| China ranks the first place in the world in the cultivation area, total yields and volume of purchase of tobacco leaves. Though the quality of tobacco leaves has been improved in recent years, differences still exist from advanced tobacco producing countries. Based on the reality and current status of the production of tobacco leaves in China, to tackle the key problems related to the quality of tobacco leaves is in accordance with the essential demands for providing quality raw materials for nicotian industry, increasing the income of tobacco cultivating farmers and expanding export of tobacco products. The quality of tobacco leaves not only depends on the species (internal factor) but also on the growth environments, among them, soil plays a significant role. The contents of potassium and nicotine are the important quality indices that are influenced by the soil moisture and temperature. It is of importance for increasing the quality of tobacco leaves to probe the influences of hydrothermal conditions on the contents of potassium and nicotine. By using Lou soil occurring in central Shaanxi as test material, this paper studied systematically the effects of soil moisture and temperature on the transformation of soil potassium, plant growth, plant potassium absorption and nicotine accumulation of plant. Hydrothermal coupling effect was firstly analyzed and investigated by thermodynamic and kinetic methods. Models and dynamic equations which could reflect dynamic changes of related factors were established of the content of readily available potassium in soil, potassium contents in tobacco leaves and plants, nicotine content in the leaves and plants, biomass of the plants. The phenomenon of zero values was analyzed of hydrothermal coupling effect of the content of readily available potassium in soil and potassium and nicotine contents in leaves and plants and plant growth. Related zero value soil moisture of thermal effect and zero value temperature of hydro-effect were obtained. Active energy, active free energy and active enthalpy could be used as energy coefficients to characterize hydrothermal effect of soil potassium transformation, plant growth, and potassium absorption and nicotine accumulation of the plants. Optimum hydrothermal conditions were determined to increase the contents of potassium and nicotine in tobacco leaves in the soil tested. Different hydrothermal conditions were found in increasing; inhibiting and coordinating dually the transformation of soil potassium, plant potassium absorption and nicotine accumulation. The main results were as follows. 1. The transformation of soil potassium was related not only to hydrothermal conditions, but also to the hydrothermal coupling effect. Application of nitrogen and phosphorus could decrease the coupling effect. Zero value soil moisture of thermal effect of soil potassium transformation was obtained form the established hydrothermal coupling effect of soil readily available potassium: 9.85 % in nitrogen and phosphorus treated soil and 8.13 % in untreated soil; zero value soil temperature of hydro-effect of potassium transformation: 6.46℃in nitrogen and phosphorus treated soil and 7.25℃in untreated soil. There existed significant differences in the contents of readily available potassium in the tobacco planted soil and unplanted soil. The characteristic hydrothermal curve of the content of readily available potassium appeared as "∧"shape with peak value. The moisture content corresponding to the peak values in the isotherm curve was 18 %, and the temperature corresponding to the peak values in the iso-moisture curve was 25℃. The hydro-effect of soil readily available potassium was strong at the peak temperature value of 25℃, but weak at higher soil moisture content (22 %, 25 %). From the equation of hydrothermal coupling effect, it could calculate that the zero value temperature of thermal effect of soil readily available potassium was 25.78℃, and the zero value soil moisture of hydro-effect was 17.84 %, close to real measured ones, indicating the adaptability of hydrothermal model of the content of soil readily available potassium. 2. Under certain conditions, the hydrothermal coupling effect of potassium absorption in tobacco leaves (plants) appeared fair facilitating effect; however, under higher temperature and moisture, the hydrothermal coupling effect showed inhibition to plant potassium absorption, appearing a double water and temperature stress. One of the reasons for the inhibition of high temperature (30~35℃) was the decrease of hydro-effect of plant potassium absorption. The optimum soil moisture and temperature were 18 % and 25℃. These hydrothermal conditions could not only meet the demands for the plant growth and potassium absorption, but also better hydro-thermal effect of potassium absorption appeared under these conditions. Similar results of optimum moisture and temperature were also obtained from the hydrothermal coupling model of potassium absorption in tobacco leaves and plants (18.46 %, 25.07℃), indicating the adaptability of the model. The zero value temperature of hydro-effect of plant potassium absorption was 4.16℃, and the zero value moisture of thermal effect was 8.97 %, which were close to "zero life temperature"(4.4℃)and the range of soil wilting moisture (8.6 %~10.5 %), indicating the rationality of the zero value coefficients. 