BIOLOGIA PLANTARUM 55 (1): 187-190, 2011 187 BRIEF COMMUNICATION Effects of cadmium on gas exchange and phytohormone contents in citrus M.F. LÓPEZ-CLIMENT, V. ARBONA, R.M. PÉREZ-CLEMENTE and A. GÓMEZ-CADENAS* Departamento de Ciencias Agrarias y del Medio Natural, Universidad Jaume I, Campus Riu Sec, E-12071 Castellón, Spain Abstract The effect of increased Cd 2+ concentrations in the watering solution on citrus physiology was studied by using two citrus genotypes, Cleopatra mandarin and Carrizo citrange. Cadmium content in roots and leaves was tested together with measurements of leaf damage, gas exchange parameters, and hormonal contents. Citrus roots efficiently retained Cd 2+ avoiding its translocation to the shoots and Cleopatra mandarin translocated less Cd 2+ than Carrizo. With increasing Cd 2+ concentration all gas exchange parameters were decreased more in Carrizo than in Cleopatra mandarin. Cd-induced increases in abscisic acid and salicylic acid contents were observed in leaves but not in roots of both genotypes. Additional key words: abscisic acid, net photosynthetic rate, jasmonic acid, salicylic acid, stomatal conductance, transpiration rate. ⎯⎯⎯⎯ Cadmium ions are easily absorbed by plant roots. In some species Cd 2+ remains in roots (Rascio et al. 2008, Jiang et al. 2009) whereas in other species, such as tobacco, Cd 2+ is translocated to the leaves. Nevertheless, the Cd 2+ content is usually much higher in roots than in leaves (Wagner 1993). Grafting experiments have been performed in Nicotiana species indicating that the responsible mechanisms for limited translocation are located in the roots (Lugon-Moulin et al. 2004). In the case of citrus, scarce information can be found on the ability of plants to translocate Cd 2+ from roots to aerial parts. The symptoms of Cd 2+ toxicity are well documented in several herbaceous species (Di Toppi and Gabbrielli 1999, Jiang et al. 2009). Visible effects of exposure to the high metal doses are growth inhibition and leaf chlorosis. The photosynthetic apparatus, the water balance and the stomatal opening are also seriously disturbed (Rascio et al. 2008). Oxidative stress has often been discussed as a primary effect of Cd 2+ exposure (Markovska et al. 2009, Rodriguez-Serrano et al. 2009). Recently, it has been shown that microtubular cytoskeleton is one of the targets of Cd toxicity in root tip cells (Xu et al. 2009). Among responses to Cd 2+ , synthesis of phytochelatins has been extensively studied (Ben Ammar et al. 2008) and constitutes a specific plant response to metal toxicity. Another group of well-known responses is the up- regulation of stress proteins (Schutzendubel et al. 2001) or the accumulation of proline (Schat et al. 1997). Only a few studies have indirectly implicated the stress hormones jasmonate and ethylene in the transcriptional control of pathogen-related and heat shock proteins under Cd 2+ excess, probably to protect cells against damage induced by Cd 2+ treatment (Rodriguez-Serrano et al. 2009). Despite the lack of studies on the effect of Cd 2+ on citrus physiology, the response of this crop to other environmental stresses has been extensively studied. It is known that Cleopatra mandarin, a commercial rootstock, is able to restrict Cl - transport to the aerial part whereas leaves of Carrizo citrange (another widely used rootstock) becomes rapidly intoxicated in the presence of high concentrations of NaCl (López-Climent et al. 2008). In response to different environmental stresses a crucial role for abscisic acid (ABA) and jasmonic acid (JA) has been suggested (Arbona and Gómez-Cadenas 2008). ⎯⎯⎯⎯ Received 7 August 2009, accepted 26 April 2010. Abbreviations: ABA - abscisic acid; CC - Carrizo citrange; CM - Cleopatra mandarin; JA - jasmonic acid; SA - salicylic acid. Acknowledgements: This work was supported by the Spanish Ministerio de Ciencia e Innovación and the Generalitat Valenciana through grants No. AGL2007-65437-C04-03/AGR and ACOMP/2009/091, respectively. Hormone determinations were performed in the central facilities (Servei Central d’Instrumentació Científica, SCIC) of Universitat Jaume I. M.F. López-Climent was recipient of a fellowship from Universitat Jaume I. * Corresponding authors; fax: (+34) 964728216, e-mail: aurelio.gomez@uji.es