The most common parameters used for stress measurement are Fv/Fm and Yield (ΔF/Fm’). Both parameters are very robust and have volumes of supporting research regarding their value and sensitivity in plant stress measurement. Fv/Fm is a dark adapted measurement that reveals maximum leaf PSII photosynthetic capacity under existing stress conditions. Yield is taken with current lighting conditions and taken at steady state PSII photosynthetic levels to reveal actual achieved photosynthetic efficiency under existing stress conditions. Both parameters have been shown to correlate well with to CO2 fixation rates under most stress conditions. Their sensitivity to stress can vary with the type of stress and in some cases with the type of plant. Some types of stress do not immediately affect PSII and therefore Yield and Fv/Fm do not normally detect these types of stress until levels are moderate, severe or even at starvation levels. In some cases novel assays have been developed to solve these problems. Solutions now exist for difficult to measure early water stress and early nitrogen stress.

Other modulated parameters are valuable for stress measurement as well such as Electron Transport Rate (ETR) and the various quenching parameters involved in photo-protection, state transitions and photo-inhibition. In some cases they are more sensitive than Yield or Fv/Fm because different types of stress can affect different mechanisms in photosystem II. For further details please refer to the stress guide.

To augment stress detection, normalized parameters have been developed from an older method called OJIP. These newer more advanced OJIP normalized parameters allow one to detect and measure stress types that have given modulated methods difficulty. The Performance Index, written as PI, has been shown to be the most useful and offers a complimentary solution for water stress, some nutrient stress types, some herbicide types, and heat stress.

While both modulated fluorometers and OJIP fluorometers have some limitations with regard to stress measurement, when they are combined they offer the most formidable stress solution. For more information on parameters for specific types of stress measurement, and references regarding stress measurement, click "To Receive a Stress Guide with References".