3. Under favorable hydrothermal conditions, hydrothermal coupling effect couldaccelerate plant nicotine accumulation, under unfavorable hydrothermal conditions, nicotine content in tobacco leaves also increased by hydrothermal enhancement. The optimum hydrothermal conditions for nicotine accumulation were 18 % (W), 30℃(T)respectively from the model of hydrothermal coupling, indicating the adaptability of the model. Zero value temperature of hydro-effect of nicotine content in plant was 5.58℃, zero value soil moisture of thermal effect was 10.05 %, close to "zero life temperature"(4.4℃)and the range of soil wilting moisture (8.6 %~10.5 %), indicating the rationality of the zero value coefficients. 4. The potassium transformation in tobacco unplanted soil was in accordance with Elovich kinetic model (Y = a + blnt), the coefficients a and b were related to soil moisture, temperature and hydrothermal coupling effect. The value of b in nitrogen and potassium treated soil was higher than that in untreated soil. Kinetic model of potassium transformation was established including water, temperature and hydrothermal coupling coefficients. The characteristic of dynamic changes of potassium transformation in tobacco planted soil was in accordance with the multinomial equation Y = ax3+bx2+cx+d. Values of dY/dt and d2Y/dt2 were all negative, indicating that the changing rate of readily available potassium in soil as well as the changing rate along with the change of time had tendencies of declining, in accordance with changing patterns in the curve of experimental data, and also indicating the feasibility of using multinomial equation to characterize dynamic changes of soil potassium transformation. 5. Power function Equation lnM = lna + bln(tM = atb)could be used to describe dynamic changes of biomass of tobacco plants. The coefficient b presented the apparent increasing rate of biomass of tobacco plants, which increased along with the increase of moisture within the moisture range of tested soil, and increased with the increase of temperature within the temperature zone of 15~30℃. Over 30℃, such as 35℃, the increase rate of plant biomass decreased a little bit, but still higher than those in the temperature zone of 15~25℃, indicating that high temperature (35℃) was still favorable to the increase of plant biomass. During the vigorous growing period, increasing soil moisture could promote the increase of plant growth, meanwhile, under high moisture condition, higher temperature could also increase the plant growth, i.e., hydrothermal coupling had significant effect on the growth rate of tobacco plant. Hereby, the equation of dynamic accumulation of plant biomass was established including parameters of moisture, temperature, and hydrothermal coupling. 6. The analysis of the relationships between plant biomass accumulation and moisture, temperature as well as hydrothermal coupling effect, within tested moisture (14 %~25 %), the biomass of the plant increased along with the increase of soil moisture, in the temperature range of 15~30℃, the biomass increased with the increase of temperature. From thehydrothermal model of plant biomass, zero value temperature of hydro-effect of plant biomass was T0 = 9.62℃, and zero value soil moisture of thermal effect of plant biomass was W0=12.30%. 7. That the dynamic changes of potassium content in the leaves was in accordance with multinomial equation Q = at3+bt2+ct+d, and the values of dQ/dt and d2Q/dt2 were all negative, showing that the increase rate of potassium content in the leaves tended to decline along with the time change, matching the changing tendency of the curve obtained from the experiment, and indicating the adaptability of the multinomial equation. Under certain hydrothermal conditions, d2Q/dt2 (negative) increased along with the increase of culturing time. Especially, during the late growth period, the increase rate of the negative value of d2Q/dt2 increased rapidly, indicating that the increase of biomass growth had a "diluting effect"to the potassium content. In the temperature range of 15~25℃, however, the increase rate of plant biomass was slow, while the increase rate of potassium content in the leaves enhanced in some scopes. However, in the temperature range of 15~25℃, the increase rate of plant biomass was relatively slow, the increase rate of potassium in the leaves enhanced, "diluting effect"was inhibited, showing that the characteristic of dynamic