| As the premature bolting restricts the quality and yield of the Welsh onion greatly, it should be controlled in the production of the Welsh onion. However, in order to shorten the breeding cycle and speed up the breeding process during the breeding of Welsh onion, the premature bolting needs to be strengthened. Therefore, it is of great significance to study the bolting mechanism and to master the key technology of bolting controlling in Welsh onion in cultivation and breeding. For this purpose, this experiment carried out the studies on the relationship between the plant index, nutritional status, seeding date, endogenous hormones and accumulated temperature and the bolting of Welsh onion as well as the effects and control of seedling age, low temperature and plant growth regulators treatment on the bolting of Welsh onion. The main results are as follows:1. By determining the nutrition indicators and the contents of 4 kinds of endogenous hormones in both the bolting and unbolting Welsh onions, it was found that nutritional accumulation was required during the bolting of Welsh onion before the winter; the low N content and high level of C/N were helpful to the bolting and flowering of the Welsh onion before the winter; after the plant lived through the winter, the P and K contents were positively correlated with the bolting in the Welsh onion plant; high ABA content promoted the bolting of Welsh onion, and for the IAA, GA3 and ZR contents, the low levels before the winter and high levels after the winter but before the bolting was favorable to the bolting of the Welsh onion.2. The earlier the seeding date was, the greater the plant height and stem diameter were, with more leaves, higher bolting rate and bolting index, and the earlier the Welsh onion bolts. The plant height, stem diameter and the leaf number were positively correlated with the bolting rate before the winter. Besides, the accumulated temperature value before the winter was also positively correlated with the morphology indicator (MI) and bolting rate of Welsh onion. The higher the accumulated temperature value was before the winter, the more favorable it was for the plant development, as well as for the promotion of the bolting rate and early bolting of the Welsh onion.3. The unbolted Welsh onion under the natural conditions bolted normally in the greenhouse, and the earlier the seeding date was, the higher the bolting rate was. Difference existed among the tested varieties. The highest average bolting rate was found in Zhangqiu Dawutong followed by Lucong 3, and the variety of Lucong 1 showed strong bolting resistance. The effects of all kinds of experimental factors on the bolting of Welsh onion was sequenced as: treatment time>variety>seedling age>temperature. The shorter it was treated under low temperature, the higher the bolting rate was. The greatest bolting rate was found in Zhangqiu Dawutong. The older the seedling age was, the higher the bolting rate was. The highest bolting rate was found under 5℃, followed by 9℃, and the poorest result occurred under 1℃. The best combination to promote the bolting of Welsh onion was M3P2W2S1, namely the Zhangqiu Dawutong sowed on September 15th, treated for 15d at 5℃.4. The critical physiological seedling age of bolting in Welsh onion was 13cm of plant height, with 0.3cm of stem diameter and about 3 full leaves. The critical active accumulated temperature before the winter was 970℃(≥5℃)and 770℃(≥10℃)and the effective accumulated temperature was 610℃(≥5℃and 540℃(≥10℃). The plants meeting the standard above completed the vernalization and then bolted after the low temperature accumulation of 0～10℃and 0～7℃at 400℃and 110℃, or after the effective low temperature accumulation of 0～10℃and 0～7℃at 210℃and 80℃, respectively.5. The bolting of Welsh onion was most significantly affected by the seeding date before the winter, while least significantly affected during the period of growth regulator application. Compared with the control group, the bolting of Welsh onion was significantly affected by in IAA-treated group and MH-treated group, i.e., it was significantly promoted when treated with IAA, and significantly inhibited when treated with MH. In this experiment, the best combination to promote the bolting of Welsh onion was A1B2C1D2, namely the Welsh onion was sowed on August 31st (the first seeding date), and sprayed with IAA 30mg·L-1 after the winter (March 5th); the best combination to inhibit the bolting of Welsh onion was A2B1C4D3, namely the Welsh onion was sowed on September 20th (the second seeding date), and sprayed with the MH 300 mg·L-1 before the winter (October 30th).
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