| Nicotine content is one of the important indicators for quality of flue-cured tobacco. As mineral nutrition is the key factor to influence nicotine content of flue-cured tobacco, understanding of the role and its physiological mechanism of mineral nutrients in nicotine synthesis is significant to develop an effective nicotine regulation technology. Aimed to solve the problems of high level nicotine content of flue-cured tobacco (FCT) and bad chemical coordination of FCT quality in tobacco production area of northwest Hubei Province, this study studied the effects of phosphorus and potassium nutrition levels, nitrogen fertilizer forms, and boron nutrition level on nicotine synthesis and formation of tobacco quality during the topping stage by use of soil pot experiments, hydroponic experiments,15N tracing experiments, and field experiments. The main results were summarized as follows:1. The effects of P and K level on tobacco's nicotine synthesis, nutrient uptake and distribution before and after topping were studied with soil-pot experiment. The results suggested that P fertilizer improved nicotine accumulation8%-69%of upper parts of FCT, correspondingly increased the activity of putrescine-N-methyltransferase (PMT), arginine decarboxylase (ADC), and ornithine decarboxylase (ODC) in roots, and the intake of N, P and K. Meanwhile, the increase of potash fertilizer decreased the nicotine accumulation by8%-45%, the activity of PMT, ADC and ODC in roots, and the uptake of N and P, but increased the uptake of K. The result indicated that the P/K nutrition of FCT could control the nicotine synthesis and accumulation through the regulation of nicotine synthase activity and the influence of nutrition accumulation.2. The influence of different N fertilizer forms and its coordination to nicotine content and relevant nicotine synthase activity were investigated with soil-pot experiment. The results indicated that100%ammonium-N was beneficial to nicotine accumulation most, while50%nitrate-N/50%ammonium-N or30%manure/35%nitrate-N/35%ammonium-N was beneficial to control the nicotine content. After topping, the nicotine content of middle and upper leaves of FCT was the highest with100%ammonium-N while the lowest with50%nitrate-N/50%ammonium-N or30%manure/35%nitrate-N/35%ammonium-N. After topping, the increase amplitude of middle and upper leaves' nicotine content and the activity of PMT, ODC and ADC in roots was the lowest with50%nitrate-N/50%ammonium-N.3. The soil-pot experiment indicated that the N content in all parts of FCTleaves was increased along with the increase of nitrate nitrogen. The N contents in root, stem and leaf were the highest with the application of100%nitrate-N, and the lowest with100%ammonium-N. There was a better uptake of K for all leaves with100%nitrate-N, and30%manure/35%nitrate-N/35%ammonium-N than100%ammonium-N, and an obvious effect in dry matter accumulation in leaves with100%amide N, and30%manure/35%nitrate-N/35%ammonium-N. The worst effects for dry matter accumulation were with100%nitrate-N and100%ammonium-N.4. The soil pot research with15N isotope tracer found that the utilization of N fertilizer was highest with the application of100%nitrate-N and30%manure/35%nitrate-N/35%ammonium-N, up to38%, while lowest with100%ammonium-N, down to25%only. The N nutrition absorbed by all leaves in the whole N content was the highest with100%nitrate-N, middle with100%ammonium-N, and the lowest with30%manure/35%nitrate-N/35%ammonium-N.5. The activity of amylase (AMS), nitrate reductase (NR), sucrose invertase (INV) and glutamine synthetase (GS) was on a declining curve in leaves after topping. The enzyme activity of AMS and NR was the highest and lowest with the application of100%nitrate-N and100%ammonium-N individually before and after topping. The activity of GS was the highest with100%nitrate-N, and the lowest with50%ammonium-N and30%manure/35%nitrate-N/35%ammonium-N before and after topping. The activity of INV had a sharp reduction after topping with100%ammonium-N, and30%manure/35%nitrate-N/35%ammonium-N.6. The hydroponic experiments of FCT showed that the leaves'nicotine content and amplification was the highest with200μmol/L B treatment, and the leaves' nicotine content increased with the rise of B level before and after topping of variety Zhongyan100. However, for variety K326and Eyan1, the leaves'nicotine content and its amplification were the highest with10μmol/L B treatment. The change of level B had a certain influence on the activity of nicotine synthesis enzymes (PMT, ODC and ADC) for different varieties.7. The hydroponic experiment showed that along with the change of B level, the activity of NR, INV, and glutamine synthetase (GS), the chlorophyll content, and the net photosynthetic rate were in a rise curve to different varieties in leaves after topping, while the activity of AMS and the concentration of CO2in leaves intercellular were in a declining curve. In general, the influence of topping to those physical signs was heavier than the change of level B. 8. The accumulation of N, B and K was mainly in leaves of FCT. Zhongyan100, K326, Eyan1had a strong ability of N and B uptake, and the N and B content in leaves increased along with the increase of B level. However, the accumulation B in leaves of Eyan1was obviously lower than that of Zhongyan100and K326. The whole leaves'K to different varieties above-ground was the highest with B level of50μmol/L, and the lowest with B level of200μmol/L.9. The leaves' nicotine content had an obvious negative correlation with the utilization rate of N fertilizer, amylase, nitrate reductase and invertase activity, and an obvious positive correlation with PMT, ODC, ADC activity and CO2concentration in leaves intercellular in roots. The leaves' nicotine content also had a negative correlation with net photosynthetic rate and chlorophyll content, but not obviously.10. In field conditions, the research of application effect with several methods of the foliage conditioner was carried out. The demonstration of field experiment showed that the foliage conditioners in research had an effect in promoting the tobacco-plant growth, shortening the growing stage, strengthening the tobacco-plant's resisting performance, and improving the economic benefit and tobacco quality.
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