Introduction

Plant life, in all its diversity, is the result of the combination of up to 30 chemical elements (Ågren, 2008). The ultimate way in which such elements combine and give rise to the elemental composition of plants depends on processes of nutrient uptake, use, storage and translocation (Baxter, 2009). Although some of these processes are conserved across the phylogeny, many are species specific, leading to an ‘elemental fingerprint’ for each taxon (i.e. the ‘ionome’, sensu Lahner et al., 2003 or ‘elementome’ sensu Li et al., 2008). The elemental composition of plant taxa is recently gaining recognition as a fundamental concept in ecology, as it encompasses each taxon nutritional requirements, which are ultimately the result of its function and life strategy (Peñuelas et al., 2019). The concentrations of the different elements can be considered as axes of variation within a multidimensional space, encompassing functional information and shaping the ‘stochiometric niche’ (sensu González et al., 2017), or ‘biogeochemical niche’ (sensu Peñuelas et al., 2019). Identifying the different factors that determine species elemental composition is, therefore, a current key goal in ecology (Jeyasingh et al., 2017; Peñuelas et al., 2019).

There is ample evidence that evolutionary history is a strong determinant of plant elemental composition (Thompson et al., 1997; Broadley et al., 2004; Watanabe et al., 2007; Neugebauer et al., 2018). The phylogenetic signal seems to be generally stronger in macroelements than microelements, which are mostly determined by environmental factors (Zhao et al., 2016; de la Riva et al., 2018). Previous studies have evaluated phylogenetic effects on plant elemental composition by partitioning elemental variance across different taxonomic levels (i.e. order, family, genus, species; e.g. Broadley et al., 2003; Watanabe et al., 2007). For instance, Hao et al. (2015) detected stronger effects at the subfamily than at the genus level. While indicative of phylogenetic relatedness, taxonomic levels do not correspond to phylogenetic distances or divergence times (Magallón & Castillo, 2009; Massoni et al., 2015). Consequently, new analytical methods that partition elemental variability across the divergence time of species are required to have an integrated overview of phylogenetic effects on plant elemental composition.

The environmental conditions in which individual plants thrive have also an impact on their elemental composition (Han et al., 2011; Zhang et al., 2012; Sardans et al., 2016). For example, both mean annual precipitation (MAP) and mean annual temperature (MAT) correlate with plant elemental concentrations (Zhang et al., 2012; Sardans et al., 2016). Soil is considered one of the main drivers of plant elemental composition (Marschner, 2012). Contrastingly, most previous studies have found only a limited effect of soil properties on the elemental composition of plants growing in the wild (Thompson et al., 1997; Zhao et al., 2016). The resolution level of soil data included in these analyses was likely too coarse to account for meaningful plant–soil interactions (Thompson et al., 1997). Although sampling of soil adjacent to plant individuals included in elemental analyses may improve the evaluation of plant–soil interaction effects on plant elemental composition (Stein et al., 2017), studies incorporating this approach are still scarce (but please refer to Salmerón-Sánchez et al., 2014; Stein et al., 2017; Pillon et al., 2019).

Species adaptation to certain environmental conditions can also strongly alter the elemental composition of plants (Huang & Salt, 2016). This is particularly true for plants growing on atypical substrates, which have to cope with soil nutrient imbalances (van der Ent et al., 2018; Matinzadeh et al., 2019; Merlo et al., 2019). Some species tolerate atypical soils by excluding phytotoxic elements or nutrients found in excess to keep elemental homeostasis (van der Ent et al., 2018; Matinzadeh et al., 2019; Merlo et al., 2019). However, specialisation to atypical substrates often involves shifts in plant elemental composition (Verboom et al., 2017), frequently related to extreme accumulation of excess elements (Pillon et al., 2010; van der Ent et al., 2018; Merlo et al., 2019). For example, species adapted to serpentine soils are commonly metal hyperaccumulators, reaching markedly high concentrations of Ni, Zn, Cd, Co, Mn, Al or Pb depending on soil pH (Stein et al., 2017; van der Ent et al., 2018). Halophytes accumulate several orders of magnitude higher Na concentrations than co-occurring species (Matinzadeh et al., 2019). Similarly, some species exclusive to calcareous soils show markedly higher concentrations of Ca and Mg compared with neighbouring plants (Hao et al., 2015). The flora of atypical soils is, therefore, a perfect system to evaluate the relevance of ecological adaptations on the elemental composition of species. However, to our knowledge, no previous attempts have been made to estimate the relative contribution of such adaptations on the elemental composition of plants. Ascertaining if they are the consequence of certain pre-adaptations of soil specialists (implying a strong phylogenetic signal), or simply an expression of micro-evolutionary processes with low phylogenetic signal, remains a critical issue to unveil how soil specialisation evolved.

