Photophysical processes of energy conversion in thylakoid membranes of Chlamydomonas reinhardtii mutants D1-R323H, D1-R323D, and D1-R323L

Chlamydomonas reinhardtii mutants D1-R323H, D1-R323D, and D1-R323L showed elevated chlorophyll fluorescence yields, which increased with decline of oxygen evolving capacity. The extra step K ascribed to the disturbance of electron transport at the donor side of PS II was observed in OJIP kinetics measured in mutants with a PEA fluorometer. Fluorescence decay kinetics were recorded and analyzed in a pseudo-wild type (pWt) and in mutants of C. reinhardtii with a Becker and Hickl single photon counting system in pico- to nanosecond time range. The kinetics curves were fitted by three exponentials. The first one (rapid, with lifetime about 300 ps) reflects energy migration from antenna complex to the reaction center (RC) of photosystem II (PS II); the second component (600–700 ps) has been assigned to an electron transfer from P680 to Q A , while the third one (slow, 3 ns) assumingly originates from charge recombination in the radical pair [P680 +• Pheo −• ] and/or from antenna complexes energetically disconnected from RC II. Mutants showed reduced contribution of the first component, whereas the yield of the second component increased due to slowing down of the electron transport to Q A . The mutant D1-R323L with completely inactive oxygen evolving complex did not reveal rapid component at all, while its kinetics was approximated by two slow components with lifetimes of about 2 and 3 ns. These may be due to two reasons: a) disconnection between antennae complexes and RC II, and b) recombination in a radical pair [P680 +• Pheo −• ] under restricted electron transport to Q A . The data obtained suggest that disturbance of oxygen evolving function in mutants may induce an upshift of the midpoint redox potential of Q A /Q A − couple causing limitation of electron transport at the acceptor side of PS II.