Investigating changes in the redox state of Photosystem I at low pH.

Changes in the redox state of Photosystem I (PSI) were studied in spinach leaf discs suspended in buffers of different pH (pH 7.5, 6.5, 5.5 and 4.5). By measuring absorbance changes at 820 nm, it was observed that under normal conditions, the electrons were supplied by Photosystem II (PSII) for the photo-oxidation of P700 while in the presence of DCMU when electrons coming from PSII are blocked, cyclic electron flow (CEF) around PSI was the major source for the absorbance changes observed at 820 nm. This was supported by complete inhibition in the reduction of both single turnover (ST) area and multiple turnover (MT) area, in the presence of DCMU, which is generally filled up by the electrons coming from PSII. In the absence of DCMU, the intersystem electron pool or plastoquinone (PQ) pool was increased at low pH which was probably due to enhanced cyclic electron flow around PSI. Our results also suggest that at low pH, in the absence of DCMU, the major contribution for faster dark re-reduction of P700(+) is attributed mainly by PSII and CEF PSI while in the presence of DCMU, the significant contribution is provided by CEF PSI and other stromal components.

Cyclic electron flow around photosystem I is enhanced at low pH.

Earlier studies have shown that at low pH (pH 5.5), PS II fluorescence decreases with concomitant increase in PS I fluorescence (Singh-Rawal et al., 2010). In order to shed light on the reasons of the above stated change, spinach leaf discs were treated with buffers of different pH (7.5, 6.5 and 5.5)and decrease in the photochemical quantum yield of PS II,Y(II) and increase in the photochemical quantum yield of PS I,Y(I) was observed. We observed an enhanced protection against over-reduction of PS I acceptor side at low pH (5.5) treated leaves. This was obviously achieved by the rapid build-up of trans-thylakoid pH gradient at low light intensities and was directly associated with a steep increase in non- photochemical quenching of chlorophyll fluorescence and a decrease in the electron transport rate of PS II. Our results suggested a strong stimulation of cyclic electron flow around PS I at pH 5.5 which directly supports protection against over-reduction of the PS I acceptor side.

Low pH-induced regulation of excitation energy between the two photosystems.

Earlier studies have proposed that low pH causes state transitions in spinach thylakoid membranes. Several Arabidopsis mutants (stn7 incapable in phosphorylation of LHC II, stn8 incapable in phosphorylation of PSII core proteins, stn7 stn8 double mutant and npq4 lacking PsbS and hence qE) were used to investigate the mechanisms involved in low pH induced changes in the thylakoid membrane. We propose that protonation of PsbS at low pH is involved in enhancing energy spillover to PS I.

Computational analysis of fluorescence induction curves in intact spinach leaves treated at different pH.

Effects of change in pH have been investigated on spinach leaf discs by measuring fluorescence induction kinetics using plant efficiency analyzer (PEA). On the basis of computational analysis of the results, we have reported that acidic pH causes a significant inhibition of the donor and the acceptor side of PS II. Energy flux models have been presented using the software Biolyzer HP 3. Effects of pH were investigated on the antenna size heterogeneity of PS II and a relative change in the proportions of α, ß, and γ centers was observed.