changes of potassium content in the leaves mainly depends on the dynamic changes of biomass increase. But in the stage of slow increase rate of plant biomass, the rate of potassium absorption played an important role in the increase of potassium in the leaves. The characteristic dynamic changes of potassium content in plant could be described by a zero level equation q = a + bt, in which the values of a and b were related to soil moisture and temperature, a was negatively related to soil moisture and temperature, b was positively related to the soil moisture and temperature. Dynamic equation of potassium content in plant was established including factors of water or temperature. Furthermore, dynamic equation of hydrothermal coupling of potassium accumulation was also established, including the factors of water, temperature and hydrothermal coupling. 8. The changes of nicotine content in the leaves along with the change of time could be simulated by a zero level dynamic equation y = bt + a, in which a and b were related to soil moisture and temperature. From this equation, the dynamic equation of potassium content in the leaves was established, including water factor or temperature factor, and the dynamic equation of hydrothermal coupling of nicotine content, including factors of water, temperature and hydrothermal coupling. The characteristic of the dynamic changes of nicotine content was in accordance with the power function equation Y = atb. In the equation, a and b were related to soil water and temperature, the higher the soil water content, the higher the values of a and b. In the temperature range of 15~30℃, a and b increased with the increase of temperature. Hereby, the dynamic equation of hydrothermal coupling of nicotine content was establishedincluding factors of water, temperature and hydrothermal coupling. The results showed that 18 % moisture content and temperature of 30℃were the optimum hydrothermal conditions not only for nicotine accumulation, but also for obtaining the maximum increase rate of nicotine content. 9. To conclude the results of hydrothermal investigation: 25℃was the optimum temperature for the promotion of the content of readily available potassium in tobacco planted soil, of the contents of potassium in tobacco leaves and plants; 30℃was the optimum temperature for the promotion of tobacco plant biomass, of the contents of nicotine in the leaves and plants; 18 % was the optimum soil moisture content for the promotion of content of readily available potassium in tobacco planted soil, of the contents of potassium in the leaves and plants. Increase of temperature could enhance the hydro-effect of the content of readily available potassium in soil (15~35℃), of the potassium content in plants (15~25℃), of the nicotine content in plants (15~30℃), of the plant biomass (15~35℃). Increase of moisture could enhance thermal effects of the content of readily available potassium in soil (14 %~25 %) and of the plant biomass (14 %~25 %). The optimum temperature could promote the related hydro-effects, and the optimum moisture could promote the related thermal effect. When soil moisture was at the drought level (14 %), the thermal effect of potassium content in the leaves was strong, and the thermal effect of nicotine content in the leaves was stronger than when the soil water content was waterlogging (22 % ~ 25 %). Increase of moisture could inhibit the strength of thermal effects of potassium content in the leaves (14%~25%), of the nicotine content in the leaves (18 % ~ 25 %). The ability of the plant potassium absorption decreased by 19 %, and the ability of nicotine accumulation decreased by 6 % in waterlogging soil (22 % ~ 25 %). High temperature (30℃,35℃) and moisture (22 % ~ 25 %) could significantly inhibit the ability of plant potassium absorption, appearing a double stress. 10. Studies showed on energy characteristics of the hydrothermal effects of potassium transformation in soil, plant growth, plant potassium absorption and nicotine accumulation by Arrhenius and Transition State theory that active energy (E), free active energy (ΔG≠) and active enthalpy (ΔH≠) could be used as characteristic energy coefficients. The results of the studies of the regular changes of ΔG≠和ΔH≠under different hydrothermal conditions evidenced the influence of hydrothermal effect. Based on the comparison of E andΔH≠, an order was given: plant potassium absorption > nicotine accumulation of plants > plant growth. Though the value ofΔG≠was relatively small, in the same temperature zone, the potassium absorption rate of plant was lower according to ΔG≠/T value, all these values correspondedto the physiological processes of the plants, indicating the feasibility of using active energy, active thermodynamic coefficients to characterize the some features of the plants during their growth from the point of energy.
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