全文获取类型
收费全文 | 67170篇 |
免费 | 3499篇 |
国内免费 | 5784篇 |
出版年
2023年 | 774篇 |
2022年 | 1122篇 |
2021年 | 1325篇 |
2020年 | 1627篇 |
2019年 | 1969篇 |
2018年 | 1949篇 |
2017年 | 1753篇 |
2016年 | 1799篇 |
2015年 | 1739篇 |
2014年 | 3006篇 |
2013年 | 5013篇 |
2012年 | 2379篇 |
2011年 | 3266篇 |
2010年 | 2434篇 |
2009年 | 3344篇 |
2008年 | 3397篇 |
2007年 | 3423篇 |
2006年 | 3007篇 |
2005年 | 2872篇 |
2004年 | 2416篇 |
2003年 | 2258篇 |
2002年 | 1929篇 |
2001年 | 1581篇 |
2000年 | 1358篇 |
1999年 | 1348篇 |
1998年 | 1307篇 |
1997年 | 1214篇 |
1996年 | 1132篇 |
1995年 | 1103篇 |
1994年 | 1074篇 |
1993年 | 968篇 |
1992年 | 963篇 |
1991年 | 782篇 |
1990年 | 762篇 |
1989年 | 726篇 |
1988年 | 643篇 |
1987年 | 648篇 |
1986年 | 483篇 |
1985年 | 915篇 |
1984年 | 1160篇 |
1983年 | 791篇 |
1982年 | 945篇 |
1981年 | 716篇 |
1980年 | 651篇 |
1979年 | 554篇 |
1978年 | 359篇 |
1977年 | 330篇 |
1976年 | 291篇 |
1975年 | 217篇 |
1974年 | 224篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
991.
Thierry Becquer Denis Merlet Jean-Pierre Boudot James Rouiller Francis Gras 《Plant and Soil》1990,125(1):95-107
Nitrate uptake and leaching were measured during one year in a declined fir forest on the Vosges highlands (eastern France),
in order to investigate whether excess nitrification could be responsible for a deleterious acidification of the ecosystem.
Nitrate uptake by the vegetation was active mainly from spring to early fall, and then reached about 66 kg N ha-1. No significant leaching loss occurred during the growth period of the vegetation. Significant nitrate leaching occurred
in winter (about 17 kg N ha-1). During fall and winter the nitrification rate was of the same magnitude as values reported for other ecosystems, and, thus,
was not considered to be abnormaly strong. No abnormal temporal discoupling of nitrate production and nitrate uptake occurred
in the ecosystem, and forest decline must therefore have some other cause. 相似文献
992.
The interactive effect of salinity and presoaking in ascorbic acid or phyridoxine on germination, seedling growth, and some
relevant metabolic changes ofLupinus termis andVicia faba seeds were studied. Germination studies indicated that broad bean tolerated NaCl salinity up to 240mM NaCl and lupin to 200mM NaCl. The lengths of roots and shoots and their water content, as well as dry matter yield, remained more or less unchanged
up to the level of 80mM NaCl.
Salinity induced marked progressive increases of carbohydrates and proline in broad bean and soluble protein in lupin seedlings,
irrespective of the salinity level used.
The other organic solutes (soluble protein in broad bean and carbohydrates in lupin seedlings) remained more or less unchanged
at low and moderate levels of NaCl. However, under the higher salinity levels, in lupin the losses in carbohydrates were accompanied
by increases in soluble protein, whereas in broad bean an opposite effect was obtained. The level of 40mM NaCl had a pronounced stimulatory effect on the all the variables studied.
Presoaking seeds in either ascorbic acid or pyridoxine counteracted the adverse effects of salinity on germination and seedling
growth as well as on some metabolic mechanisms of lupin and broad bean plants.
The importance of these processes to the salinity tolerance of broad bean and lupin have been discussed. 相似文献
993.
