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 CdCl 2 (0-200 M). Cadmium was cytochemically revealed with the method of physical development in leaves from the 0, 10 and 50 M 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.
Tree water relations and their dependence on microclimate and soil moisture were studied over several months in young oaks (Quercus robur L.) subjected in large lysimeter-based open top chambers to environments with a controlled soil water supply. Automated single point dendrometers and the recently developed leaf patch clamp pressure (LPCP) probe were used for monitoring water-related stem radius variations (ΔW) and turgor-dependent leaf patch pressures (Pp). Both parameters showed distinct diurnal patterns with sharp negative and positive peaking of ΔW and Pp, respectively, after solar noon and recovery to initial levels in the evening. During the day, varying solar radiation was responsible for short time fluctuations of Pp in the range of minutes to hours reflecting feedback regulation of leaf turgor by sunlight driven stomatal movements. At longer timescales, i.e. days to months, atmospheric vapour pressure deficit (VPD) and soil water content (SWC) were the main determinants of ΔW and Pp. Daily minimum and maximum values of ΔW and Pp decreased and increased, respectively, with increasing VPD or decreasing SWC and recovery of ΔW and Pp in the evening was impeded by low SWC. In well-watered oaks, daily positive peaking of Pp preceded daily negative peaking of ΔW; these time lags gradually increased with increasing soil drought, suggesting hydraulic uncoupling of stem and leaves.
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