On the determination of QB-non-reducing photosystem II centers from chlorophyll a fluorescence induction

The relative height of the so called F-pl (`plateau') appearing between the O and P steps in the initial chlorophyll a fluorescence rise (FR) under low exciting light, is frequently used for a determination of the relative amount of Q(B)-non-reducing photosystem II centers ([PSII-Q(B)-non](rel)). On the basis of theoretical approach it has been already suggested that the F-pl level can be influenced, among others, by the intensity of exciting light. In this contribution, we present a calculation of [PSII-Q(B)-non](rel) from the FRs measured at different intensities of exciting light (2-115 mumol photons in m(-2) s(-1)). We have found that the higher intensity of exciting light, the higher is the estimation of [PSII-Q(B)-non](rel) (8-45%); thus the numerical expression of [PSII-Q(B)-non](rel) using the F-pl level in the FR is not plausible. Our experimental results were also successfully simulated by a simple mathematical model. We recommend to use as low intensity of exciting light as possible for an estimation of [PSII-Q(B)-non](rel) from the FR. As Q(B)-reducing PSII centers contribute to the F-pl level even at very low intensities of exciting light, we further recommend to use this method for an approximate estimation of the changes in [PSII-Q(B)-non](rel), rather than for the numerical estimation of [PSII-Q(B)-non](rel). (C) 2003 Elsevier Science Ireland Ltd. All rights reserved.