Dear Ahmad,

In most of such induction experiments the first saturation pulse is given after about 1 s, which would allow you to see Fp. However, since you give this pulse at time 0 you wipe out Fp. If you look at the literature from the late sixties the Kautsky kinetics were labeled P-S-M-T, where S is the dip before your Fp and M is your Fp and T is the steady state. In most of the literature state transitions are blamed for this kind of phenomena, but I am not sure that is correct. You can also keep in mind that the activation of the Calvin-Benson cycle takes more than 1 min, so your Fp is observed during this activation process. So you can also think in terms of first activating the acceptor side of PS I, then you exhaust the pool of NADP which causes another temporary block in the electron transport chain before Calvin-Benson cycle activation. To say more, you would have to establish if the phenomenon is due to a change in qP (redox change) or for example qE (lumen pH related fluorescence yield change) or other form of NPQ. I realize that is not a complete answer to your question, but it may give an idea.

Dear Ahmad sb here these as far my inferior knowledge are

F0 = Minimum Chl fluorescence yield under drought and control

Fm = Maximum Chl fluorescence yield under drought

Fp = Maximum chlorophyll fluorescence yield when actinic radiations are switched on under drought OR steady state Chl fluorescence yield in light adapted state under drought

Biologically it means

For P1, higher Fp was recorded under drought compared to control which suggests non-photochemical quenching was enhanced for P1 under drought. It'll result a higher drought tolerance in P1.

While for P2, lower Fp was observed under drought compared to control which indicates non-photochemical quenching of P2 was decreased under drought compared to control. It'll result in higher drought sensitivity in P2.

Conclusion: P1 is more drought tolerant compared to P2.

Kindly let me know whether it is helpful.

Dear Ahmad,

To me your question is in the first place about nomenclature. If you want to see Fp, you should just turn on the actinic light (starting with a dark acclimated sample) choosing maybe 200 µmol photons m-2 s-1 and then you will see that in about 0.5 s the fluorescence will reach a maximum value after which the fluorescence starts to decline again (so, no saturation pulses). The peak that was called peak or P or Fp. This phenomenon was first described by Hans Kautsky in 1931 and, therefore, is called Kautsky effect. In your figure you do not see this Fp because it is hidden under your saturation pulse. The fluorescence rise to Fp is due to the reduction of the electron transport chain and an associated closure of the PS II reaction centers. At P you start to activate the acceptor side of PS I which allows the flow of electrons towards the Calvin-Benson cycle. As a consequence the electron transport chain becomes more oxidized again and some of the PS II reaction centers re-open (fluorescence goes down). In parallel, the proton translocation over the membrane associated with this electron flow causes an acidification of the lumen and can induce non-photochemical quenching (qE), which also lowers the fluorescence. The saturation pulses allow you to separate between these two effects: photochemical and non-photochemical quenching. Having activated the acceptor side of PS I does not yet mean that you activated the Calvin-Benson cycle. In the steady state that has happened but on the way there, there is still a small increase in the fluorescence, which in the literature is called M. That is your peak. If you would after a first illumination turn off the actinic light and restart the experiment 2-3 min later, you would probably see that your peak has disappeared and that the induction is much faster. You cannot just say on the basis of such an experiment that a higher or lower peak is due to drought. You would have to show that the effect develops during a drought stress treatment and that this is less so in known tolerant varieties than in known sensitive varieties and then at a point you can propose that there is a relationship between drought and this phenomenon. Helmut Lichtenthaler has developed a parameter called Rfd which is based on the fluorescence decline beyond Fp. You can maybe have a look if that is an interesting parameter for you. It has the advantage that it is an established parameter.

Best regards,

Gert Schansker