引言（又称前言或绪论）属于整篇论文的引论部分（开场白），介绍论文写作的背景、目的、主要研究成果及与前人工作的关系等，交代目前的研究热点、存在的问题及作者所做工作的意义，目的是引导读者进入论文的主题，让读者对论文中将要阐述的内容有心理准备。引言有总揽论文全局的重要性，是论文非常难写的部分之一。

引言内容的安排可以有较大的伸缩性，但基本内容应包括研究背景、存在的问题和研究目的等。通常先介绍范围较宽泛的一般性事实，为说明研究工作与过去工作的关系，需要回顾国内外研究历史（文献回顾或文献综述），并对研究情况作横向比较，写明前人在本课题相关领域所做的工作及存在的空白或不足。然后将重点逐渐转入与论文所探讨的问题有密切联系的主题，指出有某个问题或现象仍值得进一步研究，进而将焦点转到要探讨的研究问题上。最后阐述研究目的，将作者的研究任务具体化，还可根据情况说明作者在已有工作基础上的贡献或创新。对篇幅较长、结构复杂的论文，其引言的结尾部分还应有简略说明研究的主要结论及论文构架的内容。

对引言的篇幅无硬性的统一规定，应视论文篇幅及内容表达需要来确定，一般为200～600字，长的可以达到700～800字或1000字，甚至更多，短的可以不到100字，比较短的论文可以不单列“引言”一节，在论文正文前只写一小段文字即可起到引言的作用。

引言规范写作的基本要求是：内容全面，逐次展开；开门见山，不绕圈子；言简意赅，突出重点；尊重科学，实事求是。引言的规范写作应遵循以下原则：

1.引言写作要求和内容逐渐展开，不要将引言写成摘要的注释，不要重复摘要的内容，也不要在引言中展开讨论。

2.要视论文类别及研究领域来确定文献回顾的长度（通常至少需要一个或两个段落），如果研究主题为许多其他学者曾经探讨的问题，则应该引用、讨论较多的文献；相反，如果作者只是讨论别人最近才提出的问题，则只要引用、讨论少量的文献即可。

3.要区分所指出的“存在的问题”的类别，并采用相应的叙述方法。一般来说存在的问题主要有以下类别：（1）以前的学者尚未研究或处理不够完善的重要课题；（2）过去的研究衍生出的有待探讨的新问题；（3）以前的学者曾经提出的互不相容而且需要进一步研究才能解决的问题；（4）可以扩充到新的题目或领域中的过去的研究成果；（5）可以扩展到新的应用范围内的以前提出的方法或技术。

4.要慎重而又保留地叙述前人工作的欠缺及自己研究的创新。可以使用“限于条件”、“目前研究甚少”等谦虚用语，但不必对自己的研究或能力过谦。不宜用“才疏学浅”、“水平有限”、“恳求指教”和“抛砖引玉”等客套用语；也不要自吹自擂，抬高自己，贬低别人，除非是事实的情况下，一般不用“首次发现”、“首次提出”、“有很高的学术价值”、“填补了国内外空白”、“达到国际先进水平”等评价式用语。

5.要繁简适度地阐述研究背景，对探讨问题的本质和范围的阐述要准确、简洁、清楚，内容选择不能过于分散、琐碎，措词要精炼。由于读者一般已具备相关的专业基础知识，因此复述潜在读者早已明白的一般性知识不仅没有必要，而且容易使人厌烦，但对必要信息的叙述过于简略，则容易使读者感到突兀。通常不必将一般教科书中的已有知识写进引言。

6.要引用最相关的文献，优先引用相关研究的经典、重要和最具说服力的文献，力戒刻意回避引用最重要的相关文献，甚至是对作者研究工作具有某种重要“启示”性意义的文献，也不要不恰当地大量引用作者本人的文献。

7.要采取适当的方式强调作者在本次研究中的重要发现或贡献，让读者顺着逻辑的演进阅读全文，不要故意制造悬念。

8.要使用规范的名词、名称、术语和缩略语，不要随意使用非公知公用的术语和缩略语。非公知公用的术语和缩略语首次出现时应对其给予解释或定义，以帮助编辑、审稿专家及读者们阅读与理解。

9.要适当的使用“我们”或“作者”之类的名词，明确地指出作者所做的工作，以避免难以区分别人和作者所做的工作或引起误解。

例1(www.daowen.com)

Introduction:Agriculture in Iran is highly dependent on irrigation water，as about 70%of the agricultural products come from irrigated crops［1］.In Iran，the grass conserved as silage is the most noticeable source of winter forage available for feeding dairy cattle.Planting this crop after wheat and barley is common in some parts of the country（e.g.Isfahan，which is located in the Gavkhuni River Basin（GRB））［2］.Water supply uncertainty and financial constraints in arid regions are the two important issues that farmers are subjected to.These problems are especially severe in the case of summer crops，such as silage maize，grown during June to October and has high water requirement.Therefore，there is an increasing competition for water to obtain maximum production.The only way to keep supply and demand in balance in GRB is to reduce allocations to agriculture.Improving WP，based on more production per unit of water used in agriculture，is vital［3］.Gheysari et al.［4］focused on the response of silage maize to variable irrigation under arid and semi-arid conditions in Iran.Their results showed that the biomass of maize was increased as a function of the amount of applied water.Bekele and Tilahun［5］revealed that all deficit irrigations increased the water use efficiency of onion from a minimum of 6%by stressing the crop during the first growth stage to a maximum of 13%by partially stressing the crop at 75%ETc of the optimum application throughout the growing season.The main advantage of using a crop yield model is the capability of predicting crop yield in response to deficit irrigation levels so that field expenses can be saved to collect the experimental data.Many complicated growth models have focused on maize in water-scarce areas.Some of these models have not yet been tested under deficit irrigation in arid conditions.Some widely acceptable maize models are Hybrid model，CERES［6］，and DSSAT，which simulate the growth of maize crop under water-limited conditions［7］.Nearly all these models are complicated and require a large number of parameters.Cavero et al.［8］believes that the CROPWAT model should be used with caution due to maladaptation of simulated and observed evapotranspiration.

