ИСТИНА

After switching off illumination of a leaf, chlorophyll a fluorescence yield first drops but then fluorescence transiently increases and then decreases in about two minutes. The postillumination bump in the fluorescence curve is called the F0 rise and it was earlier explained by assuming that NADPH and reduced ferredoxin accumulate in the light and reduce the plastoquinone pool which equilibrates with the QA electron acceptor in the dark. We tested the hypothesis about reduction of QA by reduced plastoquinones. For this, Arabidopsis leaves were illuminated for 8 min with moderate light and after switching the light off, the measuring beam of the PAM fluorometer was chopped by switching it on only for 0.5 s every 5 s. No F0 rise was found to occur, indicating that the F0 rise requires an actinic effect of the measuring beam. Thus, F0 rise depends on reduction of QA by light. The role of reduction of plastoquinone by NADPH and/or ferredoxin is to produce plastohydroquinone that acts as an inhibitor of electron flow in the QB site. We also saw that preillumination with specific wavelengths is needed to prime the chloroplasts to a condition in which F0 rise can be induced. Next, the actinic effect of the measuring beam was replaced with a controllable system by using a chopped measuring beam and simultaneously applying continuous, low-intensity light (“postillumination”) (PPFD 0.5 to 2.5 µmol m-2s-1) on top of it. Systematic action spectroscopy showed that preillumination at 520, 630 or 690 nm followed by postillumination at 500, 560 or 650 nm produces a large F0 rise, and no F0 rise occurs if the wavelengths are switched. Furthermore, 520, 630 or 690 nm illumination was found to oxidize P700 and cause dephosphorylation of LHCII, while 500, 560 or 650 nm light caused phosphorylation of LHCII and did not oxidize P700. These data show that F0 rise occurs only if preillumination with moderate light favoring PSI is followed by illumination with low light favoring PSII. Furthermore, the data show that the relative action spectrum of PSII over PSI has a peak at 560 nm light whereas 520 and 630 nm favor PSI over PSII.