Physiological basis of differential zinc and copper tolerance of Verbascum populations from metal-contaminated and uncontaminated areas
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Metal contamination represents a strong selective pressure favoring tolerant genotypes and leading to differentiation between plant populations. We investigated the adaptive capacity of early-colonizer species of Verbascum recently exposed to Zn- and Cu-contaminated soils (10–20 years). Two Verbascum thapsus L. populations from uncontaminated sites (NMET1, NMET2), one V. thapsus from a zinc-contaminated site (MET1), and a Verbascum lychnitis population from an open-cast copper mine (MET2) were exposed to elevated Zn or Cu in hydroponic culture under glasshouse conditions. MET populations showed considerably higher tolerance to both Zn and Cu than NMET populations as assessed by measurements of growth and net photosynthesis, yet they accumulated higher tissue Zn concentrations in the shoot. Abscisic acid (ABA) concentration increased with Zn and Cu treatment in the NMET populations, which was correlated to stomatal closure, decrease of net photosynthesis, and nutritional imbalance, indicative of interference with xylem loading and divalent-cation homeostasis. At the cellular level, the sensitivity of NMET2 to Zn and Cu was reflected in significant metal-induced ROS accumulation and ion leakage from roots as well as strong induction of peroxidase activity (POD, EC 188.8.131.52), while Zn had no significant effect on ABA concentration and POD activity in MET1. Interestingly, MET2 had constitutively higher root ABA concentration and POD activity. We propose that ABA distribution between shoots and roots could represent an adaptive mechanism for maintaining low ABA levels and unaffected stomatal conductance. The results show that metal tolerance can occur in Verbascum populations after relatively short time of exposure to metal-contaminated soil, indicating their potential use for phytostabilization.
Key wordsAbscisic acid Copper Hydrogen peroxide Metal tolerance Verbascum lychnitis Verbascum thapsus Zinc
This research was supported by III43010 project funded by the Ministry of Education, Science and Technological Development, Republic of Serbia and OSI/FCO scholarship to F.M. The authors would like to gratefully thank Prof. Andrew Smith (Department of Plant Sciences, University of Oxford) for valuable suggestions and comments. Also, the authors would like to thank Dr. Mark Fricker for assistance with MATLAB, Dr. Markus Schwarzländer for help with microscopy, and Prof. John Pannell for loan of the LCA-4 portable gas analyzer.
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