THE MECHANISM OF PHOTOACTIVATION OF O2 EVOLUTION IN DARK-GROWN CONIFEROUS LEAVES

Abstract

This chapter discusses the mechanism of photoactivation of O2 evolution in dark-grown coniferous leaves. Coniferous seedlings develop chloroplasts in darkness during germination. These chloroplasts have incomplete lamellar structure, although active photosystem-I and photosystem-II electron transport is established in their thylakoid membranes. Light activation of the O2-evolving system can also be accomplished in angiosperm leaves grown under intermittent light and in algae grown in darkness. Extraction of Mn from normal chloroplasts with Tris and hydroxylamine causes a specific inactivation of O2 evolution. Both light and Mn addition are required for the recovery of the O2-evolving activity. The activation of the latent O2-evolving system is accomplished by a multi-quantum process that involves, at least, two photoevents and a dark reaction between them. In a study discussed in the chapter, the photoactivation of the latent O2-evolving system was accompanied by a significant increase in the thiol content and oxidation of an endogenous reductant. The thiol content increased during illumination of the chloroplast suspension and saturated when the maximum photoactivation was achieved.