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

. The mechanism of extracellular superoxide radical (O-2(.-),) formation an d the role of the oxidative burst in response to desiccation stress is repo rted here in bryophytes and lichens from habitats of contrasting water avai lability. . . Rates of extracellular production of O-2(.-) radicals were measured, befo re and after desiccation stress, by the oxidation of epinephrine to adrenoc hrome, determined spectrophotometrically. . Several desiccation-sensitive lichens and bryophytes that grow in very we t microhabitats produced O-2(.-) extracellularly at high rates, even when t hey 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 w as best developed in cyanobacterial lichens, a hornwort and the two thalloi d liverworts tested. Extracellular production of O-2(.-) was almost absent from all mosses tested, a leafy liverwort, a filmy fern and from desiccatio n-tolerant lichens. . Patterns of O-2(.-) production are discussed in terms of their possible r ole as a defence against pathogenic fungi and bacteria.