Light-induced changes in chlorophyll a fluorescence and cytochrome f in intact spinach chloroplasts: The site of light-dependent regulation of electron transport

Abstract

Dark-adapted intact spinach chloroplasts exhibited two peaks, P and M₁, at the early phase of fluorescence induction and a transient reduction of cytochrome f shortly after its initial photooxidation and in parallel to the appearance of P. Analysis of the peak P and the transient reduction of cytochrome f indicated that electron transport in intact spinach chloroplasts was regulated by light: electron transport was inactivated at the reducing side of photosystem I in the dark-adapted chloroplasts but rapidly reactivated by illumination. The fluorescence peak M₁ was correlated to the proton gradient formed across the thylakoid membrane. Effects on P and transient reduction of cytochromef of NO₂⁻, 3-phosphoglycerate (PGA) and oxalacetate (OAA), which can penetrate into intact chloroplasts and accept electrons at different sites after photosystem I, were studied to determine the site of the light regulation. NC₂⁻, which receives electrons from reduced ferredoxin, markedly diminished both P and the transient reduction of cytochrome.f, whereas PGA and OAA, the reductions of which are NADP-dependent, failed to affect the two transients. The ineffectiveness of PGA and OAA could not be attributed to the dark inactivation of glyceraldehyde-3-phosphate and malic dehydrogenases, because dark-adapted chloroplasts still retained sufficiently high levels of the enzyme activities. The results indicate that electron transport in intact spinach chloroplasts is regulated by light after ferredoxin but before NADP, i.e., at the reducing terminal of the electron transport chain.