Michael R. Blatt 《Planta》1990,180(3):445-455
Evidence of a role for abscisic acid (ABA) in signalling conditions of water stress and promoting stomatal closure is convincing, but past studies have left few clues as to its molecular mechanism(s) of action; arguments centred on changes in H+-pump activity and membrane potential, especially, remain ambiguous without the fundamental support of a rigorous electrophysiological analysis. The present study explores the response to ABA of K+ channels at the membrane of intact guard cells ofVicia faba L. Membrane potentials were recorded before and during exposures to ABA, and whole-cell currents were measured at intervals throughout to quantitate the steady-state and time-dependent characteristics of the K+ channels. On adding 10 M ABA in the presence of 0.1, 3 or 10 mM extracellular K+, the free-running membrane potential (V
m) shifted negative-going (–)4–7 mV in the first 5 min of exposure, with no consistent effect thereafter. Voltage-clamp measurements, however, revealed that the K+-channel current rose to between 1.84- and 3.41-fold of the controls in the steady-state with a mean halftime of 1.1 ± 0.1 min. Comparable changes in current return via the leak were also evident and accounted for the minimal response inV
m. Calculated atV
m, the K+ currents translated to an average 2.65-fold rise in K+ efflux with ABA. Abscisic acid was not observed to alter either K+-current activation or deactivation.These results are consistent with an ABA-evoked mobilization of K+ channels or channel conductance, rather than a direct effect of the phytohormone on K+-channel gating. The data discount notions that large swings in membrane voltage are a prerequisite to controlling guard-cell K+ flux. Instead, thev highlight a rise in membranecapacity for K+ flux, dependent on concerted modulations of K+-channel and leak currents, and sufficiently rapid to account generally for the onset of K+ loss from guard cells and stomatal closure in ABA. 相似文献
994.
Water-storage capacity was measured inThuja occidentalis L.,Tsuga canadensis (L.) Carr., andAcer saccharum Marsh. during the dehydration of stem segments 1.5–2.5 cm in diameter. Stem water potential was measured with a temperature-corrected
stem hygrometer and cavitations were detected acoustically. Water loss was measured by weight change. Dehydration isotherms
consistently displayed three phases. The first phase, from water potential (Ψ) 0 to about −0.2 MPa, had a high capacitance
(C>0.4kg water lost· (1 of tissue)−1· MPa−1) and we have attributed this high C to capillary water as defined by Zimmermann (1983, Xylem structure and the ascent of
sap, Springer-Verlag). The second phase from Ψ=−0.5 to about −2.0 had the lowest C values (<0.02 kg·l−1·MPa−1) and was accompanied by a few cavitation events. This phase may have been a transition zone between capillary storage and
water released by cavitation events as well as water drawn from living cells of the bark. The third phase also had a high
C (about 0.07–0.22kg·l−1·MPa−1) and was associated with many cavitation events while Ψ declined below about −2.5 MPa; we presume the high capacitance was
the consequence of water released by cavitation events. We discuss the ecological adaptive advantage of these three phases
of water-storage in trees. In moist environments, water withdrawn from capillary storage may be an important fraction of transpiration,
but may be of little adaptive advantage. For most of the growth season trees draw mainly on elastic storage, but stem elastic
storage is less than leaf elastic storage and therefore unlikely to be important. In very dry environments, water relased
by cavitation events might be important to the short-term survival of trees. 相似文献
995.
Effects of abscisic acid on the orientation and cold stability of cortical microtubules in epicotyl cells of the dwarf pea 总被引:1,自引:0,他引:1
Summary The effects of abscisic acid (ABA) on the orientation and cold stability of cortical microtubules (MTs) in epidermal cells of epicotyls of the dwarf pea,Pisum sativum L. cv. Little Marvel, were examined by immunofluorescence microscopy. The effect of ABA on the elongation of epicotyls and on the orientation of cortical MTs was opposite to that of gibberellin A3 (GA3). Treatment with ABA, which reduced the promotion of epicotyl elongation by GA3, eliminated the GA3-induced predominance of transverse MTs and resulted in a predominance of longitudinal MTs. The effect of ABA on the cold stability of cortical MTs was also opposite to that of GA3. ABA increased the cold stability of MTs, while GA3 decreased it. The predominance of longitudinal MTs brought about by ABA may have some relationship to ABA-induced inhibition of the elongation of the epicotyl. ABA may alter membrane proteins to stabilize cortical MTs and induce cold hardiness of plants.Abbreviations ABA
abscisic acid
- DMSO
dimethylsulfoxide
- FITC
fluorescein isothiocyanate
- GA3
gibberellin A3
- MT
microtubule
- PBS
phosphate-buffered saline
Dedicated to the memory of Professor Oswald Kiermayer 相似文献
996.
