Effect Of Exogenous Gibberellin （GA₃） And Indoleacetic Acid （IAA） On Carbon And Nitrogen Metabolism And Quality Of Upper Leaves Of Flue-cured Tobacco

Flue-cured tobacco（Nicotiana tabacum L.） was one of the important economic crop in China. The measures to improve quality of tobacco leaves play an essential role in determining enhancing market competitiveness and sustainable development of tobacco production. Most studies indicated the effect of plant growth regulators on the quality of tobacco leaves by regulating the carbon and nitrogen metabolism. However, many research focused on the influence of single plant growth regulator on tobacco, such as gibberellin（GA）, indoleacetic acid（IAA）, abscisic acid（ABA）, brassinolide（BR） and so on. There were rare reports on the effect of two kinds of plant growth regulators on tobacco. Therefore, in this study, a field experiment and a pot experiment were conducted to investigate the effect of exogenous application of GA₃ and IAA on the carbon and nitrogen metabolism and quality of flue-cured tobacco using Yunyan 99 as materials. The main conclusions are as follows:1. The effects of exogenous single and combination application of GA₃ and IAA on carbon metabolism in upper leaves of flue-cured tobacco were significantly different. There were increases in soluble total sugar content, starch content, invertase activities under single application of GA₃, as well as increases in reducing sugar content, total carbon content and amylase activity under the single application of IAA treatments, respectively. The effect of combination of GA₃ and IAA were better than the single treatment of them. Moreover, the combination treatment of GA₃ and IAA resulted in enhancement of the activities of amylase and invertase, increase of the content of total carbon and reducing sugar.2. The single and combination treatments of GA₃ and IAA had significant promoting effect on nitrogen metabolism in the upper leaves of flue-cured tobacco. Regardless of single or combination of GA₃ and IAA both increased the content of total nitrogen as well as nitrate reductase activity. The IAA single treatment showed better effect on reduction of the nicotine content than GA₃ treatment. The combination treatment effects were better than the single treatment. The combination treatment of GA₃ and IAA had a great effect on an increase of the total nitrogen content and a decrease of the soluble protein content（P<0.01）, and a significant difference on nicotine content decreasing（P<0.05）.3. Application of GA₃ and IAA could improve the economic characters and chemical compositions of flue-cured tobacco after baking. Regardless of single or combination of GA₃ and IAA both increased yield and the proportion of superior tobacco leaves, as well as the contents of total nitrogen, total sugar, reducing sugar and potassium, enhanced the ratio of sugar to alkali and the ratio of total nitrogen to nicotine, and decreased in the content of nicotine and starch of tobacco leaves. It is evident that the effect was better under combination than single application of them, especially for yield, the proportion of superior tobacco leaves, total sugar content and reducing sugar content of baking tobacco leaves（P<0.01）.4. The stepwise regression analysis and path analysis indicated that the contents of nicotine, total carbon, total potassium, reducing sugar, starch, soluble total sugar, total nitrogen, carbon and nitrogen ratio, leaf area at 30 days after topping had significant influences on economic character and chemical compositions of flue-cured tobacco. Moreover, the starch content at 30 days after topping was the main limiting factor of the yield of flue-cured tobacco. The total potassium content at 30 days after topping was the main factor limited the output value and the reducing sugar content after baking. The content of reducing sugar at 30 days after topping was the main limiting factor of the starch content and nicotine content after baking. The results above suggested that related indicators at 30 days after topping in relation to the carbon and nitrogen metabolism could regard as predicting factors of the variation trend of economic characters and chemical composition after baking.5. In conclusion, the combination of GA₃ 50 mg·L-1 and IAA 30 mg·L-1 was the best treatment to enhance carbon and nitrogen metabolism and quality of upper leaves of flue-cured tobacco.Based on the results, the difference and relationship between carbon and nitrogen metabolism and quality of tobacco leaves were revealed by exogenous GA₃ and IAA application after topping. Therefore, the combination application of GA₃ and IAA could serve as a useful approach for improving the quality of upper leaves of flue-cured tobacco.