| Plants has great impact on environmental quality, and always be referred as temperature regulator and air purifier to ecological system. During all the growing time,they do heat and mass transfer with surrounding, such as atmosphere and soil. All three of this make up a SPAC system, that is Soil-Plant-Atmosphere Continuum. While plants do as a heat pipe. Explore the hydrothermal coupling migration in root-soil system under different environmental conditions will do have a great importance on reveal the process of heat and mass transfer in plant heat pipe, as while as analysis the cold-resistance properties of hardy plant and quantitatively calculate the influence of plant on temperature regulation and purification.Common Camellia roots were selected as the main research object in this article.The main physical parameters of Common Camellia roots and soil were measured in different seasons by setting up the artificial climate chamber with an open top whose parameters can be adjustable, as well as the temperature distribution and sap flow velocity of stem of Common Camellia in different seasons. Based on the hydrothermal coupling migration of double drive model in soil of Philip and De Vries, the mathematical model of hydrothermal coupling migration between root and soil system was established by considering the influence of temperature on the moisture migration,as well as the modified Feddes root water absorption model and heat and mass transfer model in root system. In addition, the physical model of the system was also established by adopting image segmentation technology.The temperature and sap flow field between Common Camellia root and soil system under different working conditions were numerically calculated by using Hydrus microcosmic finite element method and mesoscopic model of lattice Boltzmann. And then the process ofhydrothermal coupling migration in system was analyzed. On the basis of plant physiological ecology, the cold-resistance properties of hardy plants could be analyzed from the direction of thermal response. Main conclusions as below:①Density of different parts of Common Camellia root is less than that of water,as a whole, it shows that the density in the main root > lateral root > root hair. Density differs a lot in each season, which is higher in summer than winter. When the moisture content is constant, both the specific heat capacity and thermal conductivity of Common Camellia root exist a saltation phenomenon within a small temperature difference around 0℃, while the hydraulic conductance reach its peak at 30℃.②The soil used in this experiment is mesoporous medium with the porosity rate of 43.18%, which is calculated by the soil adsorption-stripping isotherm from BET specific area and porosity analyzer. The moisture content under different depth of soil bed was measured by drying method, which decreases with the increase of soil depth.Meanwhile, the specific heat capacity and thermal conductivity are slightly linear increases while the temperature rises.③ Temperature in Common Camellia root-soil system maintains in the same variation trend with the atmospheric temperature, but with an obvious hysteresis. The main factors influencing the root temperature are different in each season. The experimental results show that the root temperature have a good correlation with the temperature of soil in summer, while both the atmospheric and soil temperature affect the root temperature in winter. Root temperature depends linearly on soil temperatures.④ When increase soil temperature in summer, the sap flow makes the root temperature change greatly due to the exuberant metabolism of roots, while change slowly in winter because of weak metabolism and great thermal resistance. It generally reflects the effect of physiological metabolism on temperature distribution of soil.Besides, the temperature inside Common Camellia root is higher than that of the surrounding soil, confirming the hardy plants are still have a physiological mechanism in cold winter. When the temperature difference between soot and soil is small, while obvious deviation from atmosphere temperature, the surrounding soil behave the same daily temperature change trend as that of root, verifying the plants, as intermediate connecting body of thermal and mass transfer in SPAC system, slightly effect the surrounding environment.⑤Sap flow rate of Common Camellia share the same daily change rule with lightintensity and atmospheric temperature curve, but with a hysteresis effect of 1~3h. The sap flow rate of Common Camellia gets influenced by light intensity, atmospheric temperature and soil temperature differently in different seasons. In summer, the impact decrease in turn as the atmospheric temperature, light intensity and soil temperature, while soil temperature plays the main control role in the period of soil freezing in winter and soil thawing in spring. And the sap flow of Common Camellia has “single-peak” relationship with soil temperature.⑥Root distribution characteristics and physiological metabolism have significant influence on the heat and mass transfer in soil. The moisture migration rate is speed up with the fact of root-water-uptake, and the speed changes greatly with the increase of root density. Temperature hysteresis area occurs in the soil at the bottom of the center of the root, which has a relationship with powder structure, distribution density and geotropism growth properties of Common Camellia root.⑦ When cooling soil in summer, the high-temperature region locates in the external border of root system, for vigorous water absorption and more heat of respiratory metabolism. While the high-temperature region locates in the center of main root when heating the soil in winter, for secreting more cold protection material.There is a phenomenon contrary to heat transfer theory appearing in the root tissue of Common Camellia on account of physiological metabolism.⑧Temperature field and sap flow field in Common Camellia root and soil system have the same tendency in different seasons. While the temperature gradient and water potential gradient are more uniform in winter compared with the results of summer.The closer to the root, the faster decline of soil moisture, especially the area with high-density root. Meanwhile, the rate of root water absorption increase with the rise of soil temperature, and the time that relative water content of root reaching 0.25 slightly shortened.⑨ Thermophystic parameters in hardy plants mutate in the sap phase change temperature. And the stronger the cold tolerance, the greater peak values of thermophystic parameters. In winter, the daily average temperature of hardy plant roots is higher than that of non-hardy. The root temperature of sap flow area is deviated from the surrounding area significantly in radial direction because of sap flow.⑩Hardy root-soil system hydrothermal coupling migration patterns: The degree of hydrothermal coupling migration is in the range of 0.25~0.5, and the degree inwinter is larger than that in summer, verifying the soil temperature plays the main controlling effect on the sap flow rate in winter. The degree of hydrothermal coupling migration is negatively related with system temperature and positively related with sap flow rate. The lower the temperature, the higher the flow rate and degree of coupling.11Cold-resistance properties of hardy plants can be described as follows. The soil belongs to low-temperature heat sources in summer. And the sap flow and temperature gradient are in opposite direction. The sap flow with low temperature reduces the whole plant temperature, avoiding scorching the plants. While in winter, the soil is high-temperature heat sources, the sap flow transfer from the roots to the canopy with high temperature, protecting the tissue in higher level. When the temperature gradient and water potential gradient are in the same direction, the sap flow rate increased with the increase of temperature gradient of soil, with reducing coupling degree of hydrothermal migration. At this time, both the physiological metabolism and cold-resistance are higher. By contrary, when the gradients are opposite, coupling degree of hydrothermal migration will enlarge with the increase of temperature gradient of system, and little decrease of temperature, more debase of sap flow rate.The sap flow rate decreased with the increase of temperature gradient with poor cold-resistance.This paper does the basic research of hydrothermal coupling migration in root-soil system, based on the porous medium heat transfer and moisture migration analysis, as while as the principle of cold-resistance, through the theoretical analysis, experimental research and numerical simulation. Ideal results were obtained. This is of significant theoretical importance to the study of cold-resistance properties of hardy plant and the development of plant physiology, and in addition, it is also important for scientifically cultivating species resistant to cold that is conducive to the ecological environment and systematic analysis of the global water and energy balance. |
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