AquaCrop is a crop growth model，developed by FAO，that resulted from the revision of Irrigation and Drainage Paper No.33［9］by differentiating the ETa into non-benefitial soil evaporation（Ea）and transpiration（Tr）.Detailed description of the model is given by Steduto et al.［10］.One of the important key features of AquaCrop is that the simulation takes into account harvest index response to water stress.

To date，no study has been reported in the literature on simulation of deficit irrigation of silage maize with AquaCrop.Therefore，some of the previous researches about other crops are presented as follows.Farahani et al.［1 1］investigated the application of AquaCrop model for cotton under full and deficit irrigation regimes in Syria.They suggested that the key parameters for calibration must be tested under different climates，soils，cultivars，irrigation methods，and field managements.Hsiao et al.［12］showed that transpiration efficiency is well-simulated by AquaCrop for fully irrigated scenarios.Many researchers have applied the AquaCrop model to evaluate the effect of changes in the quantity of irrigation water for cotton，quinoa，corn，sunflower，cotton，and maize in Syria，Bolivia，Spain，Italy，Spain，and the United States.All this research showed that the AquaCrop is a good model for scenario analysis to improve WP［11-16］.

In the present study，we focused on deficit irrigation scenarios for silage maize，at Nekuabad irrigation network，GRB，Isfahan province，Iran.The objectives of this study were to determine the following:

（i）simulation of silage maize B-yield，

（ii）ETa simulation，

（iii）WP and local CWPF.

This paper also presents the calibration and validation results of AquaCrop model for the simulation of crop parameters.

例2

Introduction:Rainfed regions occupy over two-thirds of the global peanut production area.In these regions，drought is a major production constraint as rainfall is generally erratic and insufficient（Wright and Nageswara Rao 1994）.Even peanut grown under irrigation may experience drought because of limited water supply or because irrigation water is applied in amounts or at frequencies that are less than optimal for plant growth.Reduction in peanut yield resulting from drought has been well documented（Nageswara Rao et al.1989，Reddy et al.2003），and drought during the pod and seed forming stages has been shown to reduce pod yield of peanut by 56-85%（Nageswara Rao et al.1989）.Drought also increases the likelihood of aflatoxin contamination and can result in seeds that are not fit for consumption（Rachaputi et al.2002，Holbrook and Stalker 2003）.Improving water access and management is difficult practically as water is a scarce resource，so breeding for drought resistance has been an important strategy in alleviating the problem.Holbrook et al.（2000）demonstrated that breeding for drought resistance can also be an effective strategy for alleviating pre-harvest aflatoxin contamination.However，progress in breeding for drought resistance has been slow because of the complexity of the trait.A better understanding of the underlying mechanisms of drought resistance should accelerate the progress in breeding for this trait.

Drought resistance may be enhanced by improving the ability of the crop to extract water from the soil（Wright and Nageswara Rao 1994）.Deep rooting，root length density（RLD）and root distribution have been identified as drought adaptive traits（Passioura 1983，Turner 1986，Ludlow and Muchow 1990，Matsui and Singh 2003，Taiz and Zeiger 2006）that can be used as selection criteria for drought resistance.Variation among genotypes for shifting root distribution downwards in response to drought has been found in cowpea（Matsui and Singh 2003），white clover（Annicchiarico and Piano 2004）and chickpea（Yusuf Ali et al.2005，Benjamin and Nielsen 2006；Kashiwagi et al.2006）.In contrast，Benjamin and Nielsen（2006）found that water deficit did not affect root distribution in soya bean.

In peanut，reduction in root growth rate by water stress has been reported（Meisner and Karnok 1992）.Robertson et al.（1980）found no significant difference in rooting density of the peanut cultivar Florunner under irrigated and non-irrigated treatments.In contrast，Pandey et al.（1984）reported that drought increased RLD in the lower soil profile of a peanut genotype.Rucker et al.（1995）found that some peanut genotypes with large root systems under non-stress conditions gave high yield under drought conditions，and they suggested that these genotypes possessed drought avoidance traits.However，the direct assessment of deep rooting，RLD and root distribution of peanut genotypes under different water regimes to see how peanut genotypes respond to drought in term of these traits has not been clearly demonstrated.

In peanut，selection for drought resistance in the past has primarily been based on biomass production and pod yield under drought conditions.The mechanisms by which the resistant peanut genotypes achieve high yield under drought are not well understood.Information on the ability of these droughtresistant peanut genotypes to alter root distribution contributing to high yield under water stress might reveal the avoidance mechanism and could result in the development of improved breeding strategies for drought resistance in peanut.We have received eight drought-resistant peanut genotypes from the International Crops Research Institute for the Semi-Arid Tropics（ICRISAT）for use in our peanut breeding programme.These genotypes were selected based on high biomass and pod yield under drought conditions.We raised the following questions：（1）Do these drought-resistant genotypes differ in root distribution？（2）What are the behaviours of these genotypes for root characters in response to drought？And（3）what are the relationships between root characters in response to drought and yield？Therefore，the objectives of this work were to assess root distributions，variations in RLD and percentage of root distribution，and the relevance of root traits for yield of droughtresistant peanut genotypes under different available soil water levels.