Beschreibung:
<jats:p> <jats:bold>Rates of extracellular superoxide radical formation were estimated in the liverwort <jats:italic>Dumortiera hirsuta</jats:italic> (SW) Nees. Initial experiments showed that <jats:italic>D. hirsuta</jats:italic> produced extracellular superoxide at high rates, even when unstressed, and that production increased considerably during rehydration following mild desiccation stress. Experiments in which desiccation was artificially induced using polyethylene glycol showed that superoxide was produced during rehydration rather than desiccation. Incubation of plants in water induced the release about 23% of the superoxide‐producing activity, although most appeared to be tightly bound to the cell surface. Experiments with metabolic inhibitors indicated that superoxide production was insensitive to the flavoprotein inhibitor DPI, but inhibited by cyanide, suggesting that peroxidases may produce the superoxide. Despite producing large amounts of superoxide, <jats:italic>D. hirsuta</jats:italic> only produced small quantities of hydrogen peroxide, the natural product of superoxide dismutation. However, experiments showed that <jats:italic>D. hirsuta</jats:italic> can reduce the concentration of exogenously supplied hydrogen peroxide from 50 µ<jats:italic>M</jats:italic> to zero within 1 h, suggesting that any hydrogen peroxide produced is rapidly metabolized. The physiological significance of superoxide production is discussed, with special reference to its possible role as a defence against pathogenic fungi and bacteria.</jats:bold> </jats:p>