Deciphering the 820 nm signal: redox state of donor side and quantum yield of Photosystem I in leaves

By recording leaf transmittance at 820 nm and quantifying the photon flux density of far red light (FRL) absorbed by long-wavelength chlorophylls of Photosystem I (PS I), the oxidation kinetics of electron carriers on the PS I donor side was mathematically analyzed in sunflower (Helianthus annuus L.), tobacco (Nicotiana tabacum L.) and birch (Betula pendula Roth.) leaves. PS I donor side carriers were first oxidized under FRL, electrons were then allowed to accumulate on the PS I donor side during dark intervals of increasing length. After each dark interval the electrons were removed (titrated) by FRL. The kinetics of the 820 nm signal during the oxidation of the PS I donor side was modeled assuming redox equilibrium among the PS I donor pigment (P700), plastocyanin (PC), and cytochrome f plus Rieske FeS (Cyt f + FeS) pools, considering that the 820 nm signal originates from P700(+) and PC+. The analysis yielded the pool sizes of P700, PC and (Cyt f + FeS) and associated redox equilibrium constants. PS I density varied between 0.6 and 1.4 mumol m(-2). PS II density (measured as O-2 evolution from a saturating single-turnover flash) ranged from 0.64 to 2.14 mumol m(-2). The average electron storage capacity was 1.96 (range 1.25 to 2.4) and 1.16 (range 0.6 to 1.7) for PC and (Cyt f + FeS), respectively, per P700. The best-fit electrochemical midpoint potential differences were 80 mV for the P700/PC and 25 mV for the PC/Cyt f equilibria at 22 degreesC. An algorithm relating the measured 820 nm signal to the redox states of individual PS I donor side electron carriers in leaves is presented. Applying this algorithm to the analysis of steady-state light response curves of net CO2 fixation rate and 820 nm signal shows that the quantum yield of PS I decreases by about half due to acceptor side reduction at limiting light intensities before the donor side becomes oxidized at saturating intensities.