Senescence induced alteration in the electron transport in wheat leaf chloroplasts

Abstract

Aging in vivo of primary wheat leaves not only induces loss in chloroplast O₂ evolution capacity but also alters the accessibility of the electron transport chain for exogenous electron acceptors and donors. The pH profile of ferricyanide Hill reaction in the presence of uncoupler methylamine shifts to the acidic side, upon leaf aging, close to the optimum pH of oxidized para-phenylenediamine-mediated photosystem-II-catalysed electron transport activity. This suggests that ferricyanide, which normally accepts electrons at more than one site in the electron transport chain, accepts electrons preferentially at a site close to that of electron acceptance by oxidized para-phenylenediamine in aged leaf chloroplasts. Leaf aging enhances the extent of inhibition by dibromothymoquinone, which suggests changes in the acceptor side of photosystem II. Leaf aging also enhances the rate of the photosystem-I-catalysed electron transport reaction supported by reduced dichlorophenol-indophenol and reduced tetramethyl-para-phenylenediamine. Furthermore, the fact that inhibition by KCN of reduced dichlorophenol-indophenol supported photosystem I activity in aged leaf chloroplasts is greater than the activity supported by reduced tetramethyl-para-phenylenediamine suggests an alteration in the site of feeding of electrons by reduced dichlorophenol-indophenol. Thus leaf aging appears to induce alterations in specific segments of the electron transport chain.