Light-induced EPR signal I and signal II in wheat etioplasts greened under intermittent light without or with subsequent continuous light

Abstract

The behaviour of light-induced electron paramagnetic resonance (EPR) Signal I and Signal II in wheat plastids lacking O 2 evolving capacity due to development under intermittent light (IL plastids) is compared with that of plastids whose water-splitting system has been photoactivated by a brief subsequent continuous light (ICL plastids). EPR spectra in the absence or presence of DCMU as well as the observations of rise kinetics over a long time course clearly reveal that in ICL plastids Signal I (identified with P 700 +) is the result of competing processes that oxidize P 700 on the one hand and reduce it on the other. In IL plastids the high Signal I level indicates a high steady-state concentration of oxidized P 700 due to the absence of normal electron flow from Photosystem II (PS II). The three kinetic components (dark, slow-decaying and fast-decaying) of Signal II are observed both in IL and ICL plastids. Signal II is not chemically induced as Signal I by ferricyanide. Protease eliminates Signal II but leaves Signal I intact. Only the fast Signal II is abolished by DCMU and electron donors to PS II. It is assumed to reflect the oxidation state of a component functioning on the PS II oxidizing side.