Zinc Toxicity and Xylem Vessel Wall Alterations in White Beans

Abstract

When white beans are exposed to excess zinc, reddish brown patches appear along the leaf veins. Ultrastructural observations of the xylem vessels in the discoloured zones show several modifications of the vessel walls including gelation of the pit membranes, coating of the lumen surface with an abnormal layer and deposition of electron-dense material in the secondary vessel walls. Histochemical studies indicate that the altered pit membranes and the coating layer stain positively for lipid, while the secondary wall deposits stain positively for phenolic compounds.