Opposite domination of cyclic and pseudocyclic electron flows in short-illuminated dark-adapted leaves of angiosperms and gymnosperms.

The present work was aimed to explain the recently reported higher O-dependent electron flow capacity in gymnosperms than in angiosperms and to search for other differences in the electron transport processes by simultaneous characterization of the relative capacities of pseudocyclic (direct or Flavodiiron proteins (Flv)-mediated O-reduction, Mehler(-like) reactions) and cyclic electron flows around photosystem I (CEF-PSI). To this end, a comparative multicomponent analysis was performed on the fluorescence decay curves of dark-adapted leaves after illumination with a 1-s saturating light pulse. In both gymnosperms and angiosperms, two or three exponential decay components were resolved: fast (t 1 ~ 170-260 ms), middle (~1.0-2.3 s), and slow (>4.2 s). The sensitivity of the decay parameters (amplitudes A1-3, halftimes t 1-3) to the alternative electron flows was assessed using Arabidopsis pgr5 and ndhM mutants, defective in CEF-PSI, Synechocystis sp. PCC 6803 Δflv1 mutant, defective in Flv-mediated O-photoreduction, different O concentrations, and methyl viologen treatment. A1 reflected the part of electrons involved in linear and O-photoreduction pathways after PSI. The middle component appeared in pgr5 (but not in ndhM), in gymnosperms under low O, and in Δflv1, and reflected limitations at the PSI acceptor side. The slow component was sensitive to CEF-PSI. The comparison of decay parameters provided evidence that Flv mediate O-photoreduction in gymnosperms, which explains their higher O-dependent electron flow capacity. The concomitant quantification of relative electrons branching in O-photoreduction and CEF-PSI pathways under the applied non-steady-state photosynthetic conditions reveals that CEF-PSI capacity significantly exceeds that of O-photoreduction in angiosperms while the opposite occurs in gymnosperms.