全文获取类型
收费全文 | 133462篇 |
免费 | 9123篇 |
国内免费 | 46篇 |
出版年
2022年 | 920篇 |
2021年 | 1989篇 |
2020年 | 1586篇 |
2019年 | 1720篇 |
2018年 | 3244篇 |
2017年 | 2802篇 |
2016年 | 4036篇 |
2015年 | 5508篇 |
2014年 | 5595篇 |
2013年 | 7270篇 |
2012年 | 8524篇 |
2011年 | 7692篇 |
2010年 | 4935篇 |
2009年 | 4185篇 |
2008年 | 5991篇 |
2007年 | 5743篇 |
2006年 | 5577篇 |
2005年 | 5285篇 |
2004年 | 5299篇 |
2003年 | 4589篇 |
2002年 | 4006篇 |
2001年 | 3481篇 |
2000年 | 3281篇 |
1999年 | 2708篇 |
1998年 | 1240篇 |
1997年 | 1035篇 |
1996年 | 1131篇 |
1995年 | 946篇 |
1994年 | 912篇 |
1993年 | 820篇 |
1992年 | 1865篇 |
1991年 | 1662篇 |
1990年 | 1589篇 |
1989年 | 1662篇 |
1988年 | 1487篇 |
1987年 | 1446篇 |
1986年 | 1348篇 |
1985年 | 1376篇 |
1984年 | 1208篇 |
1983年 | 989篇 |
1982年 | 803篇 |
1981年 | 746篇 |
1979年 | 1110篇 |
1978年 | 857篇 |
1977年 | 792篇 |
1975年 | 885篇 |
1974年 | 938篇 |
1973年 | 900篇 |
1972年 | 768篇 |
1969年 | 796篇 |
排序方式: 共有10000条查询结果,搜索用时 593 毫秒
951.
952.
On the opportunity cost of the photosynthate invested in stem elongation reactions mediated by phytochrome 总被引:5,自引:0,他引:5
Summary Seedlings of shade-intolerant species react to alterations of the light climate caused by their neighbors with morphological changes that may influence the pattern of resource acquisition and utilization at the whole-canopy level. One such change, the increased stem elongation rate that is triggered by low red (R, 660 nm) to far-red (FR, 730 nm) ratios (R:FR) in dense canopies, might reduce the amount of assimilates available for leaf area expansion or root growth, and in that way affect resource capture by the canopy. We have tested this hypothesis by comparing the growth of both isolated individuals and canopies of the weed Amaranthus quitensis under conditions differing only in the spectral distribution of the incident light. When canopies received the full spectrum of sunlight, the stems were a large proportion (40–57%) of total biomass. Filtering the FR waveband (and hence raising the R:FR ratio to eliminate the neighbors' proximity-signal) resulted in shorter canopies with lighter stems. However, the growth of leaves and roots was not promoted by this treatment, indicating that the opportunity cost of the assimilates invested in the stems was nil or very small. Filtering the FR had no effect on biomass accumulation when plants were grown as isolated individuals. The higher growth of the canopics under full spectrum could be due to a higher light interception or to a higher efficiency of light conversion into biomass. The first possibility is weakened by the observation that filtering the FR had no effect on the dynamics of soil covering by the crops. The second is indirectly strengthened by results of an experiment with isolated plants showing that stem elongation, stem growth, and total plant biomass can be increased by reducing the flux of R light received by the stems without affecting the light climate of the leaves. Further work is needed to distinguish between these two possibilities; whatever the cause, our results show that the elongation responses to decreased R:FR may lead to a net increase in canopy productivity, and do not necessarily have a negative impact on the growth of resource-harvesting organs. 相似文献
953.
The activities of the two unique enzymes of the glyoxylate cycle,isocitrate lyase (EC 4.1.3.1
[EC]
) and malate synthase (EC 4.1.3.2
[EC]
),were undetectable in petals of pumpkin (Cucurbita sp. AmakuriNankin) until the end of blooming, but they appeared duringsenescence. The activity of catalase (EC 1.11.1.6
[EC]
) increased,glycolate oxidase (EC 1.1.3.1
[EC]
) activity did not change, whilehydroxypyruvate reductase (EC 1.1.1.81
[EC]
) activity peaked at fullblooming stage and declined thereafter. After fractionationof cellular organelles on a sucrose density gradient, we detectedisocitrate lyase and malate synthase activities in peroxisomalfractions only from petals at the senescing stage. Northernblot analysis revealed that malate synthase mRNA increased duringpetal senescence. Citrate synthase (EC 4.1.3.7
[EC]
) and malate dehydrogenase(EC 1.1.1.37
[EC]
) activities were also present, while aconitase(EC 4.2.1.3
[EC]
) was not detectable in peroxisomal fractions. Moreoverthe presence of 3-hydroxyacyl-CoA dehydrogenase (EC 1.1.1.35
[EC]
)and urate oxidase (EC 1.7.3.3
[EC]
) in the peroxisomal fractionsfrom senescing petals indicates that peroxisomes could be involvedboth in the ß-oxidation pathway and in the purinecatabolism during petal senescence. (Received May 25, 1991; Accepted September 25, 1991) 相似文献
954.
