| Plant vernalization is an 'ancient' topic. More than 100 years ago, the phenomenon of vernalization was observed. It was found that many plants growing at high latitudes require exposure to prolonged periods at low temperature before flowering. Many genes related to flowering have been identified and cloned based on a series studies with the model plant Arabidopsis thaliana. Nowadays, researchers focus on how these genes' expression regulating during the transition from vegetative growth period to reproductive growth period. Recent researches showed that DNA demethylation plays an importmant role hi regulating gene expression which related to the initiation of floral bud differentiation. The relationship between DNA demethylation and flowering were mainly focused on model plants, such as Arabidopsis, though some experiments were done with Brassica campertris, Triticum aestivun. But changes of DNA methylation level after flower initiated has not reported so far. On the other hand, one of the most important outstanding questions is progeny re-produced from plants which require vernalization for flowering should reset the vernalization again, i.e. the vernalization signal is not transmitted to sexual progeny. In the present study, radish cultivars (Raphanus sativus L.), that differed from their vernalization requirements, were used to study the effect of seeding time on flowering, and the effects of low temperature treatment of germinated seeds on floral bud differentiation and flowering. DNA methylation level in stem apices of seedlings which exposed at various temperatures for different days was assayed when low or normal temperature treatment ended. Soluble sugar, sucrose, starch, soluble protein content, and esterase (EST), a -amylase, peroxidase (POD) activities and their isozyme were analyzed during floral bud differentiation. DNA methylation level hi stem apices and leaves at different stages of floral bud differentiation were assayed as well. Seeds of bom the spring cultivar 'Duanye 13' and the whiter cultivar 'Yidianhong' were treated with 0.10~1.00 m mol/l 5-azacytosine (a demethylation agent) to investigate its promotion effects on floral bud differentiation and flowering, DNA methylation level of stem apices was assayed after 5-azacytosinetreatment. All results obtained are summarized below.1. In the natural condition, days from seeding to flowering (DSF) was affected by cultivars and seeding date significantly. There was a significant interaction between cultivars and seeding date on DSF. The radish cultivars 'Baiyuchun' and 'Zaochundagen', which required a strong vernalization condition, did not flowered in the seeding year unless sown before April, but the spring radish cultivar 'Duanye 13' did flower even sown at a warm and short-day season. The shorter the DSF was, the less the leaves.2. Floral bud differentiation of bom cultivar 'Yidianhong' and 'Duanye 13' were promoted by low temperature treatment to germinated seeds. The promoting effects were better when the germinated seeds were subjected to lower temperature and/or for longer treatment duration. Only 10-day pretreatment of germinated seeds at 5℃ or 10℃ was enough to enhance the floral bud differentiation in the spring variety 'Duanye 13', whereas at least 20-day pretreatment was required for the promotion of floral bud differentiation in the winter variety 'Yidianhong'. Non-vernalized 'Duanyi 13' plants flowered 70 days after transplanting in a warm (above 15℃) and long photoperiod (16 h) growth condition, but non-vernalized 'Yidianhong' plants did not flower 90 days after transplanting in the same growth condition as 'Duanye 13'. On the other hand, low temperature pretreatment of germinated seeds inhibited the plant growth, including plant height, number of leaves as well as leaf length and width. The longer the treatment duration was, the stronger the inhibitions were.3. The amounts of total soluble sugars, sucrose and starch during the floral bud differentiation, either in stem apices or in leaves, changed r...
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