全文获取类型
收费全文 | 4261篇 |
免费 | 138篇 |
国内免费 | 262篇 |
出版年
2023年 | 64篇 |
2022年 | 98篇 |
2021年 | 114篇 |
2020年 | 91篇 |
2019年 | 99篇 |
2018年 | 81篇 |
2017年 | 84篇 |
2016年 | 96篇 |
2015年 | 112篇 |
2014年 | 130篇 |
2013年 | 221篇 |
2012年 | 69篇 |
2011年 | 114篇 |
2010年 | 65篇 |
2009年 | 150篇 |
2008年 | 174篇 |
2007年 | 173篇 |
2006年 | 171篇 |
2005年 | 164篇 |
2004年 | 148篇 |
2003年 | 137篇 |
2002年 | 126篇 |
2001年 | 87篇 |
2000年 | 88篇 |
1999年 | 89篇 |
1998年 | 72篇 |
1997年 | 77篇 |
1996年 | 79篇 |
1995年 | 97篇 |
1994年 | 81篇 |
1993年 | 45篇 |
1992年 | 63篇 |
1991年 | 68篇 |
1990年 | 48篇 |
1989年 | 38篇 |
1987年 | 38篇 |
1986年 | 42篇 |
1985年 | 81篇 |
1984年 | 88篇 |
1983年 | 60篇 |
1982年 | 75篇 |
1981年 | 73篇 |
1980年 | 67篇 |
1979年 | 66篇 |
1978年 | 63篇 |
1977年 | 73篇 |
1976年 | 58篇 |
1975年 | 63篇 |
1974年 | 66篇 |
1973年 | 66篇 |
排序方式: 共有4661条查询结果,搜索用时 46 毫秒
961.
Although all parts of the geranium plant (Pelargonium hortorum) are capable of synthesizing sterols and triterpenes and their esters in vitro from mevalonic acid-[2-14C], the aerial portions are more active than other tissues. All plant parts were shown to incorporate mevalonic acid-[2-14C] into isoprenoids for at least 3 days. The leaves and petioles had the greatest incorporation on a wet weight basis. Chopped preparations showed comparable incorporations of mevalonate whereby rootlets incorporated about one half as much as most parts; the flower petals incorporated five times the average amount. In leaves the principal sterol synthesized was sitosterol. Metabolic studies with isolated leaves indicated a fairly rapid conversion of free tetracyclic triterpenes to 4-desmethyl-sterols, while β-amyrin was synthesized at a different rate than α-amyrin. Esterified tetracyclic triterpenes exhibited only a slight amount of conversion to 4-desmethylsterols. 相似文献
962.
Alain Nicolas Jaques Bascoul André Crastes De Paulet Peter D.G. Dean 《Phytochemistry》1975,14(11):2407-2409
The incubation of [1-14C]-squalene-2(3)epoxide ([1-14C]-SO) with a cell-free extract of Alnus glutinosa gave only cycloartenol in 1% yield. 相似文献
963.
An enzyme was detected in petal extracts of Melandrium album which catalysed the transfer of the xylose moiety of UDP-xylose to the 7-hydroxyl group of isovitexin. Genetical analysis revealed that the presence of the dominant allele gx was necessary for enzymic activity. This activity was independent of the residual genetic background. Xylosyltransferase activity is also present in extracts of gGgx plants, in which the product of the enzyme is not detectable. Maximal activity was found between pH 7·0 and 7·5; MnCl2 inhibited this transfer. The enzyme had an ‘apparent Km' value of 1·0 mM for UDP-xylose and of O·04 mM for isovitexin. 相似文献
964.
The conversion of ent-kaur-16-enes to gibberellic acid in Gibberella fujikuroi is blocked by A-ring modifications. Thus ent-3β-hydroxykaur-16-en-19-yl succinate gives good conversion (46%) to the 7β-hydroxy derivative.* Under the same conditions the 3β-epimer gives 7β- or 6α-hydroxylation and the former occurs for the 3-oxo analogue. The succinoyloxy function acts as a less efficient block and ent-kaur-16-en-19-yl succinate is converted to 7β-hydroxy and 6β,7β-dihydroxy derivatives along with gibberellic acid. Hydrolysis of the succinate block of the metabolities provides the 7β, 19-diol and 6β,7β, 19-triol. Of this pair only the former was effectively metabolized to gibberellic acid in G. fujikuroi. 相似文献
965.
Biosynthesis of gibberellins A12, A15, A24, A36, and A37 by a cell-free system from Cucurbita maxima
GA12-aldehyde obtained from mevalonate via ent-kaurene, ent-kaurenol, ent-kaurenoic acid and ent-7α-hydroxykaurenoic acid in a cell-free system from immature seeds of Cucurbita maxima was converted to GA12 by the same system. When Mn2+ was omitted from the system GA12-aldehyde and GA12 were converted further to several products. Among these GA15, GA24, GA36 and GA37 were conclusively identified by GC-MS. With the exception of GA37 these GAs have not previously been found in higher plants. Another biosynthetic pathway led from ent-7α-hydroxykaurenoic acid to very polar products via what was tentatively identified as ent-6α, 7α-dihydroxykaurenoic acid. An unidentified component with an MS resembling that of a dihydroxykaurenolide was also obtained from incubations with mevalonate. 相似文献
966.