Extending over 100 million hectares worldwide, gypsum soils are amongst the most widespread atypical substrates of the world (Eswaran & Gong, 1991). They occur on all continents in areas where arid and semiarid conditions prevent gypsum from being leached (Eswaran & Gong, 1991). The high concentration of gypsum in soil leads to special physical and chemical conditions that pose serious restrictions to plant life, limiting the development of agriculture and conditioning the livelihood of millions of people (Verheye & Boyadgiev, 1997; Palacio & Escudero, 2014; Escudero et al., 2015). The moderate solubility of gypsum (c. 2.4 g l⁻¹) results in abnormally high Ca and sulphate concentrations, toxic for some plants (Ernst, 1998). Such high Ca and sulphate concentrations decrease nutrient availability in the soil due to the saturation of the soil solution with Ca²⁺ and sulphate ions (FAO, 1990), which leads to overall low nutrient retention ability (Casby-Horton et al., 2015). High soil Ca levels enhance sulphate uptake by plants, while a low N supply may impair the N : S balance for protein synthesis, further exacerbating excess sulphate accumulation in plants (Rennenberg, 1984). Similar to other atypical soils, the extremely restrictive conditions of gypsum soils contrast with the highly diversified flora they sustain, rich in endemic and specialised species (Mota et al., 2011; Musarella et al., 2018; Ochoterena et al., 2020), which is a conservation priority of international concern (European Community, 1992).

Depending on their affinity to gypsum, plants growing on gypsum soils are classified as gypsophiles (plants that grow only on gypsum soils) or gypsovags (plants that grow both on and off gypsum) (Meyer, 1986). Gypsophiles can further be segregated into: (1) wide gypsophiles, species occurring on most gypsum outcrops within a given region, which are considered specialised to gypsum and putatively belong to old gypsophilic lineages; and (2) narrow gypsophiles, locally distributed species that mostly belong to young gypsum lineages (Palacio et al., 2007; Muller et al., 2017). Previous studies on gypsum species from Spain, the Chihuahuan desert and Turkey indicate that widely distributed gypsophiles tend to show higher Ca, S and Mg foliar concentrations (elements found in excess in gypsum soils) than their neighbour gypsovags and narrow gysophiles (Duvigneaud & Denayer-De Smet, 1968; Palacio et al., 2007; Bolukbasi et al., 2016; Muller et al., 2017). For Chihuahuan desert plants, differences remained when phylogenetic effects were removed from the analyses (Muller et al., 2017). This ability could be related to plant ecological adaptation to gypsum soils (Palacio et al., 2014). However, studies specifically addressing the effect of evolutionary history on the elemental composition of gypsum plants are lacking.

The aims of this study were to:

1. Evaluate the relevance of phylogeny, affinity for gypsum soils and environmental factors (soil and climate) on the elemental composition of species from gypsum ecosystems in Iberia.
2. Assess the relationship between the ability to accumulate high foliar Ca, Mg and S concentrations and species specialisation to gypsum.

The following hypotheses were tested:

1. Phylogeny will be the most important factor explaining the variability on the elemental composition of plants from gypsum habitats in Iberia, but environmental and ecological (i.e. gypsum affinity) factors will also play a relevant role. In particular, we expect affinity to gypsum soils to explain an important proportion of the variability in S, Ca and Mg concentrations.
2. Phylogenetic effects will vary across the divergence time of taxa and trends will be different among elements.
3. Widely distributed Iberian gypsophyle species will accumulate more Ca, Mg and S in leaves than closely related gypsovags and narrow gypsophiles, independent of their phylogenetic origin.

We tested these hypotheses on a broad dataset including elemental concentrations of several key elements in 83 taxa. Our approach combined multivariate (including 11 elements), univariate (on 15 elements) and phylogenetic statistical tools to evaluate the effects on the elemental composition of plants as a whole and by each individual element separately. To this end, we used a new statistical procedure that allows evaluation of phylogenetic effects and their significance across the divergence time of taxa (multiple phylogenetic variance decomposition, MPVD).

