Laser pulse energy requirements for remote sensing of chlorophyll fluorescence

When measuring laser induced chlorophyll fluorescence the intention is to probe the photosynthetic system of the plant, that is, to measure its state without notably affecting it. This is a common measuring principle. It was not known if the 10 ns green laser pulses used for our laser induced fluorescence measurements satisfy this requirement. In this article we consider the problem from a theoretical point of view and by experiment. Two unwanted effects may play a role: unintended partial closure of the reaction centers and exciton annihilations. The primary photochemical events in the photosystem of the plant are simulated numerically. In addition the fuorescence in the dark adapted state has been measured with a range of excitation energies. Both theoretical and experimental results are compared and analyzed. Our results show that the probing laser pulse, used for measuring the laser induced chlorophyll fluorescence, should not exceed an excitation energy of 100 mJ/m2 at the plant. Additional laser pulses that could sere as an actinic light source to close all reaction centers before the measurement must have a pulse length of at least 100 ns.