What is "Fm quenching"?

Question

I'm pretty sure it has something to do with photosynthesis or photosystem II.

Thanks.

Answer 1

"Fm quenching" involves a reduction in the intensity of emission of chlorophyll fluorescence in Photosystem II (in either a leaf or subcellular preparation).  Plant scientists characterize changes in fluorescence emission in terms of processes that divert light energy trapped by chlorophyll away from fluorescence emission; hence, "quench" it.  The subscript "m" refers to "maximal" which is usually measured during superimposition of a brief, multiple-turnover, flash of light to fill the electron acceptor pools of Photosystem II and, hence, remove the interfering "photochemical" type of quenching.  Even after suppression of the latter, Fm can vary considerably due to the presence of a diverse set of processes that dissipate light energy as heat.  These processes are, for the most part, considered to be a photoprotective response designed to mitigate damage to Photosystem II during exposure to excess light.  A secondary source of quenching of Fm can be an enigmatic process called "state transitions" that involves pigment complex migration from Photosystem II to Photosystem I.  Fm quenching still is an active and incompletely understood area of photosynthesis research.