【作者】 姚立民；

【导师】 康绍忠；

【作者基本信息】 西北农林科技大学 ， 农业水土工程， 2011， 博士

【摘要】 目前,绝大多数宏观根系吸水模型,均以不同方式考虑了土壤水分对根系吸水的影响。然而,在研究旱生、盐生植物以及土地盐碱化等问题时,还需要考虑土壤盐分对根系吸水的影响。现有考虑水盐胁迫的宏观根系吸水模型,主要是在Feddes(1976)模型的基础上,经验性地考虑盐分的影响。这类模型形式简单,但动力学机制不强;由于这类模型的参数受多种因素影响,参数值往往随率定情景而变,因此局限性较大。根系吸水模型的进一步发展,需要整合更多的根系动力学函数。本论文的目的,是通过科学试验,在了解不同水盐胁迫对根系导水率和根系吸水影响规律的基础上,对根系吸水机理进行深入分析,然后根据水动力学原理建立水盐胁迫下的根系吸水动力学模型。该模型的参数选择及参数组合均基于水分运动的物理规律,能够比较全面地反映根系吸水的影响因素和深入揭示根系吸水的影响机理,从而比较真实地反映根系吸水过程,提高根系吸水条件下土壤水分的预报精度,并为其他根系吸水模型的研究提供借鉴。根系导水率是反映根系吸水能力的水力特性参数,该参数在SPAC系统水分传输研究中具有重要应用价值,它能够深入反映土壤环境因素对根系吸水的影响机理。本论文以根系导水率为切入点,通过盆栽试验研究,定量研究幼龄梭梭根系导水率对土壤水盐胁迫的响应规律,获得幼龄梭梭根系导水率对土壤水盐胁迫的动态响应模型。然后,在对根系吸水机理深入分析的基础上,以水动力学原理为依据,通过数学推导获得水、盐胁迫环境下的根系吸水动力学模型,并通过田间试验和数值模拟技术,对新建立的根系吸水动力学模型进行模拟验证。通过理论分析和田间试验,主要获得以下成果:(1)获得了梭梭根系导水率对土壤水盐胁迫的动态响应模型根系导水率是反映根系吸水能力的动力学参数,受多种环境因素影响,并通过根系内部一系列生理生化变化对环境胁迫做出响应。研究表明,土壤水分、盐分等均会引起根系解剖结构和水通道蛋白活性等变化,这些变化最终会引起根系导水率的变化。因此,根系导水率可以反映植物根系吸水不是纯粹的物理过程,而是有植物生理活动参与的复杂过程。试验结果表明,幼龄梭梭根系单位面积导水率随土壤相对有效含水量呈先增后减的抛物线规律变化;随土壤盐溶液浓度呈抛物线规律降低;随萌芽后天数呈线性规律降低;幼龄梭梭的根系总导水率随土壤相对有效含水量呈线性规律增加;随土壤盐溶液浓度呈抛物线规律降低;随萌芽后天数呈线性规律增加。水盐胁迫下根区水分运动的数值结果表明,本文获得的根系导水率对土壤水盐胁迫的动态响应模型,能够正确反映幼龄梭梭根系导水率对土壤水盐胁迫的响应规律。(2)获得了根茎交汇点水势的计算公式根茎交汇点水势(h_c)是影响根系吸水驱动力的主要因素之一。通常h_c远低于土壤水势(本论文中h_c在0.43-1.53 MPa之间变化,而土壤水势在0.16-0.34 MPa之间变化)。影响h_c的主要因素包括实际蒸腾速率、根区平均土壤水势和根系总导水率等。由于土壤水势和根系总导水率的变化一般比较缓慢,不能自由、快速地调整,因此当蒸腾速率增大时,只能通过迅速降低h_c来增大水流驱动力,从而满足植株蒸腾的需要。然而,植物木质部的生理特性决定了h_c不能无限降低。因为当木质部负压达到一定程度时(负压程度因植物种类而异),会诱导产生越来越严重的木质部栓塞,阻碍水分流动,大大削弱植株蒸腾速率,从而限制h_c进一步降低。通常植株蒸腾速率越大,则h_c越低,根系吸水的驱动力越大。(3)建立了土壤水盐胁迫下的根系吸水动力学模型该模型是在忽略根内轴向水分传输阻力的基础上,通过水分运动的物理规律来描述根系吸水过程,不仅合理考虑了土壤水、盐胁迫对根系吸水的影响,而且克服了根系吸水数值模拟模型需要繁杂的根系径向导水率、轴向导水率、根内边界条件和根内初始条件的缺点,因此使用更方便,实用性更强。由于该模型同时反映了根系吸水的动力学原理和根系导水率对环境因素的响应规律,因此,在研究根系水力再分配、根水倒流、根系夜间吸水等问题中,具有较好的应用前景。水盐胁迫下根区水分运动的数值模拟结果表明,本文建立的根系吸水动力学模型优于目前使用的F-VG模型。更多还原

【Abstract】 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.更多还原