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1.
Kato T Saito N Kashimura K Shinohara M Kurahashi T Taniguchi K 《Journal of agricultural and food chemistry》2002,50(22):6307-6312
On the basis of our findings that the germination of intact wheat seeds (with husks) belonging to dormancy varieties was restrained as compared with that of the dehusked seeds (grains), the germination inhibitors in the husks were explored. The water-soluble extracts from the husks were separated by the aid of inhibition assay experiments, resulting in the characterization of 2-phenylethyl alcohol 1, 4-vinylphenol 2 and its 2-methoxy derivative 3, and dihydroactinidiolide 4, all of which showed clear inhibition of germination at 500 ppm in aqueous solution. The related compounds 1-phenylethyl alcohol 5 and tetrahydroactinidiolide 6 were as active as 1 and 4, while no noticeable difference in activity was detected among both enantiomers and the DL-form of compounds 4-6. Clear synergistic relations were observed between 4 and 1 and also 4 and 3. Since the present inhibitors have been isolated from various kinds of seed plants, they may be responsible for the general germination inhibition in the seed plants. 相似文献
2.
Dicko MH Gruppen H Traore AS van Berkel WJ Voragen AG 《Journal of agricultural and food chemistry》2005,53(7):2581-2588
The screening of 50 sorghum varieties showed that, on average, germination did not affect the content in total phenolic compounds but decreased the content of proanthocyanidins, 3-deoxyanthocyanidins, and flavan-4-ols. Independent of germination, there are intervarietal differences in antioxidant activities among sorghum varieties. Phenolic compounds and antioxidant activities were more positively correlated in ungerminated varieties than in germinated ones. Sorghum grains with pigmented testa layer, chestnut color glumes, and red plants had higher contents, larger diversity of phenolic compounds, and higher antioxidant activities than other sorghums. Some red sorghum varieties had higher antioxidant activities (30-80 mumol of Trolox equiv/g) than several sources of natural antioxidants from plant foods. Among varieties used for "to", "dolo", couscous, and porridge preparation, the "dolo"(local beer) varieties had the highest average content and diversity in phenolic compounds as well as the highest antioxidant activities. The biochemical markers determined are useful indicators for the selection of sorghum varieties for food and agronomic properties. 相似文献
3.
氮素对不同生育期小麦植株累积硒的影响 总被引：4，自引：0，他引：4
【目的】施用氮肥是农业生产上最重要的增产措施之一,但增施氮肥对不同生育期小麦植株中硒累积、 转运和分配的影响尚不明确。本研究通过盆栽试验研究不同氮水平对小麦生长过程中植株各器官硒累积的影响,为合理施氮提高小麦硒含量提供理论参考。【方法】 试验设置N 100 mg/kg和200 mg/kg 两个水平,每个氮水平设置低硒 (Se 0.81 mg/kg)和高硒 (Se 5.02 mg/kg) 处理,分别在小麦苗期、 拔节期、 孕穗期、 抽穗期、 扬花期、 灌浆期和成熟期取样,分析不同生育期小麦对土壤外源硒的吸收速率,探讨不同硒水平下施氮量对小麦各器官硒累积、 转运和分配的影响。【结果】 1)与低氮相比,高氮促进了小麦籽粒产量增加,低硒的促进作用大于高硒,低硒和高硒时的高氮处理小麦籽粒产量较低氮处理分别提高了21.7%和13.7%。低硒时的高氮处理能提高小麦穗(特别是籽粒)和老叶中的硒含量,籽粒和颖壳硒含量较低氮处理分别提高了31.2%和13.6%,但高硒时高氮处理却导致小麦各器官(特别是籽粒和根)中硒含量下降,其中籽粒和根部硒含量较低氮处理分别下降了13.2%和17.8%。 2)相同硒水平下,高氮处理小麦根部硒占植株总硒的比例较低氮处理下降了约1/4; 在营养生长阶段,低硒时高氮处理能促进硒向小麦地上部转运,而在生殖生长阶段其能促进硒从小麦茎叶向颖壳和籽粒中转运,使得籽粒中硒占总硒比例提高了18.4%; 但高硒时高氮处理却促进硒从茎叶转运到颖壳中,致使籽粒中硒占总硒比例下降了8.0%。3)小麦根部硒含量在苗期和拔节期,抽穗期和扬花期之间增长幅度最大,说明此期间是小麦硒吸收的敏感期。【结论】小麦硒含量因生育期和外源硒水平的不同而异,施氮量也影响着小麦硒累积、 转运和分配。增施氮肥能提高硒的利用效率,建议低硒地区农业生产中补硒时要结合氮的合理施用。 相似文献
4.
