| Out of 22 onion cultivars collected from different region, three cultivars with different bolting character were screened from sowing dates experiements to be done followed research. The main physiological & Biochemical characteristics and main effects of low temperature treatment of onion bulbs and plants from bulbs and seeds on promoting bolting and flowering, and the effects of GA3 and PP333 treatmnets on premature bolting of onion were investigated in three successive years of experiment. The results were as follows:1. During thses three years, to keep these onion seeds used in expreiment under dried condition with 20 C could usefully prevent seeds vigor from decreasing, which lessen the effect of onion seeds vigor disgradation on bolting.Results of successive two years field experiments showed the yellow and the red onion cultivars were not sensitive to permatur bolting, however, the white onion cultivars were very susceptible to bolting. Bolting rate of plants sown at September 1 reached 100 %. there were signifcant correlations between bolting rate and sowing dates and the size of pre-winter plants respectively. Less plants of cultivars No.1 occurred bolting when the size of pre-winter plant was thought to be less than 6 leaves and 7.8 mm pseudostem in diameter. As for resistant bolting cultivar 84-5, Plants, with 7 leaves and 9mm pseudostem in diameter, began to be sensitive to vernalization, therefore, it is recommended to control the size of pre-winter plant less them in order to avoid premature bolting.2. For cultivar 84-5 (No.6) plants with 8 leaves and 10mm pseudostem in diameter treated at 5 Cfor 30, 50, 70, 85days, the bolting rate increased with treatment time. The plants treated for 85 days could finish vernalization and 96 per cent of plants were bolters. During treatment, nucleic acid (RNA and DNA) contents increased: RNA - DNA - RNA. Soluble protein content increased with RNA content, as well as acid phosphatase activity. POD activity firstly kept low level and rose later, however, SOD activity increased, decreased later. Endogenous hormones contents went up to max :ABA-GA1/3 -IAA-iPA. Compared with control, , plants treated for 85days contained less GAi/3 and more iPA with lower iPA/IAA, GA1/3/ABA, GA1/3/ IAA and higher iPA/ABA, which contribute to floral induction of onion. Compared with changes of Put and Spd contents, Spm content changed less.3. Treated at 5 C for 85days, bulbs and plants from bulbs of 'Jinqiu', a yellow cultivar with resistant bolting, both finished vernalization and their bolting rate were 94.6% and 96%, respectively. Bulbs treatment reduced thetime from treatment to flowering and was fit to reproduce seeds at the same year as bulbs harvest.For bulbs treated for 45, 65, 85days, low temperature treatment increased nucleic acid (RNA, DNA) content which went up to max in plants treated for 65days, while soluble protein content decreased to the lowest, less than control, soluble protein content increasd in other treated plants, ACP activity changed the same with soluble protein content. Compared with control, POD activity decreased and SOD activity increased. Among endogenous hormones, IAA content decreased with later treatment time, bulbs treated for 85days had lower levels of iPA, IAA, GAi/3, Spd, and higher levels of ABA , higher ABA/IAA, ABA/GA1/3, GA1/3/IAA, GA1/3/iPA, iPA/ABA, which would be redound to transition of flowering.When plants from bulbs were treated at low temperature for 30, 50,70 and 85days respectively, nucleic acid(DNA, RNA) content increased in treated plants before 50 days, then went down; soluble protein content reached peak value only in 30 and 70 days, higher than control, ACP activity increased. Compared with control, POD activity decreased and kept at low level, while SOD activity increased. During treatment, the summit value of endogenous hormones ratio appeared in the sequence of GA1/3/iPA - GAi/3/ABA -iPA/ABA-IAA/ABA. Plants treated for 85days contained more GAi/3, iPA, ABA , Spd and less IAA, at the same time lower ABA/I...
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