Common Continuous Excitation Fluorescence Parameters Measured | Hansatech Instruments | Oxygen electrode and chlorophyll fluorescence measurement systems for cellular respiration and photosynthesis research
Fo – Represents emission by excited chlorophyll a molecules in the antennae structure of Photosystem II. The true Fo level is only observed when the first stable electron acceptor of Photosystem II called Q_A is fully oxidised. This requires thorough dark adaptation.

Fm – The maximum fluorescence value obtained for a continuous light intensity. This parameter may only be termed as maximal if the light intensity used is fully saturating and the electron acceptor Q_A is fully reduced.

Fv – Indicates the variable component of the recording and relates to the maximum capacity for photochemical quenching. Calculated by subtracting the Fo value from the Fm value (Fm - Fo).

Fv/Fm РAn indication of the maximum quantum efficiency of Photosystem II and widely considered to be a sensitive indicator of plant photosynthetic performance. Presented as a ratio between 0 and 1, healthy samples typically achieve a maximum Fv/Fm value of approx. 0.85. Values lower than this will be observed if a sample has been exposed to some type of biotic or abiotic stress factor which has reduced the capacity for photochemical quenching within PSII. Fv/Fm is presented as a ratio of variable fluorescence (Fv) over the maximum fluorescence value (Fm) and is calculated as (Fm - Fo)/Fv.

Tfm – Indicates the time at which the maximum fluorescence value (Fm) was reached. May be used to indicate sample stress which causes the Fm to be reached much earlier than expected.

Area – The area above the fluorescence curve between Fo and Fm is proportional to the pool size of the electron acceptors Q_A on the reducing side of Photosystem II. If electron transfer from the reaction centers to the quinone pool is blocked (such as is the mode of action of the photosynthetically active herbicide DCMU), the area will be dramatically reduced.