锌与氮磷钾配合喷施对小麦锌累积、分配及转移的影响 总被引：4，自引：1，他引：3
5.
Thirty-nine wheat accessions were used to evaluate their extract phytotoxicity against annual ryegrass (Lolium rigidum Gaud.). Aqueous extracts of wheat shoot residues significantly inhibited the germination and root growth of a biotype of annual ryegrass resistant to herbicides of acetyl CoA carboxylase inhibitors (group A), acetolactate synthase inhibitors (B), photosystem II inhibitors (C), and tubulin formation inhibitors (D). The germination of the herbicide resistant (HR) biotype was inhibited by 3-100%, depending upon the wheat accession. The phytotoxic effects on ryegrass root growth ranged from 12% stimulation to 100% inhibition, compared to a control. The germination and root growth of a herbicide-susceptible (HS) biotype of annual ryegrass were also inhibited by the wheat extracts, with germination inhibited by 4-100%, and root growth by 19-100%. Bioassays with two known wheat allelochemicals showed that p-coumaric acid and propionic acid significantly inhibited the growth of both HR and HS biotypes of annual ryegrass. The two compounds completely inhibited the root growth of HR ryegrass at concentrations greater than 5.0 mM. In comparison with p-coumaric acid, propionic acid was more inhibitory to seed germination, shoot, and root growth of both ryegrass biotypes. The root growth of the HR biotype was more sensitive when exposed to wheat extracts, to p-coumaric acid, and to propionic acid. The results suggest that residues of certain wheat cultivars with strong allelopathic potential could provide a nonherbicidal alternative for the management of herbicide-resistant weed species. 相似文献
6.
Seed germination on sterilized moist filter or blotting papers resting on natural soils in stacked “Nalgene” plastic specimen dishes was inhibited particularly in alkaline soils in comparison with controls under similar conditions but without soil. The same response was observed when seeds were suspended on filter or blotting papers above the soils, suggesting the influence of an inhibitory volatile factor. Seed germination was not affected in Good's buffers over the pH range of the soil samples tested indicating that inhibition on filter or blotting papers in contact with soils was not due to the direct effect of pH. The observed inhibition of seed germination is similar to that found in the germination of conidia of common soil fungi; a volatile fungistatic factor was also detected in these soils. Liming of moderately acid soils increased conidial inhibition on indirect soil contact or from volatiles: germination of seeds, however, was not affected by liming in five of six soils tested. These results indicate that seeds of higher plants as well as fungal conidia are affected by some natural soils and volatiles are at least partially responsible for inhibition. 相似文献
7.
Fungal infections of barley and wheat cause devastating losses of these food crops. The endogenous proteinase inhibitors produced by plant seeds probably defend the plants from pathogens by inhibiting the degradation of their proteins by the pathogen proteases. We have studied the interactions of barley grain inhibitors with the subtilisin-like and trypsinlike proteinases of Fusarium culmorum. The inhibition kinetics of three inhibitor proteins, chymotrypsin/subtilisin inhibitor 2 (CI-2), barley alpha-amylase/subtilisin inhibitor (BASI), and Bowman-Birk trypsin inhibitor (BBBI), have been studied in detail for the first time using fungal enzymes. The kinetic studies were performed at physiological pH values to mimic in vivo conditions. Numerical approaches to kinetic analyses were used to calculate the inhibition constants, because the data analyses were complicated by some inhibitor turnover and the instability of enzymes and substrates. All were slow, tight-binding inhibitors that followed either a two-step mechanism (CI-2 and BASI) or a single-step mechanism (BBBI) under the conditions investigated. The overall Ki values derived were approximately 50 pM, 1 nM, and 0.1 nM for CI-2, BASI, and BBBI, respectively. The main difference between the CI-2 and the BASI inhibitions was accounted for by the stabilities of their final complexes and the rate constants for their second dissociation steps (9 x 10(-6)/s and 3 x 10(-4)/s, respectively). Understanding the inhibition mechanisms will be valuable in designing improved strategies for increasing the resistance of the grains to fungal infections. 相似文献
8.
