Induction of chlorophyll a fluorescence in isolated spinach chloroplasts at liquid nitrogen temperature

Abstract

1. 1. The induction of chlorophyll a fluorescence was measured at 690 nm in isolated spinach chloroplasts at liquid N 2 temperature. The effects of preillumination of the chloroplasts with varied durations of flashes were investigated. When the degree of fluorescence induction was represented by the extent of the variable component of fluorescence, a 10-μs flash at a saturating intensity eliminated 13% of the induction, and a 20-ms flash eliminated 50%. 2. 2. An oxidant, ferricyanide, markedly decreased the fluorescence yield at the steady-state level but little at the initial rise level. 3. 3. The dark recovery of the fluorescence induction at liquid N 2 temperature was found to be slow and only partial. The maximum recovery was reached after 20 min of dark time. 4. 4. The following scheme is presented for the reaction at the reaction center of System 2 at liquid N 2 temperature. ▪ X and X − are the primary electron acceptors in the oxidized and reduced forms, respectively. Y and Y + are the primary electron donors in the reduced and oxidized forms, respectively. The (X Y) state is converted to (X − Y +) by consuming the excitation energy. The (X − Y +) state formed is degradated either to (X Y) with loss of energy or to (X − Y) with reduction of Y + by a secondary electron donor. It is suggested that all the reaction centers are in the (X − Y) state at the maximum fluorescence yield. Kinetic analysis indicates that the rate constants of Reaction 1 and 2 at liquid N 2 temperature are 130 s −1 and 20 s −1, respectively, and the life time of the (X − Y +) state is 4.5 ms.