 1000 years. Changes in the phenology of primary meristems converged toward increases in length of the growing season. Has the phenology of secondary meristem changed during the last century, and to what extent?Methods
Timings of wood formation in black spruce, Picea mariana, were monitored for 9 years on a weekly timescale at four sites in the boreal forest of Quebec, Canada. Models for assessing xylem phenology were defined and applied to reconstruct onset, ending and duration of xylogenesis between 1950 and 2010 using thermal thresholds on chronologies of maximum and minimum temperatures.Key Results
All sites exhibited increasing trends of both annual and May–September temperatures, with the greatest changes observed at the higher latitudes. Phenological events in spring were more affected than those occurring in autumn, with cambial resumptions occurring 0·5–0·8 d decade−1 earlier. The duration of xylogenesis has lengthened significantly since 1950, although the models supplied wide ranges of variations, between 0·07 and 1·5 d decade−1, respectively.Conclusions
The estimated changes in past cambial phenology demonstrated the marked effects of the recent increase in temperature on the phenological traits of secondary meristems. In the long run, the advancement of cambial activity could modify the short time window for growth of boreal species and dramatically affect the dynamics and productivity of trees in these temperature-limited ecosystems. 相似文献143.
144.
利用RACE技术从已有的青杄均一化cDNA文库中克隆得到青杄(Picea wilsonii)PsbO基因cDNA的全长,并用生物信息学的方法对该cDNA的核苷酸序列、氨基酸序列的相似性,及编码蛋白PwPsbO的理化性质、亲水性/疏水性、二级和三级结构,PwPsbO基因进化树等进行了预测和分析.采用实时荧光定量PCR(RT-qPCR)技术,测定了青杄不同组织中的PsbO基因的相对表达水平.结果发现:青杄PsbO基因编码346个氨基酸,预测相对分子质量为36.6 kD,理论pI值为5.29,属于可溶性蛋白.二级和三级结构预测表明其含有较多的α螺旋、β折叠和随机卷曲结构.RT-qPCR发现该基因在青杄针叶和茎中表达量最高.这些结果为青杄中PsbO基因的初步功能研究奠定了基础. 相似文献
145.
Root inoculation with a forest soil streptomycete leads to locally and systemically increased resistance against phytopathogens in Norway spruce 总被引:2,自引:0,他引:2
Soil streptomycetes are commonly antagonistic against plant pathogens. However, interactions involving increased defense responses in the host plant, leading to suppression of plant disease development, have not yet been detailed. Here, the mechanisms were studied of disease suppression by Streptomyces sp. GB 4-2 against Heterobasidion root and butt rot in Norway spruce (Picea abies) seedlings. GB 4-2 promoted mycelial growth of the phytopathogenic fungus, germination rate of fungal spores, extension of germ tubes and early colonization of outer cortical layers of the plant root. Reduced colonization of the inner cortical cell layers was accompanied by the induction of cell wall appositions, and increased xylem formation in the vascular cylinder emerged after bacterium-fungus coinoculation. Bacterial treatment led to decreased water content in roots and needles and increased photosynthetic yield (F(v)/F(m)) and peroxidase activities in needles. The infection of needles by Botrytis cinerea was reduced by bacterial pretreatment. Complex interactions of GB 4-2 with Norway spruce and Heterobasidion abietinum were discovered. The bacterium promoted the growth of the phytopathogenic fungus but induced plant defense responses. Host responses indicate that GB 4-2 induces both local and systemic defense responses in Norway spruce. 相似文献
146.
Although tree species typically exhibit low genetic differentiation between populations, ecotypes adapted to different environmental conditions can vary in their capacity to withstand and recover from environmental stresses like heat stress. Two month old seedlings of a Picea abies ecotype adapted to high elevation showed lower level of thermotolerance and higher level of tolerance to oxidative stress relative to a low elevation ecotype. Protein expression patterns following exposure to severe heat stress of the two ecotypes were compared by means of 2-DE. Several proteins exhibiting ecotype and tissue specific expression were identified by MS/MS. Among them, small heat shock proteins of the HSP 20 family and proteins involved in protection from oxidative stress displayed qualitative and quantitative differences in expression between the ecotypes correlated with the observed phenotypic differences. On the basis of these results, it can be speculated that the observed interpopulation polymorphism of protein regulation in response to heat stress could underlie their different capacities to withstand and recover from heat stress. These local adaptations are potentially relevant for the species adaptation to the conditions predicted by the current models for climate change. 相似文献
147.
148.
Cheng-Hao Li Bao-Guang Liu Tae-Dong Kim Heung-Kyu Moon Yong-Eui Choi 《Plant biotechnology reports》2008,2(4):259-265
Picea koraiensis, called Korean spruce, is an evergreen tree and found mostly in northeast Asia. In this study, plant regeneration via somatic
embryogenesis from open-pollinated immature zygotic embryos of nine genotypes of elite trees was established. Immature zygotic
embryos were cultured onto RJW medium modified from 505 medium with 21.48 μM NAA, 2.22 μM BA, and 2.32 μM KT. The average
frequency for all nine genotypes was 74.2%. Embryogenic calluses of the nine genotypes of elite trees were subcultured on
RJW basal medium containing 8.06 μM NAA, 1.11 μM BA, and 1.16 μM kinetin. The calluses of three lines, 3#, 9#, and 2#, were actively proliferated but others were not. Somatic embryogenesis was induced from the embryogenic callus in genotypes
of 3#, 9#, and 2# on RJW medium with ABA and 60 g l−1 sucrose. Cotyledonary somatic embryos were subjected to a drying process. The drying of embryos by uncapping the culture
bottle for 5 days on a clean bench resulted in a high frequency of germination of somatic embryos (87% in RJW medium). However,
plantlet conversion from germinated embryos was greatly reduced and the optimal medium for plant conversion was 1/2 WPM or
1/2 BMI medium. In conclusion, we have, for the first time, established a plant regeneration system via somatic embryogenesis
in the Korean spruce, which can be applied for rapid micropropagation of elite trees. 相似文献
149.
150.
Mark W. Sumarah Gregory W. Adams Joanne Berghout Gregory J. Slack Aaron M. Wilson J. David Miller 《Mycological Research》2008,112(6):731-736
We have studied Picea glauca (white spruce) endophyte colonization and its affect on the growth of Choristoneura fumiferana (spruce budworm). Here we examine the spread and persistence of a rugulosin-producing endophyte and rugulosin in needles from trees maintained in the nursery, as well as in trees planted in a test field site. Additionally, we report toxicity of rugulosin against three P. glauca needle herbivores: C. fumiferana, Lambdina fiscellaria (hemlock looper) and Zeiraphera canadensis (spruce budmoth). Reduction in body weight for both the C. fumiferana and L. fiscellaria were observed at 25 and 50 μm, respectively, and head capsules were reduced at 100 and 150 μm. Z. canadensis larvae did not perform as well in tests due to an Aspergillus fumigatus infection, but were shown to be lighter when tested with 100 and 150 μm compared with controls. The endophyte and its toxin were shown to spread throughout the nursery-grown seedlings. After 3.5 and 4.5 y post-inoculation (one and two years in the test site), the inoculated endophyte and its toxin had remained present with an average rugulosin concentration of 1 μg g−1. 相似文献