Fig. 3: Chlorophyll fluorescence traces of algae and images of higher plants.

a A typical chlorophyll fluorescence trace of Phaeodactylum tricornutum (FACHB-863). NPQ was induced either with high light illumination (AL, 50 s–510 s), or by adding HAc in the dark (the pH of the medium was 5.5) at 1400–2000 s. The fluorescence quenching was abolished upon addition of KOH in weak actinic light (10 μmol photons m⁻² s⁻¹). b A typical chlorophyll fluorescence trace of the Porphyridium purpureum (FACHB-840). NPQ was first induced with high light illumination (50 s–180 s), then a second round of quenching was induced by adding HAc (pH of the medium: 5.5) at 400 s–650 s after full recovery of fluorescence. The second round of fluorescence quenching was recovered by adding KOH (the pH of the medium was 7.5) in weak actinic light (10 μmol photons m⁻² s⁻¹). c Chlorophyll fluorescence trace of Physcomitrella patens. Adding HAc (pH of the medium: 5.5) caused chlorophyll fluorescence quenching and the addition of KOH (the pH of medium 7.5) abolished the quenching. d Similar chlorophyll fluorescence trace of Chlorella pyrenoidosa (FACHB-9) as obtained in (c). The addition of HAc (pH of medium 5.5) caused chlorophyll fluorescence quenching and the addition of the KOH (titration of the medium pH back to 7.5) deactivated the quenching. e The NPQ images of wild-type Arabidopsis Col-0 (left) and npq4 mutant (right) after stable NPQ (250 s) was reached under actinic light (AL 400 μmol photons m⁻² s⁻¹). For each plant, two leaves were filtrated with 0.1 M acetic acid, while another two leaves were infiltrated with an equal amount of H₂O as control. f The npq images of Triticum aestivum (left) and Sorghum bicolor (right) after a stable NPQ was reached under actinic light (400 μmol photons m⁻² s⁻¹). For each leave, two parts of a leaf were infiltrated with 0.1 M acetic acid, while another two parts were infiltrated with an equal amount of H₂O as control. All experiments were performed in triplicate.