Characterization of the photosynthetic system by measuring the laser-induced chlorophyll fluorescence in the msec range from medium distances

A continuous He-Ne laser with an acousto-optical modulator as a high-speed optical shutter generates a pulse with a length of 2 msec and a maximum power density of 600 W/m2. According to the chlorophyll fluorescence-induction kinetics, the fluorescence intensity of plants reaches its initial value (F0) and continues to rise during the excitation pulse. This increase can be described by an exponential function and a linear term, which depend on the power density at the laser spot, on the grade of inhibition of photosynthetic activity by herbicides or environmental stress, and on the duration of the dark adaptation time. A single measurement with the msec pulse method requires only a short time because the period of dark adaptation, necessary for the induction kinetics, can be reduced to several 100 msec. A modification of the device allows measurements at distances between 0.5 m and 3 m.