Research On The Characteristic Of Vernalization And Bolting Of Welsh Onion(Allium Fistulosum L.)and Their Mechanism

Bolting reduces the quality and commercial yield of welsh onion(Allium fistulosum L.) in production. Understanding factors related to inflorescence initiation and development is important for preventing flowering in welsh onion production, thus shows the economic value of the study. We did experiment at both field and growth chambers to investigate the factors of cultivar, sowing date and transplant site and their interactions on bolting of welsh onion. We analysed the seeding age, low temperature and duration of low temperature needed for welsh onion to get through vernalization requirement. After vernalization, the temperature and photoperiod requirements for welsh onion boltingwere studied. The plant nutrition, endogenous hormones and isoenzyme changes were detected durning vernalization and flowering period of welsh onion. We also cloned two important flowering related genes: AfFT and AfLFYand studied their expression level in welsh onion. At last we studied the regulatory network of flowering in the model grass Brachypodium distachyon. The main results are as follows:1. The results from the two experimental years field experiment showed that the cultivar, sowing date and transplant site all significantly influenced the bolting ofwelsh onion. ‘JinGuan’ and ‘TieGanWuTong’ cultivars could be considered as early-bolting varieties,‘XiaHei’ and ‘Lin Bang’ cultivars could be considered as late-bolting varieties. Seedling characteristics are closely related to the bolting characteristics of plants. In our experiment, because the seedling age and the active temperature during nursing were different for the 3 sowing dates treatment, the morphological development of the seedlings was different on the transplant date, thus, the sowing date had an influence on the bolting of the welsh onion. Temperature in the three transplant sites were different, high temperature(plastic tunnel) is better for growth and flowering of the welsh onion than low temperature(open field), as long as the high temperature is below the devernalization temperature, so welsh onion bolted earlier in the plastic tunnel than in the open field.2. The seedling age, low temperature and the duration of low temperature requirements for welsh onion to get through vernalization period were studied by growth chamber experiment. The smallest seedling age for welsh onion to feel low temperaturewas 50 calendar days, 3 leaves stage, and pseudostem diameter above 2.17 mm; The temperature for welsh onion to feel vernalization was between 15/100 C and 5/00 C, the optimum temperature was 10/50C; With the optimum seedling age and vernalization temperature, at least 15 days was needed for middle bolting type of welsh onion to get through the vernalization period and start flower bud differentiation.3. The temperature and photoperiod requirements forwelsh onion bolting were determined bygrowth chamber experiment. The optimum temperature was 25/200 C, flower showing was earlier and bolting rate was higher than other tested temperatures; welsh onion grew under long-day photoperiod flowered earlier than under short-day photoperiod, but under the right temperature, welsh onion could flower regardless, thus, welsh onion belongs to day-neutral plants.4. Biomass and biochemical substances level during vernalizaiton and bolting periods in welsh were analysed. In the winter, except soluble protein and C/N value between bolted and unbolted welsh onion were different, other tested indexes all had the same change patterns, that means their change was because welsh onion grow in cold environment, had no relationship with vernalizaion. The content of soluble protein in bolted welsh onion raised while in unbolted welsh onion the content had a decrease, that means high level of soluble protein benefited welsh onion’s flower bud differention. The C/N value of bolted welsh onion was increased progressively, the appropriate C/N value is the basis of flower bud differentiation, The C/N value of unbolted welsh onion decreased at the late winter, so plant did not flower at the next spring; After winter, as the vegetable growth period was finishing, welsh onion started reproductive growth period. The lowest dry matter content in bolted welsh onion was at March 21,welsh onion flower process might have high energy cost. Before bolting, the soluble protein and sugar content reached the highest value in bolted welsh onion, soluble amino acid content had a decrease before bolting, reached the lowest level. Amino acid is the precursors of protein, the decrease of amino acid was because new protein was created before welsh onion flowering, high level of soluble sugar content was the energy base of welsh onion flower process. ABA、GA and ZR all showed the highest level before flowering, whereas IAA reached the lowest level. That means ABA、GA and ZR had positive effect on welsh onion bolting while low level of IAA was good for welsh onion bolting. SOD and POD activity had no relationship with vernalization and bolting of welsh onion.5. We isolated homolog genes of two important flower genes: FT and LFY, from welsh onion and got their whole sequences by homology cloning and RACEtechniques. By comparison, AfFT and AfLFY shared high identical sequences with other homolog genes. The phylogenetic tree analysis made a further evidence that AfFT and AfLFY in welsh onion are homolog genes of FT and LFY. We examined their expression patterns in leaf, apical meristem point and flower in different life period of welsh onion by quantitative real time PCR. Our data suggested that AfFT had the highest expression level before flowering in leaves, and there were little expression in meristem and flower organs. These were evidences that, AfFT genemay like other FT homolog genes, iscreated from leaves then AfFT protein transfer to apical meristem points to increase flower. AfLFY had really low expression level in leaves, main expression organs were apical meristem points and flowers. The highest expression level showedbefore flowering.AfLFYmay increase flower at apical meristem point area.6. The regulatory network of flowering in the model grass Brachypodium distachyonwasstudied. The results showed that BdVRN1 and BdVRN3 had the same positive feedback circuit in non-core pooid Brachypodium distachyon and core pooid wheat and barley. Although functional analyses strongly support that BdVRN2 is a repressor of flowering. Unlike in wheat and barley, VRN1 amiRNA knockdown lines did not show the predicted increase in VRN2 expression, which meansBd VRN1 does not negatively regulate BdVRN2. Also rather than decreases in response to cold as in winter wheat and barley, BdVRN2 expression level actually increases in Brachypodium distachyon. Together, these data support a model in which orthologous VRN2 genes have retained a repressive flowering function during the diversification of pooids, but that co-option of VRN2 into the network of genes regulated during vernalization occurred after the divergence of non-core Pooideae Brachypodium distachyon and core Pooideae wheat and barley. Possibly allowing further diversification of core pooids into even colder, more seasonal climates, of the temperate north.