Localization and effects of cadmium in leaves of a cadmium-tolerant willow (Salix viminalis L.). II. Microlocalization and cellular effects of cadmium

Leaves largely control the phytoextraction of cadmium in terrestrial ecosystems but can be injured by cadmium toxicity. Cadmium sinks at the cell and tissue level vary between different species of plants. Cadmium has several still debated direct and indirect toxic effects on leaf physiology. In this study, the cadmium microlocalization and the associated structural changes were investigated in leaves of a tolerant clone of Salix viminalis to assess cadmium distribution, stress and tolerance. Rooted stem cuttings were exposed during 13 weeks in hydroponics to increasing concentrations of CdCl2 (0-200 μM). Cadmium was cytochemically revealed with the method of physical development in leaves from the 0, 10 and 50 μM treatment. The resulting cellular injuries and defence reactions were analyzed with several histochemical techniques using light and fluorescence microscopy. The main cadmium sink was in the pectin-rich layers of the collenchyma cell walls of the veins. Active storage was indicated by homogeneous cell wall thickenings with cellulose and proanthocyanidins. Cadmium microlocalization and cell injury in the conducting phloem indicated metal cycling. In the leaf blade, oxidative stress and accelerated cell senescence increased in those areas of the mesophyll with a low cadmium content. Local cadmium accumulation in veinlets near the leaf edges caused tannin plugging in xylem and necroses in the surrounding mesophyll and upper epidermis. When sinks approached saturation, random accumulation of cadmium appeared at sites in the leaf blade. Higher exposure to cadmium also enhanced the intensity of stress reactions. The role of different markers in metal binding and stress mitigation is discussed.