Plant stress measurement
CO2 concentration changes in leaf chambers are measured to provide carbon assimilation values for leaves or whole plants.
Research has shown that the rate of photosynthesis is directly related to the amount of carbon assimilated by the plant.
Because these systems are effective in measuring carbon assimilation and transpiration at low rates, as found in stressed plants,[4] they are often used as the standard to compare to other types of instruments.
[6][3][7] Chlorophyll fluorescence emitted from plant leaves gives an insight into the health of the photosynthetic systems within the leaf.
The most commonly used protocols include those aimed at measuring the photosynthetic efficiency of photosystem II, both in the light (ΔF/Fm') and in a dark-adapted state (Fv/Fm).
Chlorophyll fluorometers are, for the most part, less expensive tools than photosynthesis systems, they also have a faster measurement time and tend to be more portable.
Fv/Fm tests whether or not plant stress affects photosystem II in a dark adapted state.
"The majority of fluorescence measurements are now made using modulated fluorometers with the leaf poised in a known state."
It may be used in photochemistry to produce ATP and NADPH used in photosynthesis, it can be re-emitted as fluorescence, or dissipated as heat.
In general, the greater the plant stress, the fewer open reaction centers available, and the Fv/Fm ratio is lowered.
This is a measurement of antennae fluorescence using a modulated light intensity that is too low to drive photosynthesis.
Next, an intense light flash, or saturation pulse, of a limited duration, is used, to expose the sample, and close all available reaction centers.
It is a measurement ratio that represents the maximum potential quantum efficiency of Photosystem II if all capable reaction centers were open.
According to Maxwell and Johnson, it takes between fifteen and twenty minutes for a plant to reach steady-state photosynthesis at a specific light level.
In the field, plants in full sunlight, and not under canopy, or partly cloudy conditions, are considered to be at steady state.
If different leaf absorption ratios are an issue, or they are an unwanted variable, then using Y(II) instead of ETR, may be the best choice.
[19][20] OJIP or OJIDP is a dark adapted chlorophyll fluorescence technique that is used for plant stress measurement.
[citation needed] These are instruments that use light transmission through a leaf, at two wavelengths, to determine the greenness and thickness of leaves.
The research done to date uses either[clarification needed] ten and thirty measurements on test and well fertilized crops, to provide average values.
Chlorophyll content meters are sensitive to both nitrogen and sulfur stress at usable levels.