中文翻译：

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最近和古代的进化事件塑造了土壤特有植物的元素组成：伊比利亚石膏植物的系统发育分析

 介绍

植物生命的多样性是多达 30 种化学元素结合的结果（Ågren， 2008 ）。这些元素结合并产生植物元素组成的最终方式取决于养分吸收、使用、储存和易位的过程（Baxter， 2009 ）。尽管其中一些过程在整个系统发育过程中是保守的，但许多过程是物种特异性的，导致每个分类单元的“元素指纹”（即“离子组”，意义拉纳等人， 2003年或“元素组”意义李等人， 2008 ）。植物类群的元素组成最近被认为是生态学的基本概念，因为它涵盖了每个类群的营养需求，而这些营养需求最终是其功能和生命策略的结果（Peñuelas等， 2019 ）。不同元素的浓度可以被视为多维空间内的变化轴，包含功能信息并塑造“化学计量生态位”（ sensu González等人， 2017 ）或“生物地球化学生态位”（ sensu Peñuelas等人， 2019 ）。因此，识别决定物种元素组成的不同因素是当前生态学的一个关键目标（Jeyasingh等， 2017 ；Peñuelas等， 2019 ）。

有充分的证据表明，进化史是植物元素组成的重要决定因素（Thompson等， 1997 ；Broadley等， 2004 ；Watanabe等， 2007 ；Neugebauer等， 2018 ）。宏观元素的系统发育信号似乎普遍强于微量元素，这主要是由环境因素决定的（Zhao等， 2016 ；de la Riva等， 2018 ）。先前的研究通过划分不同分类水平（即目、科、属、种；例如Broadley等人， 2003 ；Watanabe等人， 2007 ）的元素差异来评估对植物元素组成的系统发育影响。例如，郝等人。 （ 2015 ）发现亚科的影响比属水平的影响更强。虽然表明系统发育相关性，但分类水平并不对应于系统发育距离或分歧时间（Magallón & Castillo, 2009 ；Massoni et al ., 2015 ）。因此，需要新的分析方法来划分物种分化期间的元素变异性，以全面了解系统发育对植物元素组成的影响。

植物个体生长的环境条件也会对其元素组成产生影响（Han et al ., 2011 ；Zhang et al ., 2012 ；Sardans et al ., 2016 ）。例如，年平均降水量（MAP）和年平均温度（MAT）都与植物元素浓度相关（Zhang等， 2012 ；Sardans等， 2016 ）。土壤被认为是植物元素组成的主要驱动因素之一（Marschner， 2012 ）。相比之下，大多数先前的研究发现土壤性质对野生植物的元素组成影响有限（Thompson等， 1997 ；Zhao等， 2016 ）。这些分析中包含的土壤数据的分辨率水平可能太粗略，无法解释有意义的植物-土壤相互作用（Thompson等， 1997 ）。尽管元素分析中包含的植物个体附近的土壤采样可能会改善植物-土壤相互作用对植物元素组成影响的评估（Stein等， 2017 ），但采用这种方法的研究仍然很少（但请参阅Salmerón-Sánchez）等人， 2014 ；Stein等人， 2017 ；Pillon等人， 2019 ）。

物种对某些环境条件的适应也会强烈改变植物的元素组成（Huang & Salt, 2016 ）。对于生长在非典型基质上的植物来说尤其如此，它们必须应对土壤养分失衡（van der Ent等人， 2018 ；Matinzadeh等人， 2019 ；Merlo等人， 2019 ）。一些物种通过排除过量的植物毒性元素或养分以保持元素稳态来耐受非典型土壤（van der Ent等人， 2018 ；Matinzadeh等人， 2019 ；Merlo等人， 2019 ）。然而，对非典型基质的专门化通常涉及植物元素组成的变化（Verboom等， 2017 ），通常与过量元素的极端积累有关（Pillon等， 2010 ；van der Ent等， 2018 ；Merlo等） ., 2019 ).例如，适应蛇纹石土壤的物种通常是金属超富集植物，根据土壤 pH 值，镍、锌、镉、钴、锰、铝或铅的浓度显着升高（Stein等， 2017 ；van der Ent等， 2018 ）。盐生植物积累的钠浓度比共生物种高几个数量级（Matinzadeh等人， 2019 ）。 同样，与邻近植物相比，一些钙质土壤特有的物种显示出明显更高的钙和镁浓度（Hao等， 2015 ）。因此，非典型土壤植物区系是评估生态适应与物种元素组成相关性的完美系统。然而，据我们所知，以前没有尝试过估计这种适应对植物元素组成的相对贡献。确定它们是否是土壤专家某些预适应的结果（意味着强烈的系统发育信号），或者仅仅是具有低系统发育信号的微进化过程的表达，仍然是揭示土壤专业化如何进化的关键问题。

