Hello, I hope somebody could help in clarifying this point.
I am following an experiment on young olive plants in southern Italy, and I will take chlorophyll fluorescence measurements using a Fluorpen (PSI).
I will use the OJIP protocol and I am in the process of setting the saturating light intensity: I understand that in order to obtain proper Fmax measurements in dark adapted samples, one should make sure that the intensity of the initial flash of high intensity light is strong enough to temporarily close all Reaction Centers.
My questions is: which is the procedure to find the saturating light intensity ?
I already performed a series of OJIP measurements setting the light intensity from 300 to 3000 micromoles of PAR but it is not clear for me which value I should choose.
According to the Fluorpen instruction manual, one should choose the light intensity which results in the maximum Fv/Fmax ratio.
In my case this occurs at 1200 micromoles of PAR, while this ratio decreases at higher sat pulse intensities.
I also found a different suggestion in a recent review by M. Tsimilli-Michael (Revisiting JIP-test, Photosynthetic 57, 90.107, 2019): to test different light intensities (I) and to select the value for which Fpeak/I gets saturated. In my case this ratio reaches a peak at 1200 micro moles of PAR after which it starts decreasing, but I cannot see it reaching a steady value.
Am I following the right procedure ? Actually, I am not sure that 1200 micro moles could actually be a strong enough light to close all RCs in olive plants, since the “midday" PAR intensity can be over 2000 in this season.
I am looking forward to reading your comments.
Thank you
Dear Stefano,
given that saturating light pulse is given for a very short time, you need a strong amplitude to fully saturate your PSII reaction centres. Usually, for FvFm you need at least 3000 to 8000 micromol m-2 s-1
https://www.jove.com/t/59838/evaluation-photosynthetic-behaviors-simultaneous-measurements-leaf
Thank you for your answers. Yes I feel that it is reasonable to think that a strong light pulse of 3000 or more micromoles of PAR “should” be strong enough.
However, maybe I should rephrase my question:
“ How can I make sure that the saturating flash of light produced by the fluorimeter is strong enough to close all RCs? "
This is important to know whether we are measuring Fmax or Fp (Peak of fluorescence of the OJIP measurement).
So, how do I test if the "saturating” flash of light emitted by the Fluorpen is actually saturating ?
Dear Stefano,
On the one hand, the manufacturer should provide the PAR value emitted by the diode. On the other hand, you can visit colleague at the nearest physics / optics department and ask for an exact measurement for your instrument. You can also try to measure with an ordinary PAR meter, although the measurement will be probably burdened with a significant error. Either way, if you measure a value above 3000 everything is ok.
Plants have evolved in sunlight (~2000 PAR), so 3000 and more are likely to saturate the photosynthetic apparatus.
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