The biochemical and ultrastructural changes in "green islands" (GIs) on detached Avena sterilis leaves caused by the macrodiolide (8R,16R)-(-)-pyrenophorin in the dark were examined. In the absence of light, leaf segments retained their photosynthetic pigments for 96 h after treatment with (8R,16R)-(-)-pyrenophorin (70 muM), whereas in the untreated leaves complete senescence, loss of photosynthetic pigments and cell disorganization were observed 72 h after detachment. Proteolytic enzyme activity in treated tissues with pyrenophorin remained at low levels for 96 h after treatment and protein dissipation was lower in the treated than in the untreated. Although tissues in "GIs" seem macroscopically healthy, electron microscopy observations revealed structurally disorganized cells filled with granular, electron-dense material. Chloroplasts were severely damaged and contained a large number of plastoglobuli. Similar ultrastructural changes were also observed in A. sterilis tissues treated with the phytotoxin under illumination, indicating a mechanism operating both under illumination and in the dark.