Can leaf apoplastic manganese and silicon concentrations explain Si-enhanced manganese tolerance of Vigna unguiculata (L.) Walp.?

Abstract

Effects of Si supply on Mn toxicity symptoms, Mn and Si concentrations in the leaf apoplast were studied in a Mn-sensitive cowpea cultivar (Vigna unguiculata (L.) Walp., cv TVu 91) in solution-culture experiments. When 1.44 mM Si was supplied concurrently with 50µM Mn, the Mn toxicity symptoms were completely avoided without decreasing the bulk-leaf Mn concentration. Si-treated plants exhibited lower Mn concentrations in the apoplastic washing fluids (AWF) and higher amounts of Mn adsorbed on the cell walls compared with plants treated with Mn in the absence of Si supply. On the other hand, distinct Mn toxicity symptoms were observed for the plants pretreated with 1.44 mM Si and then exposed to 50µM Mn without concurrent Si supply. The severity of Mn toxicity symptoms of these plants was not significantly correlated with the Mn concentration in AWF but was highly significantly correlated with the Si concentrations in AWF. These results suggest that Si supply alleviated Mn toxicity symptoms not only by the decrease of apoplastic Mn concentration but also by an interaction of soluble apoplastic Si with Mn phytotoxicity.