PhiPS2 protocol

Leaf gas exchange was assessed with a portable infrared gas analyzer (LI-6400; LI-COR Inc., Lincoln, USA) equipped with a fluorometer (LI-6400-40, LI-COR Inc., Lincoln, USA). Data was periodically collected from the youngest fully expanded leaf (5–9 h after photoperiod initiation), never extending a total period of 90 min. Block temperature was set at 28 ◦C, light intensity was set at 300 µmol photons m−2 s−1 and CO2 concentration was kept at 400 ppm with an air flow rate of 500 µmol s−1. Fs and Fm’ represent the steady state and maximum fluorescence measured under light adapted conditions.