Fluorescence clamp: A direct measure of fluxes into and out of the antenna pool of Photosystem II.

Fluorescence clamp (FC) is a method of directly measuring the fluxes out of Photosystem II antenna. This is achieved by a feed-back loop which controls the light intensity of light emitting diodes in order to keep the amplitude of modulated chlorophyll fluorescence constant, and by taking the intensity or the current fed into the light emitting diodes as a measure of the fluxes. Saturating flashes serve to distinguish between fluxes into thermal deactivation and into the photosynthetic electron transfer chain (ETC). As FC is only active in the light period of the measuring light, the background signal (induced by actinic light) is compensated by a second feed-back loop in the dark period of the measuring light. Equations are provided for the interpretation of the FC signals. This includes the quenching parameters of chlorophyll fluorescence, the flux into the electron transfer chain and the redox state of Q(A). Experiments are presented which show that traditional fluorescence (LC) and FC measurements yield the same results. However, the FC method provides a better presentation of fluxes as the scaling factor (flux/signal) is constant for all states of Photosystem II. This leads to a simpler analysis of quenching mechanisms. Examples are given which show that the co-existing quenching mechanisms with different effects on photochemical and non-photochemical fluxes can be better identified by FC rather than by LC.