全文获取类型
收费全文 | 255篇 |
免费 | 6篇 |
国内免费 | 16篇 |
专业分类
277篇 |
出版年
2024年 | 2篇 |
2023年 | 9篇 |
2022年 | 4篇 |
2021年 | 4篇 |
2020年 | 11篇 |
2019年 | 7篇 |
2018年 | 2篇 |
2017年 | 6篇 |
2016年 | 5篇 |
2015年 | 6篇 |
2014年 | 7篇 |
2013年 | 22篇 |
2012年 | 4篇 |
2011年 | 4篇 |
2010年 | 3篇 |
2009年 | 3篇 |
2008年 | 16篇 |
2007年 | 14篇 |
2006年 | 16篇 |
2005年 | 10篇 |
2004年 | 14篇 |
2003年 | 11篇 |
2002年 | 6篇 |
2001年 | 6篇 |
2000年 | 6篇 |
1999年 | 8篇 |
1998年 | 11篇 |
1997年 | 8篇 |
1996年 | 3篇 |
1995年 | 5篇 |
1994年 | 2篇 |
1993年 | 6篇 |
1992年 | 1篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 5篇 |
1985年 | 3篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 13篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有277条查询结果,搜索用时 15 毫秒
1.
Leaf chlorophyll (Chl, A, B) and total soluble protein were assayed in greenhouse-grown 1.5-year-old trees of 2 citrus types, trifoliate orange (Poncirus trifoliata (L.) Raf.) and sour orange (Citrus aurantium L.) exposed to 12 h (day/night) photoperiods in growth chambers under high (30°/21°C, day/night; noncold-hardening) and low (16°/5°C; cold-hardening) temperature regimes. Trees were sprayed 2 × per week for 5 weeks with one of the following solutions at 100 M: napthaleneacetic acid (NAA), paclobutrazol (2RS, 3RS)-1-(4-chlorophenyl-4,4-dimethyl-2-(1,2,4-triazol-1-yl)pentan-3-ol) (PPP333), benzyl-adenine (BA), abscisic acid (ABA), gibberellic acid (GA3), minerals only (N, P, K, S, Ca, Mg) and BA (+) minerals. NAA, PP333, ABA and GA3 decreased Chl A, B and soluble protein in both citrus types under cold-hardening conditions in contrast to increases with the use of BA and BA (+) minerals especially in trifoliate orange. Both BA and GA3 increased Chl A, B and protein synthesis under high temperature in both citrus types. Under noncold-hardening temperatures, GA3 enhanced Chl A, B but sharply reduced foliar protein concentration. Dieback of both cultivars following exposure to temperatures down to –6.7°C was decreased 7% by NAA sprays during noncold-hardening temperatures. Cold tolerance of noncoldhardened trifoliate orange trees was also improved with ABA and PP333. Foliar sprays of NAA (sour orange) and PP333 and BA (+) minerals (trifoliate) increased cold tolerance of cold-hardened trees by 8%. Results indicate that spray applications of growth regulators influence physiological factors associated with foliar functioning and cold tolerance in citrus during different temperature regimes.Summary Growth promoters (BA) and inhibitors (NAA) have the potential to promote cold hardines through either a strong stimulatory effect on foliar physiology or a marked inhibition of growth in general. This suggests that each growth regulator may possess an independent role in the cold-hardiness phenomenon and may also interact with physiological processes other than soluble protein and chlorophyll metabolism. The relationship between soluble protein levels in citrus foliage and the degree of cold hardiness remains uncertain and is essentially unresolved pending more specific qualitative research.University of Florida Agricultural Experiment Station Series No. 7446.This paper reports the results of research only. Mention of a trademark of a proprietary product does not constitute a recommendation for use by the U.S. Department of Agriculture to the exclusion of other products that may also be suitable. 相似文献
2.
Nature and properties of humic acid prepared from different sources and its effect on nutrient availability 总被引:1,自引:0,他引:1
Summary Effect of sources, moisture levels and extractants on extraction, analytical properties, IR-spectra, macromolecular characteristics, interaction with cations and physiological properties of the extracted material (humic acid) were studied. Among the three variables, extractant influenced the degree of humification, IR-spectra, nature and properties of humic acids to a greater extent though the other two had significant contribution. Sodium pyrophosphate found to be an ideal extractant for humic acid. Reduced viscosity of humic acids increased with dilution particularly below 0.1 g/dl and decreased with increasing H+ concentration between pH 6.5 and 8.5 irrespective of source, moisture level and extractant. The amount of proton released and drop in initial pH of humic acids on addition of different metal ions indicated formation of mono-, di-hydroxy metal-HA complexes which broke down at pH 8.0 and above. HA-Fe2+ and HA-Cu2+ complexes were found to be most stable. Humic acids from different sources varied significantly in their effect on available nutrient status of soils though, the latter is main determining factor particularly for calcium and magnesium. 相似文献
3.
