Elongation of onion (Allium cepa L.) roots was highly stimulated by ascorbate (ASC) and its natural precursor I-galactone-y-lactone (CL). When incubation media were supplemented with lycorine (Lyc), an inhibitor of the ASC biosynthesis, root growth was negligible even in the presence of ASC or CL. ASC completely inhibited in vitro guaiacol peroxidase activities that were isolated from both the apoplast and the cell wall. However, ferulic-acid-dependent peroxidase from the cell wall was partially inhibited by ASC, whereas ferulic acid peroxidase activity from the apoplastic fluid was completely inhibited by ASC as long as ASC was present in the assay medium. ASC content in cells was increased by preincubations with ASC or CL, whereas Lyc reduced it. On the other hand, ASC or CL treatments decreased both apoplast and cell-wall-bound peroxidase activities, whereas Lyc had a slight stimulating effect. These results are discussed on the basis of a possible control of root elongation by ASC via its action on peroxidases that are involved in the regulation of cell-wall extensibility.ASC and its free radical AFR stimulate the elongation of onion (Allium cepa L.) roots in parallel to a high vacuolization of meristematic cells (Hidalgo et al., 1989;Navas, 1991;González-Reyes et al., 1994b). ASC is required for the progression of the G1 and G2 phases of the cell cycle (Liso et al., 1984). Current explanations for these observations include (a) the hyperpolarization of the plasma membrane and acidification of the apoplast induced by ASC and AFR (González-Reyes et al., 1992) after the activation of a plasmalemma NADH-AFR oxidoreductase activity that stimulates nutrient intake (Gonzalez-Reyes et al., 1994b), and (b) the maintenance, in a reduced state of Hyp-rich proteins, that is needed for the progression of the cell cycle (Arrigoni et al., 1977;De Gara et al., 1991a). Two recent reviews analyze these hypotheses in detail (Arrigoni, 1994; Cór-doba and González-Reyes, 1994).Root growth and elongation leads to an irreversible increase in cell volume. Thus, the cell-wall architecture is modified in such a way that a relaxation of the crossed bonds that link severa1 components of the cell wall must 95-0560).occur. Cell-wall loosening allows the increase of the cell surface and the intake of water by the protoplast, and changes in the cell-wall architecture during cell elongation have been recently reported (McCann and Roberts, 1994). Cell-wall-bound and/ or apoplastic peroxidases have been involved in the formation of isodityrosine bonds between glycoproteins as extensin, or diferulate bridges between polysaccharide polymers (Biggs and Fry, 1987). These peroxidase-driven cross-linking reactions of cellwall polymers stiffened the cell wall during growth, thus reducing the rate of elongation (Fry, 1986). To comprehend the regulation of cell-wall extensibility during growth, focus must be placed on the biological control of peroxidase activities at the cell wall. This includes the synthesis and secretion of peroxidases, the supp...