Correlation of Absorbance Changes and Thylakoid Fusion with the Induction of Oxygen Evolution in Bean Leaves Greened by Brief Flashes

Publisher Website
Google Scholar
Abstract
Dark-grown bean leaves (Phaseolus vulgaris) greened for several days in a repetitive series of brief xenon flashes were studied during the initial induction period when O2 evolution first appears. The induction of O2 evolution requires actinic irradiation (e.g., 2 .mu.w[microwave]/cm2 of red light) and goes to completion in about 8 min with a half-time just under 3 min. Absorbance measurements on the intact leaves showed that a change of a carotenoid pigment, monitored at 505 nm, was closely correlated with the rate of O2 evolution during the induction period. Inhibitor studies showed that the absorbance change persisted in the presence of a number of inhibitors which blocked O2 evolution. EM revealed that the primary thylakoids which were unfused in the flashed leaves before induction became fused in pairs or groups of 3 during the 8-min induction period. The 505-nm absorbance change of the carotenoid pigment is probably correlated more directly with the fusion process than with O2 evolution. Heat treatment (45 C for 5 min) or infiltration with 0.8 M tris, which prevented the fusion process, also prevented the absorbance change. If the leaves were preilluminated for 8 min with very weak red light (20 .mu.w/cm2) which induced no O2 evolution, absorbance change, or thylakoid fusion, there was an immediate burst of O2 evolution at the onset of actinic irradiation and the induction period, as noted by O2 evolution or by the 505-nm absorbance change, was reduced to 2 min (half-time of 40 s). The electron transport system in the flashed leaves apparently is blocked at the Mn site between H2O and photosystem II, and the photoactivation of Mn into the thylakoid membranes occurs during the low light, photoactivation process. After the electron transport chain is thus repaired, ion-pumping mechanisms driven by actinic light may lead to steady-state photosynthesis as well as to thylakoid fusion.