石膏土在全世界范围内延伸超过 1 亿公顷，是世界上分布最广泛的非典型基质之一（Eswaran 和Gong， 1991 ）。它们出现在各大洲干旱和半干旱条件阻止石膏浸出的地区（Eswaran 和Gong， 1991 ）。土壤中高浓度的石膏会导致特殊的物理和化学条件，严重限制植物的生命，限制农业的发展并影响数百万人的生计（Verheye & Boyadgiev, 1997 ; Palacio & Escudero, 2014 ; Escudero等）等， 2015 ）。石膏的中等溶解度（ c . 2.4 gl ^-1 ）导致异常高的钙和硫酸盐浓度，对某些植物有毒（Ernst， 1998 ）。由于土壤溶液中 Ca ²⁺和硫酸根离子饱和，如此高的 Ca 和硫酸盐浓度会降低土壤中的养分利用率（FAO， 1990 ），从而导致整体养分保留能力较低（Casby-Horton等人， 2015 ） ）。高土壤钙含量会增强植物对硫酸盐的吸收，而低氮供应可能会损害蛋白质合成的氮：硫平衡，进一步加剧植物中过量的硫酸盐积累（Rennenberg， 1984 ）。与其他非典型土壤类似，石膏土壤的极端限制性条件与其所维持的高度多样化的植物群形成鲜明对比，富含特有和特有的物种（Mota等人， 2011 年；Musarella等人， 2011 年）。, 2018 ; Ochoterena等人， 2020 ），这是国际关注的保护优先事项（欧洲共同体， 1992 ）。

根据它们与石膏的亲和力，在石膏土壤上生长的植物被分类为 gypphiles（仅在石膏土壤上生长的植物）或 gypsovags（在石膏上和石膏外生长的植物）（Meyer， 1986 ）。满洲星还可进一步分为：（1）宽满洲星，出现在给定区域内大多数石膏露头上的物种，它们被认为是石膏特有的，并且被认为属于古老的满洲星谱系； (2) 狭义满天星，局部分布的物种，大多属于年轻的石膏谱系（Palacio等， 2007 ；Muller等， 2017 ）。先前对来自西班牙、奇瓦瓦沙漠和土耳其的石膏物种的研究表明，广泛分布的石膏菌往往比邻近的石膏菌和狭窄的石膏菌表现出更高的钙、硫和镁叶面浓度（石膏土壤中发现的过量元素）（Duvigneaud & Denayer- De Smet， 1968 ；Palacio等， 2007 ；Bolukbasi等， 2016 ；Muller等， 2017 。对于奇瓦瓦沙漠植物，当从分析中去除系统发育效应时，差异仍然存在（Muller等， 2017 ）。这种能力可能与植物对石膏土壤的生态适应有关（Palacio et al ., 2014 ）。然而，缺乏专门解决进化历史对石膏植物元素组成影响的研究。

这项研究的目的是：

1. 
   评估系统发育、石膏土壤亲和力和环境因素（土壤和气候）与伊比利亚石膏生态系统物种元素组成的相关性。
2. 
   评估叶面积累高浓度 Ca、Mg 和 S 的能力与石膏物种特化之间的关系。

测试了以下假设：

1. 
   系统发育将是解释伊比利亚石膏生境植物元素组成变异性的最重要因素，但环境和生态（即石膏亲和力）因素也将发挥相关作用。特别是，我们预计与石膏土壤的亲和力可以解释硫、钙和镁浓度变化的重要部分。
2. 
   系统发育效应会随着类群的分化时间而变化，并且元素之间的趋势也会不同。
3. 
   广泛分布的伊比利亚满天星物种在叶子中比密切相关的满天星和狭义满天星在叶子中积累更多的钙、镁和硫，与它们的系统发育起源无关。

我们在广泛的数据集上测试了这些假设，其中包括 83 个类群中几个关键元素的元素浓度。我们的方法结合了多变量（包括 11 个元素）、单变量（15 个元素）和系统发育统计工具来评估植物整体元素组成和单独元素对植物元素组成的影响。为此，我们使用了一种新的统计程序，可以评估系统发育效应及其在类群分化时间内的显着性（多重系统发育方差分解，MPVD）。