Electron transport in thylakoids isolated from barley leaves infected by the powdery mildew fungus ( Erysiphe graminis DC. ex Merat f.sp. hordei Marchal)

summary Barley leaves infected with powdery mildew fungus ( Erysiphe graminis DC. ex Merat f.sp. hordei Marchal) show an accelerated loss of chloroplast photochemical activities and electron transport components, compared to the losses which occur during senescence of non‐infected controls. Concentrations of electron earners and rates of light‐saturated photochemical activities were measured at selected times after inoculation, in an endeavour to identify rate‐limiting components of the electron transport chain in infected leaves. It has previously been demonstrated that the rate‐limiting step in non‐infected leaves is the transfer of electrons from PQ to the cyt f/b 6 complex. When compared on a fresh weight basis, the concentration of all electron carriers (P700, cyt f, cyt b 559 H.P., and the photoreducible pool of PQ), and all photochemical activities (H 2 O → PS2 → PS1, H 2 O → PS2, DONOR → PS1), decreased more rapidly in infected leaves, when compared with non‐inoculated controls of the same age, starting from 4 d after inoculation. When expressed on a chlorophyll basis, concentrations of the electron carriers and rates of the photochemical activities were identical for infected and control leaves of the same age. These data suggest that the rate‐limiting step of non‐cyclic electron transport is the same in both infected and control leaves (PQ→cyt b 6 /f complex). It is proposed that in infected leaves, superimposed on normal chloroplast senescence, there is an accelerated loss of electron carriers and associated photochemical activities resulting from the breakdown of whole regions of thylakoid membranes.