| Now most of root water uptake models are about the effect of soil water, however, the effects of both soil water and salinity stress should also be taken into account in the models when investigating xerophil, halophyte and soil salinization. Current models for root water uptake under soil water and salinity stress mainly considered the effect of soil salt stress empirically on the basis of Feddes (1976)model. This kind of models have simple form but less dynamic principle, thus they need further improvement to take more dynamic root functions into account.The aim of this paper is going deeply into the root water uptake mechanisms through scientific investigation on the basis of understanding the effect of soil water and salinity stresses on root water conductivity and root water uptake, and then deducing a root water uptake dynamic model under soil water and salinity stresses according to water dynamic principles. The parameters selection and combination of this model are all based on the physical law of water movement. These parameters can better reflect the influencing factors of root water uptake and reveal the influencing mechanisms of root water uptake, and factually reflect root water uptake process, improve the accuracy of soil moisture prediction in the root zone, and give references to the study of root water uptake model.Root hydraulic conductance is a hydraulic characteristic parameter which can reflects the ability of root water uptake. It has important significance for the research of SPAC water transfer, and can reflect the mechanisms of soil factors influencing root water uptake. This research start from root hydraulic conductance. We quantificationally investigated the response of root hydraulic conductance to soil water and salinity stresses using pot plant, and obtained the dynamic response model for root hydraulic conductance to soil water and salinity stresses. Then, based on the analysis root water uptake mechanisms, according to hydrodynamic principles, we obtained a root water uptake dynamic model under soil water and salinity stresses. Finally, the newly established root water uptake dynamic model was tested through field investigation and numerical simulation.The major outcomes of this study were obtained through mathematical analysis and field investigation.(1)The dynamic response models for root hydraulic conductance of Haloxylon ammodendron sapling to soil water and salinity stresses was obtained Root hydraulic conductance is a parameter which can reflect the ability of root water uptake. When the root hydraulic conductance was influenced by environmental factors, a series of physiological and biochemical changes in the roots occur as responses to environmental stress. Several factors such as soil water, salinity and nutrients lead to changes of root anatomical structure and aquaporin activity, and these changes are the intrinsic causes for the change of root hydraulic conductance. Thus the root hydraulic conductance can reflect that root water uptake is not a mere physical process but a complex process with plant physiological activity involved.The investigation showed that root hydraulic conductance per unit root surface area of Haloxylon ammodendron sapling increased at first and then decreased according to parabolic curve with relative soil water availability; decreased according to parabolic curve with soil salt concentration; decreased linearly with the days after budbreak. The entire root hydraulic conductance of a Haloxylon ammodendron sapling increased linearly with relative soil water availability; decreased according to parabolic curve with soil salt concentration; increased linearly with the days after budbreak.(2)The expressions of root-stem junction water potential was obtainedWater potential at root-stem junction (h_c)is the main contributor to the driving force for root water uptake. Generally, h_c is far lower than soil water potential (h_c varies between 0.43-1.53 MPa and soil water potential varies between 0.16-0.34 MPa in this study). The main factors influencing h_c include actual transpiration rate, soil water potential in the root-zones and total root hydraulic conductivity etc. Because the soil water potential and the total root hydraulic conductance change slowly and they cannot be regulated freely and quickly, the only means to satisfy transpiration demand is to lower h_c rapidly and increase the water driving force when transpiration rate increased. However, the physiological properties of xylem can further prevent the decline of h_c. This is because when the negative pressure of the xylem reaches a certain value (the negative pressure of the xylem varied with plant species), the induced xylem embolism can block water flow in xylem conduit, and then greatly reduces transpiration rate and further prevents the decline of h_c. In general, when plant transpiration is stronger, the water potential at root-stem junction is lower, thus the driving force of root water uptake is much higher.(3)A dynamic model of root water uptake for Haloxylon ammodendron sapling under soil water and salinity stresses was establishedThis model describes the root water uptake process using physical laws of water movement on the basis of ignoring axial water transfer resistance in root, not only correctly thought over the influences of soil water and salinity stresses on root water uptake, but also overcomed the shortcoming of root water uptake numerical simulation models which need multifarious root radial conductivity, axial conductivity, boundary condition and initial condition in root. So it is more convenient and practical for use. Due to the model reflected root water uptake dynamic principle and the response rule of root hydraulic conductivity to environmental factors, so it has prosprous future in studying root hydraulic redistribution, root reverse flow, and nocturnal root water uptake. The results of water transfer numerical simulation in root zone under water and salinity stresses showed that the newly established dynamic root water uptake model is better than F-VG model. |
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