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JIP-test荧光数据及其它生理生态数据主成分综合分析(PCA)实例解析

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近年来,快速叶绿素荧光诱导动力学曲线(OJIP曲线)及其数据分析方法JIP-test在植物科学研究中的应用越来越广泛(BussottiF, et al., 2020; KalajiH M, et al., 2017; Pontes. D, 2019; Tsimilli-michael M, 2020)OJIP曲线可以更直观地表现出差异,JIP-test则提供丰富的参数,由于其测定方便简单,逐渐成为科研工作者们研究光合作用原初光化学反应的有力工具。

在植物科学实验中,测定的实验数据非常多,比如光合作用参数、植物生长指标、各种酶活性以及分子实验数据等,再加上JIP-test本身提供的几十种参数,丰富实验数据的同时,也会给后期的处理带来很大的工作量。因此,采用准确的数据处理分析方法尤其重要。
主成分分析法(PCA)是数据挖掘中常用的一种降维算法。所谓降维,就是把具有相关性的变量数目减少,用较少的变量来取代原先变量。在植物科学研究的实际应用中,各种参数相互之间会有影响,通过PCA分析处理后,会得到有限的几个主成分,由其代表实验参数就可以说明实验问题了,也就是所谓的降维(KalajiH M, et al., 2018;Goltsev V, et al., 2012)

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JIP-test提供丰富的参数,PCA进行数据降维处理,两者结合,能够快速处理并分析大量的实验数据,(i)揭示影响实验的主要参数,并可(ii)聚类不同处理之间的差异,也可用于(iii)大数据分析并预测植物生长变化下面通过三篇文章来详细介绍二者的结合应用。

1. 解析参数间的相关性,筛选出可禁用词汇解释问题的参数(Jurczyk B,2015)

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近年来,全球范围内短期内涝等自然灾害频发,并且随着北半球高纬度地区秋冬季降水量的增加,这种情况的出现可能会更加频繁。研究结果表明,淹水温度是影响植物对该胁迫反应的重要因素。该试验研究了耐寒性不同的四种高羊茅在低温下对土壤水分过剩的光合机构响应,旨在验证Rubisco活性改变引起的叶片水溶性碳水化合物浓度变化是否会影响土壤水分过剩条件下的光适应。

通过研究低温淹水对叶绿素 a 荧光参数、水溶性碳水化合物(WSC)Rubisco活化酶基因表达(RcaA)Rubisco活性(RA)的影响,并进行PCA主成分分析,以减少需要进行详细分析的参数数量,并筛选出能禁用词汇解释问题的参数。

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图1. 主成分分析的向量图,显示了低温对照和低温淹水被调查变量之间的相关性

淹水胁迫会直接导致植物水下组织供氧不足,缺氧后植物会加速使用碳储备进而导致碳源供应不足。主成分分析证实,由图1可以看出,淹水胁迫后,被测变量之间的关系发生显著性改变。在对照条件时,水溶性碳水化合物与能量传递效率相关参数(ETo/TRo、ETo/ABS、ETo/RC)有很高的正相关性,说明WSC的积累在对照条件下是不受限制的淹水后,WSC与能量耗散效率(DIo/CS、DIo/RC)呈正相关,说明能量转移的干扰可能限制了WSC的浓度
另一方面,WSC与描述单个活性反应中心效率的参数高度相关,揭示了类囊体膜可能也因淹水受到损伤。此外,qP和RcaA在对照植株中的表达之间的强相关性可能表明这两个性状的调控机制相似,可能与ADP/ATP比值有关。在淹水条件下,qP和RcaA的表达不相关,提示另一个因素可能调节RcaA转录水平。
总的来说低温淹水后,酶活性剧烈下降,光反应阶段吸收的光能过剩,维持较高的WSC含量能够激活光合作用适应寒冷的热耗散机制,有助于耗散掉过剩光能。
2. 聚类分析不同处理之间的差异(Zhiponova M, 2020)

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光是控制植物生长发育的主要因素。光不仅推动植物光合作用,光质和光照时间还驱动着植物主要的发育变化,如光形态发生、开花的光周期诱导、向光性、避荫以及防御等。为了评估光照条件对植物生理状态的影响,该研究在豌豆植株的早期发育过程中使用正常白光(W)、白色阴影(WS)、高光强蓝/红/远红组合光(BR)和低光强蓝/红/远红组合光(BRS)四种光照射,采用JIP-test来评估与光吸收和电子传输有关的PSII参数,并通过PCA技术聚类分析不同光照之间的差异。

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图2. JIP-test参数和不同处理的主成分分析(Plant variants: W – white light; WS – white light with shadow; BR – blue and red light; BRS – blue and red light with shadow)

