HYDROGEN-PEROXIDE PRODUCTION BY ROOTS AND ITS STIMULATION BY EXOGENOUS NADH

H2O2 production by roots of young seedlings was monitored using a non- destructive in vivo assay at pH 5.0. A particularly high rate of H2O2 production was measured in the roots of soybean (Glycine max L. cv. La brador) seedlings which were used for further investigation of the phy siological and enzymological properties of apoplastic H2O2 production. In the soybean root H2O2 production can be stimulated 10-fold by exog enous NADH or NADPH. This response displays typical features of a pero xidase-catalyzed oxidase reaction using NAD(P)H as electron donor for the reduction of O-2 to H2O2. Comparative measurements showed that the NADH-induced H2O2 production of the roots resembles the H2O2-forming activity of horseradish peroxidase with respect to NADH and O-2 concen tration requirements and sensitivity to inhibition by KCN, NaN3, super oxide dismutase and catalase. NADH-induced H2O2 production can be obse rved with similar intensity in all regions of the root, in agreement w ith the distribution of apoplastic peroxidase activity. In contrast, t he activity responsible for the basal H2O2 production in the absence o f exogenous NADH was mainly confined to a short subapical zone of the root and differs from the NADH-induced reaction by insensitivity to in hibition by superoxide dismutase and a strikingly lower requirement fo r O-2. It is concluded that the basal H2O2 production of the root is m ediated by an enzyme different from peroxidase, possibly a plasma memb rane O-2(-)-producing oxidase.