Analyses for phytate by an indirect precipitation method and for the minerals calcium (Ca), zinc (Zn), iron (Fe), copper (Cu),
and manganese (Mn) by atomic absorption spectrophotometry were carried out on 100 foods available in New Zealand. Foods with
1% phytate (dry weight basis) included untoasted muesli, rolled oats, wheat germ, wheat bran, soybean, and some soy products.
Most breads contained between 0.35 and 0.60% phytate; legumes on average had 0.62% phytate, as did snack bars. There was a
wide variation in Ca and Zn contents: There was a tenfold variation in Ca content among the legume products, whereas there
was a seventyfold variation in Zn content among the cereals. The phytate: Zn molar ratio, which is presumed to indicate the
biovailability of Zn, was above 20∶1 for two-thirds of the cereals and almost all of the snack bars; it was above 15∶1 for
one-third of the breads, almost all of the legumes, and half of the legume products. These high phytate: Zn molar ratios,
as well as some Ca: phytate molar ratios above 6∶1, indicate that there might be a reduced biovailability of Zn in many of
the foods analyzed in this study. 相似文献
4.
It is widely held that there was a phosphate compound in prebiotic chemistry that played the role of adenosine triphosphate and that the first living organisms had ribose-phosphate in the backbone of their genetic material. However, there are no known efficient prebiotic synthesis of high-energy phosphates or phosphate esters. We review the occurrence of phosphates in Nature, the efficiency of the volcanic synthesis of P4O10, the efficiency of polyphosphate synthesis by heating phosphate minerals under geological conditions, and the use of high-energy organic compounds such as cyanamide or hydrogen cyanide. These are shown to be inefficient processes especially when the hydrolysis of the polyphosphates is taken into account. For example, if a whole atmosphere of methane or carbon monoxide were converted to cyanide which somehow synthesized polyphosphates quantitatively, the polyphosphate concentration in the ocean would still have been insignificant. We also attempted to find more efficient high-energy polymerizing agents by spark discharge syntheses, but without success. There may still be undiscovered robust prebiotic syntheses of polyphosphates, or mechanisms for concentrating them, but we conclude that phosphate esters may not have been constituents of the first genetic material. Phosphoanhydrides are also unlikely as prebiotic energy sources.
Correspondence to: S.L. Miller 相似文献
5.
Donald M. Hunten 《Journal of molecular evolution》1979,14(1-3):71-78
Summary Photolysis of H2O in the atmosphere near the surface is a copious source of OH, HO2, and probably superoxides, some of which are likely to condense on the surface and migrate through the pores. The processes have been modeled in detail for their atmospheric interest. The models successfully account for the rarity of CO and O2, the notable variability of ozone, and the escape flux of hydrogen. Though only qualitative estimates can be made of surface deposition rates and lifetimes, the suggested amounts are in the range inferred by Viking. The OH rapidly destroys any organic molecules that are present as vapors.Analogous reactions involving adsorbed water have been studied by Huguenin. These processes can be driven by the much larger photon fluxes at longer ultraviolet wavelengths. The suggested explanations, and many of the experiments, make it likely that peroxides, superoxides, and adsorbed OH are all present. Both kinds of process, and their combinations, seem in principle able to explain the absence of all organic molecules and the variety of observed oxidants. Since they operate planetwide, there is a strong suggestion that the observed conditions are typical. Oases of higher than average humidity may in fact be even more hostile than the average region, because water under Martian surface conditions is anything but benign.Laboratory simulation of the atmospheric processes must pay careful attention to scaling.Curiously, similar OH densities occur at the Earth's surface, The notable differences are food for thought, and ideas about the origin of life may be particularly affected.Based on an invited paper at the Second International Colloquium on Mars, Pasadena, January 15–18, 1979 相似文献
6.
Harold P. Klein 《Journal of molecular evolution》1979,14(1-3):161-165
Summary Several ground-based investigations have been carried out since the Viking biology results were received from Mars. Many of these have resulted in reasonable simulations of the Martian data, using as analogues of Mars either strong oxidants, UV-treated materials, iron-containing clays, or iron salts. The ambiguity between the GCMS experiment, in which no organic compounds were found on Mars, and the Labeled Release experiment, in which added organics were decomposed, may well be accounted for by these simulations. 相似文献
7.
