New Fluorescence Parameters for the Determination of QA 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 qP, 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, qL, based on a Stern-Volmer approach using a lake model, which estimates the fraction of open PS II centers. We suggest that qL 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 ΦII, the yield of PS II. In contrast to qL, we show that the ΦII 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, ΦII, 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, ΦNPQ, and the yield for other energy losses, ΦNO.

Keywords: antenna; chlorophyll a fluorescence; electron transport; fluorimeter; photosynthetic unit; quantum yield.