Quinovic acid glycosides from Uncaria guianensis. 总被引:1,自引:0,他引:1
A M Yépez O L de Ugaz C M Alvarez V De Feo R Aquino F De Simone C Pizza 《Phytochemistry》1991,30(5):1635-1637
From the bark of Uncaria guianensis, two new quinovic acid glycosides, quinovic acid 3 beta-O-beta-D-quinovopyranoside and quinovic acid 3 beta-O-beta-D-fucopyranosyl-(27----1)-beta-D-glucopyranosylester, have been isolated, in addition to known quinovic acid 3 beta-O-[beta-D-glucopyranosyl-(1----3)-beta-D-fucopyranosyl]-(27----1)- beta-D-glucopyranosylester and quinovic acid 3 beta-O-beta-D-fucopyranoside. Their structures were elucidated by spectral and chemical studies. 相似文献
955.
Ethylene Production and 1-Aminocyclopropane-1-Carboxylic Acid Conjugation in Thermoinhibited Cicer arietinum L. Seeds 总被引:1,自引:1,他引:0
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The effect of supraoptimal temperatures (30°C, 35°C) on germination and ethylene production of Cicer arietinum (chick-pea) seeds was measured. Compared with a 25°C control, these temperatures inhibited both germination and ethylene production. The effect of supraoptimal temperatures could be alleviated by treating the seeds with ethylene. It was concluded that one effect of high temperature on germination was due to its negative effect on ethylene production. This inhibitory effect of high temperature was due to increased conjugation of 1-aminocyclopropane-1-carboxylic acid to 1-(malonylamino)cyclopropane-1-carboxylic acid and to an inhibition of ethylene-forming enzyme activity. 相似文献
956.
957.
The infection of cucumber leaves by Colletotrichum lagenarium was studied using cytological methods. Its progress in untreated plants was compared with that in plants in which systemic resistance had been induced by pre-infecting the first true leaf with the same fungus. In induced plants, a reduction of fungal development was observed at the leaf surface, in the epidermis, and in the mesophyll. On the leaf surface, formation of appressoria was slightly reduced. In the epidermis, enhanced formation of papillae beneath appressoria, and possibly increased lignification of entire cells, correlated with reduced development of infection hyphae. Papillae contained callose, identified by staining with aniline-blue fluorochrome and digestion with -1,3-glucanase, as a main structural component. In the mesophyll, reduced fungal development provided evidence for the existence of an additional induced defence reaction. The results imply that preinfection elicited a systemic, multicomponent defence reaction of the host plant against the fungus.Dedicated to the memory of Professor H. Grisebach 相似文献
958.
Jeanine Brederoo Piet de Wildt Corrie Popp-Snijders Robin F. Irvine Alan Musgrave Herman van den Ende 《Planta》1991,184(2):175-181
InChlamydomonas eugametos gametes, phosphatidylinositol 4-phosphate (PtdInsP) and phosphatidylinositol 4,5-bisphosphate (PtdInsP2) comprised 0.4 and 0.3% of the whole-cell phospholipids. They were concentrated in the plasma membrane around the cell body and were present in low concentrations in the flagellar membrane. When gametes were fed32PO
4
-
, the label was rapidly incorporated into PtdInsP and PtdInsP2 and only slowly incorporated into structural lipids such as phosphatidylethanolamine and phosphatidylglycerol. Similarly, when a pulse of32PO
4
-
was chased with PO
4
-
, the label was rapidly lost from the polyphosphoinositol lipids but not from the structural lipids. The major fatty acids in the polyphosphoinositides were C-22 carbon polyenoic acids (70%). The significance of these results in relationship to intracellular signalling via inositol phosphates and Ca2+ is discussed.Abbreviations InsP3
inositol 1,4,5-trisphosphate
- mt–/mt+
mating-type plus or minus
- PtdA
phosphatidic acid
- PtdEtn
phosphatidylethanolamine
- PtdGro
phosphatidylglycerol
- PtdIns
phosphatidylinositol
- PtdInsP
phosphatidylinositol 4-phosphate;
- PtdInsP2
phosphatidylinositol 4,5-bisphosphate
- TCA
trichloroacetic acid
We thank Frank Schuring for Fig. 5A and Susan Kenter, Hans Kruisselbrink, Saskia Bijvank and Nelleke Corbett for their enthousiastic assistance. 相似文献
959.
Renmei Ren Kanhui Yan Yanjie Su Hanjun Qi Bin Liang Wunyong Bao Frans B. M. de Waal 《Primates; journal of primatology》1991,32(3):321-327
Observations were made following 130 spontaneous aggressive incidents in two small breeding groups of captive golden monkeys
(Rhinopithecus roxellanae roxellanae). Participants were observed both during the first 10 min following these incidents and during matched control observations.
An increased contact rate was observed between opponents following the aggressive incident. Post-conflict contacts were characterized
by a number of behavior patterns: open mouth, rapid grooming, embrace, and crouching. Adult males played an important role
as mediator in agonistic disputes among females: males intervened in 93.6% of female fights. It is speculated that this intervention
behavior is related to the species' organization into one-male units. 相似文献
960.
Parasitism of the isopod Progebiophilus bruscai Salazar-Vallejo & Leija-Tristán over the common mud shrimp, Upogebia dawsoni Williams, was analysed in the Ensenada and Bahia de La Paz, Baja California Sur, Mexico. Mud-shrimps were collected in three similar sites that differ in grain size and in the anthropogenic organic matter enrichment. Four-hundred-nine mud-shrimps were collected; the largest abundance was registered in the organically enriched site, but they were significantly smaller and more heavily parasited than the animals from the site lacking such organic enrichment. The size of the parasite is clearly dependent on the size of the mud-shrimp. This is the first study of the upogebiid-bopyris relationship in Mexico. 相似文献