K. Gebruers C. M. Courtin H. Goesaert S. Van Campenhout J. A. Delcour 《Cereal Chemistry》2002,79(5):613-616
Twenty‐three wheat samples from 19 different European wheat cultivars (Triticum aestivum L.) were tested for their quantitative and qualitative variation in inhibition activity against family 11 endoxylanases of Aspergillus niger, Bacillus subtilis, and Trichoderma viride and a family 10 endoxylanase of A. aculeatus. Under the experimental conditions, the A. aculeatus enzyme was not inhibited by the wheat extracts, the A. niger and B. subtilis endoxylanases were affected to a similar extent, while the T. viride enzyme was much more inhibited. The inhibition activities in the different wheat samples against the A. niger, B. subtilis, and T. viride endoxylanases varied between 36.0 and 11.7, 34.0 and 12.9, and 86.2 and 46.6 IU/100 mg of dry whole meal, respectively. One IU (inhibition unit) corresponds to the amount of inhibitor resulting in 50% inhibition of endoxylanase activity under the conditions of the assay. The inhibitor activities were linearly related, indicating that the levels of different endoxylanase inhibitors with different endoxylanase specificities in the dormant wheat grains are also linearly related or that one (or more) of these inhibitors are predominantly present or has much higher specific activity, consequently causing almost all of the inhibition activity measured. Wheat flour accounted for ≈57% of the total inhibition activity in wheat grains, while the shorts and bran fractions each contained ≈21% of the total activity. On dry weight basis, the inhibition activities were about three times higher in shorts and about two times higher in bran than in flour. The results obtained may be useful in explaining differences in functionality of different endoxylanases in biotechnological processes in which wheats of different cultivars, or fractions thereof, are used as well as in screening endoxylanases for applications in wheat‐based processes. 相似文献
9.
Maturing wheat grains represent major sinks for solutes in the phloem, while the xylem sap entering the ear is mainly delivered to the glumes. Nutrients and assimilates transported in the phloem may either be exported from source organs or loaded into the sieve tubes by a xylem‐to‐phloem transfer in the stem. The transfer of zinc from xylem to phloem in the peduncle of wheat was investigated by feeding radiolabelled zinc into the cut xylem of detached shoots. The label accumulated in grains of shoots with an intact phloem but not in those steam‐girdled below the ear. These findings indicate that zinc fed in low concentrations (0.1 ‐ 10 μM) entered the ear mainly via the phloem. After feeding high zinc concentrations (100 ‐1000 μM), the label was mainly retained in the stem and steam‐girding below the ear was rather ineffective. These results led to the conclusion that at these high concentrations zinc was still eliminated from the xylem sap, but loaded only in a minor percentage into the phloem. The interactions between the two long distance transport systems may play an important role in the regulation of zinc transport to the maturing grains of cereals. 相似文献
10.
The phyla Leguminosae contains protease inhibitors, lectins, chitinases, and glycohydrolases as major defense proteins in their seeds. Electrophoretic analysis of the seed proteins of tamarind ( Tamarindus indica L.), an agri-waste material, indicated the unusual presence of two major proteins comparable to overexpression of recombinant proteins. These proteins were identified by amino-terminal analysis to be (1) Kunitz-type trypsin inhibitor and (2) class III endochitinase (34000 Da). These two proteins were purified to apparent homogeneity by a single-step chitin bead affinity chromatography and characterized. The Kunitz inhibitor was specific toward inhibiting trypsin with a stoichiometry of 1:1. The 33000 +/- 1000 Da protein, accounting for >50% of the total seed protein, is an acidic glycoprotein exhibiting a very low endotype hydrolytic activity toward chitin derivatives. SDS-PAGE followed by densitometry of tamarind seed germination indicates the disappearance of the chitinase with the concomitant appearance of a cysteine endopeptidase. On the basis of its abundance, accumulation without any pathogenesis-related stimulus, temporal regulation, amino acid composition, and very low enzyme activity, this 34000 Da protein designated "tamarinin" physiologically serves as the major storage protein. 相似文献
11.
