Phylogenetic variation in heavy metal accumulation in angiosperms

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? New Phytologist (2001) 152: 9–https://www.360docs.net/doc/3c11740339.html, 9

https://www.360docs.net/doc/3c11740339.html, ? New Phytologist (2001) 152: 9–27

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10Materials and Methods

Selecting primary data

A recent angiosperm phylogeny (Soltis et al ., 1999) was used to partition variation in shoot metal content of six heavy metals: Cd, Cr, Cu, Ni, Pb and Zn. The primary data were obtained from 44 studies (Appendix 7) in which direct comparisons of shoot metal content were made in two or more angiosperm species. These studies did not include herbarium specimens or studies in which plants were grown in unde?ned substrates or under noncomparative conditions. For a particular metal, the studies conformed to the largest set in which at least one plant species was contained within another study, and in which no subsets of unlinked data arose. Different treatments within these studies were treated as independent studies.Generating comparative data using residual maximum likelihood analyses

For each metal, the original data from the 44 studies were log e -transformed and a residual maximum likelihood (REML)analysis was performed on these transformed data (Broadley et al ., 1999). These procedures adjusted for differences in

between-study variances and means, respectively, to generate the mean relative shoot metal content for each plant species from studies performed under different experimental conditions (Table 1). Hierarchical, nested ANOVA was used to partition variation in relative shoot metal content between the classi?cation levels of species, genus, family and order for each metal. All statistical analyses were performed using Genstat 5(Genstat 5 Committee, 1997).Testing comparative hypotheses

Hypotheses were tested that, independent of phylogeny,traits impacting on Cd, Cr, Cu, Ni, Pb and Zn contents of plant shoots are associated. Comparative data were obtained by deriving independent contrasts for relative shoot metal content, using the method of comparative analyses of independent contrasts (CAIC) (Purvis & Rambaut, 1995).This approach is based on the logic of comparing pairs of species (or higher nodes) within a phylogeny that share an immediate common ancestor. For each such pair of nodes,the difference, or contrast, between the relative shoot metal contents is calculated, providing an observation that is independent of the position of the nodes within the phylogeny, and independent of any evolutionary events

Fig. 1Mean relative shoot metal contents of angiosperm orders. Data were obtained from log e -transformed data from 44 studies (Appendix 7). These data were subject to residual maximum likelihood (REML) analyses and hierarchical ANOVA. Mean values are normalised for each metal with mean and standard deviation values for each metal presented prior to normalisation. The phylogeny is based on multi-gene sequence data (Soltis et al., 1999). The number of hyperaccumulator taxa in these orders are those listed at 1st January 2000 (Reeves & Baker, 2000; RD Reeves & AJ Baker, unpublished).

Research13 prior to that common ancestor. It is these independent con-

trasts that are then compared to assess whether evolutionary

changes impacting on the shoot content of different metals are

correlated. Analyses were performed on the log

means, derived from the REML analyses, and a punctuated

evolutionary model was assumed by setting equal branch

lengths within the phylogeny (Purvis & Rambaut, 1995).

T wo approaches were used to test comparative hypotheses.

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more than one metal. For example, the evolution of traits in?uencing the shoot content of Cr, Cu, Ni, Pb and Zn also impacts on shoot Cd content, and the evolution of traits in?uencing shoot content of Cd, Cr and Cu impacts on shoot Pb content.

The PCA allows the hypothesis that traits impacting on heavy metal accumulation in plant shoots are associated to be tested for all six metals simultaneously. As the ?rst two principal components accounted for between 73% and 80% of the variation in each of the six analyses, the correlations between the loadings in these ?rst two dimensions were assumed to estimate the true correlations, the remaining dimensions merely describing noise. The loadings on these ?rst two dimensions are presented graphically for each of the six analyses in Fig. 2. Correla-tions between metals, calculated from these loadings, are pre-sented in Box 2. These correlations indicate strong positive associations between Ni and Cr, Cu and Cd, Zn and Cd, and Zn and Cr and a lack of association between Pb and Ni, Cr or Zn.

Signi?cance of pair-wise regressions of contrasts of relative shoot metal concentration. The explanatory metal de?nes the phylogeny. Regressions were constrained through the origin to test the null hypotheses of no association between pairs of metals. Values

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https://www.360docs.net/doc/3c11740339.html,

Research15 Box 2Correlations between contrasts of shoot metal concentration of six heavy metals. Values calculated as cosines of the angles between the loadings from the six principal components analysis con?gurations (Fig.

phylogeny. Mean values shown in bold with upper and lower 95% con?dence limits

Cr

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of environmental variables was removed by the REML

By quantifying the in?uence of phylogeny on shoot metal

accumulation, the information reported here can be used to

improve predictions of soil-to-plant metal transfer. For

example, many soil-to-plant contaminant transfer models rely on

transfer coef?cients or transfer factors to relate the concentra-

tion of a contaminant in the soil to that in the shoot of a plant

Research17 a high K : Ca ratio, as exempli?ed by families in the Apiales

and Asterales.

The observation that the shoot contents of most metals

(except Pb) are correlated, could result from phylogenetic

in?uences on a nonspeci?c physiological trait such as the

ratios of organic/inorganic matter or tissue fresh weight/dry

weight. In future analyses it would be worthwhile therefore to

include data relating to potentially unlinked elements such as

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Annual Review of

Angiosperm phylogeny inferred

from multiple genes as a research tool for comparative biology. Nature

A comparative

study of leaf nutrient concentrations in a regional herbaceous ?ora. New

Cation channels in the plasma membrane of rye roots.

Research19 Table A1continued

Species

Euphorbia myrsinites

Festuca pratensis

Festuca rubra

Filago eriocephala

Filago vulgaris

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4, 16, 19, 21, 22, 28, 42

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Species

Minuartia ophiolitica

Nigella arvensis

Ononis spinosa

Petroselinum crispum

Picris hieracioides

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36

36

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2

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Research23 Table A4continued

Species

Centaurea rupestris

Cerastium ligusticum

Ceterach officinarum

Chamaedaphne calyculata

Cichorium intybus

Consolida orientalis

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Research25 Table A5continued

Species

Raphanus sativus

Sambucus ebulus

Scrophularia canina

Secale cereale

Silene gallica

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2

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5, 28, 38

1

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Research27 1997. Phytoextraction of cadmium and zinc from a contaminated soil.

Journal of Environmental Quality

10ElNawawy AS, ElBagouri IH, Al-Daher R, Khalafawi S. 1994.

growth and uptake of mineral elements at an oily sludge landfarming site

in Kuwait. Resources, Conservation & Recycling

11Gabbrielli R, Pandol?ni T, Vergano O, Palandri MR. 1990.

Comparison of two serpentine species with different nickel tolerance

strategies. Plant & Soil

12Gambi OV. 1992.