Energy‐Dependent Quenching of Chlorophyll a Fluorescence in Isolated Chloroplasts

Abstract

A light‐induced, energy dependent quenching of chlorophyll a fluorescence in the presence of 2,3,5,6‐tetramethyl‐p‐phenylene diamine (diamino‐durene) and 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea is described for isolated chloroplasts, and is related to the formation of the high‐energy state of photophosphorylation. The quenching is distinguished from the normal type, operative through the redox state of the Photosystem II quencher, by its light‐intensity dependence, reversibility in the dark and sensitivity to uncoupling agents. The sensitivity of the quenching to uncouplers and ionophorous antibiotics closely parallels that of photophosphorylation. A similar quenching is described for other electron transport systems in the absence of dichlorophenyl‐dimethylurea, when it is distinguished from Q‐quenching on a time‐basis. The pH‐optimum for the uncoupler‐reversible quenching is about pH 8.0 for all electron flow systems used. This apparent discrepancy with the pH optimum for XE‐formation is discussed. Two other types of slow quenching seen during the Hill reaction with ferricyanide are described. One is found to be uncoupler enhanced and is related to acceleration of electron transport giving rise to a lower level of reduced Q in the steady state; the other is an irreversible loss of total fluorescence yield, which is most pronounced in coupled chloroplasts, and it is suggested that it is due to photochemical destruction of bulk pigment molecules via a side reaction of the excited state of chlorophyll. Copyright © 1970, Wiley Blackwell. All rights reserved