| Increasing the usability of upper leaves of flue-cured tobacco has been continuously studied by tobacco agriculture and industry. There were still some problems in upper leaf of tobacco, for example, the tissue structure was close, variegated color was heavier, the nicotine content of upper leaf was higher, the remaining starch was too much, the intrinsic chemical components was maladjustment, the irritation was heavier, the usability was low. Studying how to improve the usability of tobacco was very much significant to improve the tobacco overall quality,the structure of grade and the economic efficiency too. The changes of physiology and biochemical characteristics of upper leaf of flue-cured tobacco during the process of curing were studied in the paper. The physiological mechanism of curing with stem was conducted. The results showed that:1. The total water loss in tobacco leaves with stalk was low in the first 48 hours and then decomposed sharply while the bound water decreased gradually with the proceeding of curing. The peak of free water loss came earlier than the total water. The free water increased a little from 24 to 48 hours. The ratio of total water loss in DJ(Treatment 1, upper leaf harvested and cured with stalk) was significantly lower than that in WJ(Treatment 2, upper leaf harvested and cured without stalk) between 24h and 72h, which showed that the free water from the stalk transferred to the leaf. The tobacco leaves with stalk had much more water in tissue structure after the yellowing stage, which made the changes of physiology and biochemistry maintain the long time and was benefit for the transformation of the organic material.2. The ratio of degradation of chlorophyll in DJ was higher than that in WJ during the early of yellowing. The ratio of chlorophyll and carotene in DJ was also higher than that in WJ between 36h and 60h, which showed that the leaves with stalk yellowed a little faster. But the leaves with stalk had a longer yellowing time because it had much more water. The activities of amylase had obvious difference during the curing. The activities of amylase in DJ kept gradually increasing before the color-fixing stage and were higher than that in WJ in the whole curing. The activities of PPO in DJ was lower than that in WJ, so the enzymatic browning reaction of DJ was a little lower and the color of the leaves was much better. The activities of SOD in DJ were higher than that in WJ, but the activities of POD and the content of MDA were opposite. This showed that the protection of SOD made a leading role and the action of membrane lipid peroxidation inside the leaves with stalk was lower, which was benefit for the formation and accumulation of material basis of aroma. The content of free praline was also lower than that of WJ, which showed that the stress of DJ was a lower degree.3. The changing of the content of starch of DJ was opposite to the contents of the total sugar and the reducing sugar. But the total sugar was the same as the reducing sugar. The total sugar and reducing sugar in DJ and WJ increased quickly in the early curing and then decreased a little. The content of starch in DJ was lower than that in WJ during the whole process. But they were lower than those in WJ. This showed curing with stalk was benefited for the metabolism of carbon compounds. The contents of total nitrogen and nicotine had no obvious change during the curing. The contents of nitrate and nitrite of DJ and WJ increased quickly in the first 72 hours and then decreased rapidly, which was more than that of the flesh tobacco leaves. The content of nitrate in DJ was a little lower than that in WJ, but the nitrite was opposite, which indicated that curing with stalk was benefit for the metabolism of the nitrate. 4.The contents of potassium and chlorine of DJ and WJ had a increasing tendency but the potassium had a little decreasing in 48～72h and the chlorine in the late curing stage, which of DJ were more than that of WJ and the rate of increase was 9.3% and 15.9%.The experiment of injection of different concentration of KCL solution showed that the ions of potassium and chloride were transported between the stalk and leaves during curing process.5. The contents of MDA of DJ and water ratio were negative-correlation at 5%, which showed that with the water gradually lost, the leaves was gradually aging under conditions of stress, the membrane was damaged by the increasing extent, the permeability of membrane increased gradually, which increased the membrane lipid peroxidation. The correlation between water and PPO was positive significant, which showed that with the water gradually lost the activities of PPO gradually decreased. Therefore reasonably regulating of the baking temperature and humidity environment, appropriately controlling the rate of water loss, the activity of PPO could be effectively restrained in the prevention of tobacco browning.The contents of the total sugar and the reducing sugar were positive-correlation at 1%. The contents of the starch and the total sugar or the reducing sugar were negative-correlation at 1% or 5%, which indicated that there were complex relations of transformation among them.6. The apparent quality of cured tobacco of DJ was better than that of WJ. For example, the tissue structure was loose, the status was medium, the oily was rich, the color of intensity was strong, the proportion of orange tobacco leaves was higher, variegated leaves was few. The curing with stalk could reduce the weight per leaf, the weight per unit leaf area and the thickness of leaf and increase the filling power of tobacco leaf. The equilibrium moisture contents between DJ and WJ were on obvious difference. The contents of the total sugar,the reducing sugar,starch,the ratio of total nitrogen and nicotine and the ratio of total sugar and nicotine were lower than those of WJ. The contents of the nicotine,potassium and chlorine of DJ were a little more than those of WJ, but they were still in the suitable range. The contents of the total nitrogen and the ratio of potassium and chlorine had no obvious difference. The smoking quality of the two treatments had on significant difference. The curing with stalk could improve the proportion of high and middle grad tobacco and increase the average leaf prices, which could significantly increase the economic benefits.In general, the changes of the physiological and biochemical characteristics of the curing of upper tobacco leaves with stalk were benefit for the formation of quality. The apparent quality was improved, the contents and the ratio of the interior chemical matters inside the leaves were appropriate, the contents of starch,the weight of per leaf and the thickness were decreased, the usability and the economic characters were improved. Therefore, the harvesting and curing of upper leaf with stalk had good value for extension and application. But this was a new technology, how to grasp the harvest time and maturity and optimize the curing process and the impact of energy-saving and cost all needed to continuously explore and perfect. |
