Figure 4.

Evolution of photosynthetic parameters in wild type 4A+ cultures upon shift from 20 μmol photons m⁻² sec⁻¹ low light to 200 μmol photons m⁻² sec⁻¹ (200 μE, grey bars) or 500 μmol photons m⁻² sec⁻¹ (500 μE, white bars) high light. Samples were taken from low light grown cultures (0 h) and 1‐, 4‐ and 24 h after the shift to high light. Changes of the nonphotochemical quenching parameter (NPQ; a, b) and of the maximal PSII quantum efficiency (FV/FM, c) were determined by PAM fluorometry. Prior to the measurements, samples were dark incubated for 10 min. NPQ was measured during and after a period of illumination with 800 μmol photons m⁻² sec⁻¹, as indicated by the black‐and‐white bar. Parameters shown in panels (d–f) were determined by absorption spectroscopy. PSI/PSII ratios (d) were calculated from the ratios of the increase in the trans‐thylakoid electric field (ΔΨ) caused by the transfer of one charge across the membrane by either photosystem. ΔΨ caused by the transfer of one charge across the thylakoid membrane by PSI was measured in the presence of PSII inhibitors DCMU (40 μm) and HA (1 mm). Turnover rates of overall photosynthetic electron transfer (RPSI, e) and turnover rates of cyclic electron flow around PSI (RCEF, f) were normalized to the amplitude of one charge transferred across the membrane by PSI. RCEF was measured in the presence of PSII inhibitors DCMU and HA. At 24 h, RCEF is significantly higher in cultures kept under 500 μmol photons m⁻² sec⁻¹ than in cultures kept under 200 μmol photons m⁻² sec⁻¹ (two‐way anova, Bonferroni correction: P < 0.001). Values shown represent means of nine independent biological replicates, except for RCEF: five replicates. Results are shown ± standard deviation.