The Multiphasic Nature of Nonphotochemical Quenching: Implications for Assessment of Photosynthetic Electron Transport Based on Chlorophyll Fluorescence

Defining a quantitative relationship between chlorophyll a fluorescence yield and Photosystem II (PS II) function is important to photosynthesis research. Prior work [Peterson and Havir (2003) Photosynth Res 75: 57–70] indicated an apparent effect of psbS genotype on the in vivo rate constant for photochemistry in PS II (kP0). The nuclear psbS gene encodes a 22-kDa pigment-binding antenna protein (PS II-S) essential for photoprotective nonphotochemical quenching (NPQ) in PS II. Ten Arabidopsis thaliana lines were chosen for study, encompassing effects on PS II-S expression level and/or structure due to single-site amino acid substitution. Short-term (i.e. seconds) irradiance-dependent changes in steady state fluorescence yields Fo and Fm(open and closed centers, respectively) were evaluated for compliance with the reversible radical pair (RRP) model of PS II. All lines (including normal Nicotiana tabacum and Zea mays) deviated from the RRP scheme in the same way indicating that psbS genotype per se does not alter interactions between the antenna and reaction center and thereby affect kP0. Rather, observed departures from RRP model behavior are consistent with overestimation of Fm due to perturbing effects of the saturating multiple turnover flash employed in its measurement. Reversal of direct quenching of singlet states by plastoquinone during the flash could occur but by itself cannot account for the anomalous covariation in Fo and Fm. Reduction of the PS II acceptor side apparently either amplifies the rate constant for fluorescence or suppresses that of xanthophyll-dependent thermal deactivation (qE). A procedure was devised that considers Fo when correcting maximal fluorescence values for measurement bias. A high degree of consistency in assessment of PS II quantum yield based on corrected fluorescence parameters and simultaneous CO2 exchange measurements was noted under both steady state and transient conditions (360 μl CO2l−1, 1% O2).