Genetic Testing of Red and White Onions

The dark-red onion (A. cepa L.) cultivar ‘Xiu Qiu’ (fourth generation inbred lines from the dark-red ‘Shanxi’ variety) and the white onion cultivar ‘Ring Master’(fourth generation inbred lines from the white ‘Xinjiang’ variety) were used in this study. We performed a series of genetic tests to confirm whether white onions were dominant or recessive; as the F₁ color of (‘Xiu Qiu’ × ‘Ring master’) was pink, the genotype of the white parent was i/i, and the phenotypes of the F₂ segregating population were white, yellow, pink, and dark-red (Supplementary Fig. S2).
Fresh, undamaged 7 cm diameter samples weighing between 280 g and 320 g were collected on the 40^th day after swelling from a test field at the Beijing Academy of Agriculture and Forestry Vegetable Research Center (Fig. 1a,b). Samples were washed, excess water was absorbed with blotting paper, and lengths between 1 cm and 2 cm from the top and the bottom of each were excised with a scalpel. Outer onion bulb layers were then divided into three parts for experiments; all materials were immediately frozen in liquid nitrogen (N) and stored at −80 °C prior to RNA extraction and flavonoid analyses.