| Cabbage (Brassica oleracea L. var. Capitata L.), 2n=2x=18, originated in the coast of the Mediterranean and was introduced into China in the 16th century. It has become a kind of the world important vegetables that could produce and supply annually because of its wide adaptability, stress resistance, coupled with its simple cultivation techniques, high yield, resistance to storage and transport, high nutritional value and many edible ways. However, the leafy head cracking of spring cabbage not only affects the appearance of quality and commercial quality, but also is susceptible to disease, causing the decay of ball leaves, having poor storage property and decreased quality. It seriously affects the storage and transport of cabbage and causes huge economic losses. Therefore, resistance to cracking become an important breeding target for many breeders. The target is that the leafy head will not crack when the cabbage reaches maturity and the rate of cracking ball should be no more than 15% after the maturity. During the "Eleventh Five-Year Plan" in order to meet the demand of production and market strengthening the resistance to cracking was proposed.How to further improve and enhance the quality of resistance to cracking and breeding the new variety of spring cabbage resistant to cracking is becoming very crucial. However, to date, a unified standards of grading and methods of identification about the cracking of spring cabbage has not been put forward. As a result, our research is extremely urgent and will not only have great significance and far-reaching effects on the breeding of the spring cabbage that is resistant to cracking but also provide references for the future breeding work and cultivation production.This thesis includes the following three parts:1.Cultivate inbred lines of spring cabbage in the greenhouse under the conventional conditions and investigate the time, rate and severity of the leafy head cracking, then analyze the characteristics of head-splitting. Though the research about transmission electron microscope (TEM) and scanning electron microscope (SEM) observation of cracking leaves it provided the theoretical basis for the identification and grading standards of the leafy head cracking and theoretical guidance on the cultivation technology in the production process.2.Different grading standards of head-splitting about spring cabbage was established based on the affects of different greenhouse cultivation on the head-splitting and pictures of grading standards were also worked out combined.3.Several spring cabbage germplasm were identified in the second year in order to screen anti-cracking materials for breeding of varieties resistant to cracking.In this study, following conclusions were drawn:1.The cracking always occurs in the late vegetative period for most of the inbreds that are easy to split and the head-splitting of anti-cracking inbreds always occurs in the middle and on the top of the leafy head after the mature stage. Different inbred lines had a significantly difference of the severity about the head-splitting which is closely related to the characteristics of inbred lines themselves.2.Grading standards of head-splitting in spring cabbage was set:∑= a/H +b/C+ h/2A。Grade 0:Σ= 0; Grade 1: 0 <Σ≤20%; Grade 3: 20% <Σ≤50%; Grade 5: 50% <Σ≤100%; Grade 7: 100% <Σ≤180%; Grade 9:Σ> 180%.3.Environmental conditions of greenhouse cultivation are not conducive to head-splitting of spring cabbage. The water stress caused by the weather changes in the open field cultivation and there is severe head-splitting in adverse weather conditions.4.The anti-cracking inbreds have well developed vascular tissues in mesophyll cells, more conduits in the secondary xylem, tightly packed cells which is uniform in size, small single cells, small gaps between cells, thick cell walls, high lignification, increased mechanical strength, large adhesion between cells and elasticity between mechanical tissues and are uneasy to crack.. On the contrary, it would be easy to crack.5.There is significant differences in the leaf cell structures between the risistant and easy to crack cabbage inbred lines which could be used as one of the indicators for identification of anti-cracking for spring cabbage. The cracking of mesophyll cells is caused by the swelling of hydrating and the internal growing pressure, therefore, attention should be paid to changes of the water in the plant.6.By investigate the severity of cracking in the 15 days after the maturity under the appropriate environmental conditions of spring cabbage, the index of head-splitting was calculated and it could be used to divide the anti-cracking level of spring cabbage as well as identify several germplasm of cabbage.
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