Callose deposition in leaves of cowpea (Vigna unguiculata [L.] Walp.) as a sensitive response to high Mn supply

First macroscopic visible symptoms of Mn toxicity in cowpea (Vegna unguiculata [L.] Walp.) plants grown in solution culture were dark brown spots on the older leaves. Close to these spots, large quantities of substances which fluorescence with aniline blue were deposited, indicating formation of (1,3)-β-glucan (callose). Callose formation in the leaf epidermis was a more sensitive indicator of Mn toxicity than the appearance of macroscopic symptoms, or the Mn concentration in the leaf.