Role of leaf apoplast in silicon-mediated manganese tolerance of Cucumis sativus L.

Silicon (Si) supplied as sodium silicate (1.8 mm) clearly decreased symptoms of manganese (Mn) toxicity in Cucumis sativus L. (cv. Chinesische Schlange) grown in nutrient solution with low to elevated Mn concentrations (0.5-1000 mum). Despite approximately the same total Mn content in the leaves, plants not treated with Si had higher Mn concentrations in the intercellular washing fluid (IWF) compared with plants treated with Si, especially in the BaCl2- and DTPA-exchangeable fraction of the leaf apoplast. The Mn concentration of the IWF correlated positively with the severity of Mn-toxicity symptoms and negatively with the Si supply. Furthermore, in Si-treated plants less Mn was located in the symplast (< 10%) and more Mn was bound to the cell wall (> 90%) compared with non-Si-treated plants (about 50% in each compartment). Manganese present in Si-treated plants is therefore less available and for this reason less toxic than in plants not treated with Si. It is concluded that Si-mediated tolerance of Mn in C. sativus is a consequence of stronger binding of Mn to cell walls and a lowering of Mn concentration within the symplast. These results support the role of Si as an important beneficial element in plant nutrition.