Loss of the trans-thylakoid proton gradient is an early event during photoinhibitory illumination of chloroplast preparations

Abstract

During photoinhibitory illumination of isolated thylakoids or intact chloroplasts from spinach, an initial increase of Photosystem I electron transport was observed while proton uptake associated with PMS-mediated cyclic electron transport was rapidly lost. The latter reaction was at least as light-sensitive as Photosystem II electron transport, normally considered to be the primary target for light stress. Thus under both moderate and extreme light stress, loss of the proton gradient associated with cyclic electron transport around Photosystem I was an early event. In accordance with this observation, photoinhibitory light very rapidly caused inactivation of cyclic photophosphorylation. There was no kinetic correlation between light-induced degradation of the D1 protein and collapse of the proton gradient. Notably, under anaerobic conditions when D1 protein degradation does not occur, loss of proton uptake still occurred. Low temperatures (3°C) provided partial protection against the photodamage, but a subsequent increase of the temperature to 25°C resulted in a total loss of the proton uptake in total darkness. The proton gradient could not be re-established by addition of DCCD. Moreover, there were no changes in the polypeptide composition of CF 1 or any impairment of the ATPase activity during photoinhibitory illumination. The mechanism of the light-induced loss of the proton gradient and its correlation to other effects of light stress at the molecular and physiological levels are discussed.