Mercury tolerance study in holm oak populations from the Almadén mining district (Spain) J. Rodríguez-Alonso*, M.J. Sierra, M.A. Lominchar, R. Millán* Departamento de Medio Ambiente, Centro de Investigaciones Científicas, Tecnológicas y Medioambientales (CIEMAT), Avenida Complutense 40, E-28040 Madrid, Spain A R T I C L E I N F O Article history: Received 28 June 2016 Received in revised form 4 October 2016 Accepted 7 October 2016 Available online 8 October 2016 Keywords: Mercury Holm oak Almadén Heritable trait Upper tolerance A B S T R A C T The Almadén area (Central Spain) has been the mining district with the largest mercury exploitation of the world since Roman times. In this place, the natural vegetation could have undergone heritable adaptations to mercury caused by the long exposure times to this toxic element. The aim of this work is to study if the main arboreal species of the district (Quercus ilex) has been able to develop a heritable tolerance to mercury. For this purpose, six holm oak populations (three from the Almadén mining district and three from an uncontaminated area) were selected. From these populations, hundreds of acorns were collected and then were sown and grown subjected to three mercury solutions (5, 25 and 50 mM Hg). The response to the treatments was evaluated in relation to germination rates, biomass, Hg accumulation and nutrient uptake. On the whole all populations behaved in the same way when subjected to the different treatments. Just one population showed a worse development, probably as a consequence of the negative quality site where the acorns were collected. None of the mercury treatments injured seriously to none of the seedlings, thus Q. ilex seems to be a species more tolerant to mercury than other woody species that have been studied previously (Moreno-Jiménez et al., 2007, 2009). ã 2016 Elsevier B.V. All rights reserved. 1. Introduction Mercury (Hg) is known to be one of the most toxic elements of the world. In fact, the World Health Organization considers mercury as one of the ten chemist elements more dangerous for the human health. For this reason, in EU the mercury exportation was forbidden since the 15th of March of 2011 (EC, 2008). However, mercury is not only a risk for the human health, it is also a global pollutant able to damage the environment, affecting both flora and fauna. With respect to the plants, many studies have related how the exposure to mercury can damage their development in many ways: reduction of seed germination (Gupta, 1991; Al-Helal, 1995), reduction of the coleoptile growth (Ling et al., 2010), production of a hypertrophy of the roots (Patra and Sharma, 2000), disturbance in photosynthesis (Israr et al., 2006), reduction of the seedling growth (Setia and Bala, 1994), reduction in pollen fertility even inhibition in the flowering (Kumar and Srivastava, 2012), produc- tion of disturbances in protein bio-degradation and energy metabolism (Wu et al., 2012), increase of oxidative stress and cell death (Ortega-Villasante et al., 2005), etc. Consequently, the presence of mercury can become a life- limiting factor to the development of the plants. However, plants can adapt to damaging agents as mercury. In general, there are three ways of adapting to life-limiting factors (Matesanz and Valladares, 2014): migration, fit via phenotypic plasticity and evolution. Evolution occurs when a given factor is limiting for life and reproduction of a given population. In the course of time, the best adapted plants take the place of the sensitive ones and the pressure of that hostile environment, generation after generation, is able to make up – thanks to heritable traits – new species, ecotypes or populations if the isolation conditions are suitable. An optimum environment to test whether the natural vegeta- tion has adapted via evolution to the presence of mercury is the Almadén mining district (Ciudad Real, Spain). This area is considered a geological anomaly in the world. There is no smallest place that contains so many deposits of mercury (Hall et al., 1997). Furthermore, the mercury exploitation in the Almadén district began in Roman Empire and has lasted until the beginning of XXI century (Higueras et al., 2006) what it implies centuries of extraction, manipulation and transport of mercury ore. Therefore, the presence of mercury is a historic feature of the Almadén mining * Corresponding authors. E-mail addresses: javier.rodriguez-alonso@ciemat.es, jaroalmel@hotmail.com (J. Rodríguez-Alonso), rocio.millan@ciemat.es (R. Millán). http://dx.doi.org/10.1016/j.envexpbot.2016.10.005 0098-8472/ã 2016 Elsevier B.V. All rights reserved. Environmental and Experimental Botany 133 (2017) 98–107 Contents lists available at ScienceDirect Environmental and Experimental Botany journal home page: www.elsevier.com/locat e/envex pbot