Hyperaccumulation of metals and metalloids is a rare phenomenon, currently known in only about 720 plant species; yet, it has a broad geographic and phylogenetic distribution (Reeves et al., 2017). Hyperaccumulators are defined on the basis of exceptionally high concentrations of a given metallic element in their foliar tissue, exceeding specified criteria that are typically 2-3 orders of magnitude higher than is generally found in most plants and at least one order of magnitude higher than in other plants growing on metalliferous soils (Reeves, 2003;van der Ent et al., 2013). These high concentrations are toxic to normal plants, but hyperaccumulators have sufficient metal tolerance to survive and to reproduce on metalliferous soils, and the majority of such species are restricted to these habitats (Pollard et al., 2014). Hypotheses that have been proposed to explain the evolution of hyperaccumulation (Boyd and Martens, 1992;Boyd, 2014) include tolerance or disposal of substrate metals, drought resistance, allelopathy, "inadvertent" accumulation of the metal via mechanisms for uptake of another element, and defense against herbivores and pathogens.Regulation of elemental uptake, including processes of exclusion, accumulation, and hyperaccumulation, has broad implications for plant biochemistry, physiology, ecology, and evolution. The extreme elemental concentrations found in hyperaccumulators have made them useful systems for study of nutrient acquisition, transport, and homeostasis (van der Ent et al., 2013), plant-herbivore interactions PREMISE: Hyperaccumulation of heavy metals in plants has never been documented from Central America or Mexico. Psychotria grandis, P. costivenia, and P. glomerata (Rubiaceae) have been reported to hyperaccumulate nickel in the Greater Antilles, but they also occur widely across the neotropics. The goals of this research were to investigate the geographic distribution of hyperaccumulation in these species and explore the phylogenetic distribution of hyperaccumulation in this clade by testing related species.METHODS: Portable x-ray fluorescence (XRF) spectroscopy was used to analyze 565 specimens representing eight species of Psychotria from the Missouri Botanical Garden herbarium. RESULTS:Nickel hyperaccumulation was found in specimens of Psychotria costivenia ranging from Mexico to Costa Rica and in specimens of P. grandis from Guatemala to Ecuador and Venezuela. Among related species, nickel hyperaccumulation is reported for the first time in P. lorenciana and P. papantlensis, but no evidence of hyperaccumulation was found in P. clivorum, P. flava, or P. pleuropoda. Previous reports of hyperaccumulation in P. glomerata appear to be erroneous, resulting from taxonomic synonymy and specimen misidentification.CONCLUSIONS: Hyperaccumulation of nickel by Psychotria is now known to occur widely from southern Mexico through Central America to northwestern South America, including some areas not known to have ultramafic soils. Novel aspects of this research include the successful predi...

In Mexico, ultramafic complexes are present in different regions from the northwest (Baja California Norte) to the southeast (Chiapas). In this paper, we present the results of the exploration of three ultramafic (serpentine) habitats in central and southern Mexico: Cuicatlán–Concepción Pápalo (Oaxaca), Tehuitzingo–Tecomatlán (Puebla), and San Juan de Otates (Guanajuato). Previous geology studies showed that these complexes are mainly made up of serpentinized peridotites. Soil analyses demonstrated typical ultramafic characteristics such as high content of Mg in relation to Ca, and high concentrations of Fe, Cr, Co, and Ni. Soil samples from Oaxaca and Puebla had similar Ni contents around 2300 mg kg−1, while samples of Guanajuato showed the lowest Ni levels with an average of 200 mg kg−1 as well as for other metals such as Co, Cr, Mn, and Zn. During this study, 83 plant specimens were collected, of which 52 were identified at genus level and 40 at species level. The collected plants belong to 19 different families such as Anacardiaceae, Fabaceae, Acanthaceae, Asteraceae, Sterculiaceae, and Verbenaceae which are also widely present in other ultramafic areas in Iran, Brazil, Sri Lanka, and Costa Rica. Only two Mexican endemic species are included in the collection. Ni hyperaccumulators were not detected at any of the studied sites. Therefore, hyperaccumulation, as a tolerance mechanism of the flora in response to ultramafic geochemical stress, does not seem to be developed in Central Mexico, as observed in the close Costa Rican site of Santa Elena.

kindly supplied a hand-held XRF instrument for herbarium scanning. This research was partly undertaken at P06 at at Deutsches Elektronen Synchrotron (DESY), a member of the Helmholtz Association (HGF). The research leading to this result has been also supported by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. The micro-XRF instrumentation used in this study was cofunded by ICEEL (Carnot Institute)-CREGU-LabEX Ressources 21 (ANR-10-LABX 21-LABEX RESSOURCES 21) and FEDER. Special acknowledgments are made to MSc Blanca V. Juarez Jaimes (MEXU herbarium) for plant identification, to MSc. Jorge A. Ramírez Espinosa, Ing. Francisco Navarrete Torralba and all the students from the Universidad Tecnológica de la Selva (UTS) for their valuable contribution during the field surveys and for their current investment in this research. The authors are also grateful to Haley Disinger for the enrichment of the database (XRF screening data from the MO herbarium in Saint Louis), to Vanessa Invernon for her support at the Paris herbarium (Muséum National d'Histoire Naturelle in Paris) and to all the technicians from MEXU herbarium for their valuable cooperation.We would like to thank Kathryn Spiers and Jan Garrevoet for their assistance during the experiments.We wish to thank Professor Alan Baker (The Universities of Melbourne and Queensland, Australia) for reviewing the paper and suggesting improvements to the text.