| Tobacco weather fleck is a kind of un-infectious diseases of tobacco leaf, which decrease the quality and the yield of tobacco leaf. The disease has been becoming one of the main tobacco diseases from 1980's in China.A study was conducted to determine the physiological factors and environment factors of tobacco weather fleck occurring by natural and control conditions, and to determine the genetic model by a 1/2P*(P+1) diallel cross with 6 parents. The results was listed out below:1. In 24 hours tobacco weather fleck disease can be caused by treating tobacco with higher ozone(650×10-9 ) and lower temperature of 10℃ at the same time.But single ozone or lower temperature of 10℃ can not cause the disease, so lower temperature is the inducing factors of tobacco weather fleck disease, and ozone is the causing factor.2. In both years of natural disease investigation in the field .it was found that different gene types of cured tobacco has different resistance to weather fleck disease. The disease indexes of cultivars of RG17.K326.K346 were higher than others, and these cultivars presented susceptible to the disease; The disease index of YUYAN3 is middle:The cultivars of NC89. YUNYAN87, YUNYAN85, ZHONG YAN90 appeared lower disease index and presented higher resistance to tobacco weather fleck disease. The results was confirmed by the rate of the disease occurring under control conditions.3. Investigation the stoma density and stomatal conductance of tobacco years in the field, the results of both years indicated that the change regulars of under-epidermic stoma density of and stomatal conductance of tobacco leaf among different stalk positions presented upper> cutlers> lower. The gene types of cured tobacco possess higher under-epidermic stoma density of lower leaf presented higher disease index of tobacco weather fleck and be susceptible to ozone. Similarly, the gene type of cured tobacco possess higher stomatal conductance of tobacco leafpresented the same result.4. It was also found that the activity of antioxidant enzymes(SOD, CAT, POD) of tobacco leaf decreased when tobacco was treated by lower temperature, but increased when treated by ozone. Activity of SOD increased significantly, activity of POD decreased typically, and activity of CAT indicated a slight increase in tobacco leaf when tobacco leaf was caused the weather fleck disease by treated with ozone and lower temperature. The conclusion was drew that SOD is one of the main antioxidant enzymes to eliminate the damage by ozone, CAT and POD also played some roles.5. The contents of free amino acids and total nitrogen of tobacco leaf treated by lower temperature decreased, but the contents of potassium and total soluble sugar of tobacco leaf increased. Other while all of the contents of free amino acids and total nitrogen and potassium and total soluble sugar of tobacco leaf decreased after the tobacco was caused weather fleck disease by ozone and lower temperature. Total soluble sugar and potassium and free amino acids were connected with the resistance to weather fleck. A gene type of flue-cured tobacco which contained higher total soluble sugar and potassium and free amino acids was resistance to weather fleck. It was not found some relations between total nitrogen content of tobacco leaf and the resistance to weather fleck disease. The chill-tolerance of variety indicated tobacco disease resistance to weather fleck to some extent.6. From diallel cross analysis on inheritance of disease resistance to tobacco weather fleck, the disease index of tobacco weather fleck was found genetically consilient to the Additivity-Dominance model. For the inheritance of disease index, the dominant genes are mostly positive, and recessive genes are mostly negative. Narrow heritability of disease index was high, so it is probably to breed high resistance variety by hybridization.
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