Apoplastic peroxidases, ascorbate and lignification in relation to nitratesupply in wheat stem

The content of ascorbate in the apoplast and symplast and the effect of asc orbate on coniferyl alcohol oxidation by apoplastic soluble and wall-bound peroxidases in the stems of wheat (Triticum aestivum L.) grown at high (9.1 mmol/L) and low (1.2 mmol/L) nitrate concentrations were studied. Ascorbat e inhibited coniferyl alcohol net oxidation by apoplastic soluble peroxidas e completely. The duration of the ascorbate-caused lag phase in coniferyl a lcohol oxidation was proportional to ascorbate concentration and inversely proportional to coniferyl alcohol peroxidase activity. The oxidation of con iferyl alcohol by wall-bound peroxidase in the presence of ascorbate (5-80 mu mol/L) had no lag phase and oxidation rate decreased exponentially with increasing ascorbate concentration. The 7.5-fold higher nitrate level did n ot influence total ascorbate+dehydroascorbate concentration, bur ascorbate concentration and redox state were higher in the cells of the plants grown at nitrate excess. Only dehydroascorbate but no ascorbate was found in the apoplast of wheat stems. Coniferyl alcohol peroxidase activity was higher a nd dt hydroascorbate concentration was lower in the apoplast of plants grow n at a low nitrate level. Accordingly, conditions for lignification are mor e favourable in plants grown in nitrate deficiency compared with plants gro wn at a high nitrate level.