60] Energy-dependent reverse electron transport in chloroplasts

Abstract

This chapter describes the energy-dependent reverse electron transport in chloroplasts. ATP hydrolysis, which is latent in chloroplasts, can be activated by preillumination in the presence of dithiol reagents inducing proton uptake from the medium into the inner thylakoid space with the consequent creation of a transthylakoid pH gradient in the dark. This light-triggered ATPase further drives the reversal of electron transport toward photosystem. Reverse electron flow can also be driven by an artificially induced pH gradient. Both ATP and an artificially induced pH gradient have also been shown, under appropriate conditions, to induce luminescence from photosystem II. There is a basic difference between ATP and acid-base-driven reactions. The former are steady state reactions which are maintained as long as the ATPase is active, while the latter are transient reactions driven by a rapidly formed (and rapidly decaying) transthylakoidal pH gradient. This chapter describes the experimental techniques developed to measure ATP and acid-base driven reverse reactions including the reverse electron flow induced reduction of Q, the primary acceptor of photosystem II, and oxidation of cytochrome f. The chapter also elaborates the reverse electron flow induced luminescence.