Chlorophyll fluorescence
[1][2] Upon illumination of a dark-adapted leaf, there is a rapid rise in fluorescence from Photosystem II (PSII), followed by a slow decline.
[3] The increase in fluorescence is due to PSII reaction centers being in a "closed" or chemically reduced state.
Closed reaction centres reduce the overall photochemical efficiency, and so increases the level of fluorescence.
This is achieved by stopping photochemistry, which allows researchers to measure fluorescence in the presence of non-photochemical quenching alone.
To reduce photochemical quenching to negligible levels, a high intensity, short flash of light is applied to the leaf.
Heat dissipation cannot be totally stopped, so the yield of chlorophyll fluorescence in the absence of non-photochemical quenching cannot be measured.
Fluorescence level of dark-adapted sample when all reaction centers of the photosystem II are open.
Fluorescence level of dark-adapted sample when a high intensity pulse has been applied.
Fluorescence level of light-adapted sample when all reaction centers of the photosystem II are open; it is lowered with respect to
Fluorescence level of light-adapted sample when a high intensity pulse has been applied.
A steady-state fluorescence level decreased (= quenched) by photochemical and non-photochemical processes.
As such, it can give a measure of the rate of linear electron transport and so indicates overall photosynthesis.
Closure of reaction centers as a result of a high intensity light will alter the value of
Fluorescence can measure the efficiency of PSII photochemistry, which can be used to estimate the rate of linear electron transport by multiplying by the light intensity.
A powerful research technique is to simultaneously measure chlorophyll fluorescence and gas exchange to obtain a full picture of the response of plants to their environment.
One technique is to simultaneously measure CO2 fixation and PSII photochemistry at different light intensities, in non-photorespiratory conditions.
A plot of CO2 fixation and PSII photochemistry indicates the electron requirement per molecule CO2 fixed.
Fluorescence analysis can also be applied to understanding the effects of low and high temperatures.
Based on several years of research and experimentation, polyphenols can be the indicators of nitrogen status of a plant.
For instance, when a plant is under optimal conditions, it favours its primary metabolism and synthesises the proteins (nitrogen molecules) containing chlorophyll, and few flavonols (carbon-based secondary compounds).
On the other hand, in case of lack of nitrogen, we will observe an increased production of flavonols by the plant.
"[15] The development of fluorometers allowed chlorophyll fluorescence analysis to become a common method in plant research.
By modulating the measuring light beam (microsecond-range pulses) and parallel detection of the excited fluorescence the relative fluorescence yield (Ft) can be determined in the presence of ambient light.
Crucially, this means chlorophyll fluorescence can be measured in the field even in full sunlight.
[23] In addition, the parameters qE, and pNPQ have been developed to measure the photoprotective xanthophyll cycle.
Consistent further development into imaging fluorometers facilitate the visualization of spatial heterogeneities in photosynthetic activity of samples.
These heterogeneities naturally occur in plant leaves for example during growths, various environmental stresses or pathogen infection.
High performance imaging fluorometer systems provide options to analyze single cell/single chloroplast as well as sample areas covering whole leaves or plants.
In particular, recent advances in the area of laser-induced fluorescence (LIF) also provide an opportunity of developing sufficiently compact and efficient sensors for photophysiological status and biomass assessments.
Requiring no 15- 20 min dark adaptation period (as is the case for the Kautsky effect methods[29]) and being capable to excite the sample from considerable distance, the LIF sensors can provide fast and remote evaluation.
