Effects of high zinc concentration on poplar leaves: A morphological and biochemical study

Plants growing under zinc excess store it in leaves, the organs in charge with carbon assimilation. Therefore, leaf modifications can help to understand the plant responses to environmental stresses. In this study, we tested the hypothesis that high zinc concentration can affect leaf anatomy and physiology of Populus alba cv. Villafranca, a tree species with interesting perspectives of application in phytoremediation. Plants were supplemented or not with 300 mg zinc per kg of soil and grown for six months. Green and macroscopically symptom-less leaves from poplars of each treatment were analyzed by light and electron microscopy (TEM. SEM-EDS) and by Raman spectroscopy. LHCII and D1-D2 protein expression were considered to evaluate PSII functioning. Zinc was histochemically detected in the leaves. Zinc strongly altered leaf morphology and ultrastructure, and negatively affected PSII activity decreasing D1 and D2, but not LHCII, protein expression. Zinc was mostly detected in the cell walls of the xylem and of the parenchyma cells surrounding the bundles of zinc treated plants, but not in the controls. A significant increase in the number of calcium oxalate crystals was observed in the leaves of zinc treated plants, but the latter metal was not incorporated in crystals, suggesting an increase of free calcium following zinc accumulation in the cell walls