FLASH-INDUCED CONDUCTANCE CHANGES IN CHLOROPLAST THYLAKOID LAMELLAE -A PATCH-CLAMP STUDY

A patch-clamp method was used for measuring flash-induced currents (ph otocurrents) in Peperomia metallica chloroplasts at different holding potentials (V-h) in a 'whole-thylakoid' configuration. The applied hol ding potentials modified the kinetics of the photocurrents dramaticall y. A partially reversible, transient decrease in the magnitude of the holding current was observed with a minimum at 50-200 ms after the fla sh followed by a slow relaxation in 1-10 s. Under similar conditions t he chloroplast input (seal) conductance was followed by phase-sensitiv e detection with sine-wave potential modulation. The seal conductance was found to decrease transiently by 0.3-5% after a single-turnover fl ash and much more upon continuous illumination or repetitive flashes. The kinetics of these conductance changes were comparable to the curre nt profile as revealed by subtracting the current traces measured in t he absence and presence, respectively, of a holding potential. A major part of the conductance changes are suggested to be intimately relate d to alterations in the electrical properties of the lateral conductan ce phases along the thylakoid lamellae. The changes are extremely sens itive to agents that modify ion transport and exchange processes in th e membrane and its adjoining phases, The patch-clamp method is unique for quantifying these conductance changes in intact chloroplasts on a ms time scale. (C) 1997 Elsevier Science S.A.