FvFm

Fv/Fm is a commonly used parameter in plant physiology and photobiology that describes the maximum potential efficiency of photosystem II (PSII) photochemistry in dark-adapted leaves. It is defined as (Fm − Fo) / Fm, where Fo is the minimal fluorescence yield of PSII in the dark and Fm is the maximal fluorescence yield produced by a saturating light pulse. Fv is the difference between Fm and Fo. The ratio is unitless and typically around 0.83 in healthy, unstressed C3 plants, with values varying by species and condition.

Measurement of Fv/Fm is performed using a chlorophyll fluorescence fluorometer. Plants are dark-adapted for approximately 15–30 minutes to ensure PSII reaction centers are open. Fo is measured with a weak, modulated measuring light, followed by a brief saturating pulse to obtain Fm. The resulting Fv/Fm reflects the maximum quantum yield of PSII photochemistry in the dark-adapted state.

Interpretation and applications: Lower Fv/Fm values indicate damage or inhibition of PSII, commonly due to photoinhibition, drought, cold or heat stress, nutrient deficiency, or environmental stress. Values near or below ~0.75 can signal stressed or damaged tissues, whereas healthy plants in favorable conditions approach ~0.83. Fv/Fm is widely used to screen plant and algal stress responses, evaluate cultivar tolerance, and study photoinhibition dynamics. It is an indicator of potential photosynthetic performance but does not measure actual photosynthetic rate under current light conditions.

Limitations include species and developmental differences, the need for standardized dark adaptation, and potential recovery after stress. For light conditions or dynamic states, related metrics such as Fv′/Fm′ or ΦPSII are used to assess actual PSII efficiency during photosynthesis.