L Pouvreau H Gruppen S R Piersma L A van den Broek G A van Koningsveld A G Voragen 《Journal of agricultural and food chemistry》2001,49(6):2864-2874
Protease inhibitors from potato juice of cv. Elkana were purified and quantified. The protease inhibitors represent ca. 50% of the total soluble proteins in potato juice. The protease inhibitors were classified into seven different families: potato inhibitor I (PI-1), potato inhibitor II (PI-2), potato cysteine protease inhibitor (PCPI), potato aspartate protease inhibitor (PAPI), potato Kunitz-type protease inhibitor (PKPI), potato carboxypeptidase inhibitor (PCI), and "other serine protease inhibitors". The most abundant families were the PI-2 and PCPI families, representing 22 and 12% of all proteins in potato juice, respectively. Potato protease inhibitors show a broad spectrum of enzyme inhibition. All the families (except PCI) inhibited trypsin and/or chymotrypsin. PI-2 isoforms exhibit 82 and 50% of the total trypsin and chymotrypsin inhibiting activity, respectively. A strong variation within the latter activities was shown within one family and between protease inhibitor families. 相似文献
12.
不同时期追氮对冬小麦植株氮素积累及转运特性的影响 总被引：2，自引：2，他引：2
采用盆栽和大田相结合,并应用15N示踪技术,研究了不同时期追氮对两个不同穗型冬小麦品种植株氮素积累及转运特性的影响。结果表明,成熟期小麦植株各部位氮素积累量和分配比例均表现为子粒茎鞘+叶根系或颖壳+穗轴;子粒中氮素积累量以拔节期追氮处理最高,氮素在子粒中的分配比例以抽穗期追氮最高,在根系中的分配比例则以全部底施处理最高。小麦植株吸收追施15N的比例为16.45%～26.6%,兰考矮早八和豫麦49-198分别以返青期追氮和拔节期追氮吸收的比例最高;子粒中氮素来自15N的比例均以返青期追氮最高,分别为27.16%和22.20%,但和拔节期追氮处理差异不显著。随着追氮时期推迟,氮的花后同化量、花后贡献率增加,而花前贡献率呈下降趋势;全氮对子粒贡献率表现为花前转运的贡献大于花后同化的贡献,但抽穗期追氮处理中,15N对子粒的贡献率表现为花后同化率大于花前转运贡献率。综合考虑子粒产量、蛋白质含量以拔节期追氮较为合适。 相似文献
13.
Slow-binding inhibition of mushroom (Agaricus bisporus) tyrosinase isoforms by tropolone. 总被引：2，自引：0，他引：2
A kinetic study of the inhibition of mushroom tyrosinase by tropolone has been made. Three tyrosinase isoforms were used: two commercial tyrosinases from Fluka and Sigma (isoelectric points of 4. 3 and 4.1, respectively) and one purified isoform from mushroom strain U1 (isoelectric point of 4.5). Tropolone is a slow-binding inhibitor of these mushroom tyrosinase isoforms. Increasing tropolone concentrations provoked a progressive decrease in both the initial velocity and the final (inhibited) steady-state rate in the progress curves of product accumulation. A rapid formation of an enzyme-inhibitor complex, which further undergoes a slow reversible reaction, could take place since the inhibition of the different isoforms was partially reversed by the addition of CuSO(4). The kinetic parameters that described the inhibition by tropolone were evaluated by nonlinear regression fits. Incubation experiments of the different isoforms with tropolone demonstrated that this inhibitor only could bind to the "oxy" form of tyrosinase which justifies a mechanism previously proposed to explain the inhibition of tyrosinase by slow-binding inhibitors. 相似文献
14.
