Fv/Fm – An indication of maximum quantum efficiency of Photosystem II. It is widely considered a sensitive plant photosynthetic performance indicator. Presented as a ratio between 0 and 1, healthy samples typically achieve a maximum Fv/Fm value of approx. 0.85. Lower values 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.
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 Qa is fully oxidized. This requires thorough dark adaptation.
Area – The area above the fluorescence curve between Fo and Fm is proportional to the pool size of the electron acceptors Qa 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.