High rates of extracellular superoxide production in bryophytes and lichens, and an oxidative burst in response to rehydration following desiccation

Summary The mechanism of extracellular superoxide radical (O 2 · − ) formation and the role of the oxidative burst in response to desiccation stress is reported here in bryophytes and lichens from habitats of contrasting water availability. Rates of extracellular production of O 2 · − radicals were measured, before and after desiccation stress, by the oxidation of epinephrine to adrenochrome, determined spectrophotometrically. Several desiccation‐sensitive lichens and bryophytes that grow in very wet microhabitats produced O 2 · − extracellularly at high rates, even when they were not stressed. In addition, some species showed a powerful burst of O 2 · − production during rehydration following desiccation. Production of high levels of O 2 · − and the existence of an inducible oxidative burst was best developed in cyanobacterial lichens, a hornwort and the two thalloid liverworts tested. Extracellular production of O 2 · − was almost absent from all mosses tested, a leafy liverwort, a filmy fern and from desiccation‐tolerant lichens. Patterns of O 2 · − production are discussed in terms of their possible role as a defence against pathogenic fungi and bacteria.