Characterization of two Acanthoscelides obtectus alpha-amylases and their inactivation by wheat inhibitors 总被引：1，自引：0，他引：1
Franco OL Melo FR Mendes PA Paes NS Yokoyama M Coutinho MV Bloch C Grossi-de-Sá MF 《Journal of agricultural and food chemistry》2005,53(5):1585-1590
Wheat alpha-amylase inhibitors represent an important tool in engineering crop plants against bean bruchids. Because Acanthoscelides obtectus is a devastating storage bean insect-pest, we attempted to purify and characterize its gut alpha-amylases, to study their interaction with active proteinaceous inhibitors. Two digestives alpha-amylases (AoA1 and AoA2) were purified from gut larvae, showing molecular masses of 30 and 45 kDa for each one, respectively. The stoichiometry interaction between these alpha-amylases with two wheat inhibitors (0.19 and 0.53) showed a binding complex of 1:1 enzyme:inhibitor. In vivo activities of these inhibitors against A. obtectus were also evaluated using a rich ammonium sulfate inhibitor fraction (F(20)(-)(40)) and purified inhibitors after reversed phase high-performance liquid chromatography columns. Incorporation of three different inhibitor concentrations (0.25, 0.5, and 1.0% w/w) into artificial seeds showed that addition of the purified 0.19 inhibitor at the highest concentration (1.0%) reduced the larval weight by 80%. Similar data were observed when 0.53 inhibitor was incorporated at 0.5%. When the concentration of purified 0.53 was enhanced to 1.0%, no larvae or adult emergence were observed. Our data suggest that these alpha-amylase inhibitors present great potential for use in Phaseolus genetic improvement programs. 相似文献
15.
Jabbar Fallahi Parviz Rezvani Moghaddam Mahdi Nassiri Mahallati Mohammad Ali Behdani Mahsa Aghhavani Shajari Mohammad Behzad Amiri 《Archives of Agronomy and Soil Science》2013,59(6):791-801
In order to study the effects of seed nitrogen content and biofertilizer priming on germination indices of wheat seeds under salinity stress, a factorial experiment based on a completely randomized design with four replications was conducted in 2009. Experimental factors consisted of: (1) the application of different nitrogen fertilizer rates (0, 55, 110 and 165 kg ha?1 N) on parent plants; (2) priming of achieved seeds by biofertilizers (Nitragin, Biophosphorus and distilled water); and (3) different levels of salinity produced by NaCl (0, ?0.4, ?0.8 and ?1.2 MPa). Germination percentage, germination rate, mean germination time, germination index, radicle and plumule length, radicle and plumule dry weight and radicle number per seedling were measured. Nitrogen application increased seed nitrogen content in parent plants. All germination indices decreased with increasing in salinity levels. Biofertilizer priming, especially Nitragin, had a positive effect on germination percentage, radicle number and radicle and plumule length in most salinity levels. The highest values for germination factors were related to achieved seeds from parent plants that were treated with 110 kg ha?1 N. Overall, application of middle levels of N fertilizer (55 and 110 kg ha?1 N) on parent plants combined with seed priming with Nitragin biofertilizer improved the germination indices of wheat under salinity stress. 相似文献
16.
Yunxian Mak Robert D. Willows Thomas H. Roberts Colin W. Wrigley Peter J. Sharp Les Copeland 《Cereal Chemistry》2009,86(3):281-289
Germination in the soft wheat (Triticum aestivum L.) cultivar Rosella was followed for three days after imbibition by proteomic analysis of the germ tissue. Two‐dimensional electrophoresis was performed in triplicate for proteins extracted from embryos dissected from mature grain and from grains germinated for 1, 2, and 3 days. For this period, 63 proteins (in 86 spots) in the germ were identified as decreasing in abundance, 35 proteins (in 60 spots) as increasing in abundance, and 28 proteins (in 39 spots) as exhibiting no significant abundance change. Proteins with significant abundance changes are discussed in relation to physiology; these include proteases, amylases and amylase inhibitors, enzymes in lipid metabolism, proteins related to water stress, cell wall hydrolases, oxalate oxidase, and H+‐ATPases. Functions associated with proteins synthesized during the germination period are inconsistent with the embryo of mature grain being fully primed for germination. 相似文献
17.
为进一步探明甘肃贝母种子后熟脱休眠机理,本研究对其种子不同后熟阶段水提取液不同质量浓度下小白菜种子的生物活性进行测定。结果表明,甘肃贝母种子水提取液对小白菜种子萌发及生长具有明显影响,影响程度随其所处后熟阶段的不同而异。在形态后熟阶段,与对照相比,水提取液浓度为2.5 mg·mL-1时,对小白菜种子发芽率和发芽势均有促进作用,但随着水提取液浓度的提高,小白菜种子萌发和幼苗生长均表现明显的化感抑制效应,化感效应指数RI值逐渐降低,当水提取液浓度为40 mg·mL-1时,化感抑制效应达到显著水平(P<0.05)。在生理后熟阶段表现出不同的化感效应,各处理组小白菜种子的发芽率均低于对照,但随着水提取液浓度的提高,化感效应呈现先增大后减小的趋势,当水提取液浓度为5 mg·mL-1时,小白菜苗高、根直径与单苗鲜重均较对照显著提高(P<0.05)。综上表明,甘肃贝母种子内源化感抑制物质在后熟不同阶段具有明显差异,从形态后熟转入生理后熟后,种子内源化感抑制物明显减少。本研究结果为进一步探究甘肃贝母种子中内源抑制物质的种类特性及生产实践中打破其种子休眠提供了科学依据和理论支撑。 相似文献
18.