Robert L. Huguenin Karen J. Miller William S. Harwood 《Journal of molecular evolution》1979,14(1-3):103-132
Summary A laboratory study of the interaction of H2O frost with samples of the minerals olivine (Mg,Fe)2SiO4 and pyroxene (Mg,Fe)SiO3 at –11°C to –22°C revealed that an acidic oxidant was produced. Exposure of the frost-treated minerals to liquid H2O produced a sudden drop in pH and resulted in the production of copious O2(g) (as much as ~ 1020 molecules g–1). Exposure of frost-treated samples to 5 ml of 0.1M HCOONa solution resulted in the rapid oxidation of up to 43% of the formate to CO2(g). These reactions were qualitatively similar to the chemical activity observed during the active cycles of the Viking lander Gas Exchange and Labeled Release Biology experiments. Attempts to identify the oxidant by chemical indicators were inconclusive, but they tentatively suggested that chemisorbed hydrogen peroxide may have formed. The formation of chemisorbed peroxide could be explained as a byproduct of the chemical reduction of the mineral. The following model was proposed. Hc was incorporated into the mineral from surface frost. This would have left behind a residual of excess OH–
(ads) (relative to surface H+). Electrons were then stripped from the surface OH–
(ads) (due to the large repulsive potential between neighboring OH–
(ads)) and incorporated into the crystal to restore charge balance and produce a chemical reduction of the mineral. The resultant surface hydroxyl radicals could then have combined to form the more stable chemisorbed hydrogen peroxide species. While the chemisorbed peroxide should be relatively stable at low temperatures, it should tend to decay to O(ads) + H2O(g) at higher temperatures with an activation energy of 34 kcal mole–1. This is consistent with the long-term storage and sterilization behavior of the Viking soil oxidants. It is possible that as little as 0.1–1% frost-weathered material in the Martian soil could have produced the unusual chemical activity that occurred during the Viking Gas Exchange and Labeled Release experiments.This paper contains the material given in invited presentations at the COSPAR Meeting, Innsbruck, Austria, 5–7 June 1978 and at the Second Conference on Simulation of Mars Surface Properties, NASA Ames Research Center, 17–18 August 1978 相似文献
8.
Summary Calculations indicate that the maximum daily solar radiation reaching the Martian surface is about 325 cal/cm2 during southern hemisphere summer at latitude of about 40°S. In the ultraviolet region of the spectrum, the radiation reaching the surface at wavelengths greater than 2800 Å is within 10% of the radiation incident on the atmosphere. There is significant extinction of radiation in the spectral region near 2500 Å in mid and high latitudes due to absorption of radiation by ozone; radiation reaching the surface may be reduced to one one-thousandth of that incident on the atmosphere during winter. Virtually no radiation of wavelengths less than 1900 Å reaches the surface because of absorption by the large column abundance of carbon dioxide. Daily and latitudinal distributions of radiation are presented for wavelengths of 3000, 2500 and 2000 Å. 相似文献
9.
Lynn Margulis Peter Mazur Elso S. Barghoorn Harlyn O. Halvorson Thomas H. Jukes Isaac R. Kaplan 《Journal of molecular evolution》1979,14(1-3):223-232
Summary The results of the Viking Biology experiments are best explained by non-biological phenomena: The interaction of the reagents with the materials comprising the regolith. Conditions of water activity, temperature, availability of carbon sources and others in most regions of the planet are too extreme for survival and growth of any known Earth microorganisms. Although the possibility persists that some very unusual form of life is somewhere on that planet the evidence is best interpreted as negative. Even though there is no evidence for current life on Mars, whether or not life ever originated there is not known. 相似文献
10.
Seed dimorphism in Salicornia europaea: Nutrient reserves 总被引:1,自引:0,他引:1
Franz-Arnold Austenfeld 《Physiologia plantarum》1988,73(4):502-504
Median and lateral seeds of Salicornia europaea L. were separately analysed for their sizes and nutrient reserves. The mean air-dry weight of a single median and lateral seed was 0.31 and 0.25 mg, respectively. The composition as well as the concentration of the nutrient reserves were similar in both seed types. The bulk of the cations was derived from K+ , followed by Mg2+ , Na+ and Ca2+ . The chloride content was somewhat higher than the sodium content, and phosphate was equalled by acid soluble Ca2+ and Mg2+ . Starchy compounds and sucrose were present in equal amounts, each of them accounted for about 50% of the carbohydrates. Glucose and fructose were less than 1%. Protein-nitrogen (ethanol-insoluble N) was about 34 g (kg dry seeds)−1 . About 7 g (kg dry seeds)−1 was ethanol-soluble nitrogen, of which 10% was derived from amino acids. The total lipid content was more than 290 g (kg dry seeds)−1 , 65% were calculated to be glycerides. More than 90% of the fatty acids consisted of linoleic and oleic acids, the majority (72%) of which was linoleic acid. 相似文献