*5.1. Plant Material and Growth Conditions*

The experiment was conducted in LED climate rooms located in the basement of the College of Grassland Science, Qingdao Agricultural University, Qingdao, China. Light intensity and spectral irradiance (λ = 350–800 nm) were measured by HR550 (Hipoint Inc., Gaoxiong, Taiwan), and the spectral distributions are shown in Figure S1. The photoperiod was 12 h with a 25 ◦C day temperature, 20 ◦C night temperature and a relative humidity of 60%.

*M. sativa* L. cv. Zhongmu 1 was chosen for the studies on phenotypic responses to growth conditions. Before the experiment, alfalfa seeds were surface-sterilized by 75% ethanol for 1 min and rinsed with deionized water for 5 min and germinated on wet sterile Whatman No. 1 filter paper in a daily 8 h light from white fluorescent tubes (Sanpai Corporation, Shanghai, China) with a mean photon flux density of 60 µmol m−<sup>2</sup> s −1 (400–700 nm) at 25 ◦C. After seed coat rupture and cotyledon expansion at 5 days, 10 uniform-sized sprouting alfalfa seedlings were transplanted into a separate plugged hole in a foam sheet floating in a 3.3 L plastic container filled with half-strength Hoagland's solution. These containers were placed in the LED climate room with a light intensity of 500 µmol m−<sup>2</sup> s −1 . When the first trifoliate leaf was well-developed, the pre-cultured seedlings were transferred to five light treatments. Photosynthetic photon flux density (PPFD) was 100 µmol m−<sup>2</sup> s −1 (L100), 200 µmol m−<sup>2</sup> s −1 (L200), 300 µmol m−<sup>2</sup> s −1 (L300), 400 µmol m−<sup>2</sup> s −1 (L400) and 500 µmol m−<sup>2</sup> s −1 (L500), and light quality was the same in all treatments. The highest light intensity was chosen as it was used to grow alfalfa seedlings under laboratory conditions, and the lowest light intensity can be considered comparable to the natural shade under a closed oat forage canopy under clear-sky conditions (82–116 µmol m−<sup>2</sup> s −1 , unpublished analysis of solar radiation penetrating closed canopy in the alfalfa–oat intercropping system by W. Tang). Four containers were placed in each light treatment, and each container had 10 seedlings. The plastic containers were moved daily to avoid boundary effects, and Hoagland's solution was renewed every 3 days and kept aerated by air-spraying. Every treatment was performed with four replicates.
