Cadmium and H 2 O 2 -induced oxidative stress in Populus × canescens roots Andres Schützendübel a , Petia Nikolova a,b , Claudia Rudolf a , Andrea Polle a, * a Forstbotanisches Institut, Abteilung I: Forstbotanik und Baumphysiologie, Georg-August Universität Göttingen, Büsgenweg 2, 37077 Göttingen, Germany b Present address: University of Forestry, 10 Kliment Ochridski blvd., 1756 Sofia, Bulgaria Received 9 January 2002; accepted 13 March 2002 Abstract Clonal, hydroponically grown poplar plants (Populus × canescens, a hybrid of Populus tremula × Populus alba) were exposed to Cd or H 2 O 2 to find out whether Cd-induced injury was related to the disturbance of the cellular redox control in root tips. Cd exposure resulted in an inhibition of antioxidative enzymes (superoxide dismutase, EC 1.15.1.1; catalase, EC 1.11.1.6; ascorbate peroxidase, EC 1.11.1.11; monodehydroascorbate radical reductase, EC 1.1.5.4; glutathione reductase, EC 1.6.4.2) but had fewer effects on dehydroascorbate reductase (EC 1.8.5.1) activities. Glutathione concentrations decreased, whereas ascorbate remained unaffected by Cd. Five micromoles of Cd were subinjurious in short-term experiments and stimulated root growth. Fifty micromoles of Cd retarded shoot growth faster than root growth, caused a more severe loss in antioxidative capacity than 5 μM Cd and resulted in an accumulation of H 2 O 2 in roots. Exposure to H 2 O 2 had an effect on antioxidative enzymes similar to that found under the influence of Cd, but caused GSH accumulation, and loss of ascorbate. The present data indicate that both agents acted via the disturbance of the cellular redox control. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Antioxidative enzymes; Heavy metal stress; Hydrogen peroxide; Oxidative stress; Populus; Root physiology 1. Introduction Cadmium (Cd) is highly toxic to animals and plants. In plants, exposure to Cd causes reductions in photosynthesis, water and nutrient uptake [29]. As a consequence Cd- exposed plants show various symptoms of injury such as chlorosis, growth inhibition, browning of root tips, and finally death [15]. Cd pollution of soils is of growing concern because Cd and other heavy metals are emitted by traffic, metalworking industries, mining, and as a by- product of mineral fertilisers [22]. In addition to anthropo- genic emissions, Cd is also released into the environment by natural sources such as volcanoes and continental dusts or by weathering of the underlying bedrock. Since the presence of Cd or other heavy metals prevents the development of a normal vegetation cover, biotechnological efforts are under- way to develop more stress-tolerant species. For this pur- pose, it is important to understand the mechanisms of Cd toxicity and tolerance in plants. It is well known that exposure of plants to Cd induces the synthesis of phytochelatins [27,39]. These peptides bind heavy metals in the cytosol and sequester them in the vacuole [18,27]. The tripeptide glutathione (γ- glutamylcysteinylglycine) is thought to play a central role in the Cd detoxification because it is the precursor for the synthesis of phytochelatins. The degree of Cd tolerance was correlated with inherent glutathione levels and the cellular capacity to synthesise thiol compounds [14,35,38,40,41]. In hybrid poplar (P. × canescens, a hybrid of P. tremula × alba) with modified GSH concentrations, increased Cd accumu- lation in leaves but no ameliorating effects on shoot growth were found in the presence of Cd [3]. Upon exposure to Cd, most species initially show a decrease in GSH, probably because of an enhanced demand Abbreviations: APX, ascorbate peroxidase; CAT, catalase; DAR, dehy- droascorbate reductase; GR, glutathione reductase; GSH, reduced gluta- thione; GSSG, glutathione disulphide; MDAR, monodehydroascorbate radical reductase; SOD, superoxide dismutase * Corresponding author. E-mail address: apolle@gwdg.de (A. Polle). Plant Physiol. Biochem. 40 (2002) 577–584 www.elsevier.com/locate/plaphy © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 0 9 8 1 - 9 4 2 8 ( 0 2 ) 0 1 4 1 1 - 0