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81.
Using a combination of gas-exchange and chlorophyll fluorescence measurements, low apparent CO2/O2 specificity factors (1300 mol mol?1) were estimated for the leaves of two deciduous tree species (Fagus sylvatica and Castanea sativa). These low values contrasted with those estimated for two herbaceous species and were ascribed to a drop in the CO2 mole fraction between the intercellular airspace (Ci) and the catalytic site of Rubisco (Cc) due to internal resistances to CO2 transfer. Cc. was calculated assuming a specificity of Rubisco value of 2560 mol mol?1. The drop between Ci and Cc was used to calculate the internal conductance for CO2 (gi). A good correlation between mean values of net CO2 assimilation rate (A) and gi was observed within a set of data obtained using 13 woody plant species, including our own data. We report that the relative limitation of A, which can be ascribed to internal resistances to CO2 transfer, was 24–30%. High internal resistances to CO2 transfer may explain the low apparent maximal rates of carboxylation and electron transport of some woody plant species calculated from A/Ci curves. 相似文献
82.
The response of adaxial and abaxial stomatal conductance in Rumex obtusifolius to growth at elevated atmospheric concentrations of CO2 (250 μmol mol?1 above ambient) was investigated over two growing seasons. The conductance of both the adaxial and abaxial leaf surfaces was found to be reduced by elevated concentrations of CO2. Elevated CO2 caused a much greater reduction in conductance for the adaxial surface than for the abaxial surface. The absence of effects upon stomatal density indicated that the reductions were probably the result of changes in stomatal aperture. Partitioning of gas exchange between the leaf surfaces revealed that increased concentrations of CO2 caused increased rates of photosynthesis only via the abaxial surface. Additionally, leaf thickness was found to increase during growth at elevated concentrations of CO2. The tendency for these amphistomatous leaves to develop a distribution of conductance approaching that of hypostomatous leaves clearly reduced their maximum photosynthetic potential. This conclusion was supported by measurements of stomatal limitation, which showed greater values for the adaxial surfaces, and greater values at elevated CO2. This reduction in photosynthesis may in part be caused by higher diffusive limitations imposed because of increased leaf thickness. In an uncoupled canopy, asymmetrical stomatal responses of the kind identified here may appreciably reduce transpiration. Species which show symmetrical responses are less likely to show reduced transpirational rates, and a redistribution of water loss between species may occur. The implications of asymmetrical stomatal responses for photosynthesis and canopy transpiration are discussed. 相似文献
83.
Effects of patchy stomatal closure on gas exchange measurements following abscisic acid treatment 总被引:9,自引:5,他引:4
K. A. MOTT 《Plant, cell & environment》1995,18(11):1291-1300
Gas exchange data and images of leaf fluorescence were collected concurrently as stomata responded to abscisic acid (ABA) application. When 10?5kmolm?3 ABA was applied to the transpiration stream in a short pulse, stomatal conductance (gs), photosynthesis (A) and intercellular CO2 concentration (Ci) decreased rapidly after a short lag period and became approximately constant after 2h. There was an apparent reduction in the A versus c1 relationship as stomata closed, but the data returned to the A versus C1 curve while stomatal conductance was constant or slowly rising during the second hour after ABA treatment. Larger amounts of ABA administered during the pulse caused larger deviations from the A versus c1 relationship. When 10?7kmolm?3 ABA was applied continuously through the transpiration stream, gs, A and Ci decreased, but there was no substantial deviation from the A versus c{ curve. Fluorescence images were patchy as stomata closed for all experiments, but became slowly more uniform during the time that gas exchange was returning to the A versus Cj curve. The distribution of con-ductance among patches was not bimodal, and larger devi-ations from the A versus ct curve had greater ranges of pixel values and more pixel values representing low values of Cj during stomatal closure than did experiments show-ing small or no deviation. Estimates of A and gs from fluo-rescence images compared favourably with measured val-ues in most cases, suggesting that the patchy distributions of fluorescence were caused by patchy distributions of stomatal conductance and that apparent reductions in the A versus ct relationship were the result of these patchy stomatai distributions and not direct effects of ABA on mesophyll functioning. The data show that stomatal patches can be temporary and that patchiness may not be reflected in gas exchange data if the range of stomatal con-ductances is not large. These observations may explain some of the discrepancies among previous studies concerning the effect of ABA on the A versus Ci relationship. 相似文献
84.
