| The economic benefit of tobacco(Nicotiana tabacum L.) production depends on not only tobacco production costs, but also tobacco quality. Tobacco is a potassium (K)-loving crop. In all its essential nutrients, K is recognized as the "quality element", and thus a key factor in tobacco quality. In this experiment, a soil pot experiment was carried out to study the effects of two K nutrition levels (0.24 g/kg,0.96 g/kg) on nicotine content, nicotine synthesis as well as nutrient accumulation and distribution during the topping stage of flue-cured tobacco (N. tabacum L. K326). Meanwhile, the characterization of arginine decarboxylase (ADC) and ornithine decarboxylase (ODC) of two species of tobacco (Eyan 1 and K326) was analyzed in vitro. The main results were as follows:1. Potassium significantly reduced nicotine content in shoot of flue-cured tobacco. Compared to low-K treatment, high-K treatment decreased nicotine content of upper leaves, middle leaves, lower leaves, phloem and xylem respectively by 20.0%～44.4%, 30.6%～41.4%,22.2%～39.5%,8.99%～35.3% and 8.44%～40.1% respectively. The topping increased the nicotine content of tobacco. Compared to the control (no topping), the nicotine content in upper leaves, middle leaves, lower leaves, phloem and xylem increased by 129%,10.9%,7.76%,64.3% and 25.4% respectively at 7d after topping.2. High K application decreased the activity of ADC and ODC in roots of flue-cured tobacco. Compared to low-K treatment, ADC and ODC activity of high-K decreased by 4.40%～21.3% and 10.9%～28.8% respectively, in which ODC activity was reduced even more apparent. However, topping increased the activity of ADC and ODC. Compared with the no-topping, ADC and ODC activity increased by 52.4% and 45.9% for high-K, and 47.4% and 23.2% for low-K respectively.3. Different K nutrition had no significant effect on reducing sugar content of upper leaves, middle leaves, lower leaves and phloem of flue-cured tobacco, and compared with the low-K treatment. high-K application significantly increased reducing sugar content by 8.86% in xylem. Before topping, high K nutrition improved the accumulation of reducing sugar in upper leaves most obvious, being 29.8%. But after topping, reducing sugar content had a downward trend in different parts of flue-cured tobacco. Compared with no topping, reducing sugar content in different parts of flue-cured tobacco decreased by 0.008%-0.013%,11.0%-61.9%,0%-62.0%,37.4%-43.4% and 32.7%-41.9% and 21.9%-26.7%,5.8%-20.5%,19.3%-64.6%,17.6%-30.8% and 14.8%-24.0% at 7d after topping for low-K. Results indicate that high K nutrition was more beneficial to the increasing of reducing sugar in the phloem and xylem with time, which were the parts of the transport site of photosynthate, indicating that high K nutrition be more conducive to the transport of photosynthate.4. Potassium significantly reduced N content in shoot of flue-cured tobacco. Before topping, N content in upper leaves, middle leaves, lower leaves, phloem and xylem in high-K treatment decreased by 7.77%-22.1%,19.7%-48.7%,5.34%-40.5%, 5.26%-30.6% and 5.57%-22.0% respectively, and at 7d after topping, N content in upper leaves, middle leaves, lower leaves decreased by 9.27%,10.6% and 3.22% for low-K treatment, and by 10.9%,12.4% and 1.21% for high-K treatments respectively.Potassium application significantly decreased P content in shoot of flue-cured tobacco. Compared with low-K treatment, P content in middle leaves, lower leaves, phloem, xylem decreased by 4.00%-37.9%,14.3%-41.2%,0.49%-27.5% and 21.8%-60.7% respectively. After topping, the P content in only upper leaves gradually decreased with time, by 0%-24.4% for high-K,6.82%-18.2% for low-K.Potassium application significantly increased K content in shoot of flue-cured tobacco. Compared with low-K treatment, upper leaves, middle leaves, lower leaves, phloem, xylem increased by 2.85%-23.2%,8.89%-51.6%,31.3%-66.7%,25.8%-65.1% and 34.7%-70.1% respectively for high-K treatment. Potassium content during topping had no significant change.5. Under the condition of this study, ODC from Eyanl and K326 had optimum pH of 6.0 and 7.4 respectively, and ADC of 8.0 and 7.3 respectively. The optimum reaction concentration of substrate for ODC and ADC was 50 mmol/L. The favorable reaction temperature of two species of ODC and ADC was 35℃～40℃. In addition, the activity of ODC remained more than 80% activity upon the exposed temperatures of 50℃for 30 min, and ADC more than 70%, which suggested that the stability of ODC and ADC be high.Activity of ODC in Eyanl had a slight activation effect under 0.1 mg/L of As (V), the additions of As (Ⅲ) and As (V) of>0.1 mg/L suppressed significantly ODC and ADC activities in vitro in this study. The differences in responses of heavy metal stresses existed in ODC and ADC derived from different plants. The activities of ODC and ADC in K326 were most heavily inhibited by As(Ⅲ) with the decreases by 33.25%～55.21% and 14.94%-68.86% respectively as compared with their controls, wheareas by As (V) with the decreases by 45.78%～52.29% and 25.32%～40.32% respectively. |
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