Liyana-Pathirana C Dexter J Shahidi F 《Journal of agricultural and food chemistry》2006,54(17):6177-6184
The effects of pearling on the content of phenolics and antioxidant capacity of two Canadian wheat classes, namely, Canada Western Amber Durum; Triticum turgidum L. var. durum; CWAD) and Canada Western Red Spring; Triticum aestivum L.; CWRS) were examined. The antioxidant activity of wheat phenolics was evaluated using oxygen radical absorbance capacity (ORAC), inhibition of photochemiluminescence (PCL), Rancimat method, inhibition of oxidation of low-density lipoprotein, and DNA. The phenolic composition of wheat extracts was determined using high-performance liquid chromatography. The antioxidant capacity of both pearled grains and byproducts significantly decreased as the degree of pearling increased. Among grains, the unprocessed whole grains demonstrated the highest antioxidant capacity. The byproducts always demonstrated higher antioxidant capacity compared to the pearled grains, regardless of the wheat class. The resultant byproducts from 10-20% pearling possessed the highest antioxidant capacity. Processing of cereals may thus exert a significant effect on their antioxidant activity. The concentration of grain antioxidants is drastically reduced during the refining process. As phenolic compounds are concentrated in the outermost layers, the bran fractions resulting from pearling may be used as a natural source of antioxidants and as value-added products in the preparation of functional food ingredients or for enrichment of certain products. 相似文献
19.
Macías FA Simonet AM Pacheco PC Barrero AF Cabrera E Jiménez-González D 《Journal of agricultural and food chemistry》2000,48(7):3003-3007
A collection of 11 natural and synthetic podolactones have been tested as allelochemicals in a range between 10(-4) and 10(-9) M, and their potential use as natural herbicide models is discussed. Their effects on the germination and growth of the dicots Lactuca sativa (cv. Nigra and cv. Roman), Lepidium sativum, and Lycopersicon esculentum and the monocots Allium cepa, Hordeum vulgare, and Triticum aestivum as standard target species have been studied. An important inhibitory effect on the germination and growth of all tested species (average = 90%) was produced by compounds 9-11 at 10(-4) M. The specific structural requirements related to their activities are studied. On the basis of these results, their use as potential natural herbicide models is proposed. 相似文献
20.
Takahiro Ogura Michio Sunairi Mutsuyasu Nakajima 《Soil Science and Plant Nutrition》2013,59(1):217-227
The effects of 2-methylisoborneol (MIB) and geosmin, the main sources of soil odor produced by microorganisms, on plant seed germination were examined for 15 kinds of Brassicaceae seeds including radish (Raphanus sativus var. radicula). Authentic MIB or geosmin was used at the concentration of 100 mg L-1 in the assay, the effects were quantitatively indexed based on four parameters of cumulative seed germination: the final germination percentage, germination onset, weighted mean rate, germination index. MIB and geosmin inhibited germination of all kinds of seeds tested for these parameters. Radish variety “Comet” was the most affected of all of the tested plant seeds. The estimated 50% inhibitory concentration values on germination of “Comet” were 70.5 and 7.5 mg L-1 for MIB and geosmin, respectively. We also conducted the primary characterization of the inhibition by MIB and geosmin on “Comet” seeds. Our findings were as follows: (1) onset of seed imbibition was not affected; (2) germination of seeds pre-imbibed in the control solution until approx. 2 h before germination onset was inhibited by subsequent MIB or geosmin treatment; (3) growth of seedlings was not significantly affected; (4) the germination was restored by stratification, gibberellin A3 treatment, or seed coat removal; and (5) light irradiation affected the degree of inhibition. These results suggest that inhibition by MIB and geosmin is restricted to the germination process, at least in the early stage of growth, that they act as growth -regulating substances. 相似文献