The relationship between the quantum yield of photosynthetic electron transport and quenching of chlorophyll fluorescence

Measurements of the quantum yields of chlorophyll fluorescence and CO2 assimilation for a number of plant species exposed to changing light intensity and atmospheric CO2 concentrations and during induction of photosynthesis are used to examine the relationship between fluorescence quenching parameters and the quantum yield of non-cyclic electron transport. Over a wide range of physiological conditions the quantum yield of non-cyclic electron transport was found to be directly proportional to the product of the photochemical fluorescence quenching (qQ) and the efficiency of excitation capture by open Photosystem II (PS II) reaction centres (Fv/Fm). A simple fluorescence parameter, ΔφF/φFm, which is defined by the difference in fluorescence yield at maximal φFm, and steady-state φFs, divided by φFm, can be used routinely to estimate changes in the quantum yield of non-cyclic electron transport. It is demonstrated that both the concentration of open PS II reaction centres and the efficiency of excitation capture by these centres will determine the quantum yield of non-cyclic electron transport in vivo and that deactivation of excitation within PS II complexes by non-photochemical processes must influence the quantum yield of non-cyclic electron transport.