对获得的JIP-test参数进行主成分分析表明,尽管不同处理之间存在重叠,但它们对光合机构的影响差异还是很容易区分的。
PC1根据PSII活性分离出不同的处理,较低的值表示更高的PSII性能(低光吸收、高光化学和电子传输效率);PC2则对应PSI活性,较高的值表明PSI性能较高。
W处理表现出PSI和PSII的禁用词汇综合性能;WS处理表现出PSI和PSII的禁用词汇综合性能;BR处理表现出受损的PSII和完整的PSI活性;BRS处理表现出低PSI和完整的PSII性能。
结合其他生理数据和主成分分析可以揭示光合作用与开花的关系。具有高PSII表现(-PC1)的W和BRS处理在其发育后期发育出相同数量的花,而具有抑制PSII活性(+PC1)的WS和BR植株发育后期没有开花。
研究结果表明,PIABS在PC1上最相关,可作为预测豌豆开花数量的最准确指标。
3. 通过PCA技术对大样本试验进行数据分析(Bussotti F, 2020)

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在大规模生态调查中,为了达成筛选目的和效率,一般使用有限的参数来对样本进行快速、简单的评价和生理分类。人们提出了许多形态、化学和生理指标来评价生态系统中的植物状况。其中,叶绿素的瞬时荧光分析(JIP-test)被认为特别适用于大型生态调查,并能在短时间内筛选出许多样品。

JIP-test提供了五十多个参数来评估植物光合机构的光化学性质和功能,这些参数可以描述光化学过程在能量吸收、俘获和电子传输方面的不同阶段。
该研究采用主成分分析法(PCA)对过去在野外条件(森林、人工林和牧场)和实验室条件中获得的43987个测量数据进行分析,目的是探讨JIP-test参数之间的关系,以选择最合适的参数来捕捉植物光合效率的变异性及其对逆境的响应。

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本研究中分析的最大数据集源自FunDivEUROPE项目(Functional Significance ofthe Forest Diversity in Europe, European Union, 7th FrameworkProgram)此项目中分析了整个欧洲的森林生态系统,从地中海到欧洲北部地区,涵盖了丰富的差异树种组成。其中包括天然高大森林(In Italy, Spain, Romania,Poland,Finland, Baeten et al., 2013)和人工林场(In Finland and Germany, seeVerheyen et al.,2016)

通过PCA技术分析发现,所选的JIP-test参数形成了三个很好分离的簇。其中两个位于PC1(Cluster 1&2)上,一个(Cluster 3)位于PC2上。每一组参数描述了不同的生理过程:光能捕获和光化学阶段(Cluster 1)、耗散(Cluster 2)和热阶段(Cluster 3)。基于PCA分析,该研究认为样品的整体光合性能可以用PITOT来表示,或者用Fv/Fm和ΔVIP共同来表示。

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图3. 所选JIP-test参数的主成分分析

在大多数情况下,植物的光合性能可用Fv/Fm和ΔVIP来描述。经过验证,Fv/Fm和ΔVIP能够有效地代表各种调查(野外和实验室)、气候和时间跨度、植物物种和功能群(针叶树和阔叶树物种、草本植物)中样本的变异性。因此,可用于探索性调查,以筛选大样本植物的光合性能,以及它们对不同生态条件的适应性。
在林业或生态学调查方面,以JIP-test为代表的植被叶绿素荧光特性的大规模野外调查对验证无人机或卫星遥感观测结果具有重要意义,遥感观测数据和野外实地调查数据之间的衔接将是今后生态学研究的一个重要领域(Bussotti F, 2020)
总述
以上实例说明,PCA分析与JIP-test结合应用越来越广泛,大大提高了数据分析效率,能够快速判断实验处理后的主要变化,并分析主要影响因素,从而对实验材料进行预判。近年来,PCA在植物科学研究中的应用呈上升趋势,相信科研工作者们会开发出更多更好的应用方向。

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如何实现对叶绿素a荧光数据(JIP-test参数)、其它生理参数和基因、蛋白等分子数据组成的大数据库进行PCA分析?

通常我们可以使用学术界常用的商用数据分析软件进行PCA分析,如SPSS Statistics(IBM Corp)、Statistica(StatSoft Inc. 2011)和SAS(SAS Enterprise Miner; SAS Institute, Cary, NC)等。

在全球互联网化的大趋势下,也涌现出一批使用体验更佳、分析更加智能化的在线数据分析工具,如SPSSAU(QingSi Technology Ltd 2016-2020)、ClustVis(Metsalu, Tauno et al. 2015)等。

此外以R语言和Python为代表的计算机程序设计语言可以实现对大数据的快速智能处理、计算和制图,使用R语言和Python对JIP-test荧光数据进行PCA数据分析也已有非常成熟的语言包进行应用。

下期文章我们将以IBM SPSS Statistics 26为例详细介绍JIP-test荧光参数PCA分析操作方法,敬请期待!

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