Occurrence of a 3′-phosphoadenosine 5′-Phosphosulphate synthesizing system In two Ochromonas species
The presence of an enzyme capable of incorporating 35SO42? into 3′-phosphoadenosine 5′-phosphosulphate has been demonstrated,in Ochromonas danica and O. malhamensis. This system probably includes the enzymes ATP:sulphate adenyltransferase. E.C. 2.7.7.4 and ATP:adenylsulphate 3′-phosphotransferase, E.C. 2.7.1.25. 相似文献
967.
Ivan F. Bird Martin J. Cornelius Alfred J. Keys Charles P. Whittingham 《Phytochemistry》1974,13(1):59-64
Improved conditions for extraction and assay increased rates of sucrose synthesis from uridine diphosphate glucose (UDPglucose) plus fructose 6-phosphate (F.6.P) catalysed by leaf extracts 20-fold. Rates of 17.9, 25·0, 9·2 and 27·7 μmol/hr/g fr. wt respectively were obtained from pea shoots, spinach, wheat and bean leaves. Chloroplasts isolated from pea shoots, in which half the plastids were intact, contained less than 4% of the total UDPglucose-fructosephosphate glucosyltransferase, more than 30% of the ribulose diphosphate (RuDP) carboxylase, and more than 40% of the total chlorophyll of the leaf. Although some of the UDPglucose-fructose-phosphate glucosyltransferase was associated with particles smaller than chloroplasts at least 85% of the enzyme was not precipitated at 38 000 g. UDPglucose pyrophosphorylase, also thought to be essential for sucrose synthesis, was distributed between the cell fractions in a similar manner to UDPglucose-fructosephosphate glucosyltransferase. It is concluded that sucrose synthesis in pea shoots and spinach leaves occurs mainly, in the cytoplasm. 相似文献
968.
Using appropriate precursors, the two unnatural alkaloids 4(5)-[N-isocaproylaminomethyl]imidazole and 3-[2-N-isovalerylaminoethyl]pyrazole were produced by Dolichothele sphaerica. The former compound represents an unnatural alkaloid formed by the simultaneous introduction of two unnatural precursors, namely isocaproic acid and 4(5)-aminomethylimidazole. The latter compound represents an aberrant alkaloid formed by the introduction of a precursor of different heterocyclic entity, 3-aminoethylpyrazole. 相似文献
969.
Phytochrome-induced increases in enzyme activities for phenylalanine ammonia-lyase (EC 4.3.1.5) and chalcone isomerase (EC 5.5.1.6), and in amounts of the related end products, anthocyanin and the flavonol, quercetin, were measured in cotyledons of mustard (Sinapis alba L.). There was no correlation between the activities of these enzymes and the rate of anthocyanin accumulation; however, some correlation was found with the quercetin accumulation rate. Since anthocyanin and flavonol accumulation is spatially separated in mustard (flavonols in the upper epidermis, anthocyanin in the lower epidermis), it was possible to measure anthocyanin-associated phenylalanine ammonia-lyase independently. This activity correlated well with the accumulation rate for anthocyanin during the first few hours after induction. The phytochrome effect on anthocyanin formation differed from that on quercetin formation: anthocyanin was strongly induced by continuous far-red light and by both continuous red light and red light pulses, whereas quercetin was only effectively induced by continuous far-red light.Abbreviations CHI
chalcone isomerase
- PAL
phenylalanine ammonia-lyase 相似文献
970.
Gibberellins A1, A8, A20 and A29 were identified by capillary gas chromatography-mass spectrometry in the pods and seeds from 5-d-old pollinated ovaries of pea (Pisum sativum cv. Alaska). These gibberellins were also identified in 4-d-old non-developing, parthenocarpic and pollinated ovaries. The level of gibberellin A1 within these ovary types was correlated with pod size. Gibberellin A1, applied to emasculated ovaries cultured in vitro, was three to five times more active than gibberellin A20. Using pollinated ovary explants cultured in vitro, the effects of inhibitors of gibberellin biosynthesis on pod growth and seed development were examined. The inhibitors retarded pod growth during the first 7 d after anthesis, and this inhibition was reversed by simultaneous application of gibberellin A3. In contrast, the inhibitors, when supplied to 4-d-old pollinated ovaries for 16 d, had little effect on seed fresh weight although they reduced the levels of endogenous gibberellins A20 and A29 in the enlarging seeds to almost zero. Paclobutrazol, which was one of the inhibitors used, is xylem-mobile and it efficiently reduced the level of seed gibberellins without being taken up into the seed. In intact fruits the pod may therefore be a source of precursors for gibberellin biosynthesis in the seed. Overall, the results indicate that gibberellin A1, present in parthenocarpic and pollinated fruits early in development, regulates pod growth. In contrast the high levels of gibberellins A20 and A29, which accumulate during seed enlargement, appear to be unnecessary for normal seed development or for subsequent germination.Abbreviations GA(a)
gibberellin An
- GC-MS
combined gas chromatography-mass spectrometry
- HPLC
high-performance liquid chromatography
- PFK
perfluorokerosene
- PVP
polyvinylpyrrolidone 相似文献