Hg2+, Cu2+, and Pb2+-induced changes in Photosystem II photochemical yieldand energy storage in isolated thylakoid membranes: A study using simultaneous fluorescence and photoacoustic measurements

Simultaneous fluorescence and photoacoustic measurements have been used to study the effects of metal ions (copper, lead, and mercury) during dark inc ubation of thylakoid membranes. The values of the chlorophyll fluorescence parameters Fo (initial fluorescence yield with the reaction centers in the open state), Fm (maximal fluorescence yield), Ft (steady state fluorescence yield) and the calculated parameters, Phi(o) (maximal quantum yield of Pho tosystem II photochemistry) and Phi(t) (actual quantum yield of Photosystem II photochemistry), strongly decreased in the presence of the metal ions c oinciding with an increase in the non-photochemical deexcitation rate const ant k(N). It was observed that photosynthetic energy storage measured by ph otoacoustic spectroscopy also decreased but a large portion of energy stora ge remained unaffected even at the highest metal ion concentrations used. A maximal inhibition of photosynthetic energy storage of 80% and 50% was obt ained with Hg2+ and Cu2+ treated thylakoids, respectively, while energy sto rage was insensitive to Pb2+. The results are consistent with the known pre dominant inhibition of the donor side of Photosystem II by the metal ions. The insensitive portion of energy storage is attributed to the possible rec urrence, of cyclic electron transport around Photosystem II that would depe nd on the extent of inhibition produced on the acceptor side by the metal i on used.