Interaction of ozone pollution and light effects on photosynthesis in a forest canopy experiment 总被引:6,自引:3,他引:3
Ozone pollution may reduce net carbon gain in forests, yet data from mature trees are rare and the effects of irradiance on the response of photosynthesis to ozone remain untested. We used an open-air system to expose 10 branches within the upper canopy of an 18-m-tall stand of sugar maple (Acer saccharum Marsh.) to twice-ambient concentrations of ozone (95nmol mol?1, 0900 to 1700, 1 h mean) relative to 10 paired, untreated controls (45nmol mol?1) over 3 months. The branch pairs were selected along a gradient from relatively high irradiance (PPFD 14.5 mol m?2 d?1) to deep shade (0.7mol m?2 d?1). Ozone reduced light-saturated rates of net photosynthesis (Asat) and increased dark respiration by as much as 56 and 40%, respectively. Compared to sun leaves, shade leaves exhibited greater proportional reductions in Asat and had lower chlorophyll concentrations, quantum efficiencies, and leaf absorptances when treated with ozone relative to controls. With increasing ozone dose over time, Asat became uncoupled from stomatal conductance as ratios of internal to external concentrations of carbon dioxide increased, reducing water-use efficiency. Ozone reduced net photosynthesis and impaired stomatal function, with these effects depending on the irradiance environment of the canopy leaves. Increased ozone sensitivity of shade leaves compared to sun leaves has consequences for net carbon gain in canopies. 相似文献
85.
86.
The hydrological structure of the French coastal part of the eastern English Channel is strongly linked with tidal regimes and riverine input. Two distinct water masses are separated by a frontal area and drift along the coast in SW–NE direction. These two water masses are well-mixed during the entire year. We studied the seasonal dynamic of nitrogenous nutrients, chlorophyll a and organic particulate carbon and nitrogen at two stations, characteristic of these water masses, during the year 1994. Results show (i) a winter stock of nitrate and ammonium, (ii) a pre-bloom period corresponding to the use of ammonium, (iii) a high bloom period of short duration using nitrate, (iv) a post-bloom period with little phytoplanktonic activity probably limited by nutrients other than nitrogen and (v) an autumnal period of reconstitution of stock. The essential difference between the two stations is the importance of winter stock of nutrients and of bloom chlorophyll a concentration, with the coastal station richer than the offshore one. An assumption about the nitrogen available for new production in this area gives a value of 57% of the winter stock of inorganic nitrogen. 相似文献
87.
K.D. Clark T.M. Hennessey D.L. Nelson R.R. Preston 《The Journal of membrane biology》1997,157(2):159-167
Paramecium tetraurelia responds to extracellular GTP (≥ 10 nm) with repeated episodes of prolonged backward swimming. These backward swimming events cause repulsion from the stimulus
and are the behavioral consequence of an oscillating membrane depolarization. Ion substitution experiments showed that either
Mg2+ or Na+ could support these responses in wild-type cells, with increasing concentrations of either cation increasing the extent of
backward swimming. Applying GTP to cells under voltage clamp elicited oscillating inward currents with a periodicity similar
to that of the membrane-potential and behavioral responses. These currents were also Mg2+- and Na+-dependent, suggesting that GTP acts through Mg2+-specific (I
Mg) and Na+-specific (I
Na) conductances that have been described previously in Paramecium. This suggestion is strengthened by the finding that Mg2+ failed to support normal behavioral or electrophysiological responses to GTP in a mutant that specifically lacks I
Mg (``eccentric'), while Na+ failed to support GTP responses in ``fast-2,' a mutant that specifically lacks I
Na. Both mutants responded normally to GTP if the alternative cation was provided. As I
Mg and I
Na are both Ca2+-dependent currents, the characteristic GTP behavior could result from oscillations in intracellular Ca2+ concentration. Indeed, applying GTP to cells in the absence of either Mg2+ or Na+ revealed a minor inward current with a periodicity similar to that of the depolarizations. This current persisted when known
voltage-dependent Ca2+ currents were blocked pharmacologically or genetically, which implies that it may represent the activation of a novel purinergic-receptor–coupled
Ca2+ conductance.
