1. 10 µg Chl + 1 mM NaN 3 10 µg Chl in 50% D 2 O Singlet oxygen detection by EPR spectroscopy using TEMPD as spin probe. Shown are typical spectra recorded after 2 min illumination with red light at 500 µmol quanta m -2 s -1. The samples contained 10 µg/ml pure chlorophyll a and b (Chl a/b = 3). Chl was dissolved in methanol and measurements were performed in a buffer containing 0.3 M sorbitol, 50 mM KCl, 5 mM MgCl 2, 25 mM HEPES pH 7.6. If indicated, azide was added, or water was replaced by 50% D 2 O. Suppl. Fig. 1

2. Spin trapping with 4-POBN/EtOH. Thylakoids from flag leaves of Lomerit from the 14th of June 2014 were chosen. 10 µg Chl ml -1 were illuminated for 2 min with red light (RG 630) at 500 µmol quanta m -2 s -1. The spin trapping assay contained 50 mM 4-POBN, 4% ethanol and 50 µM FeEDTA, when indicated. Suppl. Fig. 2

3. Spin trapping with DEPMPO. Thylakoids from flag leaves of Lomerit from the 14th of June 2014 were chosen. 40 µg Chl ml -1 were illuminated for 2 min with red light (RG 630) at 500 µmol quanta m -2 s -1. The spin trapping assay contained 10 mM DEPMPO, 1 mM DPTA in 0.3 M sorbitol, 20 mM HEPES (pH 7.6). When indicated, 50 U SOD or 100 µM methylviologen (MV) were added. Suppl. Fig. 3

4. magnetic field, mT no inhibtor + DCMU + DNP-INT Spin trapping with 4-POBN/EtOH, effect of inhibitors Thylakoids from flag leaves of Lomerit from the 14th of June 2014 were chosen. 10 µg Chl ml -1 were illuminated for 2 min with red light (RG 630) at 500 µmol quanta m -2 s -1. The spin trapping assay contained 50 mM 4-POBN, 4% ethanol, 50 µM FeEDTA, 10 µM DCMU and 100 µM DNP-INT, when indicated. Suppl. Fig. 4

5. ROS detection by spin trapping EPR using 4-POBN/EtOH as spin trap. Thylakoids (10 µg Chl) Pure chlorophyll (10 µg Chl) Shown are the signals of the typical 4-POBN/α-hydroxyethyl adducts recorded after 2 min illumination with red light (RG 630) at 500 µmol quanta m -2 s -1. The samples contained thylakoid membranes (10 µg Chl/ml) or pure chlorophyll (10 µg Chl/ml Chl a/b = 3). Chl was desolved in methanol and measurements were performed in a buffer containing 0.3 M sorbitol, 50 mM KCl, 5 mM MgCl 2, 25 mM HEPES pH 7.6 Suppl. Fig. 5