**4. Materials and Methods**

#### *4.1. Plant Materials*

*Rhododendron pulchrum* Sweet used in this study was cultivated at Yangzhou University (32◦23 N, 119◦25 E), which located in Jiangsu Province, China. The purple (purple petals, purple blotches) flower variety of *R. pulchrum* is "Zihe", the pink (pink petals, purple blotches) and light pink (light pink petals, purple blotches) flower varieties are "Fenhe", and the white (white petals, no or very few blotches) flower variety is "Baihe". To explore the petal color regulation mechanism of *R. pulchrum*, we selected well-grown plants that had been planted for more than three years and collected full-bloom flowers having four different petal colors. The samples of purple, pink, light pink and white petals are labeled as A, B, C, and D, respectively (Figure 8). Fresh petals were collected on 24 April 2021, and flowers of the same color gradient from three individual plants were pooled as one sample. They were then frozen and ground in liquid nitrogen and subsequently stored in ultralow-temperature freezer under −80 ◦C for subsequent analysis.

**Figure 8.** Color comparison among four phenotypes of *Rhododendron pulchrum* Sweet. The samples of purple, pink, light pink and white petals are labeled as (**A**–**D**).
