Physiology of manganese toxicity and tolerance in Vigna unguiculata (L.) Walp.

In cowpea (Vigna unguiculata (L.) Walp.) tolerance of manganese (Mn) excess depends on genotype, silicon (Si) nutrition, form of nitrogen (N) supply, and leaf age. The physiological mechanisms for improved Mn leaf-tissue tole rance are still poorly understood. On the basis of the density of brown spots per unit of leaf area and the ca llose content which are sensitive indicators of Mn toxicity, it was confirm ed that cultivar (cv.) TVu 1987 was more Mn-tolerant than cv. TVu 91, young leaves were more Mn-tolerant, Si improved Mn tolerance, and NO3--grown pla nts were more Mn-tolerant than NH4+-grown plants. A close positive relationship existed between the bulk-leaf Mn content and the vacuolar Mn concentration from the same leaves. Since no clear and cons istent differences existed between leaf tissues differing in Mn tolerance, the results suggest that accumulation of Mn in the vacuoles and its complex ation by organic anions do not play a role in Mn leaf-tissue tolerance in c owpea. A near linear relationship was found between leaf Mn contents and concentra tions of free (H2O-soluble) and exchangeable-bound (BaCl2-extractable) Mn i n the apoplastic washing fluid (AWF) extracted from whole leaves by an infi ltration and centrifugation technique. There were no differences in apoplas tic Mn concentrations owing to genotype and form of nitrogen nutrition. How ever, Si decreased the Mn concentration in the AWF. With increasing bulk-le af Mn contents, concentrations of organic anions in the AWF also increased. The results suggest that complexation of Mn by organic anions in the leaf apoplast contribute to Mn tolerance due to genotype and more clearly due to NO3-N nutrition. Cell wall-bound peroxidase activity increased with leaf age and was higher in the Mn-sensitive cv. TVu 91 than in cv. TVu 1987. This was in agreement with a higher H2O2 production rate in cv. TVu 91. Also, a lower ratio of re duced to oxidized ascorbic acid in the AWF revealed that in Mn-sensitive le af tissue, the apoplastic reduction capacity was lower than in Mn-tolerant leaf tissue when genotypes and leaves of different age were compared. We interpret our results as strong circumstantial evidence that Mn toleranc e depends on the control of the free Mn2+ concentration and of Mn2+-mediate d oxidation/reduction reactions in the leaf apoplast.