New Fluorescence Parameters for the Determination of Q_A Redox State and Excitation Energy Fluxes

Abstract

A number of useful photosynthetic parameters are commonly derived from saturation pulse-induced fluorescence analysis. We show, that q_P, an estimate of the fraction of open centers, is based on a pure ‘puddle’ antenna model, where each Photosystem (PS) II center possesses its own independent antenna system. This parameter is incompatible with more realistic models of the photosynthetic unit, where reaction centers are connected by shared antenna, that is, the so-called ‘lake’ or ‘connected units’ models. We thus introduce a new parameter, q_L, based on a Stern–Volmer approach using a lake model, which estimates the fraction of open PS II centers. We suggest that q_L should be a useful parameter for terrestrial plants consistent with a high connectivity of PS II units, whereas some marine species with distinct antenna architecture, may require the use of more complex parameters based on intermediate models of the photosynthetic unit. Another useful parameter calculated from fluorescence analysis is Φ_II, the yield of PS II. In contrast to q_L, we show that the Φ_II parameter can be derived from either a pure ‘lake’ or pure ‘puddle’ model, and is thus likely to be a robust parameter. The energy absorbed by PS II is divided between the fraction used in photochemistry, Φ_II, and that lost non-photochemically. We introduce two additional parameters that can be used to estimate the flux of excitation energy into competing non-photochemical pathways, the yield induced by downregulatory processes, Φ_NPQ, and the yield for other energy losses, Φ_NO.