Photosynthetic photon flux density (PPFD) response curves were generated by a GFS-3000 portable photosynthesis system (WALZ, Effeltrich, Germany). The photosynthetic parameters were determined on the fifth or sixth fully expanded leaves. In a gas exchange chamber, PPFD was controlled using a LED radiation source and increased from 0 to 3000 f.Lmol m−2 s−1 . Five replications were used for each group.