Stimulation of microsecond-delayed fluorescence from spinach chloroplasts by uncouplers and by phosphorylation

Delayed fluorescence, as measured with a laser phosphoroscope, is stimulated not inhibited by uncouplers during the first 100 μs after the light is turned off. This is true only wen uncouplers cause an increase in the rate of electron transport. When ADP and P_i cause an increase in the electron transport rate, microsecond-delayed fluorescence is also increased. Indeed, there is a complex quantitative relationship between the rate of electron transport and the initial intensity of delayed fluorescence under a wide range of conditions.

Uncouplers or ADP and P_i also increase the rate of decay of delayed fluorescence so that after about 150 μs they become inhibitory, as already reported by many authors.

Microsecond-delayed fluorescence continues to rise with rising light intensities long after the rate of reduction of exogenous acceptor is light-saturated.

These observations suggest a correlation of the rate of electron transport both with the intensity of the 5–100 μs-delayed fluorescence and with the rate of decay in the intensity of delayed fluorescence. The data imply that the decrease in intensity of millisecond-delayed fluorescence which has often been noted with uncouplers is probably not due to the elimination of a membrane potential. It seems more likely that the decrease in millisecond-delayed fluorescence is a reflection of the rate of disappearance of some other electron transport-generated condition, a condition which is uncoupler-insensitive. Certainly stimulations of microsecond-delayed fluorescence by electron transport which has been uncoupled by gramicidin suggest that ion gradients are not an essential component of the conditions responsible for delayed fluorescence.