*4.2. Chlorophyll* a *Fluorescence*

Images of chlorophyll *a* fluorescence values were obtained using the IMAG-MAX/L imaging PAM system (Heinz Walz GmbH, Effeltrich, Germany), under which six plants were measured simultaneously. Plants that had been light-adapted under growth conditions were first dark-adapted for ≥20 min, after which minimal (Fo) and maximal (Fm) chlorophyll *a* fluorescence emissions were measured. Plants were then adapted to growth light conditions (≥30 min) for complete photosynthetic induction, after which they were shade-adapted at 90 μmol m−<sup>2</sup> s−<sup>1</sup> PAR in the imaging PAM for four minutes before chlorophyll *a* fluorescence emission under actinic light (Fs) and maximal fluorescence from the light-adapted leaf (Fm') were determined. Saturating beam duration was 0.7 s and saturating beam intensity was ~1300 μmol m−<sup>2</sup> s−<sup>1</sup> when determining Fm and ~2700 μmol m−<sup>2</sup> s−<sup>1</sup> in the case of Fm'. All measurements were conducted between 8:30 h and 14:00 h (growth lights switched on at 7:00).

From chlorophyll *a* fluorescence images, in ImagingWin (v2.47, Heinz Walz GmbH) four circular areas of interest (AOI) were selected per plant. In ImagingWin, AOI with several pre-defined diameters can be selected. AOI were chosen to cover as much total plant leaf area as possible while avoiding parts of the picture not covered by plant material; consequently, AOI were typically chosen to cover parts of the largest leaves of a plant (Figure S4A). From each area of interest, an average value of Fo, Fs, Fm and Fm' was obtained. The four values were later averaged to represent one biological replicate. Photosystem II maximum quantum efficiency was calculated as Fv/Fm = (Fm − Fo)/Fm, photosystem II operating efficiency was calculated as ΦPSII = (Fm' − Fs)/ Fm', and non-photochemical quenching was calculated as NPQ = (Fm − Fm')/Fm'.