997.
Howard Griffiths Mark S. J. Broadmeadow Anne M. Borland Clive S. Hetherington 《Planta》1990,181(4):604-610
Short-term measurements of instantaneous carbon-isotope discrimination have been determined from mass-spectrometric analyses of CO2 collected online during gas exchange for the epiphytic bromeliad Tillandsia utriculata L. Using this technique, the isotopic signature of CO2 exchange for each phase of Crassulacean acid metabolism (CAM) has been characterised. During night-time fixation of CO2 (Phase I), discrimination () ranged from 4.4 to 6.6, equivalent to an effective carbon-isotope ratio (13C) of –12.3 to –14.5 versus Pee Dee Belemnite (PDB). These values reflected the gross photosynthetic balance between net CO2 uptake and refixation of respiratory CO2, characteristic of CAM in the Bromeliaceae. When for the relative proportion of external (p
a
) and internal (p
i) CO2 is taken into account, calculated p
i/p
a decreased during the later part of the dark period from 0.68 to 0.48. Measurements of during Phase II, early in the light period, showed the transition between C4 and C3 pathways, with carboxylation being increasingly dominated by ribulose bisphosphate carboxylase (Rubisco) as increased from 10.5 to 21.2 During decarboxylation in the light period (Phase III), CO2 leaked out of the leaf and the inherent discrimination of Rubisco was expressed. The value of calculated from on-line measurements (64.4) showed that the CO2 lost was considerably enriched in 13C, and this was confirmed by direct analysis of the CO2 diffusing out into a CO2-free atmosphere (
13C = + 51.6 versus PDB). Instantaneous discrimination was characteristic of the C3 pathway during Phase IV (late in the light period), but a reduction in showed an increasing contribution from phosphoenolpyruvate carboxylase. The results from this non-invasive technique confirm the observations that double carboxylation involving both phosphoenolpyruvate carboxylase and Rubisco occurs during the transient phases of CAM (II and IV) in the light period.Abbreviations and Symbols CAM
Crassulacean acid metabolism
-
H+
(dawn-dusk) variation in titratable acidity
-
13C
carbonisotope ratio of plant organic material, relative to Pee Dee Belemnite (vs. PDB)
-
discrimination against 13CO2,
-
p
i, p
a
internal, external partial pressures of CO2
- Rubisco
ribulose1,5-bisphosphate carboxylase
- PAR
photosynthetically active radiation
- PEPCase
phosphoenolpyruvate carboxylase
We are grateful for financial support in respect of research grants (GR3/5360, GR3/6419) and a studentship awarded by the Natural Environment Research Council, UK. 相似文献
998.
999.
The loading of amino acids and nitrate into the xylem was investigated by collection and analysis of root-pressure exudate from the cut hypocotyl stumps of seedlings of Ricinus communis L. Glutamine was found to be the dominant amino acid in the exudate and also to be the amino acid which is transferred to the xylem most rapidly and accumulated to the greatest extent. The comparison between uptake and xylem loading showed significant differences in specificity between these two transport reactions, indicating a different set of transport systems. Nitrate is transferred to the xylem at a higher relative rate than any amino acid despite the great nitrate-storage capacity of the root system. Thus the supply of nitrate to Ricinus plants leads to enhanced nitrogen allocation to the shoots. 相似文献
1000.
P. Heino G. Sandman V. Lång K. Nordin E. T. Palva 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1990,79(6):801-806
Summary Abscisic acid (ABA) has been implicated as a regulatory factor in plant cold acclimation. In the present work, the cold-acclimation properties of an ABA-deficient mutant (aba) of Arabidopsis thaliana (L.) Heynh. were analyzed. The mutant had apparently lost its capability to cold acclimate: the freezing tolerance of the mutant was not increased by low temperature treatment but stayed at the level of the nonacclimated wild type. The mutational defect could be complemented by the addition of exogenous ABA to the growth medium, restoring freezing tolerance close to the wild-type level. This suggests that ABA might have a central regulatory function in the development of freezing tolerance in plants. Cold acclimation has been previously correlated to the induction of a specific set of proteins that have been suggested to have a role in freezing tolerance. However, these proteins were also induced in the aba mutant by low temperature treatment. 相似文献