New Ways of Assessing Photosynthetic Activity with a Pulse Modulation Fluorometer
The special features of a new type of modulation fluorometer (PAM Fluorometer) are outlined in comparison to conventional measuring systems. Fluorescence is excited by 1 µs pulses of a light-emitting-diode and the pulse signal is processed via a selective window amplifier, yielding an exceptionally high dynamic range. The system is also capable of measuring P700 changes around 820 nm, using a modified emitter-detector unit. Due to its tolerance of large non-modulated background signals, the PAM Fluorometer is well suited for measurements in full sunlight and for quenching analysis by the saturation pulse method (determination of photochemical and non-photochemical quenching components). Photochemical quenching is shown to correspond closely to O2-evolution rate. However, as is examplified by the effect of ANT-2p, under special conditions an increase in photochemical quenching may go along with a decrease in quantum yield. Cyclic electron flow around PS II is suggested to be stimulated upon loss of PS II donor side capacity.
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