Separation of light-induced linear, cyclic and stroma-sourced electron fluxes to P700⁺ in cucumber leaf discs after pre-illumination at a chilling temperature

Pre-illumination of cucumber leaf discs at 4°C with low-irradiance white light (i) led to a marked decrease in the extent of photo-oxidation of P700 (the special chlorophyll pair in the PSI reaction center) in actinic light at room temperature and (ii) hastened the post-illumination re-reduction of P700⁺. Quantifying the linear, cyclic and stroma-sourced electron fluxes to P700⁺ in two actinic light regimes, we found that there was no increase in cyclic or linear electron fluxes to account for these changes. Rather, we observed a decrease in the maximum extent of P700 photo-oxidation assayed by a strong flash superimposed on continuous, background light of wavelength 723 nm, which we interpret to represent a loss of stable charge separation in PSI due to enhanced charge recombination as a result of the pre-illumination treatment. The funneling of electrons towards fewer non-damaged PSI complexes could explain the hastened post-illumination re-reduction of P700⁺, aided by a slight increase in a stroma-sourced electron flux after prolonged pre-illumination at 4°C. Quantifying the separate fluxes to P700⁺ helps to elucidate the effects of chilling of cucumber leaf discs in the light and the reasons for the hastened post-illumination re-reduction of P700⁺.