Environmental and Experimental Botany 67 (2009) 260–268 Contents lists available at ScienceDirect Environmental and Experimental Botany journal homepage: www.elsevier.com/locate/envexpbot Responses of two populations of an Iranian nickel-hyperaccumulating serpentine plant, Alyssum inflatum Nyar., to substrate Ca/Mg quotient and nickel Rasoul Ghasemi, Seyed Majid Ghaderian ∗ Department of Biology, University of Isfahan, Isfahan 81746-73441, Iran article info Article history: Received 29 January 2009 Received in revised form 24 June 2009 Accepted 26 June 2009 Keywords: Alyssum Ca/Mg quotient Hyperaccumulation Nickel Serpentine soil abstract We selected two geographically close serpentine and non-serpentine populations of a Ni- hyperaccumulating plant (Alyssum inflatum) to investigate the influence of two common factors of serpentine soils: high Ni concentrations and low Ca/Mg quotients. Soils and plants were sampled from serpentine and non-serpentine substrates, and concentrations of Ca, Mg and Ni were measured. A hydro- ponic culture was used to compare growth and elemental composition responses of serpentine and non-serpentine plants to different Ca/Mg quotients and Ni concentrations in the nutrient solution. The Ca/Mg quotient for non-serpentine soils was 15 times higher than for serpentine soils, but there was no difference in the Ca/Mg quotient of plants from the two populations. In hydroponic culture, plants from both populations were able to survive at high Ca/Mg quotients. This result suggests that serpentine plants of A. inflatum do not necessarily need a substrate with a low Ca/Mg quotient for survival. Decreases in the Ca/Mg quotient in hydroponics decreased growth. The magnitude of this decrease was significantly greater in non-serpentine plants, suggesting a greater resistance of serpentine plants to low Ca/Mg quo- tients. Total Ni concentration in serpentine soils was 13 times higher than in non-serpentine soils, but ammonium nitrate-extractable concentrations of Ni in both soil types were similar. Ni concentrations in non-serpentine plants from their natural habitat were significantly lower than in serpentine plants, but there was no significant difference in Ni accumulation by plants of the two populations in hydroponic culture. However, increased concentrations of Ni in the hydroponic medium caused similar decreases in growth of both populations, indicating that Ni tolerance of the two populations was similar. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Ultramafic (serpentine) soils are globally ubiquitous but often distributed as small areas over a much larger geological province. Vegetation developed on serpentine soils is often sparse and patchy with low numbers of species and individuals (Brooks, 1998). Ser- pentine soils differ from other soils in that they are derived predominantly from ferromagnesian minerals and are hence rich in Fe and Mg and often Cr, Ni and Co, as well as being deficient in some macronutrient elements such as Ca, N, P and K (Brooks, 1987). In addition to their chemical properties, unfavourable physical and biological conditions such as shallow and undeveloped soil, low moisture-holding capacity and low organic material content in ser- pentine soils are other reasons for the sparse plant cover. However, there are no limiting factors common to all serpentine soils (Proctor and Nagy, 1992). The term ‘serpentine syndrome’ was introduced by Jenny (1980) to explain the effects of all the edaphic factors that determine the ∗ Corresponding author. Tel.: +98 311 7932464; fax: +98 311 7932456. E-mail address: ghaderian@sci.ui.ac.ir (S.M. Ghaderian). specialised floras that grow on serpentine soils (Brady et al., 2005). Among the limiting factors that make ultramafic soils unfavourable substrates for plant growth, the most attention has been given to low Ca/Mg quotients (due to low Ca and high Mg) and high heavy metal concentrations, especially Ni (reviewed by Brady et al., 2005; Kazakou et al., 2008). These two properties of serpentine soils not only are plant growth limiting factors (to which serpentine plants must have some adaptations), but also have been considered as important determinants of endemism of some serpentine plants (Pollard et al., 2002; Bradshaw, 2005; Brady et al., 2005; Grace et al., 2007). Indeed, one of the most interesting aspects of the plant biology of serpentine soils is the high degree of endemism in many serpentine floras (Jenny, 1980). This suggests that serpen- tine endemic plants have distinct requirements for some properties of serpentine soils. Although the full identities of the determining factors for endemism on serpentine soils are still to be explored, the most important hypotheses relate to Ca/Mg status and the presence of high concentrations of heavy metals (such as Ni) in serpentine soils. Bradshaw (2005) proposed that serpentine plants are Ca-sensitive and need a low Ca/Mg quotient in their substrate. Grace et al. (2007) ascertained that Ca/Mg quotient is a causal fac- tor in endemism of plants on serpentine soils. On the other hand, a 0098-8472/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.envexpbot.2009.06.016