Received: 28 October 1996/Revised: 24 December 1996 相似文献
88.
The mitochondrial outer membrane channel, VDAC, is thought to serve as the major permeability pathway for metabolite flux
between the cytoplasm and mitochondria. The permeability of VDAC to citrate, succinate, and phosphate was studied in channels
reconstituted into planar phospholipid membranes. All ions showed large changes in permeability depending on whether the channel
was in the open or in the low conductance, ``closed' state, with the closed state always more cation selective. This was
especially true for the divalent and trivalent anions. Additionally, the anion flux when the voltage was zero was shown to
decrease to 5–11% of the open state flux depending on the anion studied. These results give the first rigorous examination
of the ability of metabolites to permeate through VDAC channels and indicate that these channels can control the flux of these
ions through the outer membrane. This lends more evidence to the growing body of experiments that suggest that the outer mitochondrial
membrane has a much more important role in controlling mitochondrial activity than has been thought historically.
Received: 4 November 1996/Revised: 8 January 1997 相似文献
89.
The effect of ethanol on maxi Ca2+-activated K+ channels (BK channels) in GH3 pituitary tumor cells was investigated using single-channel recordings and focusing on intracellular
signal transduction. In outside-out patches, ethanol caused a transient concentration-dependent increase of BK-channel activity.
30 mm (1.4‰) ethanol significantly increased mean channel open time and channel open probability by 26.3 ± 9% and 78.8 ± 10%, respectively;
single-channel current amplitude was not affected by ethanol. The augmenting effect of ethanol was blocked in the presence
of protein kinase C (PKC) inhibitors staurosporine, bisindolylmaleimide, and PKC (19–31) pseudosubstrate inhibitor as well
as by AMP-PNP (5′-adenylylimidodiphosphate), a nonhydrolyzable ATP-analogue, but not by the phospholipase C blocker U-73122.
Phosphatase inhibitors microcystin-LR and okadaic acid promoted the ethanol effect. The blocking effect was released at higher
concentrations of ethanol (100 mm) suggesting a second site of action or a competition between blockers and ethanol. Our results suggest that the effect of
ethanol on BK-channels is mediated by PKC stimulation and phosphorylation of the channels which increases channel activity
and hence may influence action potentials duration and hormone secretion.
Received: 24 July 1996/Revised: 27 December 1996 相似文献
90.
Ouabain-blocked toad urinary bladders were maintained in Na+-free mucosal solutions, and a depolarizing solution of high K+ activity containing only 5 mM Na+ on the serosal side. Exposure to mucosal sodium (20 mM activity) evoked a transient amiloride-blockable inward current, which decayed to near zero within one hour. The apical sodium conductance increased in the initial phase of the current decay and decreased in the second phase. The conductance decrease required Ca2+ to be present on the serosal side and was more rapid when the mucosal Na+ activity was higher. At 20 mM mucosal Na+ and 3 mM serosal Ca2+ the initial (maximal) rate of inhibition amounted to 20% in 10 min. The conductance decrease could be accelerated by raising the serosal Ca2+ activity to 10 mM. The inhibition reversed on lowering the serosal Ca2+ to 3 μM and, in addition, the mucosal Na+ to zero. Exposure of the mucosal surface to the ionophore nystatin abolished the Ca2+ sensitivity of the transcellular conductance, showing that the Ca2+-sensitive conductance resides in the apical membrane. The data imply that in the K+-depolarized epithelia, cellular Ca2+, taken up from the serosal medium by means of a Na+-Ca2+ antiport, cause feedback inhibition by blockage of apical Na+ channels. However, the rate of inhibition is small, such that this regulatory mechanism will have little effect at 1 mM serosal Ca2+ and less than 20 mM cellular Na+. 相似文献