Description:
When plant leaves are illuminated, part of the light energy is trapped by the chlorophyll molecule, which becomes 'excited' and undergoes a shift in energy state. The energised state is unstable and the energy is rapidly released (re-emitted) via 3 competing pathways. Fluorescence describes the energy that is re-emitted as a low energy high wavelength photon of red and far red light. The remaining energy is used to drive photosynthesis or dissipated as heat. Damage to photo-system II (PSII) and reductions in photosynthetic rate will reduce the amount of fluorescence released in response to light. Thus fluorescence measurements provide information on inhibition or damage to transfer of electrons from PSII and photochemical quantum yield.
Previous experience:
Measurement of fluorescence has been used as an early indicator of physiological stress. Reductions in fluorescence usually precede visible damage. Different parameters associated with fluorescence kinetics have been related to a range of stresses such as freezing, drought and high photo-illumination following cold nights. Changes in fluorescence kinetics may be linked to acid deposition through accumulation of sulphate anions in cells (Beauregard, 1991), but the response is not specific to acid exposure.
Suitability to indicate atmospheric concentrations:
The relationship between chlorophyll fluorescence and acid concentration is not direct. The method provides a general indicator of stress that is not specific to cause and data may be difficult to interpret. Insufficient evidence is available to review the sensitivity of the method.
Suitability to indicate atmospheric depositions:
The relationship between chlorophyll fluorescence and acid deposition is not direct. The method provides a general indicator of stress that is not specific to cause and data may be difficult to interpret.
Suitability to indicate environmental impacts:
The relationship between chlorophyll fluorescence and acid concentrations/deposition is not direct. The method provides a general indicator of stress that is not specific to cause and data may be difficult to interpret.
Sensitivity to other factors:
Different parameters associated with fluorescence kinetics have been related to a range of stresses such as N concentration/deposition, freezing, drought and high photo-illumination following cold nights. Exposure to acid precipitation and drought appears to cause a larger effect than to acid precipitation when plants are not water-stressed (Mena-Petite et al., 2000). The method is strongly affected by moisture content of the plant, particularly in cryptogams, where photoassimilation is determined by H2O availability. However, it is not very sensitive to temperature within the range of normal outside conditions in UK (Wulff et al.,1994).
Timescale:
Available evidence suggests that the method is more suited for assessing short-term direct effects (e.g. days to months).
Limitations:
The method does not appear to be limited to particular habitat or species types.
Expertise in field:
The method is simple and rapid and many measurements can be undertaken in a single day, providing sufficient dark adaptation clips are available. The equipment is portable, relatively inexpensive and could be operated by Agency staff following suitable training.
Expertise in laboratory:
Data can be difficult to interpret without the help of an experienced researcher.
Cost (per unit sample): £100-500
Cost Comment: Sufficient data can be collected by trained agency staff, (or an expert) in 1 day. However interpretation of the data may take several days and may involve help from a experienced researcher. Daily costs for expert help would costs around £300 per day.
Robustness score: 
