RESEARCH PAPER Spatial distribution of cadmium in leaves and its impact on photosynthesis: examples of different strategies in willow and poplar clones F. Pietrini 1 , M. Zacchini 1 , V. Iori 1 , L. Pietrosanti 1 , M. Ferretti 2 & A. Massacci 1 1 Institute of Agro-Environment and Forest Biology of the National Research Council, Monterotondo Scalo (Roma), Italy 2 Institute for Technologies Applied to Cultural Heritage of the National Research Council, Monterotondo Scalo (Roma), Italy INTRODUCTION Cadmium (Cd) is relatively mobile in plants and inhibits various metabolic and physiological processes when trans- ported in large quantities into leaves (reviewed by Sanita ` di Toppi & Gabbrielli 1999; Joshi & Mohanty 2004; Ku ¨p- per & Kroneck 2005). Above a threshold value of accu- mulation (Pietrini et al. 2005), photosynthesis becomes sensitive to Cd as it affects CO 2 diffusion through sto- mata and CO 2 fixation (Becerril et al. 1988; Pietrini et al. 2003). The Cd ion binds easily to sulphur, oxygen and nitrogen elements of functional groups of many enzymes and structural proteins or hydrocarbons, thus impairing photosynthetic activity. Consequently, it is imperative for plants to minimize Cd translocation into leaves and its movement inside leaf cells. Some plants that tolerate Cd in micromolar concentrations in leaves counteract Cd binding by stabilising this metal in complexes with phyto- chelatins or reduced glutathione, organic acids or amino compounds. Some of these complexes are thermodynami- cally rather stable and minimize Cd interactions with photosynthetic components, while also contributing to Cd transport and sequestration in cell vacuoles (Clemens 2001; Hall 2002; Pietrini et al. 2005; Callahan et al. 2006; Vollenweider et al. 2006). The latter act as a physiological detoxification mechanism in leaf and root cells that allows the plant to accumulate relatively high amounts of Cd. This mechanism has a limited capacity, depending on Keywords Energy-dispersive X-ray fluorescence; image fluorescence; phytochelatins; Populus; Salix. Correspondence A. Massacci, Institute of Agro-Environment and Forest Biology of the National Research Council, Via Salaria Km 29,300, 00015 Monterotondo Scalo (Roma), Italy. E-mail: angelo.massacci@ibaf.cnr.it Editor F. Loreto Received: 11 May 2009; Accepted: 3 August 2009 doi:10.1111/j.1438-8677.2009.00258.x ABSTRACT The interaction of cadmium (Cd) with photosynthesis was investigated in poplar (Populus x canadensis Mo ¨nch., clone A4A, Populus nigra L., clone Poli) and willow (Salix alba L., clone SS5) clones that had different leaf metal concentrations in preliminary experiments. Plants grown in the pres- ence of 50 lm CdSO 4 for 3 weeks under hydroponic conditions were used to examine leaf gas exchange, chlorophyll fluorescence parameters and images, and for Cd detection using energy dispersive X-ray fluorescence (ED-XRF). Leaves were finally analysed for Cd and phytochelatin concentra- tions. Results showed that SS5 had the highest leaf Cd concentration and high gas exchange activity similar to that of Poli, which had the lowest Cd concentration. Leaf fluorescence images evidenced in large undamaged areas of SS5 corresponded to high values of F v ⁄ F m ,F o , FPSII, qP and NPQ, while patches of dark colour (visible necrosis) close to the main vein corresponded to low values of these parameters. In A4A, these necrotic patches were more diffuse on the leaf blade and associated with a range of fluorescence parame- ter values. ED-XRF analysis indicated that Cd was only detectable in necro- ses of SS5 leaves, while in A4A it was relatively more diffuse. Phytochelatins (PCs) were not detected in SS5, while their concentration was high in both Poli and A4A. The absence of these molecules in SS5 is thought to favour confinement of high accumulations of Cd to necrotic areas and gives SS5 the ability to maintain high photosynthesis and transpiration in remaining parts of the leaf. Plant Biology ISSN 1435-8603 Plant Biology 12 (2010) 355–363 ª 2009 German Botanical Society and The Royal Botanical Society of the Netherlands 355