Dehydroascorbate and traces of ascorbate were present apoplastically in living spruce (Picea abies) twigs. Since the proposed apoplastic ascorbate degradation pathway contains several steps that possibly generate H2O2, the effects of ascorbate and some of its degradation products were tested on apoplastic H2O2 concentrations in a cell culture of P. abies as a model and on non-enzymic H2O2 production in vitro. Ascorbate scavenged H2O2 in the culture medium of lignin-producing Picea cells and in spent and boiled spent medium; in the presence of Cu2+ or fresh medium, ascorbate led to the non-enzymic generation of H2O2. Preparations of dehydroascorbate (the initial oxidation-product of ascorbate), and diketogulonate (the hydrolysis-product of dehydroascorbate) induced H2O2 accumulation both non-enzymically and enzymically in Picea cell-suspensions. Paper electrophoresis showed that the dehydroascorbate and diketogulonate preparations contained several degradation products; some of these probably contributed to H2O2 production and/or scavenging in these experiments, and would also do so in vivo. These results indicate a complex ability of apoplastic ascorbate, dehydroascorbate, diketogulonate, and further products to modulate H2O2 concentrations, with potential consequences for the control of growth, development and lignification.